Principles of Canning.pdf

Principles of Canning Z Boz, R Uyar, and F Erdogdu,  University of Mersin, Mersin, Turkey 

2014 Elsevier Ltd. All rights reserved. This article is a revision of the previous edition article by Aslan Aziz, volume 2, pp 1008 –1016,  1999, Elsevier Ltd.

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Introduction It always has been a challenge for communities to maintain nutrition nutritional al value and quality quality attribute attributess (e.g., (e.g., taste, taste, texture, texture, �avor, and color) of food products longer. Thermal processing  is one of the most common preservation methods to make food products microorganism-free via the effect of heat and temperature. It not only provides a medium, free of pathogenic  and spoilage microorganisms to some extent, but also inacti vates enzymes, eventually deteriorating the quality attributes. Canning provides sterilization and increases the shelf life of  food food produc products ts by app applyi lying ng heat heat in herme hermetic ticall allyy sealed sealed (airti (airtight ght)) contai container ners. s. About About 50 billio billion n cans cans are manufa manufactu ctured red and consumed globally every year in the food-processing area. Two fundamentally different methodologies might be applied in cannin canningg proces process: s: retort retort proces processin singg and asepti asepticc proces processin sing. g. In the �rst process, containers containers (cans, (cans, jars, or any other retortabl retortablee containers) are �lled with the product, product, sealed sealed airtight, airtight, and thermally processed under pressure until a certain sterilization degree is achieved. In aseptic processing, limited to liquid food products, the container and product are sterilized individually, and �lling and sealing processes are carried out. Continuous heat heat proces processes ses,, such such as asepti asepticc proces processin sing, g, enabl enablee food food producers to perform thermal treatment at elevated temperatures tures for reduce reduced d times. times. Such Such proces processes ses are called called highhightemperat temperature ure short-tim short-timee (HTST) (HTST) and ultrahighultrahigh-temp temperat erature ure (UHT) processes due to the high temperatures involved. HTST  and UHT are designated to be operated at higher temperatures than other other convention conventional al pasteuriza pasteurization tion or sterilizat sterilization ion techtechniques. This leads to a reduced process time, preserving the organoleptic quality of food products. In UHT processes, the boiling point is exceeded via the increased pressure to sterilize the product, while HTST processes still might be characterized as a pasteu pasteuriz rizati ation on proces processs for possib possible le app applic licati ations ons at  temperatures below 100  C. For example, regarding the heat  proces processin singg of milk, milk, HTST HTST pasteu pasteuriz rizati ation on is carrie carried d out at  around 72  C for 15 s, while in a UHT steriliz sterilizati ation on process, process, boiling temperature is exceeded (135–145  C) for 1–10s. Retort processing, as in-container sterilization, generally is consid considere ered d to be ‘canning ’ within the food industry. Canning as a food preservation method started in early 1800s in France  when Nicholas Appert developed a new methodology to preserve and extend the shelf life of a wide variety of food products, including some vegetables, in glass jars and bottles. Even though Appert explained the process to some extent, the true foundations of the process were laid by the discovery of  Louis Pasteur, who explained that the heating process inacti vated the microorganisms, limiting the shelf life of food products. The discovery of the relationship between thermophilic bacteria and the spoilage of canned corn and peas was another milestone in canning, and the investigation of basic  biological and toxicological characteristics of   Clostridium botulinum  formed the theoretical foundation for understanding its signi�cance to establish a controlled process. Botulinum toxin

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causes botulism, resulting in permanent nerve damage, and C. botulinum botulinum   spores spores require require anaerobic anaerobic condition conditions, s, low-acid low-acid foods, relatively high moisture, and mild storage temperatures for cans given a suitable environment for their germination. Starting with Appert ’s process in glass bottles more than 200years ago in 1810, 1810, produc producing ing heat-p heat-pres reserv erved ed foods foods in hermetically sealed containers (including cylindrical tin cans) has contributed to improved nutrition and health in a significant way. The invention invention of metal metal container containerss and pressure retorts evolved into twenty-�rst-century canning technology.  The development of metal and glass containers capable of   withstanding added internal pressures was a major breakthrough through to apply processing processing temperat temperatures ures of 120  C above atmospheric pressure. The presence of headspace is required to ensure an adequate amount of vacuum during the process, and it has a signi�cant in�uence on the heat-transfer rate – especially for liquid foods and liquid–solid food mixtures – as demo demons nstr trat ated ed by the the incr increa ease sed d heat heat-t -tra rans nsfe ferr rate rate in liquid-containing cans. Canning is regarded as a universal and economical method in food processing. Even though canning has many processing  steps, the critical control point that ensures food safety and causes causes changes changes in quality quality parameter parameterss is thermal thermal processing. processing. Retort Retort systems are the most often often used equipment equipment during  thermal thermal processing. processing. Therefore, Therefore, thermal thermal processing processing and its contributi contribution on to the canning process are emphasize emphasized d in the following section before discussing the processing steps.

