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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
24
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
26
temperature- time history
20 o 18
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life from
Post-harvest quality strategies for fruit and vegetables
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Figure 1 shows how loss of storage life can be calculated from a fluctuating temperature-time history for Stored produce. Figure 1 is constructed by taking the curves for tomatoes presented by Thorne & Alvarez (1981) for temperature and deterioration rate against time and integrating the deterioration rate curve to give progressive deterioration. Typical limits for heat sensitivity and chilling have been added, together with an assumed optimum storage temperature line. The deterioration,time curve shows clearly that part of the deterioration which temperature management strategies can affect. The deterioration at optimum temperature is the minimum deterioration which can be achieved. The management of deterioration due to temperature is concerned with avoiding temperature sensitive limits and with assessing how much storage life remains after a particular temperature-time history. When the cumulative deterioration reaches 100 % there is no shelf life left. It is reasonable to suppose that similar deterioration-time curves can be generated for produce for other environmental factors. For example, storage away from optimum humidity levels, described for apples and plums by Wilkinson (1970), causes skin splitting at high moisture levels. Ryall & Lipton (1972) show that water loss occurs at low humidity levels. It should therefore be possible to predict water loss from a vapour pressure deficit-time history and to construct a cumulative wilting-time curve. Controlled atmosphere, hypobaric storage and ethylene concentrations could be assessed in the same way, giving a proper basis for management decisions.
Pathological deterioration Major post-harvest diseases of fresh fruit and vegetables and their causal micro-organisms have been described by Eckert (1977) together with estimated losses for some produce due to pathological diseases ranging from 2 % for McIntosh apples from Nova Scotia to 52 % for Sanguinello oranges from Italy. Rippon (1980) states that the capability of a microorganism to initiate a post-harvest disease depends on a number of factors which can be associated with the micro-organism, the host or the environ, ment. There are also interactions between these factors, e.g. the pathogenhost interaction. For any particular organism the properties of the host which control disease growth rates include pH, water status or nutrient availability, inhibitors of microbial growth, vulnerability o f the cell wall
28
J. E. Holt, D. Schoorl, I. F. Muirhead
to attack by pectolytic enzymes of the pathogen and the ability of the host to form morphological or chemical barriers to the development of the pathogen (Rippon, 1980). Environmental factors, including temperature, relative humidity and controlled or modified atmosphere, have major effects on disease growth rate. Mandels (1965) states that, while manifestations of growth of pathogens are varied depending on the organism as well as the environment, and that no generalised analysis can be made, some useful mathematical expressions have been shown to apply. He describes idealised growth, based on a constant rate of cell division. In this case the rate of cell division is given by: dN =kN dt where N is number of cells, t is time and k is a constant. By integrating: log N = kt + c where c is a constant. The slope of this line, k, is a measure of the growth rate and N, the number of pathogens at any time, is a measure of the cumulative deterioration up to that time. The number of cells can be expressed in exponential form as: N
= c e kt
where e = base of natural logarithms. Mandels (1965) then derives an idealised growth curve, determined by three basic phenomena. First, there is a lag phase where there is a delay in the initiation of growth after innoculation of a culture. Secondly, after cell division starts, it soon reaches a constant rate of growth or exponential phase. Lastly, due to the depletion of food or for other reasons, growth stops. The growth curve can thus be characterised by only three phases, lag, exponential and stationary. T h e above principles apply to cellular growth but similar phases exist for hyphal growth. As far as management of deterioration is concerned, the important features of the growth curve are the transition from the lag to the exponential phase and the rate of growth during the exponential phase. Other forms of growth curve may better describe the behaviour of various pathogens but it is likely that they will still exhibit these two important phases. The exact form of the mathematical description of the growth period may vary, but it is important to establish the conditions which
Post-harvest quality strategies for fruit and vegetables
29
initiate growth and also the conditions likely to promote or retard disease development. This leads to the ability to quantify the effectiveness of control measures. To avoid unacceptable levels of disease, control measures may be instituted for contributing factors, host pathogen, environment and interactions and the cumulative damage calculated from a knowledge of the latent period following treatment and of the subsequent rate of growth of the pathogens concerned. The basis for management is thus a cumulative deterioration-time curve similar to the curve derived for the loss of shelf life versus time, shown in Fig. 1. Once the percentage damage due to pathological deterioration exceeds desired standards, the produce is deemed inferior and management is concerned with taking action before this occurs. Harvey (1960) describes how decay forecasts can be used to market grapes that show a likelihood of developing a high percentage of decay and to hold for long storage those with low decay potential.
Mechanical damage Mechanical damage to horticultural produce may be categorised as bruising, slip, cracking and abrasion. The damage results from static and dynamic loads imposed on the produce throughout the distribution system during handling, transport and storage. It is characterised by cell bursting in bruising, separation of tissue along shear surfaces in slip, tearing apart in cracking and by surface scuffing and scoring in abrasion. The basic mechanism involved in mechanical damage is the'transformation of energy, e.g. in bruising due to impact, kinetic energy is dissipated by bursting of cells in stressed regions while in cracking stored energy is released by crack propagation. The mechanics of failure in fruit and vegetables is discussed by Holt & Schoorl (1982). Unlike most physiological and pathological deterioration, damage due to excessive loads can be considered instantaneous. When an apple is dropped, for example, bruising or cell bursting occurs immediately although discoloration of the bruised tissue due to enzyme reactions may take some time. Brusing itself does not increase with elapsed time from the damaging event, although bruising may influence subsequent physiological and pathological decay. Other forms of mechanical damage can also be described as instantaneous so a distribution system can be viewed as a series of discrete energy inputs for the analysis and prediction of mechanical damage.
