2. Factors related to the post-harvest system

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2.1 Technologies

The major technologies for reducing losses in horticultural products are listed below followed by a statement of probable environmental effects from the named procedure.

 

2.1.1 Gentle handling

Because of their soft texture All horticultural products should be handled gently to minimize bruising and breaking of the skin. Bruising renders the product unsaleable to most people although it usually has minor effect upon the nutritional value. The skin of horticultural products is an effective barrier to most of the opportunistic bacteria and fungi that cause rotting of the tissues. Breaking of the skin also stimulates physiological deterioration and dehydration. Careful digging and movement of roots and tubers significantly reduces postharvest losses. Careful handling of fruits and vegetables to minimize bruising and breaking of the skin likewise is a well-known method of reducing postharvest losses as is the provision of adequate shipping containers to protect the produce from bruising' and puncturing of the skin. Reducing the number of times the commodity is handled reduces the extent of mechanical damage.

Environmental effects. There are no adverse environmental effects to this technology. Thus careful digging, harvesting and handling, and appropriate packaging end transportation are environmentally count methods for reducing losses. Also, since damaged skin is the major entry point for fungal infections, some of which produce mycotoxins, gentle handling can improve the safety of the produce.

 

2.1.2 Temperature control

It is well known that cooling horticultural produce extends storage life by reducing the rate of physiological change and retarding the growth of spoilage fungi and bacteria. Cooling is the foundation of quality protection (see Figure 1 Page 12). There are several ways of reducing the storage temperature of horticultural crops:

Cooling technique Environmental effect
a) Keep out of direct rays of sun. This is an easy low-cost method with minimal effect on the environment. Almost all societies can provide shade at low economic or environmental cost.
b) Use natural cooling, e.g., harvest during the cool early morning hours, open stores for ventilation during the cool of the night, utilize the cool temperature of high altitude or a natural source of cold water when available. Minimal environmental costs.
c) Evaporative cooling obtained by drawing dry air over a moist surface. Minimal environmental and economic costs. Restricted to areas of low humidity and low-cost water.
d) Mechanical refrigeration. Energy coats and economic costs are relatively high but give most positive control of temperature. Generated heat is dumped into the environment.
e) Cool promptly after harvest. High energy coat.

Since every degree reduction from ambient temperature increases storage life' every form of cooling is beneficial even if it is not optimum cooling, i.e.' simple low-cost cooling or refreshing the product is better than no cooling at all.

The optimum storage temperature for most temperate horticultural crops is close to 0C. If they are cooled slightly below this temperature they freeze and suffer from "freezing injury" and spoil quickly. Most tropical horticulture crops however can be injured oven at temperatures above freezing point. This is called "chilling injury" and causes rapid deterioration in quality. The optimum storage temperature for most tropical horticulture crops is between 7 to 10C; for yams and bananas it is about 15C.

Although refrigerated storage is not often appropriate for some commodities such as yams, it should be considered an important element in the temperature management of a wide range of perishable crops because it gives the most positive and direct control of temperature. The popularity of refrigerated storage in some countries has suffered setbacks duo to occasional poor design of units and bad management. This has sometimes resulted in the impression that refrigerated storage is costly and unsuited for use in developing countries which is not necessary the case. Good cosign and proper management are essential ingredients in considering the introduction of refrigeration techniques as are the supporting infrastructures available within the post-harvest system. When studied on a case-by-case basis it seems likely that refrigerated storage will continue to find many successful applications to the needs of developing countries.