Microbiological Viewpoint  The objective of thermal processing is to reduce or partially  inacti inactivat vatee themicroorg themicroorgani anisms sms that that exist exist in a medium medium.. Althou Although gh thermal inactivation of microorganisms is associated with irre versible denaturation of membranes, ribosomes, and nucleic  acids, various factors determine the heat resistance of microorganisms, including the type of microorganism (e.g., spores are resistant resistant compared compared with the vegetative vegetative cells) and heat treatmen treatment  t  conditions (pH, water activity, composition of the food material). Water activity of the food product in�uences the heat  resistance of vegetative cells. In addition, moist heat is more effective than dry heat for microbial destruction because of the increased heat-transfer coef �cient of the heating medium. Canning, Canning, as one of the basic processes processes in thermal treatments, ments, reduces reduces or partially partially inactivate inactivatess the microorga microorganisms nisms.. Microorganisms can affect the quality of a canning process in three possible ways: 1. Microorganisms might survive the heat treatment treatment and cause spoilage, safety problems, and undesirable changes in the products due to an insuf �cient process. 2. Microorganisms that naturally grow or contaminate the raw  mate materi rial al may may dest destru ruct ct the the qual qualit ityy attr attrib ibut utes es befo before re processing.

Encyclopedia of Food Microbiology, Volume 2

  http://dx.doi.org/10.1016/B978-0-12-384730-0.00156-7

HEAT TREATMENT OF FOODS j  Principles of Canning

161

100000

  s   m   s    i   n   a   g   r   o   o   r   c    i   m   g   n    i   v    i   v   r   u   s    f   o   r   e    b   m   u    N

10000 1000 100 10 1

D 

0.1 0.01 0

2

4

6

8

10

12

14

Time (min) Figure 1

A typical survival curve for bacterial spores during heat treatment at a certain temperature to determine  D -value.

3. Contamination –   related to equipment and lack of  personnel hygiene –   might occur and endanger the endproduct ’s quality and safety during and after processing  (e.g., damaged cans might be subjected to contaminated  water during cooling).  The �rst and third ways that microorganisms affect the quality may be related to the pathogenic microorganisms. As soon as validity of a thermal treatment is ensured and hygiene is taken care; retort technology and canning are designed to destruct all pathogenic and most of the spoilage microorganisms in a hermetically sealed container and to create an environment inside the container that will disable the growth of  spoilage microorganisms and their spores. On the basis of this principle, canned food products are processed thermally to make them ‘commercially sterile.’ Thermal Resistance of Microorganisms

 Various researchers have studied the determination of thermalprocessing parameters in canning for many years. On the basis of their work, current technology in the canning industry has abundant resources to maintain and safely validate the critical thermal-processing procedure. Obtaining the required thermalprocessing parameters with laborious experimental procedures, however, is still a milestone for this systematic information.  Thermal resistance of a microorganism is determined by validating the purity of strains and spores and gaining a distinct  number of organisms following the thermal treatment at  a constant temperature. For this procedure, an exact number of  spores or vegetative cells are placed in sealed containers, made up of Pyrex, screw-top closed glass, or glass capillary tubes.  Then, heat treatment at a given time –temperature combination is applied. Enumeration and recovery are carried out to determine the surviving number of microorganisms or spores. Eventually, data obtained from the thermal resistance experiments are utilized to form microbial survival curves and to determine the required process parameters on the basis of the given target microorganism. Factors in �uencing the heat resistance of microorganisms can be summarized as species of the microorganism, acidity (pH) of the medium, water activity, and composition of the food product and oxygen level.