30
J. E. Holt, D. Schoorl, I. F. Muirhead
The evaluation of distribution systems for fruit and vegetables in terms of mechanical damage using energy inputs has been proposed by Holt & Schoorl (1981). Handling can be described by the number and height of drops likely to be experienced by a package as it moves t h r o u g h t h e system. Similarly, transport operations can be evaluated from r o a d vehicle-load interactions, as shown by Schoorl & Holt (1982a). In each case, the amount of damage to produce is directly related to the energy absorbed. For apples, there is a strong linear correlation between volume of bruised tissue and energy absorbed.: (Holt & Schoorl, 1977; Schoorl & Holt, 1978). Similar results have been obtained for strawberries and pears. For produce in packages, the total amount of damage and the distribution of damage between various layers in the pack can also be predicted from energy considerations (Holt et al., 1981; Schoorl & Holt, 1982b). Management of mechanical damage must be based on the control of energy inputs to the produce. For a package which is dropped, the energy available to do damage is a direct function of the height from which it is dropped. If the same package is dropped a second time, the damage is cumulative. Similarly in transport, the kinetic energy of the truck suspension and tray as the vehicle traverses a bump may be absorbed by a damage susceptible load. In the case of transport, management strategies include controlling vehicle speed, vehicle suspension and alterations to road profile. Proposed changes c a n b e readily evaluated by the change in energy inputs to the produce. Alternative handling schemes can also be quickly evaluated. For example, it is not u n c o m m o n for packages to be dropped from 300 m m or more during manual handling while the same package handled by fork-lift may never suffer drops in excess of 30 mm. The change to a fork-lift system means a reduction of bruise volume by a factor of 10. The actual bruise volume in a pack of apples for either handling method is given by: Bruise volume = cnmgh where c = bruise resistance coefficient (ml J - 1), n = number of apples in pack, rn = mass of individual apple (kg), g = gravitational constant (ms -z) and h = d r o p height (m). For a pack of 120 apples, each of 0-130 kg, bruise resistance, 10 ml J 71, dropped 0.30 m, the volume of bruised tissue would be 459 ml. Dropped from 30 m m during machine handling, the total bruise volume is reduced to 45-9 ml.
Post-harvest quality strategies Jor J~'uit and vegetables
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Interactions of physiological, pathological and mechanical deterioration In several important post-harvest diseases caused by fungi, an unusually long period elapses between initiation of infection and appearance of symptoms. Muirhead & Deverall (1981) have reviewed the literature on appressorial dormancy and conclude that termination of dormancy is associated with major changes in the state of the host--i.e, ripening, senescence and wounding. Wounding, for example, produces sites for infection although it has been found for bananas, Muirhead (1979), that the wounds must exceed a certain size for organism to take hold. Smoot (1969) reports that citrus often requires degreening with ethylene, a procedure known to increase the incidence of both stem-end rot and anthracnose decay. Low temperature storage may also promote pathological infection through chilling~injury. Smoot] also found linteractions involving both temperature and time; for instance, while stemend rot and green mould are the decays usually associated with freshly harvested Florida grown citrus, after storage or from 12 to 20 weeks at low temperatures these causal organisms are replaced by Alternaria, Colletotrichum, Penicillium italicum and others. Harvey (1978) states that temperature and relative humidity affect the healing of injuries; for example, suberisation in potatoes, and also the rate of growth of organisms. Physiological processes can also be affected by mechanical damage and infection. McGlasson (1970), in a review paper, states that ethylene production and respiration are generally stimulated by mechanical injury. On the other hand, the extent of mechanical damage is greatly influenced by the maturity state of product. Schoorl & Holt (1978) found that the bruise resistance of Jonathon, Delicious and Granny Smith apples markedly decreased with storage time. For example, the bruise resistance of Jonathon apples changed from 4.4 ml J - 1 to 10.1 mI J - 1 in 5 months at 2 °C. Eckert (1977) states that after several days of incubation, wound infection becomes very difficult, if not impossible, to eradicate. This is presumably because chemical treatments cannot reach the pathogens active beyond the original damaged tissue. The basic aim in the management of destructive interactions thus must be control of the primary factors, i.e. physiological, pathological and physical determinants, together with environment and time. In this way, interactions are prevented. The control of established interactions can only be of
32
J. E. Holt, D. Schoorl, I. F. Muirhead
limited effectiveness because of their complexity. A compromise control programme may be needed. For example, Harvey (1978) reports that the balance between the effects of temperature on the host and on the pathogen may be critical to the reduction of post-harvest losses. For potatoes a compromise temperature of 10-15 °C is maintained to allow some healing of the tubers and also to slow bacterial growth. The total deterioration of produce at any time as it moves through the distribution system is hence the sum of the deterioration due to primary sources and secondary interactions. Thus, the management of quality in distribution must predict and quantify the deterioration due to each of the contributing factors and decide whether the total deterioration is acceptable or not. This is the basis for decision making about available management strategies.
