New Food Engineering

NEW FOOD ENGINEERING RESEARCH TRENDS

NEW FOOD ENGINEERING RESEARCH TRENDS

ALAN P. URWAYE EDITOR

Nova Science Publishers, Inc. New York

Copyright © 2008 by Nova Science Publishers, Inc.

All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA New food engineering research trends / Alan P. Urwaye, editor. p. cm. Includes index. ISBN-13: 978-1-60692-828-8 1. Food industry and trade--Research. I. Urwaye, Alan P. TP370.8.N49 2007 664--dc22 2007028954

Published by Nova Science Publishers, Inc.

New York

CONTENTS Preface

vii

Chapter 1

Ionizing Irradiation of Foods Albert Ibarz

Chapter 2

Fruits and Vegetables Dehydration in Tray Dryers Dionissios P. Margaris and Adrian-Gabriel Ghiaus

Chapter 3

Ultrasound in Fruit Processing Sueli Rodrigues and Fabiano A.N. Fernandes

Chapter 4

Optimisation of the Conversion of Ergosterol in Mushrooms to Vitamin D, and Its Bioavailability Conrad O. Perera and Viraj J. Jasinghe

Chapter 5

Chapter 6

Protein Hydrolysis with Enzyme Recycle by Membrane Ultrafiltration Antonio Guadix, Emilia M. Guadix and Carlos A. Prieto The Development of the Processing of Yuba (Protein-Lipid Film) Li Zaigui, Shen Qun and Lin Qing

Chapter 7

Far-Infrared Heating in Paddy Drying Process Naret Meeso

Chapter 8

A Novel Two-Stage Dynamic Packaging for Respiring Produce: Concepts and Mathematics Tobias Thiele and Benno Kunz

Index

1 45 103

137

169

195 225

257 271

PREFACE This new book presents new research in the growing field of food engineering which refers to the engineering aspects of food production and processing. Food engineering includes, but is not limited to, the application of agricultural engineering and chemical engineering principles to food materials. Genetic engineering of plants and animals is not normally the work of a food engineer. Food engineering is a very wide field of activities. Among its domain of knowledge and action are: Design of machinery and processes to produce foods Design and implementation of food safety and preservation measures in the production of foods Biotechnological processes of food production Choice and design of food packaging materials Quality control of food production Chapter 1 - Irradiation, like other types of food treatments, is a method used to make food safer for the consumer and to increase its useful life in good conditions. In this chapter the interaction of ionizing radiation with matter and the sources of production of ionizing radiation are described. The biological effects caused by this type of radiation are also described. Likewise, the application of ionizing radiation in the food industry is described as well as the effects that it has on most food components. The inhibitory effect on microorganisms is described, as well as the effects on different kinds of foods such as meat, poultry, fish and shellfish, eggs and egg-derived products, tubers and bulbs, seeds, legumes, dry fruits, spices, seasonings and herbs, and for quarantine treatment. Finally, a short description of food treatment plants, dosimeters and certain current normative aspects of the ionizing radiation used are given. Chapter 2 - Dehydration involves simultaneous transfer of heat, mass and momentum in which heat penetrates into the product and moisture is removed by evaporation into an unsaturated gas phase. Owing to the complexity of the process, no generalized theory currently exists to explain the mechanism of internal moisture movement. In this Chapter, the investigation of momentum, heat and mass transfer phenomena, in both laboratory and large scale convective drying systems (suitable for dehydration of thermolabile products) by means of experimental measurements and numerical simulation are presented. The air flow inside complex geometry spaces, such as drying rooms containing hundreds of trays arranged in rows and columns, is analyzed by solution of 3-D momentum turbulent flow equations for different room configurations. Laboratory measurement data, concerning the space velocity distribution and the pressure field of the air flow over one tray, are provided and used for validation of turbulence models. The results of the flow investigation

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lead to practical suggestions for the improvement of the air flow uniformity inside the drying space which is very important for the quality of the product. A novel numerical code, DrySAC, able to predict the unsteady-state processes taking place in a complex drying system, was developed. Unlike other attempts to predict drying processes, DrySAC takes into account not only the drying process itself, but also the behavior of the other system equipment and the interaction between them. Drying curves, evolution of the air state parameters in characteristic points of the system and product properties are predicted during the drying of various fruits and vegetables and. As a practical validation of the code, the predicted values compared with the measured data taken in-situ showed very good agreement. When a dryer configuration is given, the numerical DrySAC code can be used for optimization of the process parameters when a dryer configuration is given. For the most of the studied cases, an air recirculation ratio of around 75 % has proved to be the optimum, giving a minimum drying time. The code can be used both for evaluation of existing dryers and for optimum design of the new units with valuable impacts in increasing the efficiency of the systems and in reduction of energy consumption. Aiming to overcome the lack of experimental data in the open literature, a laboratory drying unit was constructed and is under operation for testing and monitoring the dehydration of agricultural products. Using this facility, experimental drying curves are set up for the drying of horticulture products under controlled conditions of the drying air parameters, which are gathered by means of a data acquisition system. The laboratory experimental results are useful for the validation of numerical models which further are an essential tool for optimization and increasing the efficiency of the drying process. Drying of agricultural products remains an open research field mainly because of their delicate and hard to be established, properties. Chapter 3 - Power ultrasound has been successfully employed in the chemical industry, polymer and plastic industry for many years and its use has been growing in the food industry. Power ultrasound can produce chemical, mechanical or physical effects on the processes or products where it is applied. Taking advantage of one of the effects or their combination, power ultrasound has been used in the food industry in drying, freezing, extraction processes and enzyme inactivation. The use of ultrasound in ambient fluids is well known to cause a number of physical effects (turbulence, particle agglomeration, microstreaming and biological cell rupture) as well as chemical effects (free radical formation). These effects arise mainly from the phenomenon known as cavitation. Herein a brief review of the use of ultrasound in the food industry is presented and the main applications are discussed. A comprehensive discussion on the effects of ultrasound in the tissue structure of fruits is presented along with photomicrographs of melons submitted to ultrasound. A detailed discussion is presented concerning the use of ultrasonic waves in drying, where an ultrasonic pre-treatment can be used prior to air-drying. The methods involved in the ultrasonic pre-treatment are presented along with the results obtained for several fruits such as melons, bananas, pineapples, papayas and other. Mathematical models that can be used to simulate the process are presented. Optimization of the drying process is also discussed for ultrasonic pre-treatment and ultrasound-assisted osmotic dehydration. Chapter 4 - The conversion of ergosterol in mushrooms to vitamin D2 by exposure to ultra violet (UV) light was studied under different UV lamps (UV-A, UV-B, and UV-C) and was found to be significantly different (p 20

X-rays; β rays; γ rays; electrons and positrons Protons Neutrons, according your energy α radiation; heavy nucleus

In the same way as with the absorbed dose, for the equivalent dose a historical unit known as the rem (“Roentgen equivalent man”) has been used, which is equivalent to 10-2 J for each kg of matter: 1 Sv = 1J/kg 1 Sv = 100 rem Another variable used in the measurement of radiations is the effective dose (E), which takes into account the risk of developing cancers or hereditary effects and is measured in Sv. This effective dose is a weighted sum of the average doses received by the different tissues and organs of the human body:

E = ∑ Fi · H i

(4)

i

where Hi is the equivalent dose for an organ i, while Fi is the weighting factor of this organ, its value depending on the organ considered. Furthermore, it is important to take into account the dose that a person can accumulate over time, and for that purpose the committed dose is determined, which is the dose accumulated over a certain period of time. To achieve a better understanding of the possible danger of radiation, it should be mentioned that a dose is lethal when its value exceeds 4 Sv, and that for doses of up to 0.25 Sv no harmful effects have been observed. By means of illustration, the radiation that can be received in an X-ray of the thorax shows a value of 0.02 mSv, for a CT head scan the value is 3 mSv, and the average annual dose per person in Spain is approximately 3.5 mSv, adding all the natural and artificial contributions, while the worldwide average is 2.5 mSv. The annual average dose received by the Spanish population due to the nuclear industry is in the order of 0.015 mSv, which is equivalent to what a person would receive when undertaking a 3-hour flight, due to cosmic radiation.

2.3. Sources of Ionizing Radiation The sources of ionizing radiation are not only artificial, as there are also natural sources. It is normal to find radioactivity and radiation in nature. The presence of ionizing radiation in our world and in the whole universe is normal, and as such a very important part of Earth’s natural radiation is due to cosmic radiation which is of extraneous origin. It is thought that

Ionizing Irradiation of Foods

11

every second in the order of 2x1018 particles of very high energy reach the Earth, most of which are protons (86%) and α particles (12%), in addition to neutrons and other particles. Cosmic radiation represents about 10% of the radiation that a person receives. The dose of radiation received in this way depends on the altitude, as the atmosphere absorbs part of it, which means that at higher altitudes the radiation is greater, hence when travelling by plane the radiation is more than that received at sea level. It is estimated that at an altitude of some 10,000 m, which is a normal altitude of transatlantic flights, a dose of some 5 mSv is received; at the summit of a mountain at 6,700m the dose is 1 mSv; in cities located at 2,000m the dose is 1 mSv; while at sea level it is usually in the order of 0.03 mSv (UNSCEAR, 1988). Moreover, this radiation is due to electrically charged particles and due to the Earth’s magnetic field they are diverted, so that in the equatorial zone less radiation is received than at the poles. Therefore, dose depends on both terrestrial latitude and longitude. In the case of Spain every hour people are traversed by 105 cosmic rays of neutrons and 4·105 secondary cosmic rays (CSN, 1992a,b). Furthermore, the cosmic radiation that reaches us from outer space can interact with different atmospheric components to produce radioactive substances. Cosmic radiation is, on average, about 10% of the total dose received. Most of the dose received is due to the radiation which comes from the Earth itself. This is due to the fact that in the subsoil there are large amounts of radioactive elements, such as uranium and thorium, among others. This radiation means that the whole planet is impregnated with radioactivity, including in the human body. It is estimated that every hour 2·108 of γ radiation is received from the soil. Due to this cause the average radioactive content in Spain of different materials is estimated at being 3,000 Bq for an adult human being, 1,000 Bq for 1 kilogram of coffee, and 25,000 Bq for 25kg of fertilizer. These figures are much higher for radioactive residues from medical and industrial applications. It is estimated that for 1 kg of low activity residues the activity is 106 Bq, for those of average activity it is 108 Bq, and for high activity residues it is 1013 Bq. Radiation which comes from the natural decay of uranium is important, as it provokes the appearance of the gas radon, which passes through cracks and pores in the soil, mixing with the air. The decay of radon produces radioactive compounds which remain attached to the particles of dust contained in the air and reach the lungs, with an estimated disintegration in every person every hour of some 30,000 atoms, with the emission of α and β particles and γ rays. Furthermore, it should also be borne in mind that natural radionuclides are also taken in with the ingestion of food. Noteworthy among these is 40K, which the human body contains to such an amount that it is estimated that every hour some 15·106 atoms disintegrate, emitting high energy β particles and in some cases producing γ rays (NRPB, 1986). Besides these natural sources there are also those which are due to the processes which man carries out in his medical and industrial applications, which could be called artificial sources. Of particular note is the ionizing radiation deriving from medical activity, due to its use in diagnosis and the treatment of diseases. This radiation covers the range from X-rays to nuclear radiation. But artificial radiation does not only come from medical applications, as in industry and in daily activity there are numerous examples, such as luminous watches, smoke detectors, radiation to define the structure of welding, and many other cases. The production of electrical energy in nuclear installations is another source of artificial ionizing radiation, although in thermal power stations the combustion of coal also produces

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natural radionuclides. In Spain the dose received due to this type of activity is less than 0.001 mSv, although the people who work in these power stations can receive greater doses which nevertheless do not reach 0.01 mSv per year. Finally, mention should also go to the radiation deriving from nuclear weapons testing and accidents like that of Chernobyl, which also contribute to the dose received by the human population. This is estimated at some 0.01 mSv per year.

3. BIOLOGICAL EFFECTS OF IONIZING RADIATION The biological effects that ionizing radiation produces depend on the type of interaction that occurs with the matter on which it acts. The absorption of radiation by living organisms depends on the kind and quantity of the radiation, as well as the structure and the kind of absorbing matter. Thus different kinds of effects may be shown, although in any case the incident radiation is an energy bearer, energy which is transferred to the absorbing medium either directly or indirectly, according to the mechanisms of excitation or ionization. When the absorbed radiation produces the effect of excitation of the matter’s atoms and molecules it can cause molecular changes if enough energy is absorbed, and if this is greater than that of the atomic bonds. If the ionization process is involved the effect is more important, as changes are always produced in the atoms, and it is capable of causing alterations in the structure of the molecules on which the radiation has fallen. The ionization induced in live tissues by exposure to radiation is usually quantified by the so-called lineal energy transfer (LET), which is the amount of energy yielded per unit distance travelled by the radiation in the tissue. Radiation is classified into two categories of high and low LET. α radiation and that of neutrons are considered to be of high LET, while X-rays and β and γ radiation are considered to be of low LET. Radiation produces different effects if the doses are high or low. For high doses of radiation the effects can be of two kinds, deterministic and stochastic. Deterministic effects are those that produce immediate effects, and show a minimum dose below which these effects do not occur, but appear immediately when the dose is greater than this minimum. Stochastic effects are those of delayed appearance and are probabilistic in nature, as is the case of cancer, which may develop some years after the exposure to the radiation. Also considered as stochastic are hereditary effects, due to genetic alterations, which appear in the descendents of the organisms which have received the dose of radiation. One thing to be borne in mind is that any organism is exposed to natural radiation, and so it is difficult to evaluate the effects of exposure to low doses of radiation, as these effects could be masked by the manifestation of conditions which could be considered normal, and which may not be due to the radiation received. For these low doses the possible effects can either be genetic or cancer. The biological effects of radiation can act at different levels, on cells, tissues or on whole organisms. The biological damage as well as the acceptable doses can vary greatly with each case. According to the molecular complexity of the living organisms, the biological effect is produced for different doses of radiation (Urbain, 1986). Thus, for mammals the lethal dose is ranged from 0.005 to 0.01 kGy, for humans this dose is in the order of 4 Gy; for insects it is from 10 to 1000 Gy, for plants it is 1 kGy. For the bacteria, the lethal dose depends on if they

Ionizing Irradiation of Foods

13

are in vegetative or sporulated form; thus, in their vegetative form the lethal dose is of 0.010.0 kGy, while for the sporulated forms it is ranged from 10 to 50 kGy. The most resistant organisms are the virus whose lethal dose is ranged from 10 to 200 kGy. This indicates that the greater the molecular complexity the smaller the dose required producing biological effects. Low overall doses are capable of killing a person, or else causing significant damage, while in order to completely destroy insects, larvae and eggs, the required doses are higher, and the doses needed to destroy bacteria, fungi and yeasts are much higher still. When a cell is irradiated the radiation may act directly on the genetic material or on the macromolecules, although it may also act indirectly on the water contained in the cell, or the ionized molecules may interact with the surrounding matter. There are considered to be three different stages in the overall process, a physical stage, a chemical stage and a biochemical stage. In the first physical stage the radiation interacts with the matter, which can excite or ionize its atoms, with characteristic times of 10-15 and 10-17 s, respectively. In the chemical stage free radicals are formed, with characteristic times in the order of 10-12 s. The last is a molecular or biochemical stage, in which the free radicals recombine and can form toxic molecules. The molecules formed by direct irradiation, or radicals, and those obtained indirectly, are known as radio-induced substances. These substances can be toxic or harmful to the cell. The presence of these substances causes the cell to set in motion the so-called cell repair mechanisms, giving rise to three different possible situations. If a very large amount of toxic substances has been produced the result is death. Otherwise the cell may survive although the harm caused to the genetic material is great and does not allow the cell to reproduce; this is called reproductive failure. If the amount of genetic damage is not excessive the cell can repair part of the genetic material, allowing it to reproduce, although in this case a delay in cell division is observed, enabling the transmission of mutations to subsequent generations. Therefore, in irradiated cells two types of damage are caused, the formation of toxic substances or genetic damage. From a survival viewpoint there may be important consequences, with the loss of tissue or organ functionality, the development of cancer, sterility problems, or the transmission of mutations to offspring. On penetrating tissues charged particles (α and β) lose energy by electric interaction with the electrons in the shells of the atoms which they strike. Due to indirect effects such as the photoelectric effect, the Compton effect and pair formation, when X and γ rays hit the tissues they end up freeing atomic electrons, which produces an end result of ionization. Due to the electric interaction of the charged particles an electron is split from the atom’s shell, which produces a positively charged atom. The separated electron can ionize other atoms. Generally, both the electron and the ionized atom are very unstable and react rapidly, giving rise to new molecules, some of which are highly reactive, being as they are free radicals. These radicals may react with each other and with other molecules, causing changes in molecules which are biologically important for cell functioning. Altogether the process lasts about one millionth of a second. The biological transformations which can occur in such a short interval may destroy or modify the cells, possibly giving rise to the appearance of genetic defects or cancer. In the case of food irradiation the problem is quite a different one, as it is necessary to evaluate the effects of irradiation from a food viewpoint. Foodstuffs are biological material and irradiation can cause different effects, such as the destruction of insects and microorganisms, the production of toxic substances, it may damage genetic material, or it may reduce the nutrient content of the foods. Of these four effects, on principle, the only desired

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one is the first. The appearance of toxic substances is not an effect which is unique to irradiation, as the appearance of this kind of substance has also been observed in chemical and thermal food treatments (Raventós, 2003; Satin, 2000; Molins, 2001). With regard to genetic damage, it should be pointed out that from the food viewpoint reproductive viability is not important, although care should be taken to avoid causing any problem for the consumer. The destruction of nutrients not only occurs in irradiation treatments, but is also observed in other kinds of food treatments, such as in thermal processes.

