Post-Harvest Quality Control Strategies for Fruit and Vegetables

~tgricultural Systems 10 (1983) 21-37

Post-Harvest Quality Control Strategies for Fruit and Vegetables J. E. Holt Department of Mechanical Engineering, University of Queensland, St. Lucia (Qld) 4067, Australia

D. Schoorl Redlands Horticultural Research Station, Delancey Street, Ormiston (Qld) 4163, Australia

& I. F. Muirhead Plant Pathology Branch, Queensland Department of Primary Industries, Indooroopilly (Qld) 4068, Australia

SUMMARY Post-harvest deterioration of fruit and vegetables is central to quality management in the distribution of all produce. The causes can be categorised as physiological, pathological, physical and combinations of these three. Deterioration due to these factors can be quantified as functions of time and environment. Physiological and pathological deterioration are continuous processes, while physical damage is the result of discrete inputs of energy. Examples of the quantification of deterioration due to temperature fluctuations, pathogen attack and mechanical energy inputs are given. Interactions between these primary factors are also considered. As fruit and vegetables move through the distribution system the total quality deterioration can be determined at all times from the contributions of the primary factors and their interactions. A plot of 21 AgriculturalSystems 0308-521X/83/0010-0021/$03.00 © Applied SciencePublishers Ltd, England, 1983. Printed in Great Britain

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J, E. Holt, D. Schoorl, I. F. Muirhead deterioration against time is given, showing how various management strategies can be assessed by comparing total deterioration against acceptable limits, thus forming a basis Jor management action to either sell quickly or to introduce post-harvest control s.

INTRODUCTION Post-harvest deterioration in flesh produce is extensive and is manifested by a reduction in quality, or total loss of product with consequent reduction in monetary value. Harvey (1978) and Rippon (1980) report that post-harvest losses of 25-50 ~o of a crop are not unusual in some countries where refrigeration facilities are not available and appropriate chemical treatments are not used. Eckert (1977) reports on post-harvest decay losses of from 2 to 5 ~ in commercial shipments of a range of products if effective post-harvest treatments were not utilised. The high incidence of bruising in packaged apples has been reported by Schoorl & Williams (1972, 1973). Brecht (1980) reports that improved production practices have contributed to superior quality and yield at harvest. He points out that the impact of these advances has not been fully realised at wholesale, retail and consumer levels because of product spoilage and deterioration during distribution, and that approximately 25~o of produce harvested world wide is not consumed because of spoilage. Clearly, a framework for the management of post-harvest losses is essential if the costs of these losses are to be minimised. The major factors contributing to post-harvest deterioration are physical damage, physiological degeneration, pests and diseases. McGlasson et al. (1979), Rippon (1980) and Harvey (1978) all accept similar classification although pests were not mentioned. However, there are also important interactions between these primary sources. Further, superimposing on the inevitable deterioration of all living produce with time, inputs of these primary sources may occur at any time during distribution. Deterioration is thus additive and cumulative and total damage is the result of the action of physical, physiological, pathological and interactive sources over time. There is a great amount of information available on the physiological and pathological sources of deterioration, and some information about physical damage. What is lacking is a framework for combining and using this information for the management of deterioration during the

Post-harcest quality strategies Jor J?uit and vegetables

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distribution of horticultural produce. This paper describes the sort of information available on the primary sources of decay and shows how this information can be used to predict deterioration. It then shows how management strategies can be developed and evaluated. The management of production during the growth and life phase o f a product has become accepted practice. This paper proposes that the management of the death processes of produce similarly needs to be, and can be, practised.

MECHANISMS OF D E T E R I O R A T I O N All fruit and vegetables begin to deteriorate when they are harvested and what happens to them after this can not reverse the process; only the rate at which the deterioration takes place can be effected. The basic process of decay in fruit and vegetables is respiration, whereby matter is converted into energy, oxygen is consumed and CO 2 and heat are released. The rate at which respiration occurs controls, if somewhat indirectly, the useful 'life' of the product after harvest. Anything that increases the rate of respiration decreases the shelf-life, which may not be disadvantageous; for example, ethylene ripening of bananas. The development of disease is due to growth of microorganisms, the most serious of which cause rapid and extensive breakdown of certain fruit and vegetables, often spoiling the entire package and causing secondary infections in the advanced stages of the disease. Physiological and pathological decay are usually continuous functions of time. Mechanical injury is generally not time dependent and is manifested by rapid tissue rupture due to external loads. The ruptured tissue may then provide conditions Suited to pathogen attack. If the deterioration of produce to all these factors is to be managed, the basic mechanisms involved need to be understood and their effects quantified.