Q U A L I T Y C O N T R O L STRATEGIES The deterioration of produce after harvest is a function of both time arid rate of deterioration so that there are two basic strategies available to management to control quality--first, the time produce spends in the distribution system can be monitored and controlled and, secondly, the events which determine the rate of deterioration can again be monitored and, if need be, modified. Further, in any particular situation, it is necessary to establish acceptable amounts of deterioration, i.e. acceptable quality standards against which cumulative damage can be judged. Acceptable quality standards need to be defined in terms of those attributes of quality, both sensory and objective, which the buyer or consumer perceives to be important. Quality management during distribution thus needs to be based on continual assessment of the condition of produce in terms of set quality standards, and on taking action when needed to either sell the produce immediately or to ameliorate the factors causing deterioration. Those factors can be categorised as physiological, pathological and physical and combinations of these three. Figure 2 shows how alternative management strategies can be represented in terms of time and the rate of deterioration. The horizontal axis is the time produce spends in various handling, transport and storage operations, starting from harvest and extending to the consumer. Handling operations--stacking, lifting and sorting--are represented as
Post-harvest quality strategiesfor fruit and vegetables 100
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Alternative quality management strategies on a deterioration-time plot.
instantaneous events. The times for other activities such as storage and transport are, of course, not necessarily fixed, and are subject to management decisions. Such decisions might be prompted by either quality deterioration or price advantage for selling at a particular time. The distance up the vertical axis is a measure of the deterioration at any particular time, beginning with the initial quality at harvest and extending to the amount of deterioration that can be accepted. Beyond this level, the produce is deemed to be seriously reduced in value. The deterioration is measured as the sum of the contributions of the various factors causing deterioration, and might be conveniently expressed as a percentage figure made up of contributions from the factors, also expressed as percentages. For example, consider a consignment of apples. The decision that needs to be made is what represents 100 % deterioration for each of the three contributing factors, e.g. the amount of bruising which is acceptable, the a m o u n t of rot and the loss of shelf life, perhaps indicated by texture. The deterioration of the apples at any time is the total of the individual percentages contributed by these factors. If any particular sample has 10 % of the permissible rots, 10 % of the permissible bruising and has lost
34
J. E. Holt, D. Schoorl, I. F. Muirhead
10 ~ of texture or shelf life, then the total deterioration is 30 ~ . On this basis, another sample with no bruises and no appreciable loss in shelf life, but with 30 ~ of the permissible rot level, has deteriorated the same percentage of the total allowable and is, at that specific time, the same quality. Strategy I and Strategy II, shown in Fig. 2, are examples of plotting alternative management strategies for distribution on a time-deterioration framework. Strategy I supposes produce with initial quality at harvest represented by point A on the vertical axis. It is then stored on the farm at temperature T 1 until time t 1. During this time the produce has slowly deteriorated. At time, tl, the produce is retrieved from storage and loaded onto a refrigerated transport vehicle, using a forklift, resulting in an instantaneous energy input (drop) causing some bruising. During transport at temperature T 1 the vehicle traverses a bump at time t2, giving another energy input and causing more mechanical damage. At time t 3 the produce reaches the storage depot at the markets and is again handled by a forklift, receiving another energy input. At the markets the produce is held in cool store, again at temperature T~, and further reduction in shelf life takes place. At time t 5 the produce is removed from storage, sold and manually loaded on a retailer's truck. The manual handling results in more damage than the previous machine handlings. The produce is no longer held at a controlled temperature and completes its journey to the retail outlet at temperature T 2 > T t . The condition of the produce when it arrives at the retail shop is given by the summation of the temperature deterioration and damage due to energy inputs and is represented by point B, about 70 ~ of the acceptable deterioration level. Strategy II assumes produce with the same initial quality as that in Strategy I but with dormant infection. The produce is held in storage on the farm at temperature T 3, slightly higher than T~, resulting in a faster deterioration rate. At time t 1 it is manually loaded onto transport, and appreciable mechanical damage occurs. During transport at temperature T 3 further deterioration occurs due to a bump and manual handling, respectively. The damage present at time t 3 now stimulates the growth of the dormant pathogen and rapid deterioration begins. At time t 4 management decides that deterioration levels warrant action and a pathogen treatment is applied. F r o m then on, the storage temperature is optimised at T 4. At t s the produce is loaded into a refrigerated van by forklift and finally arrives at the retail shop in condition C, slightly lower
Post-harvest quality strategies for fruit and vegetables
35
in quality than the first consignment. If corrective action was not taken at t 4 the produce would have exceeded acceptable limits, at point:C, say, and could not have been delivered to the retail outlet in saleable condition.
CONCLUSION For horticultural produce there is enough information available in m a n y cases to provide a sound basis for the quantification of deterioration due to physiological, pathological and physical factors, as the produce moves through the distribution system. Where the information does not exist, this paper shows what sort o f information would assist in the development of management strategies.
ACKNOWLEDGEMENT Thanks are due to Mr M. Gauld for assistance in the preparation of the figures.