Numbers of microorganisms and spores, exposed to heat  for a certain period of time, logarithmically reduce proportional to the applied temperature and time. When the microbial population as a function of time is presented in semilogarithmic coordinates, a linear decrease in the microbial population with time at a constant temperature is observed: D ¼

t  log  N 0    log  N 

[1]

 where D  is decimal reduction time, and N 0 and N  are the initial and  � nal numbers of microorganisms.  D -value can be de�ned as the required time to reduce the number of microorganisms 1 log cycle (or by a factor of 10) at a given temperature. Even though the  D -value is regardless of the initial number of the population, applied temperature is of great effect, and it is a strong function of temperature. A typical survival curve for  microorganisms to determine D-value can be observed in Figure 1, and effect of temperature on  D -values is illustrated in  Figure 2.  As demonstrated in  Figure 1, the number of survivors are plotted on a semilog graph (by taking the logarithm of the number of survivors) as a function of time, and a 1 logcycle reduction determined from the slope gives the D-value 1 D ¼  . A decrease in  D -value requires a temperature Slope increase to reach the same degree of reduction, and this temperature change (Figure 3), the z-value, is the required increase in temperature to reduce the  D-value 1 log cycle (or by  a factor of 10):





 z ¼

T 2    T 1 DT  log  1 DT 2

[2]

 Table 1  gives D- and z-values of various microorganisms. Finally, sterilization value, accumulated value of lethality, L (eqn [3]) is de�ned as the time required to reduce the number  microorganism and microbial spores to a predetermined level (eqn [4]): T T ref   z

L ¼ 10

[3]

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HEAT TREATMENT OF FOODS j   Principles of Canning

1.00E + 08 1.00E + 07   s   r 1.00E   o   v    i   v   r 1.00E   u   s    f 1.00E   o   r 1.00E   e    b   m1.00E   u    N

+ 06 + 05 + 04 + 03 + 02

1.00E + 01 1.00E + 00 1.00E – 01

T 3

0

5

T 2

10

15

T 1

20

25

Time (min) Figure 2

Effect of temperature on  D -value (T 1 < T 2 < T 3).

1000 100    )   n 10    i   m    (   e   u    l   a   v   -

      D

1 z 

0.1 0.01 100

109

118

127

136

145

Temperature (°C) Figure 3

Change in  D -value as a function of temperature to determine  z -value. t 

F 0 ¼

Z 

10

t  ¼ 0

T T ref   z

 N  

dt  ¼ D$log 

0

 N 

[4]

 where T ref    is a reference temperature and t   is the thermal process time. As   eqn [4]   demonstrates, F - value might be expressed as multiples of  D-values, and the most common relationship used in canning is F  ¼ 12D for  C. botulinum in commercial sterilization for low-acid canned foods (pH > 4.6). Given that the 12D for   C. botulinum  is 2.52 min (12  0.21), the food safety requirement for a sterilization process is F 0  2.52 min. Logarithmic destruction of microorganisms leads to probability calculations in sterilization examples. For example, 0.5 kg cans with an initial concentration of   C. botulinum  spores of 102 g 1 are processed thermally to satisfy the 12D  concept, and the survival concentration becomes 1010 g 1. This �nding  means that only one living spore may survive in a 1010 g endproduct (a probability of  �nding only 1 infected can in 20 million). It is also possible to express the sterilization value at any  time–temperature combination in terms of equivalent time of  F 0  at 121  C: F  ¼ F 0 $10

T ref  T   z

[5]

On the basis of   eqn [5],  F 0 of 2.52 min at 121  C would be equivalent to 32.46 min at 110  C and 0.32 min at 130  C,  while assuming instantaneous heating and cooling to the appropriate temperatures (accumulated lethality values during  these times are ignored).  The explained methodology (eqn [4]) developed by Bigelow et al., which involves numerical integration when the process temperature change and thermal–physical properties of  the canned product is known, is a simple and accurate methodology to determine sterilization value. Even though several formula methods, for example, Ball, Stumbo, and Pham, should be developed to determine the process time or accumulated lethality for a given process, Bigelow ’s method is more convenient to apply. For this purpose, temperature change at  the coldest spot of the product (geometrical center for  conduction-heated food products) or the slowest heating zone (the SHZ between the geometrical center and the bottom surface for food products involving convection heating) should be known.  Figure 4  illustrates the location of the coldest spot  for conduction-heated and the SHZ for convection-heated canned food products with the given temperature scales. Blueshaded areas show the cold regions and red-shaded areas show  the hot regions.   Figure 4(a) demonstrates temperature contours with uniform kernels via the effect of conduction with