There are tour basic principles which must be correctly applied for successful refrigeration of perishable crops:

  1. Select only healthy products. Refrigeration does not destroy pathogens responsible for product deterioration but only slows their activity; it does not improve product quality but only maintains it. A damaged product will deteriorate more quickly than a healthy one even in refrigerated storage, hence it is pointless to submit poor quality produce to refrigeration. In addition storage under refrigeration increases the coat to the product. The storage therefore of low grade diseased produce frequently cannot achieve an adequate economic return.
  2. Timely cooling: Since refrigeration slows the development of micro-organisms and physiological changes responsible for deterioration of perishable crops, it is obvious that cooling should be applied as soon as possible after harvest. The technique of pre-cooling was developed to fill this need by cooling produce soon after harvesting down to a temperature appropriate to that product.
  3. Adhere closely to optimal conditions for temperature and relative humidity. It is well known that refrigeration provides maximum storage life it these two parameters are correctly adhered to. This fact is especially important for tropical fruits and vegetables because their optimum storage temperatures vary considerably between varieties and even between producing areas. One of the main roles of research centres in tropical countries should be to define optimum storage conditions for commodities grown under tropical conditions. There is a need to evaluate the limitations of storage of these commodities under a range of temperature conditions and to consider the implications and problems of product compatibility under conditions of mixed commodity storage.
  4. Uninterrupted cooling: Refrigeration should be applied from the point of harvest through to the point of consumption where maximum post-harvest life with high product quality is justified. This is the well known concept of the "cold chain".

 

2.1.3 High humidity

High humidity retards wilting and maintains the product in better condition. Most horticultural products store best in an atmosphere that has a relative humidity of 90% (Lutz and Hardenburg, 1968). Providing humidity has little environmental cost.

 

2.1.4 Waxing of the surface

Waxing the surface of horticultural products is a treatment used on a number of commodities including citrus fruits, apples, rutabagas and cucumbers. It retards the rate of moisture loss, and maintains turgor and plumpness and may modify the internal atmosphere of the commodity, and is performed primarily for its cosmetic effect; the wax imparts a gloss to the skin and gives the produce a more shiny appearance than the unwaxed commodity. Sometimes antiwaxing is a techniques that could probably be used more widely in developing countries with advantage. In some countries indigenous waxes may be suitable for this purpose. For example, experiments in Colombia have shown that waxing of cassava can extend the storage life from 2 to 3 days up to about 30 days by preventing discolouration in the vascular tissue. (Buckle et al 1973) Work in India has also demonstrated the efficacy of indigenously produced wax emulsion formulations in extending the storage life of different fruits and vegetables. (Dalal et al 1970)

Environmental effects. The waxes and wax formulations that are used in the U.S. are approved by the Food and Drug Administration and are kept under continuous review. Most of the ingredients in the wax mixtures are classified "Generally Recognized As Safe" (GRAS). In most cases the skin is removed and discarded before consumption in which case the wax is not ingested and should cause no special problems. However, problems might arise if unregistered formulations are used, or if the skin is eaten by humans or fed to animals.

 

2.1.5 Controlled atmosphere storage

Controlled atmosphere storage consists of placing a commodity is a gas-tight refrigerated chamber and allowing the natural respiration of the fruit to decrease the oxygen and increase the carbon dioxide content of the atmosphere in the chamber. Typically, for storage of apples the oxygen content is lowered to about 3% and carbon dioxide is allowed to increase to 1 to 5%. This atmosphere can extend the storage life of apples by several months and allows fresh apples to be marketed every month of the year. This technology requires expensive storage chambers and close supervision of the composition of the atmosphere and is unsuited for widespread use in less developed countries.

Some roots and tubers are stored in pits in the ground, known as "clamp storage". Well designed clamps tend to change the atmosphere to some extent by reducing oxygen and increasing the carbon dioxide content. Modified atmosphere storage would probably be effective for a limited number of commodities in developing countries especially if coupled with low temperature storage. Wills and Wimalasiri (Hort. Science, 14 528 1979) have recently shown that short pre-storage exposure to high carbon dioxide and low oxygen atmosphere of vegetables can extend the storage life of commodities even at ambient temperature.

Environmental effects. Since this technology only manipulates the proportions of asses that are naturally present in the air there should be no adverse environmental effect.