REFERENCES Brecht, P. E. (1980). Use of controlled atmospheres to retard deterioration of produce. Food Technology, 30(3), 45-50. Coursey, D. G. & Proctor, F. J. (1975). Towards the quantification of post harvest loss in horticultural produce. Acta Horticulturae, 49, 55-65. Eckert, J. W. (1977). Control of post-harvest diseases. In Antifungal compounds (Siegel, M. R. & Sisler, H. D, (Eds)), Marcel Dekker, New York; 269-351. Harvey, J. M. (1960). Instructions for forecasting decay in table grapes for storage. U.S. Agr. Market Serv. AMS--392; 1-12. Harvey, J. M. (1978). Reduction of losses in fresh market fruits and vegetables. Ann. Rev. Phytopathol., 16, 321 41. Holt, J. E. & Schoorl, D. (1977). Bruising and energy dissipation in apples. J. Text. Stud., 7, 421-32. Holt, J. E. & Schoorl, D. (1981). Fruit packaging and handling distribution systems:An evaluation method, Agricultural Systems, 7(3), 209-18. Holt, J. E. & Schoorl, D. (1982). The mechanics of failure in fruit and vegetables. J. Text. Stud., 113(1). Holt, J. E., Schoorl, D. & Lucas, C. (1981). Prediction of bruising in impacted multi-layered apple packs. Trans. ASAE, 24(1), 242-7.
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J. E. Holt, D. Schoorl, I. F. Muirhead
Isenberg, F. M. R. (1979). Controlled atmosphere storage of vegetables. Hort. Rev., 1, 337-94. Kader, A. A. (1980). Prevention of ripening in fruits by use of controlled atmospheres. Food Technology, 34(3), 51-54. Littmann, M. D. & Peacock, B. C. (1972). Summation of the effects of varying temperatures on the metabolism of a biological system. Qld. J. of Agric. and Anim. Se., 29(1), 67-69. Lutz, J. M. & Hardenburg, R. E. (1968). The commercial storage of fruits, vegetables, and florist and nursery stocks. Agriculture Handbook No. 66. United States Department of Agriculture 1-94. McGlasson, W. B. (1970). The ethylene factor. In: The biochemistry offruits and their products, Vol. 1 (Hulme, A. C. (Ed)), Academic Press, London, pp. 475-519. McGlasson, W. B., Scott, K. J. & Mendoza, D. B. (1979). The refrigerated storage of tropical and subtropical products. Intern. J. of Refrig., 2(6), 199-205. Mandels, G. R. (1965). Kinetics of fungal growth. In: The Fungi, Vol. 1. (Ainsworth, G. C. & Sussman, A. S. (Eds)), Academic Press, New York, pp. 599-612. Molenaar, W. H. (1981). Properties of ornamentals and requirements for its packaging. Symposium, Packaging of Horticultural Produce, Wageningen, The Netherlands, pp. 61-71. Muirhead, I. F. (1979). Latency ofColletotrichum musae in banana fruit, PhD Thesis, University of Sydney. Muirhead, I. F. & Deverall, B. J. (1981). Role of appressoria in latent infection of banana fruits by Colletotrichum musae. Physiol. Plant Pathol. 19, 77-84. Peacock, B. C. (1980). Banana ripening--Effect of temperature on fruit quality. Qld. J. of Agric. and Anim. Sci., 37(1), 39-45. Rippon, L. E. (1980). Wastage of postharvest fruit and its control. CSIRO. Food Res. Quart, 40(1), 1-12. Ryall, A. L. & Lipton, W. J. (1972). Handling, transportation and storage of fruits and vegetables, Avi, Westport, Connecticut, 1-473. Schoorl, D. & Holt, J. E. (1978). The effects of storage time and temperature on the bruising of Jonathon, Delicious and Granny Smith apples. J. Text. Stud., 8, 409-16. Schoorl, D. & Holt, J. E. (1982a). Road-vehicle-load interactions for transport of fruit and vegetables. Agric. Systems, 8, 143-55. Schoorl, D. & Holt, J. E. (1982b). Impact bruising in three apple pack arrangements. J. Agric. Eng. Res. (Submitted for publication.) Schoorl, D. & Williams, W. T. (1972). Prediction of drop-test performance of apple packs., Qld. J. of Agric. and Anim. Sci., 29, 18%97. Schoorl, D. & Williams, W. T. (1973). Robustness of a model predicting drop testing performance of fruit packs. Qld. J. of Agric. and Anita. Sci., 30(3), 24%53: Smock, R. M. (I979). Controlled atmosphere storage of fruits. Hort. Rev., 1, 301-35.
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Smoot, J. J. (1969). Decay of Florida citrus fruits stored in controlled atmospheres and air. (Chapman, H. C. (Ed)). Proceedings of 1st Citrus Symposium, Vol. 3, pp. 1285-93. Thorne, S. & Alvarez, J. S. S. (1981). The effects of irregular temperature on stored tomatoes. Symposium." Packaging of Horticultural Produce, Wageningen, The Netherlands, 73-85. Wilkinson, B. G. (1970). Physiological disorders of fruit after harvesting: Vol. 1. (Hulme, A. C. (Ed)), Academic Press, London, pp. 537-54.
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
22
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
23
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
24
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
26
temperature- time history
20 o 18
16 14 12
I
10
T'" 0,15 -
I
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deterioration rate, variable temperature ------ deterioration rate, constant temperafure(13"C)
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.
.
.
.
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.