4. IONIZING RADIATION IN THE FOOD INDUSTRY Food irradiation is considered as the process of applying high energy to a food with the aim of pasteurising, sterilizing or prolonging its commercial life, eliminating micro-organisms and insects.

4.1. Types of Ionizing Radiation The sources of ionizing radiation which are applied in the food industry are X-rays, electron beams and γ radiation. Other kinds of radiation which have been used are ultraviolet rays (UV). UV rays are obtained using lamps which contain gases at different pressures, and are characterised by a low power of penetration, of only a few millimetres because their emission spectrum corresponds to rays with a wavelength considerably longer than X-rays. X-rays constitute a much more energetic electromagnetic radiation, for which their power of penetration is higher, showing a continuous spectrum of radiation with a maximum value of 5 MeV. X-rays are usually obtained by bombarding a metal plate with a high potential electronic beam (figure 3).

Figure 3. X Rays generation.

γ radiation is produced with radioactive isotopes, which in the food industry are normally the radioisotopes of 60Co and 137Cs. At present 60Co is the most commonly used for irradiating foods with γ radiation, as it is relatively straightforward to obtain and it produces

Ionizing Irradiation of Foods

15

radiation with a greater power of penetration than that of 137Cs. The energy spectrum of γ radiation is not continuous, but rather is discreet, and depends on the radioisotope used. The energy proceeding from 137Cs is 0.66 MeV, while that from 60Co is 1.17 and 1.33 MeV. 60Co is produced in a nuclear reactor bombarded with neutrons granules of 59Co highly refined, and in the decay process β and γ radiation is produced. 137Cs is obtained as a result of the fission of 235U, producing β and γ radiation (figure 4).

Figure 4. Radioactive disintegration of

60

Co and 137Cs.

The source of irradiation of the isotope 60Co is obtained from 59Co, which is compressed in cylindrical tablets which are placed in 50cm long steel tubes. These tubes containing 59Co are placed in a nuclear reactor where they are bombarded constantly with neutrons, which produce radioactive 60Co, which is capable of producing a controlled emission of γ rays. 137Cs is extracted from the bars of used combustible of the nuclear reactors. This reprocessing of nuclear waste has become very controversial and its possible use as a source of irradiation in food is very improbable. Thus it appears that 60Co offers better possibilities for food processing; moreover this type of source shows a greater degree of effectiveness, greater penetration of γ rays and greater environmental safety, as it is insoluble in water. The electron beam is a series of electrically charged particles of high energy, of up to 10 MeV. For the electrons to have a high energy level they are led to a linear accelerator which confers them with high voltages, thereby obtaining electrons with high speed, approaching that of light. The advantage of this compared to γ radiation is that the electronic beam is produced in an electric machine and can be turned on and off like a light bulb. Nevertheless, its power of penetration is low, from some 5 to 10 cm. Table 3 shows the advantages and disadvantages of the different sources of radiation used in the irradiation of foods. When foods are irradiated with γ rays, X-rays and electron beams, a certain degree of radioactivity can be induced in them. However, this is such a small amount that it is not distinguishable from the natural radiation possessed by the food. Hence the variation in radioactivity among different non-irradiated foods is greater than any difference existing between the same food when irradiated and non-irradiated (Stewart, 2001). From a food viewpoint, of all the energy transfer mechanisms of ionizing radiation due to the incidence of photons, the most important is the Compton Effect. For the photoelectric effect to be produced the energy of the incident photon would have to be lower than that

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provided in the normal intervals of food irradiation. On the other hand, for the formation of electronic pairs higher energy levels would be required (Stewart, 2001). Table 3. Advantages and disadvantages for the different irradiation sources Source type γ-Rays (60Co 137Cs)

Electron beam (10 MeV)

X-rays (5 MeV)

Advantages - Deep penetration - Reliability of irradiation source - Easy automation

- Electric source only work when switch on - Unit control possibility - High dose rate (several kGy/s) - Absence of environmental impact - Low cost for running - Electric source only work when switch on - Unit control possibility - High dose rate (several kGy/s) - Absence of environmental impact - Low cost for running

Disadvantages - First category radioactive facility - Transport and storage radioactive sources - Activity loss of storage radioactive source - Dose rate determined by source - Permanent irradiation emission - High cost for running and security - First category radioactive facility - Limited Penetration - Need a lot of handling staff - Need automated equips - First category radioactive facility

Source: Raventós (2003).

4.2. Irradiation Dose in Foods As the radiation used in food treatment is electromagnetic in nature (X-rays and γ rays) or else accelerated electrons, the weighting factor FR (table 2) of the equivalent dose is 1, with which the absorbed dose (D) and equivalent (H) coincide. Hence in this case the absorbed dose is usually employed. For every food product the permitted doses of radiation depend on its characteristics and the aim of the treatment. This means that the dose for the elimination of insects, for pasteurisation and sterilization will be different. Hence three irradiation categories are considered according to the dose employed, either low, medium or high doses. Low doses are those which do not exceed 1 kGy, and are used in the control of insects in grain, in the control of trichina in pork and can also inhibit the decomposition of fruits and vegetables. Average doses are those in the range from 1 to 10 kGy, and are applied in the control of pathogens in meat, poultry and fish and also retard the growth of moulds on strawberries and other fruits. High doses exceed 10 kGy, and are used to kill micro-organisms and insects in spices, and also when aiming to obtain commercially sterile foods. According to the dose of radiation the treatment usually receives different names. Thus the elimination of non-spore producing pathogenic microorganisms and parasites to an imperceptible level is called radicidation. The treatment of foods with ionizing radiation aimed at increasing their average life by reducing the number of modifying micro-organisms (pasteurisation) receives the name of radurisation, while the elimination of micro-organisms by irradiation to levels of sterilization is called radapertization. Table 4 shows the doses used to irradiate foods and the applications of each case.

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17

Table 4. Food irradiation, dose and applications Dose Low

< 1 kGy

Medium

1 a 10 kGy

High

10 a 50 kGy

Absorbed dose (kGy) 0,04 – 0.10 0,03 – 0,20 0,50 – 1,00 1–3

Application Sprout inhibition of tubers and bulbs Insects, grubs and eggs sterilization Fruit and vegetables ripening process control Insect death

1–7 2 - 10 15 – 50

Radicidation (pathogen elimination) Radurization (pasteurization) Radapertization (sterilization)

10 - 50

Spices and seasonings decontamination

4.3. Changes in Irradiated Foods Irradiated foods are treated at low levels of radiation, which means that only chemical changes are possible, and that changes which would make them radioactive do not occur. The large number of investigations carried out indicates that the changes produced in irradiated foods are similar to those produced by a conventional cooking treatment. The studies show that in irradiated foods toxic or mutagenic effects do not exist, and that irradiation does not produce chemical residues in the food. Irradiation is a cold process, which means that there is only a slight temperature rise of the food during processing. There is hardly any change in the physical appearance of the irradiated foods, which do not undergo the changes in texture and colour shown by foods treated by heat pasteurisation, or by tinned and frozen foods. Certain bad tastes in meat and excessive softening of fresh peaches and nectarines have been reported. In irradiated foods some changes do occur, although they are not as important as those that occur with conventional cooking methods. When the high energy particles hit the matter, electrons are released from the atoms, giving rise to ions. The radiolytic products formed in this way can interact to form new compounds. A few of these reactions may give rise to strange tastes. The FDA concluded that “very few of these radiolytic compounds are unique to irradiated foods; approximately 90% of radiolytic compounds are natural compounds of the food” (Web and Penner, 2000).

4.4. Irradiated Food Labelling Retailed irradiated foods must bear the symbol radura (figure 5) which identifies them as such. Furthermore the sentence “treated with irradiation” must also appear. Manufacturers are permitted to add the objective of the treatment; thus, for example, it may be labelled as “treated with radiation to control deterioration”. With non-packaged fruits and vegetables every piece must be labelled, and furthermore on the shelf containing the product and clearly visible to the consumer there must be a sign indicating that they have been treated with radiation.

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For unpackaged irradiated elements sold by weight, such as spices, it is also necessary to indicate that they have been irradiated. Nevertheless, if these foods are incorporated as ingredients in other foods, it is not necessary for the final products to be labelled as irradiated. Hence, if irradiated pepper is added to a processed meat, it is not necessary for it to be mentioned on the product that this irradiated spice has been used as an ingredient.

Figure 5. Radura symbol.

5. EFFECTS OF RADIATION ON THE FOOD’S COMPONENTS Any food is composed of different compounds, of which water is by far the commonest component, so that the possibility of the incident radiation affecting a water molecule is very high. Apart from water other majority components of foods are carbohydrates, fats and proteins, although other so-called minority components should also be considered, among which vitamins and minerals feature. Of all of these components contained in foods, minerals are not affected by irradiation of the food whatever the dose absorbed. However, irradiation produces changes in the other components, which are studied in detail in the following sections.

5.1. Water Radiolysis The interaction of radiation with a molecule of water can cause it to be ionized, due to the loss of an electron, resulting in H2O+, or else its dissociation into OH- and H+, due to the breakage of the link. Excitation may also occur, whereby the water molecule moves to a state of higher energy. The different ions that appear may react with water molecules to give rise to a whole series of compounds, among which are included atomic and molecular hydrogen,

Ionizing Irradiation of Foods

19

hydroxyl radicals, hydrogen peroxide, a solvatated proton and an aqueous (solvated or hydrated) electron (Stewart, 2001). The range of molecules produced is known as radioinduced products. Of all of these, only molecular hydrogen and hydrogen peroxide are stable molecules, although they are consumed in the reactions following on from irradiation. Among the range of radio-induced products the most reactive are H2O2 and the OH- radical, which can react with adjacent molecules to form new compounds. The hydroxyl radical is a powerful oxidizing agent, while hydrogen and the aqueous electron are reducing agents, which means that the water contained in the food may well undergo both oxidising and reducing reactions. The hydroxyl radical can remove the hydrogen atoms in the C-H links of the olefins or aromatic compounds. It should be noted that the presence of oxygen in the medium favours the formation of the most reactive products and the processes of oxidation. If oxygen does not exist in the medium the amount of peroxide formed is very low, which leads to a reduction in the oxidation reactions of the food. pH is another of the variables of the medium which may affect the final result of irradiation. The H+ ions can combine with a solvatated electron to produce atomic hydrogen radical, which can combine with a hydroxyl radical freeing the aqueous electron (Diehl, 1995). Therefore, an acid medium may favour the disappearance of the aqueous electron, while a basic medium favours its formation. Acid foods will favour reduction reactions during the irradiation process. Another factor affecting the whole process is the temperature of the product being irradiated. It should be mentioned that the initial stages of ionization and excitation and the reactions of the more reactivate species are temperature independents (Swallow, 1977). If the product is frozen the intermediate reactive components of water radiolysis are trapped and, as a result, cannot react with other components of the food (Urbain, 1986). Hence the effects of irradiation in frozen foods are fewer than when the irradiation is carried out on a product containing water in its liquid form.

5.2. Carbohydrates The chemistry of the radiation of carbohydrates is quite complex, as different radiolytic compounds can be formed. The main reaction involving carbohydrates when a food is irradiated is that which occurs with the hydroxyl radicals formed by water radiolysis. The end products of these reactions are ketones, aldehydes and acids. Depending on the dose they hydrolyse and oxidise, giving rise to simpler compounds, although they may also undergo a depolymerisation, making them more susceptible to the attack of hydrolytic enzymes. The action of the hydroxyl radical on the carbohydrates begins with the separation of a hydrogen atom linked with a carbon atom, giving rise to new radicals. These radicals may undergo new disproportioning, dimerisation and dehydration reactions. All of this can lead to the appearance of a large number of radiolytic products. The irradiation of solid sugars with a low molecular mass causes the reduction of their fusion point. In some cases this leads to the appearance of browning, as occurs with the irradiation of reducing sugars like glucose and fructose, and a mix of gases like hydrogen, carbon monoxide and carbon dioxide, among others (Dauphin and Saint-Lèbe, 1977). In the case of carbohydrates with a high molecular mass, like the polysaccharides (starch, pectin,

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cellulose), radiation causes their degradation. This degradation seems to be due to the division of the glycosidic bonds, giving rise to sugars of a lower molecular mass, although the degradation may continue to give other radiolytic products like formic acid, methanol, acetone, ethanol and ethyl formiate (Dauphin and Saint-Lèbe, 1977; Sokhey and Hanna, 1993; Stewart, 2001). The presence of other food compounds exercises a protecting effect on the carbohydrates. Therefore the end result of irradiation on the carbohydrates present in a food is not the same as that which occurs in the irradiation of pure sugar solutions. Moreover, carbohydrates generally occur in foods of plant origin, and the presence of the plant cell wall will exercise a protecting effect (Stewart, 2001), which means that in the end these foods are less susceptible to the effects of radiation and that the changes produced are insignificant.

5.3. Fats Fats are mostly insoluble in water, so that the effects of radiation on them will be different from those produced on carbohydrates. If the radiation dose is lower than 35 kGy, the changes in the physical and many chemical properties of the lipids are insignificant (Urbain, 1986). The radiation of fats causes the formation of cationic radicals and excited molecules which subsequently react to give rise to new compounds. The radicals formed may break up or dimerise, or a molecular disproportioning may be produced, although the adjacent free electrons may also be fixed (Nawar, 1977, 1978, Delincée, 1983a). The number of end products obtained is wide and diverse, as fatty acids, propanediol esters, aldehydes, ketones, diglycerides, alkanes, alkenes, methyl esters, hydrocarburates and short chain triglycerides may all be formed. However these compounds are also produced when fats are thermally treated, and the amount produced is even greater than in the irradiation treatments.