HEAT TREATMENT OF FOODS j  Principles of Canning

Table 1

163

D - and  z -values of various microorganisms

Foodborne pathogens

Food spoilage microorganisms

Microorganism 

D-value (min)

Reference temperature (  C) 

z-value (  C)

Medium  

Clostridium perfringens  Clostridium botulinum  Listeria monocytogenes  Escherichia coli  Bacillus stearothermophilus  Clostridium sporogenes 

5.30 0.21 3.29 1.97 4.00 0.8–1.5 0.5 –1.00

60.0 121.1 60.0 60.0 121.1 121.1 65.6

6.74 10.0 6.33 4.67 10.0 8.8 –11.1 –

Lean ground beef – Fatty beef Lean ground beef Vegetables and milk Meat products High acid foods

Yeast and molds

Adapted from Fellows, P.J., 1988. Food Processing Technology (Principles and Practice), Ellis Horwood Ltd., Chickester, England; Chen et al. (2011); Thippareddi, H., Sanchez, M., 2006. Thermal processing of meat products. In: Sun, D.-W. (Ed.), Thermal Food Processing  –  New Technologies and Quality Issues, CRC Press  –  Taylor & Francis, Boca Raton, FL, pp. 155–196; Teixeira, A.A., 2006. Simulating thermal food processes using deterministic models. In: Sun, D.-W. (Ed.), Thermal Food Processing  –  New Technologies and Quality Issues, CRC Press  –  Taylor & Francis, Boca Raton, FL, pp. 73 –106; Holdsworth, S.D., 2004. Optimizing the safety and quality of thermally-processed packaged foods. In: Richardsson, P. (Ed.), Improving the Thermal Processing of Foods, Woodhead Publishing –CRC Press. Boca Raton, FL, USA, pp. 3 –27.

the cold spot at the geometrical center, and distorted kernels of  temperature contours by natural convection with the SHZ are observed in  Figure 4(b) . Figure 5  shows a simple example for calculation of sterilization value using the temperature data obtained at the geometrical center of a conductively heated food product in a can. Temperature change at the coldest spot of a conductively  heated can (307  409) subjected to the processing  temperature of 121  C for 90 min and then cooling at 20  C is given in Figure 5(a), and thelethality changeat thecoldest spot  is shown in  Figure 5(b) . Numerical integration using the data given in   Figure 5(b)   results in the sterilization value of  2.82 min (area- A  under the lethality curve).

Derivation of the sterilization value F 0  solely depends on two prominent assumptions: �rst, isothermal spore inactivation kinetics follow the linear logarithmic relationship to be characterized by the D-value; and, second, the temperature dependence of  D-value also conforms to a linear logarithmic  behavior (alternatively, temperature dependence of inactiva1 tion rate k-value k ¼ obeys the Arrhenius relation). Recent  D literature, however, shows that these assumptions do not  always hold, and the inactivation of bacterial spores may  follow the Weibullian model (eqn [6]) in which temperature dependence of the inactivation rate also should be taken into account in addition to time. Therefore, it is signi�cant to





Figure 4 Location of (a) coldest point for conduction heated and (b) slowest heating zone for convection heated canned products (left-hand side is the center line along the cross section ).

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HEAT TREATMENT OF FOODS j   Principles of Canning

(a)

140 120

    e     r     u      t     a     r     e     p     m     e      T

100 80 Cold spot temperature

60

Process temperature

40 20 0

0

30

60

90

120

150

180

Process time (min) (b)

0.16 0.14 0.12

    y 0.1      t      i      l     a      h      t 0.08     e      L

Lethality

0.06 0.04 A

0.02

= F 0

0 0

30

60

90

120

150

180

Process time (min) Figure 5

Temperature change at the coldest spot of a can, including (a) conductively heated food and (b) lethality change at the coldest spot.

determine the theoretical implications to apply nonlinear  kinetics for thermal processing 

 N 

log 

 N 0

¼ bðT Þ$t nðT Þ

[6]

 where b (T ) and  n (T ) are temperature-dependent coef �cients.