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80
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Calculation
of rate
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of deterioration a n d loss of storage t e m p e r a t u r e time history.
life from
Post-harvest quality strategies for fruit and vegetables
27
Figure 1 shows how loss of storage life can be calculated from a fluctuating temperature-time history for Stored produce. Figure 1 is constructed by taking the curves for tomatoes presented by Thorne & Alvarez (1981) for temperature and deterioration rate against time and integrating the deterioration rate curve to give progressive deterioration. Typical limits for heat sensitivity and chilling have been added, together with an assumed optimum storage temperature line. The deterioration,time curve shows clearly that part of the deterioration which temperature management strategies can affect. The deterioration at optimum temperature is the minimum deterioration which can be achieved. The management of deterioration due to temperature is concerned with avoiding temperature sensitive limits and with assessing how much storage life remains after a particular temperature-time history. When the cumulative deterioration reaches 100 % there is no shelf life left. It is reasonable to suppose that similar deterioration-time curves can be generated for produce for other environmental factors. For example, storage away from optimum humidity levels, described for apples and plums by Wilkinson (1970), causes skin splitting at high moisture levels. Ryall & Lipton (1972) show that water loss occurs at low humidity levels. It should therefore be possible to predict water loss from a vapour pressure deficit-time history and to construct a cumulative wilting-time curve. Controlled atmosphere, hypobaric storage and ethylene concentrations could be assessed in the same way, giving a proper basis for management decisions.
Pathological deterioration Major post-harvest diseases of fresh fruit and vegetables and their causal micro-organisms have been described by Eckert (1977) together with estimated losses for some produce due to pathological diseases ranging from 2 % for McIntosh apples from Nova Scotia to 52 % for Sanguinello oranges from Italy. Rippon (1980) states that the capability of a microorganism to initiate a post-harvest disease depends on a number of factors which can be associated with the micro-organism, the host or the environ, ment. There are also interactions between these factors, e.g. the pathogenhost interaction. For any particular organism the properties of the host which control disease growth rates include pH, water status or nutrient availability, inhibitors of microbial growth, vulnerability o f the cell wall
28
J. E. Holt, D. Schoorl, I. F. Muirhead
to attack by pectolytic enzymes of the pathogen and the ability of the host to form morphological or chemical barriers to the development of the pathogen (Rippon, 1980). Environmental factors, including temperature, relative humidity and controlled or modified atmosphere, have major effects on disease growth rate. Mandels (1965) states that, while manifestations of growth of pathogens are varied depending on the organism as well as the environment, and that no generalised analysis can be made, some useful mathematical expressions have been shown to apply. He describes idealised growth, based on a constant rate of cell division. In this case the rate of cell division is given by: dN =kN dt where N is number of cells, t is time and k is a constant. By integrating: log N = kt + c where c is a constant. The slope of this line, k, is a measure of the growth rate and N, the number of pathogens at any time, is a measure of the cumulative deterioration up to that time. The number of cells can be expressed in exponential form as: N
= c e kt
where e = base of natural logarithms. Mandels (1965) then derives an idealised growth curve, determined by three basic phenomena. First, there is a lag phase where there is a delay in the initiation of growth after innoculation of a culture. Secondly, after cell division starts, it soon reaches a constant rate of growth or exponential phase. Lastly, due to the depletion of food or for other reasons, growth stops. The growth curve can thus be characterised by only three phases, lag, exponential and stationary. T h e above principles apply to cellular growth but similar phases exist for hyphal growth. As far as management of deterioration is concerned, the important features of the growth curve are the transition from the lag to the exponential phase and the rate of growth during the exponential phase. Other forms of growth curve may better describe the behaviour of various pathogens but it is likely that they will still exhibit these two important phases. The exact form of the mathematical description of the growth period may vary, but it is important to establish the conditions which
Post-harvest quality strategies for fruit and vegetables
29
initiate growth and also the conditions likely to promote or retard disease development. This leads to the ability to quantify the effectiveness of control measures. To avoid unacceptable levels of disease, control measures may be instituted for contributing factors, host pathogen, environment and interactions and the cumulative damage calculated from a knowledge of the latent period following treatment and of the subsequent rate of growth of the pathogens concerned. The basis for management is thus a cumulative deterioration-time curve similar to the curve derived for the loss of shelf life versus time, shown in Fig. 1. Once the percentage damage due to pathological deterioration exceeds desired standards, the produce is deemed inferior and management is concerned with taking action before this occurs. Harvey (1960) describes how decay forecasts can be used to market grapes that show a likelihood of developing a high percentage of decay and to hold for long storage those with low decay potential.
Mechanical damage Mechanical damage to horticultural produce may be categorised as bruising, slip, cracking and abrasion. The damage results from static and dynamic loads imposed on the produce throughout the distribution system during handling, transport and storage. It is characterised by cell bursting in bruising, separation of tissue along shear surfaces in slip, tearing apart in cracking and by surface scuffing and scoring in abrasion. The basic mechanism involved in mechanical damage is the'transformation of energy, e.g. in bruising due to impact, kinetic energy is dissipated by bursting of cells in stressed regions while in cracking stored energy is released by crack propagation. The mechanics of failure in fruit and vegetables is discussed by Holt & Schoorl (1982). Unlike most physiological and pathological deterioration, damage due to excessive loads can be considered instantaneous. When an apple is dropped, for example, bruising or cell bursting occurs immediately although discoloration of the bruised tissue due to enzyme reactions may take some time. Brusing itself does not increase with elapsed time from the damaging event, although bruising may influence subsequent physiological and pathological decay. Other forms of mechanical damage can also be described as instantaneous so a distribution system can be viewed as a series of discrete energy inputs for the analysis and prediction of mechanical damage.