5.4. Proteins In the irradiation of proteins various types of reaction may be produced (Hanna and Shepherd, 1959; Delincée, 1983b; Simic, 1983). An important feature is the rupture of the protein bond, giving rise to peptides of shorter chains than the protein from which they originate. The amino acids that form part of the polypeptidic fraction may react with the free radicals produced by water radiolysis, which breaks the peptidic bonds. Irradiation can cause the denaturalisation of the proteins due to aggregation or disgregation of the polypeptides, or else due to the changes in their secondary and tertiary structures. However, it should be noted that the denaturalisation of proteins deriving from irradiation processes is less than that produced by thermal treatments. In order for the radiation to affect the amino acids doses of 40-50 kGy are required, and in this case certain organoleptic changes in the treated foods are brought about. Irradiation of proteins at doses up to 35 kGy causes no discernible reduction in amino acid content (Urbain, 1986). In the presence of fats, the irradiation of amino acids can give rise to aldehydes and ketones, which are products that cause bad odours in the treated food. It should be noted that irradiation has hardly any effect on enzymes. In order to inhibit their activity doses of up to 60 kGy would be needed. As the doses used in the irradiation of

Ionizing Irradiation of Foods

21

food do not approach this value, there exists the possibility of the occurrence of deteriorating enzymatic reactions in irradiated foods. If these foods are to be stored for long periods they should receive additional heat treatment for the enzymes to be inactivated.

5.5. Vitamins Vitamins are food micronutrients which are sensitive to irradiation. This sensitivity depends on the dose received by the food and the type of vitamin. It should be supposed that water-soluble vitamins are more sensitive to deterioration by radiation, due to the reactions of the hydroxyl radical produced in the radiolysis of water. The vitamins A, E, C, K and thiamine (B1) are relatively sensitive to radiation. In contrast, riboflavin, niacin, pyridoxine, pantothenic acid and vitamin D are much more stable (Diehl and Josephson, 1994). Of all of these thiamine is the most sensitive to irradiation, although its losses amount to no more than 2.5% after treatment. Generally, the loss of vitamins in irradiated foods is usually insignificant (Diehl ,1991;.Thayer et al., 1991). The sensitivity of water soluble vitamins to irradiation follows the order: vitamin B1 > vitamin C > vitamin B6 > vitamin B2 > niacin = folate > vitamin B12; and for fat soluble vitamins the order for their sensitivity is: vitamin E > carotene > vitamin A > vitamin D > vitamin K (Stewart, 2001). As occurs with some macronutrients, the presence of oxygen and the temperature affect the deterioration of vitamins caused by radiation. This deterioration of vitamins can be reduced by working at low temperatures in the absence of oxygen, with vacuum packaging or under an atmosphere of nitrogen. Table 5 shows the content of different vitamins in chicken meat after receiving different treatments. It can be seen that the irradiated chicken meat shows similar vitamin content to that treated thermally, and that for thiamine the results are even better. Table 5. Vitamin content in poultry with different treatments Vitamin Tiamine Riboflavin Piridoxine Ac. nicotinic Ac. pantotenic Biotine Folic ac. Vitamin A Vitamin D Vitamin K Vitamin B12

Source: Thayer (1990).

Vitamin concentration (mg/kg) (dry weight) Frozen Thermal γ−Irradiated 2.31 1.53 1.57 4.32 4.60 4.46 7.26 7.82 5.52 212.9 213.9 197.9 24 21.8 23.5 0.093 0.097 0.098 0.83 1.22 1.26 2716 2340 2270 375.1 342.8 354 1.29 1.01 0.81 0.008 0.016 0.014

β−Irradiated 1.98 4.90 6.70 208.2 24,9 0.100 1.47 2270 466.1 0.85 0.009

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Albert Ibarz

6. INACTIVATION OF MICROORGANISMS BY RADIATION Irradiation is an efficient method of treatment for the destruction of parasites and both pathogenic and non-pathogenic bacteria, and to lesser degree viruses. The mechanisms of inactivation, as indicated in section 3, are due to the damage that irradiation produces in the genetic material, both directly and indirectly. Thus, ionizing radiation can collide directly with the cell’s genetic material and damage its DNA, although it can also act on an adjacent molecule which subsequently reacts with the genetic material. When ionizing radiation acts on a DNA molecule it does so at random, so that it may collide with a single or double strand of the molecule. When the collision occurs with a single strand of DNA the damage caused is not necessarily lethal and may give rise to mutations; although it is also possible that the damage caused exceeds the cell’s capacity for reparation, resulting in its death. If the damage is inflicted on a double strand it may slice the DNA molecule with lethal effects for the cell, which is incapable of repairing the damage caused by the incident radiation. If the radiation does not fall directly on the genetic material but rather on an adjacent molecule, the effects are more complex. As mentioned previously, the commonest molecule is that of water, and so there is a high probability of the radiation falling on one of these molecules. The resulting molecules, or radio-induced products, may react with the nucleic acids damaging the genetic material, although they may also give rise to a series of reactions with the different molecules they encounter, finally producing a range of molecules which are toxic for the cell, with possible lethal consequences. In addition to these effects on the genetic material, irradiation may affect other cell components, such as the membrane, the enzymes and the cytoplasm. This damage may not be directly lethal, but can damage the cell sub lethally, so that it can impede the survival of the damaged cell. The different organic compounds that form part of the cell show different sensitivity towards irradiation, proportional to their molecular mass. Therefore amino acids are less sensitive than enzymes, which in turn are less sensitive than DNA molecules (Pollard, 1966). Cells which have received irradiation are damaged by some of the mechanisms described, and the cell responds to this damage with its own repair and survival mechanisms. The recovery of surviving cells after irradiation depends on pH (Fielding, et al., 1994). Generally the inactivation of micro-organisms is caused by the damage done to the DNA, and thus the survival mechanisms are centred on the reparation of this molecule. Consequently, those micro-organisms which are efficient in the repair of DNA show the greatest probability of survival, and are therefore the most resistant to irradiation. In addition to this survival mechanism there are micro-organisms which are resistant to radiation owing to the fact that they possess mechanisms for the elimination of damaged genetic material, which aids their survival. One example of a bacterium resistant to radiation is Deinococcus radiodurans, which can be found in foods which have been irradiated with doses of up to 40 kGy (Dickson, 2001; Moseley, 1976). In thermal treatments the inactivation of micro-organisms at a certain temperature follows first-order kinetics. In a similar fashion, it has been observed that the number of micro-organisms present in an irradiated food decreases with the applied dose according to first-order kinetics:

Ionizing Irradiation of Foods

N = N 0 exp(− k D )

23 (5)

where N0 is the number of micro-organisms initially present in the food, N the number of micro-organisms which survive, D is the dose applied and k is the kinetic constant of microorganism destruction by irradiation. For pasteurisation doses of between 1 and 10 kGy are required, while for the sterilization of the product doses of between 15 and 50 kGy are needed. In food irradiation treatments it is very useful to apply the variable D10, known as the decimal reduction dose, which represents the dose applied in order to reduce the number of micro-organisms to one tenth of the original. In this case N = 0.1N0, so that:

⎛ N0 ln⎜⎜ ⎝ 0,1N 0

⎞ ⎟⎟ = ln (10 ) = k D10 ⎠

(6)

which is the same as:

D10 =

2,303 k

(7)

Table 6a shows the D10 values for some of the commonest bacteria in food. It can be seen that for the spore-producing forms the D10 values are higher, which indicates that they are more difficult to destroy. This difference compared to the vegetative forms can be explained by the fact that in the spore-producing forms the water content is much greater, which would minimize the secondary effects of the radiation, leading to an increase in the resistance to radiation. D10 data is also available for some parasites and certain pathogenic viruses (Dickson, 2001) (table 6b). In the case of viruses, the contamination of the food generally originates from infected food handlers which can transmit the disease if they have contaminated the food they have prepared. The D10 values for viruses are greater than those for vegetative bacteria, and are more similar to the spore-producing forms, which indicate greater resistance to radiation. Thus, the coxsackie virus shows values of 7-8 kGy, the polio virus a value of 3, while the hepatitis A virus shows a value of 2 (Sullivan et al., 1973; Heidelbaugh and Giron, 1969; Mallet et al., 1991; Dickson, 2001).

24

Albert Ibarz Table 6a. D10 values for selected gram-positive bacteria Bacteria Gram-positive Spore formers Bacillus cereus

Clostridium botulinum Clostridium perfringens Non-spore formers Listeria monocytogenes

Staphylococcus aureus Gram-negative Aeromonas hydrophila Campylobacter jejuni Escherichia coli O157:H7 Salmonella

Shigella

Vibrio Yersinia enterocolitica

Medium

Conditions

D10 (kGy)

Distilled water Mozzarella cheese Yogurt Beef stew Water

20 – 25ºC; aerobic - 78ºC, aerobic; spores - 78ºC 20 – 25ºC; type E 20 – 25ºC

1.6 3.6 4.0 1.4 1.2 – 1.3

Chicken Chicken Ground beef Trypticase soy broth Phosphate buffer Ice cream Poultry Meat

2 – 4ºC 12ºC 12ºC 0ºC 0ºC - 78ºC 10ºC ---

0.77 0,49 0.5 – 0.9 0.21 0.18 2.0 0.42 0.86

Ground fish Ground fish Ground turkey Ground beef Ground beef Salsa Roast beef Ground beef Deboned chicken Deboned chicken Deboned chicken Deboned chicken Liquid whole egg Liquid whole egg Oysters Crabmeat Oysters Crabmeat Oysters Crabmeat Prawns Shrimps Ground beef Ground beef Minced meat

2ºC - 15ºC 0 – 5ºC, vacuum - 17ºC 2 – 5ºC 3ºC; S. typhimurium 3ºC; S. typhimurium 20ºC; S. typhimurium - 40ºC; air; S. typhimurium - 40ºC; air; S. enteritidis - 40ºC; air; S. newport - 40ºC; air; S. anatum Frozen; S. seftenberg Frozen; S. gallinarum S. dysenteriae S. dysenteriae S. flexneri S. flexneri S. sonnei S. sonnei Frozen; V. cholerae Frozen;V. parahaemolyticus 25ºC - 30ºC ---

0.16 0.274 0.19 0.307 0.241 0.416 0.567 0.55 0.533 0.534 0.436 0.542 0.47 0.57 0.40 0.35 0.26 0.22 0.25 0.27 0.11 0.1 0.2 0.39 0.10 – 0.21

Source: Dickson (2001).

Table 6b. D10 values for selected virus Virus Coxsackie Polio Echovirus Hepatitis A Rotavirus SA11

Source: Dickson (2001).

Medium Raw and cooked beef Fish MEM medium Oysters Oysters

Conditions -90 – 16ºC 0ºC -------

D10 (kGy) 6.8 – 8.1 3 4.3 – 5.5 2 2.4

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25

7. EFFECT OF IRRADIATION ON FOODS The effects produced by irradiation depend on the type of food which is being treated, as well as the characteristics of the medium. These effects depend on radio-induced processes, as they are favoured by the presence of oxygen, by an increase in pH, by an increase in temperature and by the water content. In dry and dehydrated food products direct irradiation is the most effective, as there is less water available and therefore the formation of free radicals is reduced. It is also recommendable to irradiate at freezing temperatures, as the water is not present in liquid form, which eliminates the process of radiolysis. Furthermore, it is better to irradiate vacuumpacked foods or those in a modified atmosphere, in the absence of oxygen or with very low levels of this gas. With regard to pH, the irradiation of foods with a value of less than 4.5 is safer, this requiring lower radiation doses than a higher pH. It is important to underline that irradiation, like other treatments, is only effective with foods in a healthy and hygienic state. Irradiation will never improve already degraded foods. The food’s components will vary depending on the food under consideration. Thus carbohydrates will be the majority components of foods of plant origin, while proteins and fats are predominant in foods of animal origin. A brief description of the effects of irradiation for different types of food and irradiation processes according to their objective is given below.

7.1. Irradiation of Meat and Meat Products From the sacrifice of the animal to the retail of the meat, meats and poultry are subjected to different stages of handling and processing, which could lead to their becoming contaminated with pathogenic micro-organisms such as Salmonella, Campylobacter, Listeria, Yersinia and Escherichia coli. Much intoxication by Escherichia coli have been caused by contaminated minced meats. In chicken and other poultry the most problematic intoxications are due to Salmonella, either because the food has not received suitable treatment or it has not been handled correctly. Several studies have demonstrated that the irradiation can be an effective treatment in the destruction of different pathogen microorganisms in meat (Farkas, 1987; Lee et al., 1996) Irradiation at doses of between 10 and 50 kGy eliminates the previously mentioned microorganisms, and even destroys Trichinella in raw beef and pork (Kotula, 1983; Molins, 2001). The Food and Drug Administration (FDA) in the United Stated in 1985 approved irradiation of fresh pork at 0.3 to 1.0 kGy in order to inactivate Trichinella spiralis. One of the most dangerous micro-organisms is Clostridium botulinum, due to the botulinic toxin that it produces. In order to eliminate Clostridium botulinum and the spore-producing forms of other micro-organisms it is necessary to apply higher doses of radiation. For this microorganism it is necessary a 12D value of 38 to 48 kGy in a radiation treatment in pork, ham or chicken (Kreiger et al., 1983). In order to obtain high quality meats and meat products a recommended procedure is an initial stage of packaging, followed by gentle thermal treatment to inactivate enzymes, then a stage of freezing to end with the irradiation of the product. One advantage obtained by

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irradiation in comparison to other preservation treatments is that it is possible to eliminate the preservatives which are added to many processed meat products.

7.2. Irradiation of Fish and Shellfish Products The processing of fish and shellfish using irradiation can control pathogenic and harmful micro-organisms, as well as prolong the product’s commercial life. Irradiation does not cause sensorial changes in the products. Several authors have reviewed different aspects of the irradiation on fish and shellfish products (Kilgen, 2001; Nickerson et al., 1983). Pathogenic micro-organisms present in fish and shellfish come from the water in which they are submerged. The micro-organisms which usually appear are Clostridium perfringens, Clostridium botulinum, Vibrio parahemolyticus, Vibrio cholerae, and Aeromona hydrophila. In addition to these pathogens, owing to deficient handling of the fish and shellfish products, there may also be Salmonella, Shigella and Staphylococcus aureus. Generally this type of food is sold fresh and by weight, exposed on ice, for which processing with irradiation effects the initial flora that it contains, but in subsequent handling it can become contaminated again. Therefore, although a safe product is obtained initially, if the subsequent handling is unsuitable, the global treatment will be ineffective. Shellfish, such as crustaceans, feed by filtering the surrounding water and absorbing the food that it contains. If the water is contaminated this contamination also passes on to the shellfish. A dangerous case could be contamination by bacteria like Escherichia coli and viruses found in waste waters. The doses normally applied in the irradiation of fresh and cooked fish and shellfish is from 0.75 to 1.5 kGy, while for frozen products this dose is higher, of between 2 and 5 kGy (Raventós, 2003; Kilgen, 2001). These doses are not high enough to eliminate the spores of bacteria like Clostridium botulinum, or the toxins that the product may contain. For this reason and other, the fish and shellfish products should be processed and stored in cold below 3ºC, in ice or frozen. This way, after the irradiation treatment, if the product is conserved under appropriate conditions it can increase the time of storage from 1 to 3 weeks in the case of fresh or cooked product, and if it is frozen it can duplicate the conservation time (Raventós, 2003).