C. botulinum (a heat-resistant pathogenic microorganism in this pH level) or its spores. Because pH, a term used to designate the acidity or alkalinity of a solution, has signi�cant effect, food products are classi�ed on the basis of pH for the purpose of  thermal processing:

Commercial Sterility and Effect of pH

1. High acid (pH < 4.0) 2. Acid (pH between 4.0 and 4.6) 3. Low acid (pH > 4.6 where the target organism is C. botulinum )

If a canned product satis�es the requirement of being microbiologically safe under storage conditions, it could be described as ‘commercially sterile.’  Commercial sterility is described in the Food Safety and Inspection Service (FSIS) Canning Regulations 9 CFD 318.300 and 9 CFD 381.300:  The condition achieved by application of heat, suf  �cient alone or in combination with other ingredients and/or treatments, to render the product free of  microorganisms capable of growing in the product at non-refrigerated condition (over 10  C) at which the product is intended to be held during distribution and storage. Commercial sterility implies that  any remaining microorganisms and spores will be incapable of  growth under normal storage conditions.  Two groups of microorganisms concern the canning of food products, one of which endangers the health and safety of  population. Foods with a pH above 4.6 might contain

 The relationship between pH and the thermal resistance of bacteria and bacterial spores was a milestone for canning  to classify the canned foods on the basis of their pH. Other  types of microorganisms are more heat tolerant than C. botulinum  and its spores in low acid foods. Even though these microorganisms may cause spoilage and some undesirable quality changes, they are not pathogenic to human health. For the economical perspective, the spoilage possibility of those can be tolerable to the levels of 105, whereas the level is 1012 for   C. botulinum . For canned foods, the distinctive pH value is 4.6, which is the minimum pH for the growth of   C. botulinum   as the most heat-resistant food pathogen microorganism.  A typical thermal death time in thermal processing of  shelf-stable canned foods is F  ¼ 12D, as explained

HEAT TREATMENT OF FOODS j  Principles of Canning

previously, with the D-value of   C. botulinum   (0.21 min) at  121.1  C. The primary objective of thermal processing is to inactivate C. botulinum  in products with a pH greater than 4.6 (since   C. botulinum   spores cannot germinate below pH of  4.6) and to destroy vegetative and other spore-forming  microorganisms that might cause spoilage. Besides C. botulinum , mesophilic species (like   Clostridium sporogenes, Clostridium butyricum, and   Clostridium pasteurinaum) and thermophilic species (Clostridium thermosaccharolyticum) can cause putrefactive or sacharolytic spoilage and gas formation leading to the swelling of cans. Contrary to swelling, microorganisms like   Bacillus coagulans and   Bacillus stearothermophilus cause thermophilic  � at-sour spoilage in cans. In acidic foods (pH below 4.6), however,   Clostridium barati, Clostridium perfringes, and  C. butyricum   might cause intoxication problems. These microorganisms have been reported to produce toxins in infant food formulations. Processing times and temperatures are lower in acid foods, compared with the case of low-acid foods, as microorganisms can be inactivated easier in an acid environment. Acid foods can be processed at temperature around 100  C at atmospheric  pressure without the requirement to use pressurized retorts, for   which B. coagulans, as a consideration for �at-sour spoilage, can be used as a target microorganism. Process Validation

 Thermal-processing parameters are calculated due to several factors in canning, and a thermal process is evolved by determining the following: 1. Heat resistance of the spoilage–pathogen microorganisms 2. Heat penetration rate into the product  3. Calculation of sterilization value (or sterilization value –  F , time required for reduction in a population of vegetative cells or spores) using temperature change at the coldest spot  or the SHZ of the product and thermal resistance data ( z-value) of the given microorganism 4. Validation of process time by microbiological (inoculated pack studies) or mathematical–computational methods  The length of thermal processing is determined by resistance of the target microorganism, process conditions, pH and composition of the food product, can size, and heat-transfer  mechanism (conduction or convection) occurring inside the can. To control and validate the thermal process performed, some key points are to be followed. The first and most  important one is the heat-penetration mechanism and temperature distribution within the canned food. During  a heat-penetration test, temperature of the retort and can is measured with thermocouples throughout the processing time. Because of the thermal and physical properties of food and properties of the container, the heat-transfer mechanism and heat-penetration rate might change over time. Regarding the heat penetration, the coldest spot or SHZ is de�ned as “the region that is reaching the required sterilization temperature latest and that is limiting the heat-transfer rate.” The position of  the SHZ depends on the size and shape of the can and the thermophysical properties and physical state of the food product. On this basis, the heat-transfer mechanism between heating medium and canned food should be known to