30
J. E. Holt, D. Schoorl, I. F. Muirhead
The evaluation of distribution systems for fruit and vegetables in terms of mechanical damage using energy inputs has been proposed by Holt & Schoorl (1981). Handling can be described by the number and height of drops likely to be experienced by a package as it moves t h r o u g h t h e system. Similarly, transport operations can be evaluated from r o a d vehicle-load interactions, as shown by Schoorl & Holt (1982a). In each case, the amount of damage to produce is directly related to the energy absorbed. For apples, there is a strong linear correlation between volume of bruised tissue and energy absorbed.: (Holt & Schoorl, 1977; Schoorl & Holt, 1978). Similar results have been obtained for strawberries and pears. For produce in packages, the total amount of damage and the distribution of damage between various layers in the pack can also be predicted from energy considerations (Holt et al., 1981; Schoorl & Holt, 1982b). Management of mechanical damage must be based on the control of energy inputs to the produce. For a package which is dropped, the energy available to do damage is a direct function of the height from which it is dropped. If the same package is dropped a second time, the damage is cumulative. Similarly in transport, the kinetic energy of the truck suspension and tray as the vehicle traverses a bump may be absorbed by a damage susceptible load. In the case of transport, management strategies include controlling vehicle speed, vehicle suspension and alterations to road profile. Proposed changes c a n b e readily evaluated by the change in energy inputs to the produce. Alternative handling schemes can also be quickly evaluated. For example, it is not u n c o m m o n for packages to be dropped from 300 m m or more during manual handling while the same package handled by fork-lift may never suffer drops in excess of 30 mm. The change to a fork-lift system means a reduction of bruise volume by a factor of 10. The actual bruise volume in a pack of apples for either handling method is given by: Bruise volume = cnmgh where c = bruise resistance coefficient (ml J - 1), n = number of apples in pack, rn = mass of individual apple (kg), g = gravitational constant (ms -z) and h = d r o p height (m). For a pack of 120 apples, each of 0-130 kg, bruise resistance, 10 ml J 71, dropped 0.30 m, the volume of bruised tissue would be 459 ml. Dropped from 30 m m during machine handling, the total bruise volume is reduced to 45-9 ml.
Post-harvest quality strategies Jor J~'uit and vegetables
31
Interactions of physiological, pathological and mechanical deterioration In several important post-harvest diseases caused by fungi, an unusually long period elapses between initiation of infection and appearance of symptoms. Muirhead & Deverall (1981) have reviewed the literature on appressorial dormancy and conclude that termination of dormancy is associated with major changes in the state of the host--i.e, ripening, senescence and wounding. Wounding, for example, produces sites for infection although it has been found for bananas, Muirhead (1979), that the wounds must exceed a certain size for organism to take hold. Smoot (1969) reports that citrus often requires degreening with ethylene, a procedure known to increase the incidence of both stem-end rot and anthracnose decay. Low temperature storage may also promote pathological infection through chilling~injury. Smoot] also found linteractions involving both temperature and time; for instance, while stemend rot and green mould are the decays usually associated with freshly harvested Florida grown citrus, after storage or from 12 to 20 weeks at low temperatures these causal organisms are replaced by Alternaria, Colletotrichum, Penicillium italicum and others. Harvey (1978) states that temperature and relative humidity affect the healing of injuries; for example, suberisation in potatoes, and also the rate of growth of organisms. Physiological processes can also be affected by mechanical damage and infection. McGlasson (1970), in a review paper, states that ethylene production and respiration are generally stimulated by mechanical injury. On the other hand, the extent of mechanical damage is greatly influenced by the maturity state of product. Schoorl & Holt (1978) found that the bruise resistance of Jonathon, Delicious and Granny Smith apples markedly decreased with storage time. For example, the bruise resistance of Jonathon apples changed from 4.4 ml J - 1 to 10.1 mI J - 1 in 5 months at 2 °C. Eckert (1977) states that after several days of incubation, wound infection becomes very difficult, if not impossible, to eradicate. This is presumably because chemical treatments cannot reach the pathogens active beyond the original damaged tissue. The basic aim in the management of destructive interactions thus must be control of the primary factors, i.e. physiological, pathological and physical determinants, together with environment and time. In this way, interactions are prevented. The control of established interactions can only be of
32
J. E. Holt, D. Schoorl, I. F. Muirhead
limited effectiveness because of their complexity. A compromise control programme may be needed. For example, Harvey (1978) reports that the balance between the effects of temperature on the host and on the pathogen may be critical to the reduction of post-harvest losses. For potatoes a compromise temperature of 10-15 °C is maintained to allow some healing of the tubers and also to slow bacterial growth. The total deterioration of produce at any time as it moves through the distribution system is hence the sum of the deterioration due to primary sources and secondary interactions. Thus, the management of quality in distribution must predict and quantify the deterioration due to each of the contributing factors and decide whether the total deterioration is acceptable or not. This is the basis for decision making about available management strategies.