The now technology of hypobaric storage is emerging which maintains reduced pressure in the refrigerated storage chamber by means of vacuum pumps. In this system the commodity is placed in a flowing stream of highly humidified air which is maintained at a reduced pressure and controlled temperature. Under these conditions, Bases released by the commodity that limits its storage life, are flushed away. Reports indicate that the storage life of certain fruits and vegetables is extended substantially by this procedure. The economic feasibility of this type of controlled atmosphere storage is presently being tested. This is an energy-intensive and capital-intensive technology and is perhaps unsuited for less developed countries. The major environmental effect is the high energy coat.

 

2.1.6 Field factors

Maturity at time of harvest is an important factor in the keeping quality of horticultural products. Commodities that are harvested in an immature state not only have poor eating quality but may tend to shrivel in storage and be more susceptible to storage disorders. When picked too mature the commodity is soft or fibrous, the flesh breaks down more quickly and it has a shorter storage life. There is an optimum time of harvest to give maximum storage life for fruits, vegetables and tubers.

The rootstocks used for establishing fruit orchards may affect [oases. For example, McDonald and Wutscher (1974) reported decay in grapefruit ranging from 3.3% to 27.7% depending on the rootstock. It is reported that the storage life of fresh cassava can be greatly extended by leaving part of the stalk attached to the tubers at harvest time. There are a number of other field factors that affect losses and these should be utilized as much as possible.

Environmental effects. Generally there are no adverse environmental effects in these operations.

 

2.1.7 Suberization and curing

Potatoes, sweet potatoes, yams and several other roots and vegetables have the ability to heal skin wound e when held at moderately warm conditions end high humidity for several days after harvest. The self-healing of wounds, cute and bruises is known as curing. There are two steps in the curing process. First is suberization - the production of suberin and its deposition in cell walls. The second is the formation of a cork cambium and production of cork tissue in the bruised area. The new cork tissue seals the cut or bruised areas and helps prevent the entrance of decay organisms. The healing of injuries received in harvesting and handling prolongs the storage time and reduces the incidence and spread of decay in storage.

The storage life of onions and garlic is extend-d by exposure to warm dry conditions for several days to dry the outside akin and prevent the ingress of spoilage organisms. This process if also known as curing although physiologically it is rather different and causes about 5% weight loss. Curing is carried out in the field when weather conditions are suitable; otherwise the product is subjected to forced circulation of warm dry air when first put into storage.

This is sound environmental practice. There is little effect on the environment from curing.

 

2.1.8 Genetic control of shelf life

Each variety of a horticultural crop has a limited storage life even under optimum storage conditions. The potential storage life is partly under genetic control and can be manipulated by breeding. Table 6 (Appendix 1 - Page 5) shows the normal storage life of some North American varieties of potatoes and onions under Rood storage conditions. This very wide range of storage life is typical of horticultural products; each variety has its own particular life span.

Plant breeders should be encouraged to include potential storage life as one criterion in their programme for breeding improved varieties of roots, tubers, fruits and vegetables. This is particularly needed with the breeding programmes in tropical climates where refrigerated storage capacity is in short supply. This should be a high priority method for reducing losses in horticultural products.

Farmers should be encouraged to grow varieties that hare long storage life. For example, Martin and Degras (1978) point out that different Jam varieties differ in storability from a week to several months. Extension agents and experiment stations should be encouraged to include inherent storage life as one of the considerations to be taken into account when deciding which types of crops and which varieties of those crops should be recommended to farmers.

There are no known adverse environmental effects from the efforts of plant breeders to extend the inherent storage life of horticultural crops. However, the results of plant breeders' work may need to be monitored. The U.S. Food and Drug Administration established regulations for release of new varieties of edible plans a when it wee discovered that a new potato variety that wee released in 1969 had an unusually high content of the toxic glycoalkaloids that are naturally present in potatoes. The FDA regulations apply to any plant material that provides more than 2% of the U.S. diet. The regulations require that plant breeders must establish ah two point a before releasing a new variety:

(i) that the content of the major nutrients is no lower than the average found in existing varieties of that commodity and (ii) that toxic substances naturally present in the commodity are no higher than normal for existing varieties.

 

2.1.9 Shorten the time between harvest and consumption

In developing countries a considerable amount of produce is wasted because of poor transportation systems and poor marketing procedures. Much produce is spoiled because it is stored beyond its inherent shelf life before marketing is completed.