7.3. Processing of Eggs and Egg Products by Irradiation In eggs and their derivatives, the most problematic pathogenic micro-organism is Salmonella enteriditis. Thermal pasteurisation treatments eliminate this pathogen, but the resulting product is different from the original, as the heat causes the denaturalisation of the proteins. As mentioned in the previous section irradiation is considered to be a “cold” thermal treatment, hence by irradiating this type of food the pathogenic micro-organism can be eliminated, while the food conserves all of its organoleptic properties. The dose for eliminating Salmonella is 2.5 kGy. With this dose whole eggs and derivatives (mayonnaise, creams, sauces, etc) can be irradiated, producing products which have no perceivable differences compared to their non-irradiated equivalents.

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27

The irradiation process has been applied to inactivate the Salmonella contamination in dried egg products, and a dose of 2 kGy has been recommended (Narvaiz et al., 1992; Farkas, 2001).

7.4. Irradiation of Fresh Fruits and Vegetables Diverse revision works exist on the fruits and vegetables processed with radiation (Akamine and Moy, 1983; Willemot et al., 1996; Thayer and Rajowsky, 1999; Thomas, 2001a). Once harvested fruits and vegetables start deteriorating, and for this reason postharvest treatments are normally applied to these products, based on refrigeration processes. This type of product possesses high carbohydrate content, although its water content is between 80% and 95%, and what’s more the intercellular spaces contain oxygen. Hence the irradiation of fruits and vegetables is usually carried out at relatively low doses, in order to avoid the previously mentioned problems associated with water radiolysis and subsequent oxidation of the carbohydrates due to the presence of oxygen. Irradiation at low doses can delay the ripening of some fruits and vegetables, leading to the prolongation of their useful life. With doses of 0.3 to 1 kGy a significant increase in their commercial life can be obtained (Thomas, 2001a). In the case of mangos and bananas a prolongation of their useful life by one and two weeks, respectively, has been achieved. Additionally, the irradiation of mushrooms and asparagus at doses of 1-1.5 kGy has slowed their maturation (Satin, 2000). Generally, doses of up to 0.25 kGy applied to most fruits and vegetables do not affect their organoleptic properties. Doses of between 0.25 and 1 kGy can cause significant modifications and may even cause a reduction in the vitamin content. Doses of more than 1.75 kGy enable the control of storage-related diseases. If a dose of between 1 and 3 kGy is applied it may accelerate the softening of the product, and may lead to the development of undesirable organoleptic properties. In the case of irradiation at doses of more than 3 kGy an excessive ripening is produced, with the appearance of certain disagreeable tastes (Raventós, 2003).

7.5. Irradiation of Tubers and Bulbs Tubers and bulbs are vegetables which are widely used ain human food. The most important tuber crops are potato, yam, sweet potato and ginger; while the most important bulbs are onion, garlic and shallot. Of these the potato and the onion are the products for which most information on irradiation treatments exists. Irradiation is applied to these products with the aim of reducing the losses which occur after harvesting. These losses are due to four main factors (Thomas, 2001b): physical, physiological, microbiological and entomological factors. Included among the physical factors is mechanical damage done during harvesting and handling. Physiological factors are losses caused by the formation of sprouts, the appearance of rot, and the transpiration and respiration of the product. These last two factors involve the loss of water during storage, which decreases if the product has a well-formed skin or dry outer scales. In stored potatoes greening of the skin can occur as a as a result of the formation of chlorophyll in artificially lit

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Albert Ibarz

warehouses, which is accompanied by the production of solanine, a glycoalkaloid which confers a bitter taste and which can be poisonous. In some cases during storage the starch may be converted into sugars, the presence of which causes browning problems when elaborating crisps. With regard to microbiological factors the growth of bacteria and fungi is favoured by a warm and humid environment, and can be controlled by storage at low temperatures. In addition to these factors another significant cause of losses in tubers is that owing to the attack of the moth Phthorimaea operculella (Zeller), which is very common in hot tropical and subtropical regions. Of all of these factors, one which causes the most concern is the appearance of sprouts due to germination, and in order to reduce this kind of deterioration different techniques have been applied. Thus germination is inhibited by storing potatoes at 4ºC, while with bulbs the temperature should be 0ºC. Another way of inhibiting sprout formation is by the use of chemical products like maleic hydrazide (HM; 1,2-dihidropiridacina-3-6-diona), applied before the bulbs are picked (del Rivero and Cornejo, 1971; Isenberg, 1956; Thomas, 2001b). Other chemical inhibitors applied to potatoes are Chlorpropham [CIPC; isopropyl-N-(3chlorophenyl) carbamate], Propham [IPC; isopropyl-N-phenyl carbamate] and Tecnazene [TCNB; tetrachloro nitrobenzene] applied after harvesting, and they are proved effective to extend stored life of these products at 7-10ºC (Booth and Proctor, 1972; Smith, 1977). The application of these chemical inhibitors has been questioned for the possible risks they represent for human health. The application of irradiation can prevent the germination of this type of food. As early as in 1936 it was observed that the application of X-rays could inhibit sprout formation in vegetables (Metlitsky, 1936). The application of X-rays at a dose of 4.5 Gy inhibits the germination of potatoes (Sparrow and Christensen, 1950). The works carried out on the inhibition of germination in tubers and bulbs is very extensive, including studies on the effects of the applied dose rate, the interval between harvesting and irradiation, sprout inhibition mechanisms, the influence of temperature and humidity during storage, sensorial characteristics, susceptibility of the irradiated products to rotting, and methods of irradiation detection (Thomas, 2001b). Irradiation appears to produce changes in the levels of endogenous growth hormones and affects the metabolism of nucleic acids. It has also been observed that irradiation is more effective if applied immediately after harvesting, when they are in their period of lethargy, as the dose applied to prevent germination must be increased as the time between harvesting and irradiation increases. For tubers normal treatment doses are from 70 to 150 Gy, which not only prevent germination but also wither the sprouts already formed. In the case of onions and other bulbs doses of between 20 and 90 Gy are usually applied (Thomas, 2001b), which bring about the inhibition of germination if applied within 2-4 weeks after harvesting. In general the period of lethargy is prolonged if the onions are stored at low temperatures (º0C), which enables suitable inhibition of germination if an irradiation treatment is applied during this time. Before treating with irradiation it should be ensured that the tubers are clean, dry and soil-free. In the case of bulbs it is essential that they are well hardened, with dry outer scales and neck. Furthermore, if these products are washed it is essential that they are dry when treated with irradiation.

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29

7.6. Disinfestation of Cereals, Seeds, Legumes, Dried Fruits, Nuts and other Dried Foods Many insects can cause serious problems in stored products, particularly in grains and cereals. The FAO estimates that during storage between 5% and 10% of cereals can be lost due to infestation by insects. To avoid these losses control techniques of temperature and controlled atmosphere have been used, although the use of pesticides for the control of insects in stored foods is very common. Currently a widely used product in this control is methyl bromide, as it presents a wide fumigation spectrum, although it has been put under very strict control by the developed countries. This fumigant possesses an ozone reduction potential of more than 0.2, which is the limit marked by the Montreal Protocol (Ahmed, 2001), which recommends its gradual removal and total prohibition before the year 2010. Therefore there is widespread concern to find alternative treatments to fumigation with pesticides. Disinfestation by radiation began with studies on the sensitivity to radiation of the rice weevil (Hunter, 1912). Since then there have been various studies on the irradiation of insects aimed at disinfestation (Lorenz, 1975; Ahmed, 2001). The Codex Alimentarius Commission recommends a dose of 1 kGy for the disinfestation of all agricultural foods and products (CAC, 1984), although with most dry foods this can be achieved at lower doses. In insects, as in other organisms, the effects of radiation are due to the effects that are produced in their cells. Sensitivity to radiation is directly proportional to the cells’ reproductive activity and inversely proportional to their degree of differentiation. Hence, dividing cells are more sensitive than those at the adult stage. Eggs are more sensitive to radiation, and even at sublethal doses subsequent malformations and adult sterility are observed. Insect larvae in their period of lethargy are more resistant to radiation, although these larvae die after the pupal stage. Male pupae are more resistant to radiation than females. It appears that sensitivity to radiation varies from one type of insect to another, so that, for example, the Coleoptera (beetles) are more sensitive to radiation, while the Lepidoptera (moths) are the most resistant group. The sterilization doses differ from one type to another, so that for beetles a dose of 50 Gy is sufficient, while with moths doses of some 1000 Gy are necessary. However, in order to kill the insect a dose of between 3 and 5 kGy will be needed (Ahmed, 2001). To perform a satisfactory irradiation treatment the type of insect that is to be targeted should be known, and its development stage which is most sensitive to radiation identified. Any food product treated with doses up to 10 kGy does not present a toxicological risk and such doses can be applied; nevertheless in 1983 the Codex Alimentarius Commission (table 1) recommended that in the disinfestation of cocoa beans, dates, legumes, rice, wheat and derivatives the dose of 1 kGy should not be exceeded. To summarise, the disinfestation doses applied will depend on the type of insect and product although, in general, it is considered that this objective is achieved with doses in the interval of 0.5 kGy to 3 kGy. However, it should be borne in mind that for doses of more than 1 kGy some products may be affected in their organoleptic qualities, in their vitamin content and in their starch properties. Table 7 shows the radiation doses most commonly used to control insects present in stored foods. Irradiation treatment for the disinfestation of cereals, legumes and seeds does not cause any harmful effects in them, although it does not prevent the products from being reinfested. Although there is no harmful effect on the quality of these irradiated products, it should be

30

Albert Ibarz

borne in mind that irradiation at disinfestation doses may prevent the seeds and cereals from germinating, which in some cases may be more of an inconvenience than an advantage. Disinfestation treatment by radiation has not only been applied to cereals, legumes and seeds, but also has been successfully applied to other dehydrated products such as salted, smoked and dried fish, nuts, dehydrated fruits, coffee and cocoa beans, etc. Table 7. Irradiation dose used to insect control in stored products Species Coleoptera Sitophilus oryzae S. granarius S. zeamais Tribolium castaneum T. confusum T. cadeus Rhyzopertha dominica Latheticus oryzae Oryzaephilus surinamensis O. mercator Callosobruchus analis C. chinensis C. maculatus Bruchus rufimanus Bruchidius incarnatus Trogoderma granarium Dermestes maculatus Lasioderma serricorne Nerobia rufipes Araecerus faciculatus Lepidoptera Anagastus kuehniella Plodia interpunctella Cadra cautella Sitrotoga cerealella Nemapogon granellus

Stage

Dose (kGy)

All All All All All All Larvae Adults All All All All All All All All All All All All

0.16 0.16 0.16 0.20 0.20 0.20 0.25 0.20 0.20 0.20 0.20 0.20 0.20 0.40 0.40 0.25 0.50 0.50 0.30 0.75

Larvae, pupae Larvae Larvae, pupae All All

0.60 0.45 0.45 0.60 0.50

Source: Ahmed (2001).

7.7. Irradiation of Spices, Herbs, Seasonings and other Dry Food Ingredients Among the different possibilities for the application of irradiation, perhaps that which presents an immediate application is the treatment by irradiation of dry food ingredients, as is the case of spices, herbs and plant-based seasonings (Farkas, 2001). These kinds of products are appreciated because they confer special flavours and aromas to many cooked dishes. The main problem is that they possess high levels of contamination due to bacteria, fungi and yeasts, which can contaminate the food to which they are added. Therefore it is necessary for them to receive suitable treatment to reduce this contamination.

Ionizing Irradiation of Foods

31

Until fairly recently, the destruction of micro-organisms was achieved by means of fumigation with ethylene oxide, and to a lesser degree with propylene oxide. However, this type of fumigation can cause toxicity problems, as it is carcinogenic, and subsequently its use on food has been prohibited in many European countries. Hence the best alternative for treating this kind of product is irradiation, as it does not present this problem of toxicity, achieving a degree of decontamination much greater than any other kind of treatment. High dose irradiation treatment does not cause sensorial changes or affect the functional properties of spices. Furthermore, the process of microbial reduction continues during subsequent storage. Spices and food ingredients possess low water content, enabling the use of high doses in the irradiation treatment, and in fact doses of more than 5 kGy are usually employed. For doses of between 5 and 10 kGy the bacterial population is reduced to between 1% and 1 per thousand. Doses of between 7.5 and 15 kGy do not affect the sensorial properties of spices, although they may affect herbs (Sugimoto et al., 1986; Onyenekwe et al., 1997; Farkas, 2001). Many of these products are sold already packaged, which makes it essential to identify packaging materials which are not affected by radiation. The most common treatment is the irradiation of the food once it has been packaged, which enables the destruction of the microbial load while at the same time conserving the characteristic aromas of these dry food ingredients.

7.8. Irradiation of Milk Products As mentioned earlier, irradiating milk with ultraviolet radiation, besides destroying micro-organisms also produces an increase in vitamin D, although the content of other vitamins such as vitamin B and C is reduced. This irradiation may produce bad odours, which can be prevented if the treatment is performed in an atmosphere of nitrogen. The most important contamination that can occur in cheeses is due to Listeria monocytogenes, which may be present even in pasteurised cheese. These bacteria can proliferate at low temperatures. Table 8. Irradiation of different cheese types Cheese type

Irradiation source

Purpose

Brinsen

60

Increase stored time

Camembert

60

Cottage/Camembert

60

Increase shelf-life Destruction of Listeria and Salmonella Bacterial population destruction

Cheddar

Electrons

Rind decontamination

Fresco

Electrons

Listeria elimination

Gouda

60

Organoleptic changes

Kashar

60

Increase shelf-life

Mozzarela

60

Listeria elimination

Ras

60

Bacteria elimination

Source: Calderón (2000).

Co Co Co

Co Co Co Co

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In France the irradiation of Camembert cheeses elaborated with non-pasteurised milk has been permitted. In general, irradiation used in the elaboration of cheeses enables the use of non-pasteurised milk, prolonging its useful life, while at the same time ensuring the elimination of pathogenic bacteria (Raventós, 2003). Table 8 shows different cases of the application of radiation in the elaboration of different kinds of cheeses.

7.9. Irradiation of Wines and Liqueurs With the aim of preventing the development of bacteria in wines chemical and physical agents are added. The chemical agents used are, among others, sulphurous anhydride, ascorbic acid, caproic and caprilic acids; while physical treatments are in the form of microwaves and ultrasounds; although antibiotics have also been used (nisine and piramicine) and lactic enzymes (lisozimes and cimolases). Nevertheless, studies are being carried out into the possible application of ionizing radiation in the treatment of wines. There are various objectives of radiation, one of which is to use it as an alternative to treatment with carbon dioxide to prevent the development of bacteria and viruses in bottled wines. Irradiation is effective in preventing the souring of wines and enables the prolongation of their commercial life. The ionising irradiation can be applied to improve organoleptic properties such as colour, odour and flavour. Table 9 shows the applications of irradiation in different alcoholic products. Table 9. Irradiation of different alcoholic products Product Brandy (from sweet potato) Bottled beer

Irradiation source γ−Rays

γ−Rays

Target Improvement of organoleptic quality Pathogen reduction Microbial load reduction

Wine

γ−Rays < 0.8 kGy γ−Rays

Prevent bacterial and virus growth in bottled wines Speed up aging process

γ−Rays < 0.6 kGy

Microorganisms elimination Organoleptic properties variation

γ−Rays (2.4 kGy) (70ºC, 10 min) γ−Rays

Increase of shelf-life

γ−Rays Electrons

Microbial load reduction Sour stopping

Wine (Madeira, Rakia) Wine (Romania)

wine (from rice) Grapefruit and pulp

Cork

Source: Calderón (2000).