Q U A L I T Y C O N T R O L STRATEGIES The deterioration of produce after harvest is a function of both time arid rate of deterioration so that there are two basic strategies available to management to control quality--first, the time produce spends in the distribution system can be monitored and controlled and, secondly, the events which determine the rate of deterioration can again be monitored and, if need be, modified. Further, in any particular situation, it is necessary to establish acceptable amounts of deterioration, i.e. acceptable quality standards against which cumulative damage can be judged. Acceptable quality standards need to be defined in terms of those attributes of quality, both sensory and objective, which the buyer or consumer perceives to be important. Quality management during distribution thus needs to be based on continual assessment of the condition of produce in terms of set quality standards, and on taking action when needed to either sell the produce immediately or to ameliorate the factors causing deterioration. Those factors can be categorised as physiological, pathological and physical and combinations of these three. Figure 2 shows how alternative management strategies can be represented in terms of time and the rate of deterioration. The horizontal axis is the time produce spends in various handling, transport and storage operations, starting from harvest and extending to the consumer. Handling operations--stacking, lifting and sorting--are represented as
Post-harvest quality strategiesfor fruit and vegetables 100
/ M
~
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d~
33
C I
/
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I~,---deferiorafion warranis action
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rapid pafhogen
g 50
-
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0
~ " - - "
harvest
~ ~"--
-I-
r"
I manual ...J
2f3
handling
fB
I
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A
r21
c
Jl 7-
handling
handling
f° I
retail outlet
TIME Fig. 2.
Alternative quality management strategies on a deterioration-time plot.
instantaneous events. The times for other activities such as storage and transport are, of course, not necessarily fixed, and are subject to management decisions. Such decisions might be prompted by either quality deterioration or price advantage for selling at a particular time. The distance up the vertical axis is a measure of the deterioration at any particular time, beginning with the initial quality at harvest and extending to the amount of deterioration that can be accepted. Beyond this level, the produce is deemed to be seriously reduced in value. The deterioration is measured as the sum of the contributions of the various factors causing deterioration, and might be conveniently expressed as a percentage figure made up of contributions from the factors, also expressed as percentages. For example, consider a consignment of apples. The decision that needs to be made is what represents 100 % deterioration for each of the three contributing factors, e.g. the amount of bruising which is acceptable, the a m o u n t of rot and the loss of shelf life, perhaps indicated by texture. The deterioration of the apples at any time is the total of the individual percentages contributed by these factors. If any particular sample has 10 % of the permissible rots, 10 % of the permissible bruising and has lost
34
J. E. Holt, D. Schoorl, I. F. Muirhead
10 ~ of texture or shelf life, then the total deterioration is 30 ~ . On this basis, another sample with no bruises and no appreciable loss in shelf life, but with 30 ~ of the permissible rot level, has deteriorated the same percentage of the total allowable and is, at that specific time, the same quality. Strategy I and Strategy II, shown in Fig. 2, are examples of plotting alternative management strategies for distribution on a time-deterioration framework. Strategy I supposes produce with initial quality at harvest represented by point A on the vertical axis. It is then stored on the farm at temperature T 1 until time t 1. During this time the produce has slowly deteriorated. At time, tl, the produce is retrieved from storage and loaded onto a refrigerated transport vehicle, using a forklift, resulting in an instantaneous energy input (drop) causing some bruising. During transport at temperature T 1 the vehicle traverses a bump at time t2, giving another energy input and causing more mechanical damage. At time t 3 the produce reaches the storage depot at the markets and is again handled by a forklift, receiving another energy input. At the markets the produce is held in cool store, again at temperature T~, and further reduction in shelf life takes place. At time t 5 the produce is removed from storage, sold and manually loaded on a retailer's truck. The manual handling results in more damage than the previous machine handlings. The produce is no longer held at a controlled temperature and completes its journey to the retail outlet at temperature T 2 > T t . The condition of the produce when it arrives at the retail shop is given by the summation of the temperature deterioration and damage due to energy inputs and is represented by point B, about 70 ~ of the acceptable deterioration level. Strategy II assumes produce with the same initial quality as that in Strategy I but with dormant infection. The produce is held in storage on the farm at temperature T 3, slightly higher than T~, resulting in a faster deterioration rate. At time t 1 it is manually loaded onto transport, and appreciable mechanical damage occurs. During transport at temperature T 3 further deterioration occurs due to a bump and manual handling, respectively. The damage present at time t 3 now stimulates the growth of the dormant pathogen and rapid deterioration begins. At time t 4 management decides that deterioration levels warrant action and a pathogen treatment is applied. F r o m then on, the storage temperature is optimised at T 4. At t s the produce is loaded into a refrigerated van by forklift and finally arrives at the retail shop in condition C, slightly lower
Post-harvest quality strategies for fruit and vegetables
35
in quality than the first consignment. If corrective action was not taken at t 4 the produce would have exceeded acceptable limits, at point:C, say, and could not have been delivered to the retail outlet in saleable condition.
CONCLUSION For horticultural produce there is enough information available in m a n y cases to provide a sound basis for the quantification of deterioration due to physiological, pathological and physical factors, as the produce moves through the distribution system. Where the information does not exist, this paper shows what sort o f information would assist in the development of management strategies.
ACKNOWLEDGEMENT Thanks are due to Mr M. Gauld for assistance in the preparation of the figures.