Improving transportation and marketing facilities, spreading the harvest season by growing varieties that mature at different times, and staggering the planting dates of annuals and reducing the number of steps between producer and consumer are methods that can be used to shorten the time between harvest and consumption.

 

2.1.10 Processing

Considerable quantities of fruit a and vegetables are processed by dehydration, canning and freezing in developed countries. In developing countries. In developing countries small amounts of these commodities are processed for local consumption although large volumes of some commodities are processed for export (e.g., canned pineapple).

Canning and freezing require a high capital cost, high energy costs and expensive packaging and are unsuited for widespread use in less developed countries. Dehydration or sun drying is the simplest and lowest cost method of preservation and should be more widely used in developing countries because it converts a perishable commodity into a stable item with long storage life. Some excellent quality dehydrated products can be made from roots and tubers; this kind of processing should be encouraged.

Environmental effects. Occupational hazards in the fruit and vegetable processing industry are the normal hazards associated with machinery, for which adequate safety measures are well developed. The National Institute for Occupational Safety and Health in the U.S. (NIOSH) have no complaints en safety hazards in processing plants that handle horticultural products. The U.S. Occupational Safety and Health Administration (OSHA) have no regulations specific to the fruit and vegetable processing industry other than the board guidelines that apply to industry in general. The fruit and vegetable processing industry is not on the list of occupational groups in which excess cancer incidence is reported by the U.S. National Cancer Institute.

There are occupational risks to some workers with specific horticultural products. For example, Barber and Husting (1977) report isolated cases of contact dermatitis among workers handling raw fruits and vegetables, including carrots, asparagus, mangoes, cashew fruits and nuts, and some citrus fruits. Fruit and vegetable handlers may also suffer contact dermatitis due to sensitivity to specific insecticides and fungicides. Indirect effects of handling fruits and vegetables include chapping and moniliasis from exposure to moisture, photosensitization dermatitis from sunlight, and parasitism from mites. Products that cause photosensitization include fig, rue, lime, bergamot, paranips, parsley, carrots, fennel, dill and pink rot celery. Raw pineapple fruits contain the proteolytic enzyme bromelain which causes skin irritations to workers in pineapple processing plants. This problem is overcome by supplying workers who handle cut fruit with rubber gloves.

 

2.1.11 Heat treatment

Some of the organisms that cause rotting are inhibited or killed at elevated temperatures that are below the injury threshold of the product. For example, hot water dipping of mangoes at about 50C for a few minutes kills many pathogens without adversely affecting the quality of mango. Heat treatment is however not a desirable procedure for most fruits and vegetables. When applicable, very rigid temperature controls are needed.

There is little adverse environmental effect from heat treatment. Small amounts of heat are dumped into the environment.

 

2.1.12 Sanitation

All handling, storage, cleaning and washing equipment for horticultural products should be kept in a sanitary condition in order to minimize the risk of spreading infection. Diseased or damaged units should be sorted out and properly disposed of because their presence promotes the growth of fungi and bacteria. Insects infesting cull piles may fly to good produce and introduce pathogenic organisms and increase losses. Wash water should be changed at regular intervals before it becomes heavily contaminated with fungi and bacteria and spreads infection. In some cases the wash water is treated with chlorine or some other chemical in order to reduce the count of viable organisms. The sanitation programme in the People’s Republic of China is considered an exceptionally important element of pest control.

The environmental effects of good sanitation practice are minimal.

 

2.1.13 Use of chemicals

A number of chemicals may be applied to horticultural products in order to obtain a desirable post-harvest effect. Most of these are applied after harvest, but a few are applied in the field in order to obtain a specific post-harvest response. For example, the sprouting of onions in storage can be delayed by spraying the onions with maleic hydrazide (MH) in the field while the tops are still green. Chemicals used pre harvest whose scale propose is to achieve a post-harvest effect should be included in the list of post-harvest chemical treatments.