Sterilization

Effect Bitter taste elimination Taste improvement Dark color appearance Unpleasant taste Possible organoleptic changes Improvement of organoleptic characteristics Discoloration Decrease of pigments and tannins Decrease of SO2 and permanganate content No detection in organoleptic changes Favorable development of organoleptic characteristics s Pulp irradiation produces low quality wines Prevent to unpleasant taste formation

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7.10. Irradiation as a Quarantine Treatment Due to the great demand for food generally no country is self-sufficient and must resort to importation. Imported foods more often than not can contain a high microbial load which can be hazardous. Furthermore, they may contain entomological species which do not belong to the importing country, and if appropriate measures are not taken these can reach plague proportions in a short period of time. The insects contained in foods develop in equilibrium in their natural environment, but when they are transported to another country where their natural predators do not exist they can reproduce without control and cause serious problems. In the face of the possibility of the outbreaks of plagues and their grave consequences, the laws of every country impose obligatory quarantine measures. The difference between a disinfestation treatment and a quarantine treatment should be noted. Disinfestation kills, eliminates and/or inactivates pests in a product at any level, while a quarantine treatment can achieve the disinfestation of a pest put in quarantine at a predetermined level. In some cases an exemption from quarantine can be conceded if there is the guarantee that the appropriate measures have been applied in order to achieve the effective disinfestation of the food. These treatments are normally performed in the country of origin, during transport or at the final destination. Table 10. Quarantine treatment dose by irradiation for several pests Quarantine pest

Geographic distribution

Mexican fruit fly, Anastrepha ludens West Indian fruit fly, A. obliqua Zapote fruit fly, A. serpentina Caribbean fruit fly, A. suspensa Melon fly, Bactrocera cucurbitae Oriental fruit fly, B. dorsalis

Extreme southern Texas to Guatemala Caribbean islands, Mexico to Brazil Mexico to Argentina Florida, Caribbean islands Asia, parts of eastern Africa, Hawaii India to southern China, Hawaii, N. Mariana Islands Northern Australia India to China, Laos, Singapore, Hawaii Australia, New Guinea, New Caledonia, Austral Islands, Society Islands Mediterranean, Africa, Central America and South America, Middle East, Hawaii, Western Australia, North Mariana Islands United States and Canada east of Rocky Mountains, Utah Sub-Saharan Africa, India, Asia, Australia, Oceania,, much of tropical and subtropical Americas

Jarvis fruit fly, B. jarvisi Malaysian fruit fly, B. latifrons Queensland fruit fly, B. tryoni Mediterranean fruit fly, Ceratitis capitata Plum Curculio, Conotrachelus nenuphar Sweet potato weevil, Cylas formicarius elegantulus

Dose (kGy) 0.07 0.1 0.1 0.1 0.21 0.25 0.075 0.15 0.075 0.225

0.092 0.165

Source: Hallman (2001).

One of the most widely used quarantine treatments has been ethylene dibromide, although in 1987 it was prohibited in the USA, owing to its carcinogenic potential (Ruckelhaus, 1984), and in subsequent years other countries have also prohibited it. This provided the opportunity to apply radiation as a quarantine treatment (Hallman 1999, 2000,

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2001; Johnson and Marcotte, 1999). Hence papayas treated with irradiation were initially sent from Hawaii to California, and subsequently in 1995 they were also sent from Hawaii to Chicago treated with a dose of 0.25 kGy, while in August of the year 2000 different tropical fruits were sent from Hawaii and treated in irradiation centres of New Jersey and Illinois (Hallman, 2001). Currently Hawaii possesses irradiation centres for fruit treatment, which enables importation from these islands. In Texas and California guavas irradiated with a dose of 0.15 kGy have been received, with the aim of eliminating the Caribbean fruit fly, while Florida has received approval to send irradiated fruit, and sweet potatoes have been treated with a dose of 0.165 kGy to control the sweetpotato weevil. Various studies have been carried out on different pests at different radiation doses, which are shown in table 10, while table 11 shows the absorbed doses required in order achieving quarantine safety for various groups of pests (Hallman, 2001). Table 11. Absorbed doses that might achieve quarantine security Pest Grup Aphids and whiteflies Seed weevils (Bruchidae) Scarab beetles Fruit flies (Tephritidae) Weevils (Curculionidae) Borers (Lepidoptera) Thrips Borers (Lepidoptera) Spider mites Stored product beetles Stored product moths Nematodes

Objective Sterilize actively reproducing adult Sterilize actively reproducing adult Sterilize actively reproducing adult Prevent adult emergence from third instar Sterilize actively reproducing adult Prevent adult growth from late larva Sterilize actively reproducing adult Sterilize late pupa Sterilize actively reproducing adult Sterilize actively reproducing adult Sterilize actively reproducing adult Sterilize actively reproducing adult

Dose (Gy) 50–100 70–100 50–150 50–150 80–165 100-280 150–250 200–350 200–350 50–400 100–1000 ~ 4000

Source: Hallman (2001).

8. FOOD IRRADIATION PLANTS Food irradiation plants, whatever the process they apply, consist of different elements which are common to all of them. Hence the differential stage is usually that of the treatment applied. It is important to bear in mind that the zone where the products awaiting treatment are stored is separated from the zone where the already-treated products are stored; otherwise there may be cross-contamination. In any treatment plant the following elements can be distinguished. a) Storage zone of the products awaiting treatment. Generally this is located near the loading area of the treatment devices. b) Loading zone. In this zone the products awaiting treatment are loaded in crates or on suitable supports, which are then placed on a conveyor belt. c) Conveyor belt. This is used to transport the products awaiting treatment from the storage zone to the treatment point. This belt also conveys the irradiated products to the treated products storage zone. The belt’s movement should allow the products to receive the adequate dose.

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d) Irradiation zone. This is the main element of any plant, and as it is potentially the most dangerous area it must be isolated from the plant operation personnel. For this reason the treatment chamber is covered with 2 metre thick concrete walls which protect the operators. Inside the chamber there is the source of radiation. Because water is one of the best protections against radiation deriving from the decay of 60Co or 137Cs, the radioactive source is submerged in a pool, and is raised by remote control once the food to be treated is inside the treatment chamber. In the case of the electron accelerator the machine comes on when the product enters the chamber. The trajectory followed by the product should ensure that it receives the dose calculated for the treatment’s objectives; hence the travelling speed of the conveyor belt should be such that the time spent by the product inside the treatment chamber is that required. When the product is large or very dense, it is turned over and irradiated again in order to ensure that it receives adequate treatment. e) For plants which use an electron accelerator it is necessary for them to be equipped with a chamber for the refrigeration circuit. These accelerators are usually compact, and there are different models on the market, with a differentiated range of potentials. One such model has a potential of 35 kW, which emits electrons with energies of between 3 and 10 MeV, although there are also models with potentials of 80 and 150 kW. As for the sources of 60Co o 137Cs, there should be a pool to store these sources, which consist of bars containing the capsules of radioactive material. These bars are kept in deionised water in a covered pool buried at a depth of some 4 metres underground, in order to guarantee operator safety. When the product is treated, these bars rise vertically out of the pool and once the treatment is over they are re-submerged. f) Treated product loading zone. In a part of the plant well away from the loading zone, where the treated product is received. g) Storage zone. Before definitive storage, the product must be suitably labelled and the received dose measured. Then it is taken to the warehouse where it is stored under suitable conditions, so that there is no recontamination of the product. For this reason this zone should be at some distance from the zone where the non-treated product is stored. h) Control laboratory. This is where the dosimetry in all parts of the plant is controlled. It is necessary to measure the dose received by the products, although it is also essential to measure the radiation received by the personnel manning the plant. The two basic types of ionizing radiation treatment plants differ of the source used to provide this radiation. In anyone of them the product is taken to the irradiation area by means of conveyor belt, although they can differ in the form in that the foods are transported; this way, plants based on different concepts of food transport exist. The food to be treated is packed in boxes or pallets that are transported by a conveyor until the irradiation zone; thus, depending on the product-handling concepts there are several models of plants (Kunstadt, 2001). In addition to all of these elements the plant should have a control room, from where all the operations of the plant are controlled, including the speed of the conveyor belt, the systems for raising and lowering the bars of the radioactive source, and all the safety systems of the plant. It should be underlined that this type of plant is completely automated, and that

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all of the mechanisms can be controlled from the control room. Other elements which are found in irradiation plants are offices and auxiliary service rooms.

Figure 6. Tote box irradiator (Courtesy of MDS Nordion).

Figure 7. Pallet conveyor irradiator (Courtesy of MDS Nordion).

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TROLLEY RAIL (RAIL SUPPORTING STRUCTURE NOT SHOWN FOR CLARITY)

REPRESENTATIVE HOIST MECHANISM

RETAINING MECHANISM POOL BELL SURGE TANK

FLOOR LEVEL UNDERGROUND PORTION

EARTH

SOURCE PLENUM

BELL LIMITER PLENUM GUIDE

GRAY*STAR Genesis Irradiator TM

Figure 8. Genesis Irradiator™ (Courtesy of GRAY*STAR, Inc.)).

Figure 9. Electron beam for food irradiation device.

Figure 6 shows a typical plant for a tote box irradiator, while figure 7 shows pallet irradiator. Also, there are irradiation plants where the cobalt-60 radiation source never leaves the shielded pool (figure 8). Product is moved into the pool by special product containers

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Albert Ibarz

(bells) via an overhead hoist and trolley system. At the bottom of the pool, the product is irradiated in a stationary position on two sides of a fixed dry plenum filled with inert helium that contains the source of radiation. Consequently, the irradiator is inherently safe. Instead of lifting the source out of the pool into a shielded chamber, the product is lowered into the pool adjacent to the source. To accomplish this product must obviously be kept dry, and a solution to the problem of lost efficiency that is normally associated with underwater irradiation, had to be found. Figure 9 shows a diagram of a high voltage generator of electron beams applied to the irradiation of food.

9. DOSIMETRY With irradiation treatments it is necessary to be able to determine the dose received by the food, in order to ensure that the received doses are within the limits dictated by the current legislation. A simple dosimetry system capable of covering the whole range of irradiation levels received by the food does not exist. The choice of dosimetry system depends on the requirements of the type of food and the aim of the application. For such purposes the so-called dosimeters are used, which vary according to the value of the dose they must measure. All measuring systems possess a device which shows an effect produced by the radiation, as well as a counter which measures this effect. The commonest effects caused by radiation are optical changes produced in solutions or in solid materials, and these are measured by means of spectrophotometers. For reliable measurement there must be a pre-calibration according to pre-established norms, and this must be appropriate for the situation in which it is going to be used. The products most frequently used in dose measurement are alanine, amino acids, radiometric films, cellulose triacetate, colorants, iron sulphate, K/Ag dichromate, among others, depending on the range of radiation that is to be measured (Ehlermann, 2001). Some dosimeters are routinely used to check whether the food has received radiation; generally these are substances which change colour when irradiated and which enable the user to see at a glance whether the food has been subjected to irradiation for the appropriate period. The advantage of this type of dosimeter is that it can be placed inside the packaging and can tell if the product has been irradiated. The commonest routine dosimeters are radiometric colorants and those that indicate the decomposition of plastics, the former measure the range of 0.1 to 50 kGy, while the latter measure from 5 to 50 kGy.

10. NORMATIVE Throughout the history of food irradiation the process of approval by different countries has been gradual and unequal. Hence, in the former Soviet Union the irradiation of potatoes and grains was unblocked in the years 1958/59, in Canada the irradiation of potatoes was permitted in 1960, while in the USA in the years 1963/64 it was already permitted to irradiate wheat, flour, bacon and potatoes. The Joint FAO/IAEA/WHO Expert Committee on the Wholesomeness of Irradiated Foods (JECFI) declared in 1980 that irradiation with doses of up to 10 kGy does not present toxicological problems, and does not cause nutritional and microbiological problems. However, it wasn’t until after 1984, when the General

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Standardised Codex for Irradiated Foods was published, that many countries started establishing regulations on the irradiation of food. The International Consultative Group on Food Irradiation (ICGFI) was established in the same year under the patronage of the FAO/IAEA/WHO, with one of its aims being that of advising on the application of irradiation, providing the necessary information. Table 1 shows the good practice codes of irradiation for different kinds of food, while table 12 shows the technological limits of the recommended doses for good irradiation practice, published by the consultative group ICGFI. Table 12. Advisory technological dose for good irradiation practice Food type

Purpose

Type 1: bulbs, roots and tubers

To inhibit sprouting during storage To delay ripening Insect disinfestation Shelf-life extension Quarantine control

Type 2: fresh fruits and vegetables (different to type 1)

Type 3: cereals and their milled products, nuts, oilseeds, pulses, dried fruits Type 4: fish, seafood, and their products (fresh or frozen)

Type 5: raw poultry and meat and their products (fresh or frozen)

Type 6: dry vegetables, spices, condiments, animal feed, dry herbs, and herbal teas Type 7: dry food of animal origin Type 8: miscellaneous foods, including by not limited to honey, space foods, hospital foods, military rations, spices, liquid egg, thickeners

Dose Maxima (kGy) 0.2 1.0 1.0 2.5 1.0

Insect disinfestation Microbial load reduction Reduction of pathogenic microorganisms Shelf-life extension Control of infection by parasites Reduction of pathogenic microorganisms Shelf-life extension Control of infection by parasites Reduction of pathogenic microorganisms Insect disinfestation Insect disinfestation Control of molds

1.0 5.0

Microbial load reduction Sterilization Quarantine control

> 10 > 10 > 10

5.0 3.0 2.0

7.0 3.0 2.0

10.0 1.0 1.0 3.0

Source: IAEA (1998), Molins (2001).

In 1999 some 45 countries possessed authorization for the irradiation of one or more food products (ICGFI, 1999). Unfortunately, national regulations on the irradiation of food are different from one country to the next, and furthermore there is no international agreement on packaging materials used in the irradiation of food. In the USA and the UK regulations on irradiation have been adopted based on types of food, something which has not been done in most countries. For several years now efforts have been made to promote the harmonization of the regulations existing in different countries which have approved the irradiation of food. For this purpose work is taking place in the development of a model based on the General

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Standardised Codex for Irradiated Foods, although it also incorporates diverse concepts contained in the good practice codes of irradiation (table 1); however, due to the specific needs of each country or region there are four variations of a basic model, one applicable for the Asian/Pacific region, one for Africa, a third for Latin America and the Caribbean and finally a fourth for the Near East (Molins, 2001). In a similar fashion as to what occurs on a world scale the situation in the European Union varies according to the country, and there is a heated debate about the irradiation of food. However, in 1999 directives were approved in order to permit the irradiation of different food products. Likewise, the conditions under which the irradiated products should be labelled are established as well as the types of ionizing irradiation sources permitted and the methodology for measuring the doses received by the food. Furthermore, the member states are obliged to set in motion the legal, regulatory and administrative dispositions that must be fulfilled. In August 2001 a communication of the Commission of the European communities was published regarding the foods and ingredients authorized for treatment with ionizing radiation. All of the existing regulations on irradiated foods in the European Union can be found on the corresponding web (European Commission; European Food Safety Authority-EFSA). Likewise, there are also the Directives 1999/2/CE and 1999/2/CE of the European Parliament and the Council of the European Union, published in the Official Bulletin of the European Communities (13th March 1999), relating to the laws that govern foods and food ingredients treated with ionizing radiation, including dry aromatic herbs, spices and plant-based seasonings. Moreover, in the Official Bulletin of the European Union of 11th March 2003 a list is provided of the foods and food ingredients which the member states have authorized for treatment with ionizing radiation.