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determine the temperature distribution within the product. All thermal-processing calculations are carried out for the coldest  spot or the SHZ to ful�ll the safety requirements and to consider the worst-case scenario. In addition to using thermocouples, the following methods also are considered for  process validation: 1. Microbiological methods and survival curves 2. Simulation techniques 3. Time–temperature indicators

Canned Food Production  The canning industry has a strong background due to the early  demands and improvements. Handling of raw material and containers and choosing the retort system provide the basis for  the production of canned foods. Raw Material and Containers

In the canning process, plenty of foods can be used as a raw  material, some of which require special or additional processes. For example, �sh should be cleaned or peas should be taken apart from their shells. Before canning, pretreatments might be needed –   for example, blanching of vegetables to remove respiratory gases, inhibit enzymatic reactions, promote shrinkage of product for adequate �ll, hydrate dry products, and preheat the product to assist in further vacuum formation. For all of these separate processes, new equipment was developed to ful�ll the requirements. Raw material chosen for canning should have certain properties.If a raw material and its container are of a high quality and thermal treatment is performed appropriately, the end-product   will be satisfactory for both producers and consumers. Raw  material should be grown or harvested away from hazardous  waste, including chemical resources and domestic, industrial, or  agricultural wastes. Variations in raw material properties, such as high initial microbial load, maturation level, size, and shape of  the product, might cause variations in thermal processing and result in food safety risks.  After  �lling into glass or metal containers, the exhaust  procedure to create an anaerobic environment is carried out  before the sealing and heating –cooling processes. Containers for a canned food can be metal or glass with certain fundamental properties depending on the consumer demand and available processing techniques: 1. Container and sealing parts should not have a negative effect on sensory properties of the product and performance of thermal processing. 2. Container should be resistant to mechanical, chemical, and thermal effects through the whole process, including storage. 3. Containers should be compatible to sealing hermetically. 4. Sealing material should be appropriate for the product. Metal containers are the most regularly used for canned foods with their higher thermal conductivity and thin walls, enabling heat penetration during thermal processing. There are  various types of metal containers, such as tin plate cans, twopiece cans, tin-free steel, and so on. In addition to their  advantages for a convenient processing, metal containers pose

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HEAT TREATMENT OF FOODS j   Principles of Canning

risks to alkaline, corrosive water, and food-induced corrosion.  To prevent this, a special coating called ‘ can enamel’ is applied to the can. Other types of containers, such as glass, plastic, or  semirigid and �exible containers, also are used in canning. Customers prefer certain types of containers depending on their intended use, such as a transparent body to see theinterior  and microwavable properties of plastic containers. Besides metal and glass containers,  � exible retort pouches consisting of  a three-ply laminate of polyester, aluminum foil (oxygen and light barrier), and polypropylene (inner seal) that can withstand sterilization temperatures up to 130  C also have been used in canning process. Canning Process

 As summarized, a generalized canning process contains the following steps (Figure 6): 1. Preprocessing of raw material (cleaning, sorting, peeling, slicing, blanching, preparation of brine, syrup, or oil depending on the type of raw material) 2. Preparation of the packaging material (containers) 3. Filling the raw material

4. Exhausting and sealing  5. Thermal processing in retorts and storage During these processes, appropriate sampling and inspection procedures should be applied to ensure safety during  process and storage. Thermal-Processing Equipment

During the early development of thermal processing over  100  C, saturated salt solutions were used for heat-transfer  purposes. The invention of pressurized retort systems with steam heating, however, led to thermal processing of cans in  various types of retort systems. Superheated steam over  atmospheric pressure enables to reach temperatures over  100  C with the latent heat released. A typical vertical saturated steam batch retort is shown in   Figure 7 and   Figure 8 demonstrates the process principle of a continuous rotary  sterilizer system. Because high pressures and temperatures are required during the canning process, every retort system should include  well-equipped control systems. These systems include time– temperature recording devices, pressure gauge and safety 

A general  �ow diagram of a canning process. Adapted from Downing, 1996. Canning operations. In: A Complete Course in Canning, Book I, II. CTI Publications, Inc., Maryland, USA, p. 263. Figure 6

HEAT TREATMENT OF FOODS j  Principles of Canning

Figure 7

A typical vertical saturated steam batch retort. Adapted from May (2006).