Physiological deterioration Softening, change of colour, wilting, chilling injury and sunburn are all physiological changes that are directly influenced by the produce environment, e.g. temperature, vapour pressure deficit, gas composition and light. The gaseous environment has a profound effect on the retardation of senescence, as shown by Kader (1980) and Smock (1979) for

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J. E. Holt, D. Schoorl, I. F. Muirhead

fruits and Isenberg (1979) for vegetables. Vapour pressure deficit controls the rate at which water is lost after harvest and Ryall & Lipton (1972) and Molenaar (1981) list acceptable water losses for various vegetables and flowers ranging from 4 to 19 ~o- Ethylene is intimately involved in the ripening of fruit and can cause serious disorders in leafy vegetables and flowers at concentrations of 0-05-10 ppm (Ryall & Lipton (1972) and Lutz & Hardenburg (1968)). Wilkinson (1970) describes the physiological disorders of fruit after harvesting, emphasising the effects of the environment and pre-harvest factors on those disorders. Of all the environmental effects, however, Harvey (1978) and McGlasson et al. (1979) report that refrigeration at optimum temperatures offers the greatest potential for increasing post-harvest life and reducing losses. This factor will thus be considered in more detail. Thorne & Alvarez (1981) write that it is well established that the rate of deterioration of most agricultural produce is a direct function of temperature. For each commodity there is an upper temperature limit beyond which heat damage occurs and a lower limit below which chilling and then freezing result. Ryall & Lipton (1972) and Lutz & Hardenburg (1968) have reviewed the literature on chilling and freezing injury. In the range between heat and chilling injury, the relationship between life span and temperature is of greatest concern in maintaining quality. Littmann & Peacock (1972) show that the green-life of climacteric-type fruit is a function of temperature of the form: GTo = aTemtTo -T)

where Gyo and G~ are the green-lives at temperatures, TO and T, respectively. This enables the green-life at temperature T to be converted to its equivalent at some arbitrary but standard temperature, T01 Peacock (1980) found a similar exponential relationship for banana ripening. Thorne & Alvarez (1981) state that the storage life of green tomatoes is related to temperatures by the following expression t = 9 7 e -°'13T where t = storage life in days and T = temperature (°C). These deterioration progress curves for fruit over a range of temperatures can be used to calculate cumulative loss of storage life as follows. Thorne & Alvarez (1981) describe a time-temperature-tolerance hypothesis which begins by expressing the rate of deterioration as the

Post-harvest quality strategies for fruit and vegetables

25

reciprocal of the storage life. For any given temperature-time history, a corresponding graph of deterioration rate against time can then be constructed and the area under this graph is a measure of the total deterioration t h a t has occurred, i.e.: 1

Deterioration rate = -

where t = storage life and

t

Total deterioration =

ldt

t

where t s = total storage time.

Thorne & Alvarez (1981) state that this hypothesis has been widely and successfully applied to frozen produce and demonstrate that the concepts apply to changes in colour and firmness in tomatoes stored between 12 °C and 27 °C. There is also a considerable a m o u n t of information about o p t i m u m storage temperatures for fruit and vegetables for maximum shelf life. Lutz & Hardenburg (1968) give the best commercial storage conditions (temperature and humidity) for fruit, vegetables, flowers and nursery stock. For each commodity there is thus a storage life for o p t i m u m conditions, and variations from the o p t i m u m storage temperature will incur a penalty of reduced storage life. The time-temperature-tolerance hypothesis can be used to quantify the cumulative effects of temperature variations and to provide a basis for management. The storage life, topt, at o p t i m u m temperature Topt is:

/opt = kl ek2T°pt where k I and k 2 are constants and Topt is the o p t i m u m temperature. The storage life, t, at any other temperature, T, is: t = k 1 e k2T so that the cumulative deterioration due to changing temperature conditions which can be managed, i.e. the deterioration due to the difference between experienced and o p t i m u m temperatures, for a particular storage time, ts, can be derived from the T-T-T hypothesis and is given by: Manageable d e t e r i o r a t i o n =

.]o\ t~t~(1 toptl) d t

J. E. Holt; D. Schoorl, I. F. Muirhead

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temperature- time history

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16 14 12

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deterioration rate, variable temperature ------ deterioration rate, constant temperafure(13"C)

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