Post-harvest chemicals are classified into groups below (Pantastico 1975). Many of these are not used commercially and are of research interest only:

  1. Fungicides which prevent or delay the appearance of rota and molds in the product. Examples are, sodium orthophenylphenate (SOPP), benomyl, thiabendazole (TBZ), sodium hypochlorite, and sulphur dioxide (SO2). Methyl formate (Erinol), ethyl formate and (in some countries) ethylene oxide are frequently applied to dried fruits to kill infestations of insects and molds. Sulphur dioxide and benzoic acid are frequently, and propionic acid, ascorbic acid or sorbic acid sometimes, added to processed fruit products, especially juices, to inhibit the growth of yeasts and molds.
  2. Chemicals that delay ripening or senescence. Examples are: the kinins and kinetins that delay chlorophyll degradation and senescence in leafy vegetables, gibberellins that retard the ripening of tomatoes and hold citrus fruits on the tree beyond normal maturity, and auxins that delay physiochemical deterioration of oranges and green beans.
  3. Growth retardants that inhibit sprouting and growth. Examples are maleic hydrazide which is applied pre-harvest and inhibits sprouting in a number of stored commodities, e.g., onions and potatoes. A number of chemicals are applied post-harvest to potatoes to control sprouting, for example, CIPC, TCNB and MENA. Daminozide (Alar) give a increased fruit firmness, bettor colour and early maturation in apples.
  4. Chemicals that hasten ripening and senescence. Examples are ethylene and compounds such as Ephephon that release ethylene, abscisin, ascorbic acid, hydroxyethyl hydrazine (BOH), acetylene and substances that release acetylene such as calcium carbide, and certain alcohols and fatty acids.
  5. Chemicals that may hasten or delay ripening and senescence depending on the dose and the commodity on which they are used. Examples are 2, 4-D; 2,4, 5-T; indoleacetic acid (IAA) and naphthalene acetic acid (NAA).
  6. Metabolic inhibitors that block certain biochemical reactions that normally occur. Examples are cycloheximide, actinomycin D, vitamin K, maleic acid, ethylene oxide, and carbon monoxide.
  7. Ethylene absorbants. These delay ripening and senescence because they remove the ethylene produced by the fruit. They are usually placed in clove proximity to the commodity and leave no residue on it. An example is potassium permanganate- impregnated alumina or vermiculite (fur fir).
  8. Fumigants to control insects or sometimes molds. Ethylene dibromide and methyl bromide are the most commonly used fumigants.
  9. Colouring. The use of artificial colours is sometimes permitted in order to improve the appearance of a fruit. For example, fresh grange a from Florida may have artificial colour added to the akin for cosmetic purposes. Since most people do not eat orange skins other than for marmalade it is considered to be a harmless addition.
    In warm climates ethylene is used to degreen lemons, oranges and tangerines imparting a brighter colour to the skin. Ethylene is a naturally occurring metabolite of ripening fruits.
  10. Food additives. A number of compounds are permitted to be added to processed horticultural products for preservative or functional effect. The major preservatives are sulphur dioxide, benzoic acid or benzoates, and sorbic acid or sorbates. Functional additives include antioxidants, colouring, flavouring, thickeners, emulsifiers, etc. The use of food additives in the U.S.A. is regulated by the Food and Drug Administration (FDA). Other countries have an equivalent government agency to regulate the use of additives. At the international level the Joint FAO/WHO Expert Committee on Food Additives (JECFA) formulates general principles governing the use of food additives and makes recommendations regarding their examination and control. Food additives will not be discussed further because they are only used in processing and formulating horticultural product a and are not applied to raw horticultural products. One exception is sulphur dioxide which is used to fumigate fresh grapes in cold storage in order to control growth of yeasts and molds.

There are two important differences between the use of chemicals in the field and the use of post-harvest chemicals:

  1. Smaller quantities of post-harvest chemicals are used. For example, the normal dose of CIPC for controlling sprouting of potatoes is about 30 grams per ton and the normal dose of ethylene dibromide for fumigation of fruit a and vegetables is about 30 grams per ton. These levels contrast with the use of field chemicals where doses of one to several kg. per hectare are commonly used.
  2. The chemicals are not broadcast over the field but are applied in the confined apace of the storage chamber.