REFERENCES Brecht, P. E. (1980). Use of controlled atmospheres to retard deterioration of produce. Food Technology, 30(3), 45-50. Coursey, D. G. & Proctor, F. J. (1975). Towards the quantification of post harvest loss in horticultural produce. Acta Horticulturae, 49, 55-65. Eckert, J. W. (1977). Control of post-harvest diseases. In Antifungal compounds (Siegel, M. R. & Sisler, H. D, (Eds)), Marcel Dekker, New York; 269-351. Harvey, J. M. (1960). Instructions for forecasting decay in table grapes for storage. U.S. Agr. Market Serv. AMS--392; 1-12. Harvey, J. M. (1978). Reduction of losses in fresh market fruits and vegetables. Ann. Rev. Phytopathol., 16, 321 41. Holt, J. E. & Schoorl, D. (1977). Bruising and energy dissipation in apples. J. Text. Stud., 7, 421-32. Holt, J. E. & Schoorl, D. (1981). Fruit packaging and handling distribution systems:An evaluation method, Agricultural Systems, 7(3), 209-18. Holt, J. E. & Schoorl, D. (1982). The mechanics of failure in fruit and vegetables. J. Text. Stud., 113(1). Holt, J. E., Schoorl, D. & Lucas, C. (1981). Prediction of bruising in impacted multi-layered apple packs. Trans. ASAE, 24(1), 242-7.
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J. E. Holt, D. Schoorl, I. F. Muirhead
Isenberg, F. M. R. (1979). Controlled atmosphere storage of vegetables. Hort. Rev., 1, 337-94. Kader, A. A. (1980). Prevention of ripening in fruits by use of controlled atmospheres. Food Technology, 34(3), 51-54. Littmann, M. D. & Peacock, B. C. (1972). Summation of the effects of varying temperatures on the metabolism of a biological system. Qld. J. of Agric. and Anim. Se., 29(1), 67-69. Lutz, J. M. & Hardenburg, R. E. (1968). The commercial storage of fruits, vegetables, and florist and nursery stocks. Agriculture Handbook No. 66. United States Department of Agriculture 1-94. McGlasson, W. B. (1970). The ethylene factor. In: The biochemistry offruits and their products, Vol. 1 (Hulme, A. C. (Ed)), Academic Press, London, pp. 475-519. McGlasson, W. B., Scott, K. J. & Mendoza, D. B. (1979). The refrigerated storage of tropical and subtropical products. Intern. J. of Refrig., 2(6), 199-205. Mandels, G. R. (1965). Kinetics of fungal growth. In: The Fungi, Vol. 1. (Ainsworth, G. C. & Sussman, A. S. (Eds)), Academic Press, New York, pp. 599-612. Molenaar, W. H. (1981). Properties of ornamentals and requirements for its packaging. Symposium, Packaging of Horticultural Produce, Wageningen, The Netherlands, pp. 61-71. Muirhead, I. F. (1979). Latency ofColletotrichum musae in banana fruit, PhD Thesis, University of Sydney. Muirhead, I. F. & Deverall, B. J. (1981). Role of appressoria in latent infection of banana fruits by Colletotrichum musae. Physiol. Plant Pathol. 19, 77-84. Peacock, B. C. (1980). Banana ripening--Effect of temperature on fruit quality. Qld. J. of Agric. and Anim. Sci., 37(1), 39-45. Rippon, L. E. (1980). Wastage of postharvest fruit and its control. CSIRO. Food Res. Quart, 40(1), 1-12. Ryall, A. L. & Lipton, W. J. (1972). Handling, transportation and storage of fruits and vegetables, Avi, Westport, Connecticut, 1-473. Schoorl, D. & Holt, J. E. (1978). The effects of storage time and temperature on the bruising of Jonathon, Delicious and Granny Smith apples. J. Text. Stud., 8, 409-16. Schoorl, D. & Holt, J. E. (1982a). Road-vehicle-load interactions for transport of fruit and vegetables. Agric. Systems, 8, 143-55. Schoorl, D. & Holt, J. E. (1982b). Impact bruising in three apple pack arrangements. J. Agric. Eng. Res. (Submitted for publication.) Schoorl, D. & Williams, W. T. (1972). Prediction of drop-test performance of apple packs., Qld. J. of Agric. and Anim. Sci., 29, 18%97. Schoorl, D. & Williams, W. T. (1973). Robustness of a model predicting drop testing performance of fruit packs. Qld. J. of Agric. and Anita. Sci., 30(3), 24%53: Smock, R. M. (I979). Controlled atmosphere storage of fruits. Hort. Rev., 1, 301-35.
Post-harvest quality strategiesfor fruit and vegetables
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Smoot, J. J. (1969). Decay of Florida citrus fruits stored in controlled atmospheres and air. (Chapman, H. C. (Ed)). Proceedings of 1st Citrus Symposium, Vol. 3, pp. 1285-93. Thorne, S. & Alvarez, J. S. S. (1981). The effects of irregular temperature on stored tomatoes. Symposium." Packaging of Horticultural Produce, Wageningen, The Netherlands, 73-85. Wilkinson, B. G. (1970). Physiological disorders of fruit after harvesting: Vol. 1. (Hulme, A. C. (Ed)), Academic Press, London, pp. 537-54.