REFERENCES Akamine, E.K.; Moy, J.H. In: Preservation of Food by Ionizing Radiation; Vol. 3. Josephson, E.S. and Peterson, M.S.; Eds.; CRC Press: Boca Raton, FL, 1983; pp 129-158 Ahmed, M. (2001). In: Food Irradiation. Principles and Applications; Molins, R.; Ed.; Wiley-Interscience: New York, NY, 2001; pp: 77-112 Booth, R.H.; Proctor, F.J. PANS, 1972, 18, 409-430 CAC. Codex General Standard for Irradiated Foods, Codex Alimentarius Comission, CAC/, 1984; Vol. XV, E-1, Codex Stan 106-1983, Joint FAO/WHO Food Standards Programme, FAO Rome Calderón, T. La Irradiación de Alimentos. Principios, Realidades y Perspectivas de Futuro; McGraw-Hill, Madrid, Spain, 2000 CSN (1992a). Consejo de Seguridad Nuclear. Dosis de Radiación, Madrid, Spain [http://www.csn.es/] CSN (1992b). Consejo de Seguridad Nuclear. Protección Radiológica, Madrid, Spain [http://www.csn.es/] Dauphin, J.F.; Saint-Lèbe, L.R. (1977). In: Radiation Chemistry of Major Food Components; Elias, P.S. and Cohen, A.J.; Eds.; Elsevier Scientific: Amsterdam, 1977; pp: 131-220 del Rivero, J.M.; Cornejo, J. Bol. Patol. Veg. Entomol. Agric., 1971, 31, 71-75

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Delincée, H. In: Recent Advances in Food Irradiation; Elias, P.S. and Cohen, A.J.; Eds.; Elsevier Biomedical Press: Amsterdam, 1983a; pp: 89-114 Delincée, H. In: Recent Advances in Food Irradiation; Elias, P.S. and Cohen, A.J.; Eds.; Elsevier Biomedical Press: Amsterdam, 1983b; pp: 129-147 Dickson, J.S. 2001). In: Food Irradiation. Principles and Applications; Molins, R.; Ed. Wiley-Interscience: New York, NY, 2001, pp: 23-36 Diehl, J.F; Josephson, E.S. Acta Alimentaria 1994, 23(2): 195-214 Diehl, J.F. Food Control 1991, 2 (1): 20-25 Diehl, J.F (1995). In: Safety of Irradiated Foods; 2nd ed., Marcel Dekker: New York, NY; pp: 43-88 Farkas, J. Acta Alimentaria 1987, 16, 351-384 Farkas, J. (2001). In: Food Irradiation. Principles and Applications; Molins, R.; Ed.; WileyInterscience: New York, NY, 2001; pp: 291-312 Ehlermann, D. In: Food Irradiation. Principles and Applications; Molins, R.; Ed.; WileyInterscience: New York, NY, 2001; pp: 387-414 Fielding, L.M.; Cook, P.E.; Grandison, A.S. J. Appl. Bacteriol. 1994, 76, 412-416 Fruin, J.T.; Kuzdas, C.D.; Guthertz, L.S. Proc. Eur. Meeting of Meat Research Workers, 26, 1980; Vol. I, E-20, p. 241 Hallman, G.J. Postharvest Biol. Technol. 1999, 16, 93-106 Hallman, G.J. J. Agric. Forest Entomol. 2000, 2, 1-11 Hallman, G.J. In: Food Irradiation. Principles and Applications; Molins, R.; Ed.; WileyInterscience: New York, NY, 2001; pp: 113-130 Hannan, R.S.; Shepherd, H.J. J. Sci. Food Agric., 1959, 10, 286-295 Heildelbaugh, N.D.; Giron, D.J. J. Food Sci. 1969, 34, 239-241 Hunter, W.D. J. Econ. Entomol., 1912, 5(2): 188 IAEA. Report of Joint AAEA/FAO/IAEA Regional Workshop on Present Status and Guidelines for Preparing Harmonized Legislation on Food Irradiation in the Near East, Tunis, Tunisia, Oct. 12-16. IAEA, Vienna, 1998 ICGFI. Database of Food Irradiation Clearances, 1999, (in http://www.iaea.org/icgfi) Isenberg, F.M. Proc. Am. Soc. Hortic. Sci. 1956, 63, 343-348 Johnson, J.; Marcotte, M. Food Technol., 1999, 53(6), 46-51 Kilgen, M.B. In: Food Irradiation. Principles and Applications; Molins, R.; Ed.; WileyInterscience: New York, NY, 2001; pp: 193-211 Kotula, A.W. Food Technology 1983, 37(3), 91-94 Kreiger, R.A.; Snyder, O.P.; Pflug, I.J. J. Food Sci. 1983, 48, 141-145 Kunstadt, P. In: Food Irradiation. Principles and Applications; Molins, R.; Ed.; WileyInterscience: New York, NY, 2001; pp: 415-442 Lee, M.; Sebranek, J.G.; Olson, D.G.; Dickson, J.S. J. Food Prot. 1996, 59, 62-72 Lorenz, K. CRC Crit. Rev. Food Scie. Nutr., 1975, 6, 317-382 Mallet,J.C.; Beghian, L.E.; Metcalf, T.G.; Kaylor, J.D. J. Food Safety, 1991, 11, 231-245 Metlitsky, L.V. Cited by P. Thomas. In Food Irradiation. Principles and Applications; Molins, R.; Ed.; Wiley-Interscience: New York, NY, 2001; pp: 241-272 Molins, R. In Food Irradiation. Principles and Applications; Molins, R.; Ed.; WileyInterscience: New York, NY, 2001; pp: 1-21 Moseley, B.E.B.. Photchem. Photbiol. Rev., 1976, 7, 223-274 Narvaiz, P.; Lescano, G.; Kariyama, E.; Kaupert, N. J. Food Safety 1992, 12, 263-282

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Nawar, W.W. Radiation Chemistry of Major Food Components; Elias, P.S. and Cohen, A.J. Eds.; Elsevier Scientific: Amsterdam, 1977; pp: 21-61 Nawar, W.W. J. Agric. Food Chem. 1978, 26(1), 21-25 Nickerson, J.F.R., Licciardello, J.J., and Ronsivalli, L.J. In Preservation of Food by Ionizing Radiation; Josephson, E.S. and Peterson, M.S. Eds.; CRC Press: Boca Raton, FL, 1983; pp: 12-82 NRPB Living with Radiation. National Radiological Protection Board, Her Majesty’s Stationary Office, London, UK; 1986 Onyenekwe, P.C.; Ogbadu, G.H.; Hasimoto, S. Postharvest Biol. Technol. 1997, 10, 161-167 Phillips, B.J.; Kranz, E.; Elias, P.S. Food Cosmetic Toxicol. 1980, 18, 471-475 Pollard, E.C. (1966). In Enciclopedia of Medical Radiology; Zuppinger, A. Ed.; SpringerVerlag: New York, NY, 1966; Vol. 2 Raventós, M. Indústria Alimentària. Tecnologies Emergents; Edicions UPC: Barcelona, Spain, 2003; pp: 101-127 Renner, H.W.; Graf, U.; Wurgler, F.E.; Altmann, H.; Asquith, J.; Elias, P.S. Food Chem. Toxicol., 1982, 20, 867-878 Ruckelhaus, W.D. Fed. Reg., 1984, 49(70), 14182-14185 Simic, M.G. In Preservation of Food by Ionizing Radiation; Josephson, E.S. and Peterson, M.S. Eds.; CRC Press: Boca Raton, 1983; Vol. II , pp: 1-73 Smith, O. Potato: Production, Storage and Processing; AVI Publishing: Wesport, CO; 1977 Sokhey, A.S.; Hanna, M.A. Food Struct. 1993, 12, 397-410 Sparrow, A.H.; Christensen, E. Am. J. Bot. 1950, 37, 667-371 Stewart, E.M. In Food Irradiation. Principles and Applications; Molins, R.; Ed.; WileyInterscience: New York, NY, 2001; pp: 37-76 Sugimoto, B.M.; Cherry, W.B.; Dodd, D.J. Appl. Environ. Microbiol. 1986, 34, 602-603 Sullivan, R.; Scarpino, P.V.; Fassolitis, A.C.; Larkin, E.P.; Peeler, J.T. Appl. Microbiology 1973, 22, 61-65 Swallow, A.J. In Radiation Chemistry of Major Food Components; Elias, P.S. and Cohen, A.J.; Eds.; Elsevier Scientific: Amsterdam, 1977; pp: 5-20 Thayer, D.W. J. Food Quality, 1990, 13, 147-169 Thayer, D.W.; Fox Jr., J.B.; Lakritz, L. In: Food Irradiation; Thorpe, S. Ed.; Applied Science: London, 1991; pp: 285-325 Thayer, D.W. Food Technology In Food Irradiation. Principles and Applications, Molins, R.; Ed.; Wiley-Interscience: New York, NY, 2001;, 1994, 48(5): 132-135 Thayer, D.W.; Rajowky, K.T. Food Technology 1996, 53(11), 62-65 Thomas, P. In Food Irradiation. Principles and Applications; Molins, R.; Ed.; WileyInterscience: New York, NY, 2001a; pp: 213-240 Thomas, P. In Food Irradiation. Principles and Applications; Molins, R.; Ed.; WileyInterscience: New York, NY, 2001b; pp: 241-272 UNSCEAR United Nations Scientific Committee on the Effects of Atomic Radiation. Sources, Effects and Risks of Ionising Radiation. Report to the General Assembly, United Nations. New York, 1998 Urbain, W.M. Food Irradiation, Academic Press, London, 1986 Web, M; Penner, K.P. Food irradiation. MF-246, Kansas State University, Kansas, ,2000 WHO WHO, Wholesomeness of Irradiated Foods. Technical Report Series 659, Geneva, 1981

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Willemot, C.; Marcott, M.; Deschenes, L. In: Processing Fruits: Science and Technology; Somogy, L.P. and Ramaswamy, H.P. Eds.; Technomic Publishing: Lancaster, 1996; Vol. 1, pp: 221-260

Electronic Media Resources Codex Alimentarius (FAO/OMS): http://www.codexalimentarius.net/ European Commission: http://www.europa.eu.int/comm/ Consejo de Seguridad Nuclear (CSN)-Spain: http://www.csn.es/ European Food Safety Authority (EFSA): http://www.efsa.eu.int/ Foundation for Food Irradiation Education (FFIE): http://www.food-irradiation.com/ International Consultative Group on Food Irradiation (ICGFI): http://www.iaea.orat/icgfi/ Nuclear Energy Institute: http://www.nei.org/ Ministerio de Sanidad y Consumo (Spain): http://www.msc.es/

In: New Food Engineering Research Trends Editor: Alan P. Urwaye, pp. 45-101

ISBN: 978-1-60021-897-2 © 2008 Nova Science Publishers, Inc.

Chapter 2

FRUITS AND VEGETABLES DEHYDRATION IN TRAY DRYERS Dionissios P. Margaris*1 and Adrian-Gabriel Ghiaus*2 1

Fluid Mechanics Lab., Mechanical Engineering and Aeronautics Dept., University of Patras, GR-265 00, Patras, Greece 2 Technical University of Civil Engineering – Bucharest Bd. P. Protopopescu nr. 66, RO- 021414, Bucuresti, ROMANIA

ABSTRACT Dehydration involves simultaneous transfer of heat, mass and momentum in which heat penetrates into the product and moisture is removed by evaporation into an unsaturated gas phase. Owing to the complexity of the process, no generalized theory currently exists to explain the mechanism of internal moisture movement. In this Chapter, the investigation of momentum, heat and mass transfer phenomena, in both laboratory and large scale convective drying systems (suitable for dehydration of thermolabile products) by means of experimental measurements and numerical simulation are presented. The air flow inside complex geometry spaces, such as drying rooms containing hundreds of trays arranged in rows and columns, is analyzed by solution of 3-D momentum turbulent flow equations for different room configurations. Laboratory measurement data, concerning the space velocity distribution and the pressure field of the air flow over one tray, are provided and used for validation of turbulence models. The results of the flow investigation lead to practical suggestions for the improvement of the air flow uniformity inside the drying space which is very important for the quality of the product. A novel numerical code, DrySAC, able to predict the unsteady-state processes taking place in a complex drying system, was developed. Unlike other attempts to predict drying processes, DrySAC takes into account not only the drying process itself, but also the behavior of the other system equipment and the interaction between them. Drying curves, evolution of the air state parameters in characteristic points of the system and product * *

Dionissios P. Margaris: Tel.: +30.2610.997193, Fax: +30.2610.997202, E-mail: [email protected] Adrian-Gabriel Ghiaus: Tel.: +40.21.2524280, Fax: +40.21.2526880, E-mail: [email protected]

46

Dionissios P. Margaris and Adrian-Gabriel Ghiaus properties are predicted during the drying of various fruits and vegetables and. As a practical validation of the code, the predicted values compared with the measured data taken in-situ showed very good agreement. When a dryer configuration is given, the numerical DrySAC code can be used for optimization of the process parameters when a dryer configuration is given. For the most of the studied cases, an air recirculation ratio of around 75 % has proved to be the optimum, giving a minimum drying time. The code can be used both for evaluation of existing dryers and for optimum design of the new units with valuable impacts in increasing the efficiency of the systems and in reduction of energy consumption. Aiming to overcome the lack of experimental data in the open literature, a laboratory drying unit was constructed and is under operation for testing and monitoring the dehydration of agricultural products. Using this facility, experimental drying curves are set up for the drying of horticulture products under controlled conditions of the drying air parameters, which are gathered by means of a data acquisition system. The laboratory experimental results are useful for the validation of numerical models which further are an essential tool for optimization and increasing the efficiency of the drying process. Drying of agricultural products remains an open research field mainly because of their delicate and hard to be established, properties.

1. INTRODUCTION In an extremely varied range of types and industries, drying processes are used to obtain dry products, the performance of which is often determined by morphological aspects created in the drying process. In process optimization and drier design the strong non-linearity of the equilibrium relationship and the internal resistance of water diffusion prohibit the use of effective overall mass-transfer coefficients, as it is common used in chemical engineering. The complexity of materials, in composition as well as in (multi-phase) structure, strongly limits the linking of diffusion coefficients and sorption isotherms to materials on the basis of first principles, in contrast to the data bank and group estimation methods in chemically welldefined mixtures. The exchange of mass, heat and momentum between air and product demands specific models for each drier, as it depends on geometry. Morphological properties are dependent on the material (deformation) properties which change in the transient process. Drying is generally considered an intensive energy process, and because of this it is often mentioned in national and international research programs.

1.1. Need for Drying The reasons for drying are as diverse as the materials which may be dried. However, most of the materials investigated in drying processes are agricultural and food products. A dried product must be suitable for either subsequent processing or sale, [Keey, 1982]. Although drying is considered a unit operation, it covers a rather diverse field and many configurations for drying equipment exist. Compared to other classical unit operations, this diversity in technology has been and remains an intrinsic obstacle in the development of scientific understanding of the drying process. From an economical point of view, there is only one good reason for drying, namely making profit. This requires that the drying process:

Fruits and Vegetables Dehydration in Tray Dryers • • •

47

adds value to the product (quality, specifications), is performed at minimum cost (design, control, optimization, equipment, technology, energy, other resources), and is achieved within the criteria, imposed by society (safety, environmental protection, loss prevention).