Process principle of a continuous rotary sterilizer system. Adapted from Weng, Z.J., 2006. Thermal processing of canned foods. In: Sun, D.-W. (Ed.), Thermal Food Processing  –  New Technologies and Quality Issues BocaRaton,CRC Press – Taylor& Francis, FL,pp.335 –362. Figure 8

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 valves, and steam controlling units. Once the cans are loaded, the lid must be closed tightly. A thermal process is applied with a cycle of come-up, holding, and cooling times. After desired sterility value is reached, the cooling process is carried out.  There are various types of batch retorts, such as air-steam retorts, full-water immersion retorts, crateless retorts, raining  and sprayed water retorts, horizontal retorts, and rotary retorts.  An increase in the consumption of canned foods led to the need for the development of new techniques that enable the process of more containers in a limited time. Consequently, continuous systems have been developed to produce 200– 1500 containers per minute.  There are also rotary systems in addition to the continuous cycles of retorting. These systems were designed to achieve elevated heat penetration rates with theforced convection in the different types of sterilizing food product (i.e., viscous foods, liquid–solid mixtures). Those processes in which the agitation takes place reduce the time required by forcing the natural convectioninside thecontainers and increasingthe heat-transfer  coef �cient for a safe process with less demand to heat exposure.  The retort systems operating at overpressure conditions also meet market demand for the use of microwaveable glass or  plastic containers, leading to higher quality products.

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HEAT TREATMENT OF FOODS j   Principles of Canning

Conclusion

Further Reading

 Although canning is one of the most basic and widely  commercialized preservation methods applied in food processing, some inherent disadvantages have caused new  processes to emerge. With market demand to consume better  quality, value-added products together with safety, new  thermal and nonthermal processes, such as aseptic processing, ohmic heating, microwave–radio frequency, high-pressure processing (HP), pulsed electric  � eld (PEF), and pulsed light or  ultraviolet light, have emerged for possible uses in food processing. Thermally assisted technologies such as PEFand HP are effective when integrated with other thermal processes, and therefore, are used in conjunction with other thermal systems, such as aseptic processing, to extend shelf life. Compared with these new emerging technologies, retort technology, as applied in canning, has less controllable processing conditions because of the resistance to heat-penetration and heat-transfer medium. Nevertheless, rotary systems –  especially in the processing of  liquid and solid–liquid mixture foods –  have helped obtain better quality products. Future developments in retort technology might include the following:

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1. Improvement in agitated retorts to moderate the effects of  heating by increasing heat-transfer rate into the product  2. Use of variable retort temperatures to enhance and control medium temperature inside the retort offering better quality  3. Optimization of the process with online process control, and new designs of cans to improve product quality  Optimization studies applying computational methods become prominent among these possibilities. These studies generally focused on determining variable retort temperature pro�les and controlling theprocess conditions – �uctuations to �lling gaps in the conventional thermal processes. Additionally, new container designs to reduce the destruction effect of  heat also have been reported (e.g., the development of toroid cans to provide increased rate of heat transfer).

Acknowledgment  This study was part of a research supported by the Scienti�c and  Technical Research Council of Turkey, project no: 110O066 (TOVAG-Agriculture, Forestry and Veterinary Research Grant  Committee).

See also: Geobacillus stearothermophilus  (Formerly  Bacillus  stearothermophilus );  Clostridium :  Clostridium botulinum ;  Heat Treatment of Foods:  Spoilage Problems Associated with Canning;  Heat Treatment of Foods:  Ultra-High-Temperature

Treatments; Heat Treatment of Foods  –  Principles of Pasteurization;  Thermal Processes:  Pasteurization; Thermal Processes, Commercial Sterility (Retort).