It is impossible to obtain figures for the quantities of post-harvest chemicals that are used because this is considered proprietary information by the companies that manufacture and formulate them. However, all post-harvest chemicals are classed as "minor use" by the U.S. Environmental Protection Agency because the quantities used are relatively small.

In the U.S. a company must produce experimental evidence of the toxicity, safety, and usefulness of a new agricultural chemicals before it can be registered for use as an agricultural chemical. Each use must be cleared through registration for every commodity to which it is applied. The Environmental Protection Agency has the responsibility for registering pesticides and setting tolerances. Table 7 (Appendix 1 - Pages 6-10, incl.) lists the post-harvest chemicals that are cleared for use in or on raw agricultural commodities by the U.S. Environmental Protection Agency and the tolerance for each commodity for which they are registered. This list is kept under continuous review. Any changes that are made in the list are published in the U.S. Federal Register. The FAO/WHO Codes Alimentarius Commission recently published "Guide to Codes Maximum Limits for Pesticide Residues" and plants update this fiat at regular intervals.

In addition to the chemicals listed in Table 7 a large number of materials are exempted from the tolerance in post-harvest pesticide formulations. Most of these are inert ingredients that do not affect the pest but do improve the functional properties of the pesticide formulation. Examples of these chemicals are, surfactants, solvents, diluents, synergists, preservatives, stabilizers, antioxidants, thickeners, emulsifiers, and antifoam agents. Most of these substance are Generally Recognized As Safe (GRAS) by the U.S. Food and Drug Administration.

The use of post-harvest chemicals in the U.S. is strictly controlled and monitored. The chemical suppliers usually described in detail on the label and/or in supplementary literature exactly how, when, and how much of the chemical is to be used. This is backed up by the agricultural extension service of each state which keeps in close contact with the farmer. Most states have a cadre of inspectors who regularly draw samples of horticultural products and submit them to a central analytical laboratory to assay for chemical residues. Few violation of the regulations are detected, and most samples tested are found to be well below the tolerance. Most other developed countries maintain close supervision and control of the use of post-harvest chemicals on horticultural products.

The situation may be quite different in the less developed countries where governments usually do not have the expertise or back-up analytical laboratories to monitor adequately the use of post-harvest chemicals on perishable crops. It is difficult to obtain information on this topic, but from a general knowledge of how governments in LDCs operate, it appears to be a matter that deserves investigation. Presumably, the pesticide tolerances for the major export crops (e.g., fresh bananas) are effectively monitored by the large corporations who operate this trade and by the developed countries that import these commodities. One cannot be so sanguine about the horticultural crops that are indigenously produced and consumed in the less developed countries.

The U.S. Cancer Institute has prepared a list of 26 chemicals or industrial processes associated with cancer induction in man. (Table 8 Appendix 1 - Page 11) None of the items found in this list are found in Table 7.

The International Labour Office in Geneva, Switzerland, has compiled a list of 69 compounds that are listed as carcinogens by one or more of the following countries: Australia, Belgium, Finland, Federal Republic of Germany, Italy, Japan, Sweden, Switserland, United Kingdom, Union of Soviet Socialist Republics, United States of America. (Table 9 - Appendix 1 - Page 12) None of these 69 compunds appear in Table 7.

The fact that none of the chemicals listed in Table 7 appear in either of the above two carcinogen lists coca not guarantee that they are not carcinogenic because the question of what causes cancer is not completely resolved. Apart from carcinogenicity there is also the question of other ways in which chemicals may be harmful to human health, e.g., teratogenity and mutagenicity. The question of the safety of the chemicals that are added to foods is changing rapidly because much research and regulatory attention is being devoted to this issue in a number of countries.