These three factors are essentially the main driving forces for research and development. Nevertheless, the personal motives of individual researchers should not be ignored. With well-defined physical properties, it is possible to predict the heat and mass transfer in food processing and to solve the heat and mass transfer equations. For geometrically simple systems, mass and heat transfer equations can be solved analytically, but for complex systems, a computer program is required to solve the equations numerically. Furthermore, the porous structure and the changes taking place during the drying process will give rise to further problems in the calculations. Sophisticated models for mass and heat transfer in drying materials are very important for developing a better understanding of the drying process at the microscopic scale. These rigorous models incorporate a maximum of physical relevance and are not easy to handle. Most model parameters depend on moisture content and temperature and are laborious to establish experimentally. In most cases drying kinetics is characterized by means of drying curves, which represent an overall drying behavior of the sample and from which it is rather difficult to derive reliable intrinsic material properties. Taking into account: • • • •

the large amount of dryer configurations and the many, varied materials to be dried, the modeling at different scales, from micro- (pores, molecules), meso- (particle) to macro- scale (incremental or total dryer), and and the balance between physically meaningful and practically manageable models, one can explain the great diversity in theoretical approaches of drying processes.

Modeling flow patterns and particle trajectories has shown increased interest in using Computation Fluid Dynamics (CFD) due to the availability of excellent commercial software in this field. It is without any doubt that CFD shows great potential in the practical design of dryers. The combination of CFD modeling and sophisticated drying kinetics seems to be a future objective. Stable, accurate and fast computer programs for these rigorous approaches are still hard to develop and accordingly, the need for simplified models will persist in the future. It is generally felt that technology management in industry is rather conservative. There is often a lack of qualified personnel with drying expertise, and it may be that drying is just one of their many duties, therefore not taking highest priority. Furthermore, one has to admit that in many cases the trial and error approach leads more quickly to a working solution than scientific routes. Especially for food products, enhancement of product sales can be better achieved via advertisements, commercials and attractive packaging rather than improving the product quality via more risky research efforts. On the other hand, the academic world produces a great diversity of drying models which do not always show a drive for putting theory into real industrial practice. A possible solution for bringing industry and academia closer together should take into consideration that there is a need for:

48

Dionissios P. Margaris and Adrian-Gabriel Ghiaus • •

good, reliable software development within industry, and practical laboratory methods for establishment of the relevant material properties.

The field of dehydration is full of puzzles, inconsistencies, disputed observations and conflicting interpretation. An enormous amount of well designed experimental work needs to be done, leading into many lines of theoretical inquiry and technological advance. Advances in understanding the thermodynamics of moist materials and better solutions to the equations of change would obviate the need for empiricism in selecting optimum drying conditions, [Luikov, 1970]. More detailed knowledge of the mechanism of moisture transport would undoubtedly be needed if drying is to become more of a science than an art.

1.2. Historical Features Humans have benefited from dried foods since the Cro-Magnon era. Sun and wind fostered the first dried foods. Early man copied the drying process he observed in nature. Dried grasses, seeds, fruits and nuts were gathered and stored. Samples of dried foods dating to 4000 years before present have been found in both ancient settlements in Jericho and Egyptian tombs. The first patent for dehydration was granted in England in 1780 to J. Grafer, who scalded vegetables in boiling, salt water and then dried them in a heated room. Later, the Royal Navy expedition that searched for the British arctic explorer Sir John Franklin in 1852 carried carrots and potatoes that had been dried in hot air for 20 to 30 hours, [Borgstrom, 1971]. During the Crimean War (1853-1856), an attempt was made to combat scurvy by using potatoes dried by Edwards’ patented process, which was to boil them, press them through small holes to form fine spaghetti-like threads, and dry them on steam-heated plates. Many air dried vegetables were produced during World War I, [Borgstrom, 1971].

1.3. Drying of Foods Throughout history, man has learned that removal of water increases the period of usefulness of perishable products. The lower storage and transportation costs associated with the reduction of weight and volume due to water removal have provided additional economic incentives for widespread use of dehydration processes, [Rizvi, 1986], features especially important for developing countries and in military feeding and space food formulation, [Jayaraman, 1992]. From a theoretical point of view, dehydration of porous materials, such as foods, is a rather complex process. It involves interactions not only between heat and mass transfer processes occurring within the food itself, but also between the food and the drying medium circulating around the solid matter. Successful outcome of these competing phenomena requires simultaneous solution of the separate differential equations for heat, mass and momentum transport within the food system being dried and in the external drying medium. Coupling of the two processes at the surface of the solid for general theoretical solutions to the overall drying problem, along with the dependence of the transport coefficients on the values of driving forces, presents serious computational problems. Further complications arise

Fruits and Vegetables Dehydration in Tray Dryers

49

as a result of the lack of thermo-physical property data for real foods. Clearly, much work is needed toward obtaining numerical or analytical solutions of the basic differential equations for the dehydration of real foods under practical drying conditions. The delicate characteristics of foods require the skilful operation and design of dehydration systems, and this requires an understanding of the principles of dehydration. The conditions under which a product should be dried (drying time, drying temperature, amount of heat to be supplied, amount of water vapor to be removed, etc.) vary greatly because the products to be dried are very different from each other in nature and in properties such as shape and dimensions, moisture content and temperature sensitivity, [Leniger, 1975]. Besides providing crude fiber and bulk, fruits and vegetables are indispensable sources of essential dietary nutrients, vitamins and minerals. However, due to their high moisture content (above 80 %) they are highly perishable, [Jayaraman, 1992]. Total world production of dried grapes averages about 700,000 tons per year. The major producers are: U.S.A. (35 %), Turkey (31 %) and Greece (15 %). About 40 % of total production reaches the international market and is mainly directed to the industrialized countries of Europe, where dried grapes are used as ingredients by the confectionery industry, [Riva, 1986]. The preservation of fruits and vegetables by dehydration offers a unique challenge. To achieve the desired results for dehydration of fruits and vegetables, the process must provide the optimum heat and mass transfer within the product. Only through analysis and understanding of these processes can maximum efficiency and optimum quality be achieved, [Singh, 1993]. Due to changing lifestyles, especially in the developed world, there is currently great demand for a wide variety of dried products with emphasis on high quality and freshness as well as convenience. This calls for sustained basic research on drying conditions and equipment and their influence on food qualities, [Jayaraman, 1992].

1.4. Food Quality and Safety Scientific research and development in agricultural engineering fosters optimum utilization of available human, physical and financial resources. Improved technologies for drying fruits and vegetables have been introduced to reduce losses arising from seasonal gluts. In most developing countries, less than 20 percent of agricultural output undergoes industrial processing compared with 80 or more percent in developed countries. The safety of food is essential for the health and well-being of man, and its quality for his satisfaction. Bacteria, yeasts, moulds, insects and rodents are in constant competition with man for his food supply. Foods are also subject to destruction by almost every variable in the natural environment. Heat and cold, light, oxygen, moisture, dryness and natural enzymes within the food, all tend to cause deterioration. Scientific advances and better knowledge through research have supported older and newer technologies alike in their ability to ensure the safety and quality of the processed food supply. Without knowledge of the drying mechanism there is no sound way to predict methods for increasing the drying rate or for improving product quality or retention of nutritional value.

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Dionissios P. Margaris and Adrian-Gabriel Ghiaus

1.5. Objectives of the Research While computer models have been increasingly successful in simulating an ever widening range of engineering problems, it is nevertheless essential that advances in these models are validated and verified by experimentation. Experimental measurements are themselves conditioned to the requirements of the computational models. Accordingly, it is important that scientific work on experimental facilities must be correlated with research in developing computer codes as well as with monitoring and measurements on full-scale prototypes. The orderly and progressive concurrent development of all these fields is essential for the progress of engineering sciences. This chapter presents research carried out by numerical and experimental investigation on the operation of convective drying systems used for dehydration of agricultural products. The objectives of the research have been: to investigate the air flow field inside large capacity tray drying rooms, to develop a numerical code suitable for simulating the operation of a drying system, to predict the drying time and the drying process parameters, to evaluate experimentally the drying parameters for specific products, to improve the uniformity of air distribution inside the drying room, to minimize the drying time, to increase the efficiency of the drying system by optimizing the process parameters, and finally to reduce the energy consumption.

2. FUNDAMENTALS OF DRYING PROCESSES Dehydration involves the simultaneous transfer of heat, mass and momentum in which heat penetrates into the product and moisture is removed by evaporation into an unsaturated gas phase. Owing to the complexity of the process, no generalized theory currently exists to explain the mechanism of internal moisture movement. Although it is now accepted that in most practical situations of air drying of foods the principal rate-determining step is internal mass transfer, there is no agreement on the mechanism of internal moisture movement, [Chirife, 1983]. In the case of capillary-porous materials such as fruits and vegetables, interstitial spaces, capillaries and gas-filled cavities exist within the food matrix and water transport takes place via several possible mechanisms acting in various combinations. The possible mechanisms proposed by many workers include: • • • • • • •

liquid diffusion caused by concentration gradients, liquid transport due to capillary forces, vapor diffusion due to shrinkage and partial vapor-pressure gradients (Stefan’s law), liquid or vapor transport due to the difference in total pressure caused by external pressure and temperature (Poiseuille’s law), evaporation and condensation effects caused by differences in temperature, surface diffusion in liquid layers at the solid interface due to surface concentration gradient, liquid transport due to gravity.

Fruits and Vegetables Dehydration in Tray Dryers

51

Additionally, moisture may also be transported inside a material if a suitable temperature gradient exists (thermo-gradient effect), because of thermodynamic coupling of heat and mass transport processes. Most foods are classified as capillary porous rigid or capillary porous colloids, [Bruin, 1980]. Therefore, it is often proposed that a combination of capillary flow and vapor diffusion mechanisms should be used to describe internal mass transfer. Water activity, rather than moisture content, influences biological reactions. In the regions of water adsorption on polar sites or when a mono-molecular layer exists, there is little enzyme activity. Enzyme activity begins only above the region of mono-molecular adsorption. When the moisture content of a substrate is reduced below 10 %, microorganisms are no longer active. It is necessary however to reduce the moisture content to below 5 % in order to preserve nutrition and flavor, [Charm, 1978].

2.1. Dehydration Principles In air-drying processes, two drying periods are usually observed: an initial constant-rate period in which drying occurs as if pure water was being evaporated and a falling-rate period where moisture movement is controlled by internal resistance. Figure 1a illustrates this by showing the moisture content as a function of time, where segment A-B represents the initial unsteady-state warming-up period, B-C the constant rate period and C-D-E the falling-rate period.

Figure 1. Drying curves showing: a) moisture content vs. drying time, b) drying rate vs. drying time, and c) drying rate vs. moisture content.

Figure 1b shows the drying rate as a function of time, whereas in figure 1c the drying rate is plotted against moisture content. The drying rate during the constant-rate period may be computed using either the heat transfer or mass transfer equation. As the surface of the material maintains a saturated condition and its temperature is the wet-bulb temperature of the drying air, the drying rate is given as:

52

Dionissios P. Margaris and Adrian-Gabriel Ghiaus

&v= −m

h t ⋅ A ⋅ ( Tdb − Twb ) = k p ⋅ A ⋅ (pw − pv ) h lg

heat transfer eq.

(2.1)

mass transfer eq.

& v -drying rate, h t -global heat transfer coefficient, A-area of the where the symbols are: m drying surface, Tdb -dry-bulb temperature of moist air, Twb -wet-bulb temperature of moist air, h lg -latent heat of vaporization, k p -mass transfer coefficient pressure-basis, p w -partial pressure of water vapor at saturation, and p v -partial pressure of water vapor of the drying air. The drying rate is actually the mass flux of water vapor evaporated from the product. As moisture content of the material decreases during the drying process, the drying rate has negative value. The negative sign of the drying rate in Eq.(2.1) is a consequence of the fluxes’ direction convention (positive from the drying air to the material).

(

)

The temperature difference Tdb − Twb is often defined as the wet-bulb depression. The driving force for vaporizing water from the surface is the difference between the vapor pressure of water at the temperature of the surface and the partial pressure of water in the air. Eq.(2.1) can be used to calculate the drying rate in the case of flat surfaces and can only be applied to plates, sheets, etc. In other cases, it may be better to relate the drying rate to the weight of the material rather than the drying surface area. In practice, the heat transfer equation gives a more reliable estimate of the drying rate than the mass transfer equation because its parameters can be measured directly. The first falling-rate period is the period of unsaturated surface dehydration. During this period, increasingly larger proportions of dry areas appear on the surface as drying progresses. In many food materials, the migration of moisture occurs through the mechanism of diffusion. In practice the diffusion coefficient is dependent, to some extent, on the moisture content. To estimate the average drying time during the first falling-rate period, Fick’s second law of diffusion is widely used. Assuming an idealized system with a constant diffusion coefficient, the partial differential equation for one-dimensional diffusion is given as:

∂X = D eff ∂t

⎛ ∂ 2 X C ∂X ⎞ ⋅ ⎜⎜ 2 + ⋅ ⎟⎟ z ∂z ⎠ ⎝ ∂z

(2.2)

where X is the moisture content dry-basis of the material, t is the time, D eff is the effective diffusion coefficient, z the distance, and C a constant of 0 for planar, 1 for cylindrical, and 2 for spherical geometry. Assuming a uniform initial moisture distribution and in the absence of any external resistance, the analytical solutions of Fick’s law for a slab of material are given in the form of infinite series, [Chirife, 1983]:

Fruits and Vegetables Dehydration in Tray Dryers

X − Xe 8 ∞ 1 = 2⋅∑ ⋅ exp X i − X e π n = 0 ( 2n + 1) 2

⎡ ⎤ ⎢− ( 2n + 1) 2 ⋅ π 2 ⋅ D eff ⋅ t ⎥ ⎢ z2 ⎥ ⎣ ⎦

53

(2,3)

where X is the average, X e the equilibrium and X i the initial moisture content dry-basis. For long drying times and for an un-accomplished moisture ratio less than 0.6,

[ (X − X e ) / (X i − X e )] < 0.6

(2.4)

generally only the first term of the infinite series (Equation 2.3) is used to estimate the drying rate:

X − Xe 8 = 2 ⋅ exp ( − K ⋅ t ) Xi − Xe π

(2.5)

where

K = π2 ⋅

D eff z2

(2.6)

−1 is termed the drying constant (dehydration constant) and has the unit of measurement sec . The drying analysis presented above is based on the assumption that the heat transfer effects can be neglected and drying can be treated as a purely diffusion controlled mass transport phenomenon with a constant, effective diffusion, coefficient. This approach is based on several experimental studies, which indicate the existence of very small internal temperature gradients within foods during drying, [Chirife, 1983]. When the foodstuff is a porous solid, the mass flux involved is one of vapor through the intercellular spaces and hence, there is a need to include porosity in the mass transport equation, [Lozano, 1980]:

ε⋅

&v ∂ρ v 1 ∂m ∂X + ⋅ = −(1 − ε ) ⋅ ρ ds ⋅ ∂t A ∂z ∂t

(2.7)

where ε is the porosity, ρ v the density of water vapor, t the time, A the drying area, z the thickness of the material, and ρ ds the density of the dry product. During the second falling-rate period, drying occurs at a moisture content where the equilibrium relative humidity is below saturation and heat transfer should be considered along with mass transfer because desorption of moisture requires consumption of substantial amounts of heat. The diffusion coefficient is moisture content dependent and Fick’s second law of diffusion for an infinite slab can be written as:

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Dionissios P. Margaris and Adrian-Gabriel Ghiaus

∂X ∂ ⎛ ∂X ⎞ = ⎜ D eff ⋅ ⎟ ∂t ∂z ⎝ ∂z ⎠

(2.8)

Diffusion-like theories of drying cannot describe the complete spectrum of moisture transport mechanisms that occur during the drying of granular porous medium and therefore gas-phase momentum should be included in any comprehensive theory. This incorporates the liquid- and vapor-phase continuity equations, combines the liquid-, solid- and vapor-phase thermal energy equations into a single temperature equation and makes use of Darcy’s law for the liquid phase to account for moisture transport due to capillary action, [Whitakar, 1984]. If the rate of heat transfer to the material is sufficiently high, vaporization takes place within the material and the rate of drying is determined by the heat transfer rate into the material. Among the most important parameters in the evaluation, design and specification of drying systems are the energy requirements and drying times. With few exceptions, existing drying models have taken the heat of vaporization to be constant and invariant with moisture content. This is in fact true in most drying situations. For most foods the enthalpy of evaporation does not differ significantly from the latent heat of vaporization of pure water until moisture contents of 0.1 kg w/kg ds or less are reached. Most commercial operations do not dry materials to this degree. The general case of food material dehydration involves energy inputs to meet the following energy requirements: • • •

Removal of free water through evaporation, Removal of water associated with the food matrix, Superheating of water vapor evaporated as it passes through the food.

2.2. Batch Drying Systems Batch drying systems are also called tray or compartment dryers and consist of an enclosed, insulated housing in which solids are placed upon tiers of trays in the case of particulate solids, or stacked in piles or upon shelves in the case of large objects. The drying material bed is in a static condition in which each particle rests upon another at essentially the settled bulk density of the solids phase (figure 2). During drying, there is a permanent & v ) between the air stream and the bed surface. exchange of heat ( q& ) and water vapor ( m Specifically, there is no relative motion among solid particles. Satisfactory operation of tray-type dryers depends on maintaining a constant temperature and uniform air velocity over all the material being dried. Circulation of air at velocities of 1 to 10 m/s is desirable to improve the surface heat transfer coefficient and to eliminate stagnant air pockets. Proper air flow in tray dryers depends on sufficient fan capacity, the design of ductwork to modify sudden changes in direction, and on properly placed baffles. Non-uniform air flow is one of the most serious problems in the operation of tray dryers.

Fruits and Vegetables Dehydration in Tray Dryers

55

Figure 2. Parallel air flow over a static bed of solids.

Tray dryers may be of the tray-truck or the stationary-tray type. In the former, the trays are loaded on trucks which are pushed into the dryer; in the latter, trays are loaded directly into stationary racks within the dryer. Trays may be square or rectangular, with 0.5 to 1 m2 per tray, and may be fabricated from any material compatible with corrosion and temperature conditions. When the trays are stacked in the truck, there should be a clearance of not less than 4 cm between the material in one tray and the bottom of the tray immediately above. When material characteristics and handling permit, the trays should have screen bottoms for additional drying area. Batch dryers are the most popular family of convective industrial dryers used in the food industry undertaking the processing of all materials that need to be dried for an extensive time period (usually days). Performance of these dryers depends on several factors related to dryer design, product type and on conditions of the drying air. Over a time interval from tn to tn+1, the fall in moisture content of the drying material is related to the increase in air humidity over the tray from the inlet value x in to the outlet value x out :

& a⋅ − m ds ⋅ ( X n +1 − X n ) = m

t n +1

∫ ( x out − x in ) ⋅ dt

(2.9)

tn

where m ds is the mass of dry solid, X n +1 and X n are the moisture content dry-basis at

& a the mass flow rate of air. time n+1 and n, respectively, and m

(

)

For an infinitesimal time interval t n +1 − t n → dt , the drying rate is:

& v = − m ds ⋅ m

dX & a ⋅ ( x out − x in ) =m dt

(2.10)

56

Dionissios P. Margaris and Adrian-Gabriel Ghiaus So, the outlet air humidity can be calculated as:

& m x out = x in + v &a m

(2.11)

Over a given batch, x out rises rapidly from x in as drying begins to reach a maximum value, then falls as the drying rate declines towards the end of the process. Under quasi-steady conditions, when the accumulation of heat by the moist material is & a becomes: zero, the energy balance for an intake of mass flow rate of drying air, m

& F +Q & H − h out ⋅ m &L =0 & a +Q & a −Q h in ⋅ m

(2.12)

& is the fan where h in and h out are the specific enthalpy at inlet and outlet, respectively, Q F & the heat input from the heater and Q & the heat loss from the drying room to the work, Q H L outside. The work performed by the fan is normally small. The resistance of airflow in a simple drying chamber is about 20 Pa for an air velocity of 2 m/s over the tray, [Keey, 1982]. If the fan efficiency is 80 %, the work done per unit cross-section of the dryer is 20 x 2 / 0.8 = 50 W/m2. Heat losses are more significant even at fairly low drying air temperatures. Installing suitable lagging would eliminate about two-thirds of this loss. As air passes over the wet material, the air temperature is reduced and its humidity is increased as described by the adiabatic cooling curve of the psychrometric chart. The temperature of the air leaving the tray is given by:

Tout = Tsurf + ( Tin − Tsurf ) ⋅ e − N t

(2.13)

where Tsurf is the temperature of the drying surface, and N t the number of transfer units calculated from the following equation:

Nt =

h t ⋅ L tr ρ a ⋅ w ⋅ c p ⋅ z fs

(2.14)

where h t is the global heat transfer coefficient, L tr the tray length, ρ a the air density, w the air velocity, c p the specific heat at constant pressure and z fs the free space between trays. During the constant-rate period and in the absence of radiant heat transfer, Tsurf assumes the adiabatic saturation temperature of the air. In order to maintain an economic drying operation, the warm, moist air leaving a dryer is frequently mixed with dryer, fresh air in order to reduce the heating cost of the incoming air. Air recirculation is generally in the order of 80 to 95 percent except during the initial drying stage of rapid evaporation [Perry, 1987].

Fruits and Vegetables Dehydration in Tray Dryers

57

In a batch through-circulation dryer, heated air passes through a stationary permeable bed of wet material placed on removable screen-bottom trays suitably supported in the dryer. This dryer is similar to a standard tray dryer except that hot air passes through the wet solid instead of across it. The air pressure loss in a through-circulation dryer is related to the bed characteristics by:

Δp = z bed ⋅ (1 − ε ) ⋅ ( ρ s − ρ a ) ⋅ g

(2.15)

where Δp is the pressure drop, z bed the depth of the material bed, ε the porosity, ρ s and ρ a the densities of material and air, respectively, and g the acceleration of gravity. Batch through-circulation dryers are restricted in application to granular materials which permit free flow-through circulation of air. In these cases drying times are usually much shorter than those of parallel-flow tray dryers.

2.3. Mathematical Modeling of Drying Processes The simulation of various product drying systems involves solving a set of heat and mass transfer equations which describe: a) heat and moisture exchange between product and air, b) adsorption and desorption rates of heat and moisture transfer, c) equilibrium relations between product and air and d) psychrometric properties of moist air, [Jayas, 1991]. The most rigorous methods of describing the drying process are derived from the concepts of irreversible thermodynamics in which the various fluxes are taken to be directly proportional to the appropriate “potential” [Ghiaus, 1997]. The mass balance inside the product can be written as:

∂( ρ s ⋅ X) ∂t

∂X ⎞ ⎛ = div ⎜ ρ s ⋅ D eff ⋅ ⎟ ⎝ ∂z ⎠

(2.16)

and the heat-energy balance can be set down as:

ρs ⋅ c p ⋅

∂T ⎛ ∂T⎞ = div ⎜ λ ⋅ ⎟ ⎝ ∂z ⎠ ∂t

(2.17)

The following initial and boundary conditions apply for the system:

T( z,0) = Ti −λ⋅

∂T( n, t ) & v =0 + h c ⋅ [Tdb − T( n, t )] − h lg ⋅ m ∂z

(2.18)

(2.19)

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Dionissios P. Margaris and Adrian-Gabriel Ghiaus

∂T(1, t ) =0 ∂z

(2.20)

where T(n, t) is the temperature of the slab layer ‘n’ in contact with the drying air, and T(1, t) the temperature of bottom slab layer. No general theory or equation would be valid for the prediction of drying kinetics of fruits and vegetables, and actual experimentation using specific material is essential for describing its drying behavior. Although correlation for the calculation of the heat and mass transfer coefficients has been proposed in the literature, few data are available to allow the constants in this correlation to be fixed with certainty. For a particulate bed, and air-flow parallel to the product surface, the following equation to calculate heat transfer coefficient can be used [Jayas, 1991]:

h c = 0.00327 ⋅ ρ a ⋅ w ⋅ Re −0.65 ⋅ Pr −0.66

(2.21)

In the case of through-circulated beds, where the product is particulate (e.g. grapes), for a laminar flow with Re < 350 (the characteristic length is taken as the particle diameter), the heat transfer coefficient can be calculated as:

h c = 182 . ⋅ ρ a ⋅ w ⋅ c p ⋅ Re −0.51

(2.22)

For Re > 350, the flow through the bed becomes turbulent and the following alternative correlation applies:

h c = 0.989 ⋅ ρa ⋅ w ⋅ c p ⋅ Re −0.41

(2.23)

The mass transfer coefficient (kp) is difficult to measure and is usually estimated from the Chilton and Colburn analogy, [Bird, 1960]. A widely used relationship in drying calculation, that correlates heat and mass transfer coefficients with the latent heat of water vaporization hlg, is the following, [Sereno, 1990]:

hc = 64.7 h lg ⋅ k p

(2.24)

2.4. Modeling of Agricultural Product Properties Variability in composition and physical characteristics is typical for all food products. Most thermal property models are empirical rather than theoretical, i.e. they are based on statistical curve fitting rather than theoretical derivations involving heat transfer analysis. The specific heat indicates how much heat is required to change the temperature of a material. The ratio of the heat supplied to the corresponding temperature rise is defined as the heat capacity of a body. Specific heat is given by the mass heat capacity equation:

Fruits and Vegetables Dehydration in Tray Dryers

c=

1 dQ ⋅ m dT

59

(2.25)

where m is the mass, Q the heat, and T the temperature of the body. Specific heat of a product is influenced by the product components, moisture content, temperature and pressure. For products whose composition is known, the following equation may be used:

c = 4187 . ⋅ m w + 1549 . ⋅ m p + 1675 . ⋅ m f + 1424 . ⋅ m c + 0.837 ⋅ m a

(2.26)

where m is the mass fraction and the subscripts are: w-water; p-protein; f-fat; c-carbohydrate; a-ash. Some composition values of selected foods are given in table 1. Table 1. Composition values of selected foods Food Apples, fresh Garlic Peaches Peas, raw Pineapple, raw Potatoes, raw Rice, white Spinach Tomatoes

Water ,% 84.4 61.3 89.1 78.0 85.3 79.8 12.0 90.7 93.5

Protein, % 0.2 6.2 0.6 6.3 0.4 2.1 6.7 3.2 1.1

Fat, % 0.6 0.2 0.1 0.4 0.2 0.1 0.4 0.3 0.2

Carbohydrate, % 14.5 30.8 9.7 14.4 13.7 17.1 80.4 4.3 4.7

Ash, % 0.3 1.5 0.5 0.9 0.4 0.9 0.5 1.5 0.5

Specific heat of agricultural products has been most commonly modeled with equations of the form [Sweat, 1986]:

c = C1 + C 2 ⋅ X

(2.27)

with the constants C 1 and C 2 varying with product. For moist foodstuffs, the specific heat is equal to the sum of the specific heat of water and that of the solid material, [Mohsenin, 1980]. This can be mathematically expressed as follows, [Abalone, 1994]:

c = W ⋅ c p , w + (1 − W ) ⋅ c ds

(2.28)

where W is the moisture content wet-basis, c p, w the specific heat at constant pressure for water, and c ds the specific heat of the dry product. Table 2 presents the values of specific heat for several fruits and vegetables correlated with their water content and temperature. The enthalpy of a dry solid is defined by the product of the heat capacity or specific heat c ds and the temperature excess:

h ds = c ds ⋅ ΔT

(2.29)

60

Dionissios P. Margaris and Adrian-Gabriel Ghiaus Table 2. Specific heat for some fruits and vegetables Foodstuff Apples Apples Apricots Berries Carrots Figs Grapes Peaches Pears Plums Potatoes

Water content W, % 84.1 75-85 85.4 75-85 88.2 78 81.8 86.9 83.5 85.7 77.8

Temperature T, °C 0-100 0-100 0-100 -

Specific heat c, J/kg K 3600 3730 3680 3730-4100 3770 3430 3600 3770 3600 3680 3430

Reference ASHRAE, 1986 Ordinanz, 1946 ASHRAE, 1986 Ordinanz, 1946 ASHRAE, 1986 ASHRAE, 1986 ASHRAE, 1986 ASHRAE, 1986 ASHRAE, 1986 ASHRAE, 1986 ASHRAE, 1986

In foods, thermal conductivity depends mostly on the individual components. For products whose composition is known, the following equation may be used:

λ = 0.58 ⋅ m w + 0155 . ⋅ m p + 016 . ⋅ m f + 0.25 ⋅ m c + 0135 . ⋅ ma

(2.30)

Thermal conductivity of most high moisture foods has values closer to the thermal conductivity of water. For products that are predominantly water, a model of the form:

λ = C1 + C 2 ⋅ W

(2.31)

is commonly used. For fruits and vegetables with a water content greater than 60 %, the coefficients C 1 and C 2 are 0.148 and 0.493, respectively [Sweat, 1974]. To cover widely varying temperatures, a model should account for variation in temperature. For greater accuracy, this would probably include a T term and a T2 term, because the thermal conductivity of water varies as temperature is squared. For potatoes, [Abalone, 1994], thermal conductivity can be calculated as:

λ = C1 + C 2 ⋅ T + C3 ⋅ T2

(2.32)

with C1 = 1.05 W/m K, C 2 = -1.96 102 W/m K2, and C 3 = 1.90 10-4 W/m K3. Temperature and water content are the major factors affecting thermal diffusivity. A multiple regression analysis on 246 published values, gave the following equation for the calculation of thermal diffusivity, [Singh, 1982]:

α = ( 0.057363 ⋅ W + 0.000288 ⋅ T) ⋅ 10 −6

(2.33)

Effective mass diffusion coefficient is related to various physical properties of the food such as thermal conductivity, bulk density, enthalpy, etc., as well as to environmental conditions [King, 1968]. The equations derived may be written as:

Fruits and Vegetables Dehydration in Tray Dryers

D eff =

b ⎛ ∂a w ⎞ a ⋅⎜ ⎟ ⋅ pw ⋅ ρs ⎝ ∂X ⎠ T 1+ a

61

(2.34)

where b is the vapor-space permeability, ρs the solid density, a w the water activity, X the moisture content dry-basis, p w the partial pressure of water vapor at saturation, and a is calculated from the following equation:

(

)(

2 a = λ ⋅ R w ⋅ T 2 / b ⋅ a w ⋅ p w ⋅ h st

)

(2.35)

where λ is the thermal conductivity, R w the gas constant for water vapor, T the temperature, and h st the heat of sorption. The term a / (1+ a ) in Eq.(2.34) determines the degree of mass or heat transfer control. If a >> 1 , the process is totally mass transfer controlled; if a