The whole issue of harmful chemicals in the environment is complex and not very clear at the present time. However, we can make two reasonable assupmtions about chemicals added to horticultural products:

  1. The developed countries have the expertise to engage in the debate on the harmfulness of chemicals, to evaluate the risks and benefits of their use, to enact new legislation controlling their use as new knowledge becomes available and to establish the inspection-analysis-prosecution machinery to ensure that the legislative intent is carried out. It is reasonable to assume that any proven grave rick from the use of chemicals will soon be brought under control in the developed countries.
  2. Most of the less developed countries have little of the kind of expertise listed above. There is a real risk that chemicals will be improperly used and their improper use will not be brought under control, with the potential to cause harm to human health and the environment. Therefore, the use of post-harvest chemicals in a given country should be discouraged until adequate inspection services analytical laboratories have been established to ensure that these chemicals are used safely.

Environmental effects. Misuse of certain post-harvest chemicals may lead to seriuos environmental harm. As far as can be determined little is known about the ultimate effects of post-harvest chemicals on the environment when they are correctly used. It seems to be generally assumed that since the compounds are used in small quantities in confined areas, and since most of then decompose into nonactive substances there are no adverse environmental effects. Although this is a reasonable assumption there is little concrete evidence either for or against this widely held opinion.

2.2 Pests

The major causes of lose in perishable produce after harvest are certain pathogenic fungi and bacteria. Viruses and nematodes play a minor role in postharvest losses; rodents and insects are also generally of lesser importance in contrast to the significant damage they cause in food grains.

In addition to the direct lone in quantity of food resulting from microbial infections, a partial loss results because of effect on appearance and/or quality resulting from disfiguring surface infections of fruit, root, and tuber crops. Other secondary adverse effects may include a decline in shelf life, possible contamination with mycotoxins, acceleration of ripening because of release of ethylene in pathogenesis by certain fungi, and in some instances deterioration of canned fruit crops because of the presence of heat-resistant hydrolytic enzymes formed by decay fungi in fruit tissues prior to the canning process.

The loss in the post-harvest period may originate from infections that were initiated by fungi during the growing season well in advance of harvest. Much of this pre-harvest infection involves a group of fungi that are capable of infecting healthy developing fruits either by direct penetration, e.g., anthracnose deseases caused by species of colletctrichum or by invasion via natural openings much as lenticels or stomates or through breaks in the tissue at the points of attachment of fruits to the plant. In many cases the infection process may be incomplete. Thus, sub-cuticular mycelium may be formed which remains in a latent stage until the post-harvest period when changes in susceptibility may occur and the pathogen mycelium may ramify through the tissue.

Many of the fungi (e.g., species of Penicillium, Rhizopus, and Gectrichum) and bacteria (e.g., species of Erwinia, Bacillus and sometimes Clostridium) involved in decay problems associated with the post-harvest period may be considered as opportunistic pathogens. They are usually incapable of penetrating unijured tissue or aggressively attacking vigorous healthy plants during their active growth period. However, they do have the ability to parasities fleshy plant organs when tissues are bruised, injured by insects, or otherwise placed under environmental stress. In many cases tissues are invaded by a succession of organisms which may interact in a synergistic manner.

Each species of root, tuber or fruit is affected by specific groups of fungal or bacterial pathogens. It is important that broad non-specific designations of these organisms be avoided (i.e., molds or rot organisms). The species of fungi or bacteria associated with specific decay problems should be properly identified. For example, it has been clearly shown that species of Knisopus differ markedly in their susceptibility to specific fungicides.

The main factors affecting disease development are:

  1. Most susceptibility
  2. maturity
  3. wounds and wound healing
  4. temperature of the commodity
  5. relative humidity (especially in storage)
  6. packaging
  7. handling in general
  8. concentration of inoculum

The basic methods of control involve three different approaches 1) prevention of infection 2) elimination of incipient or latent infections, and 3) prevention of spread of the pathogen in the host tissue.

Losses due to micro-organisms may be reduced by:

  1. refrigeration
  2. improved handling procedures
  3. pre- and post-harvest chemical control
    Although every effort needs to be made to minimize or reduce dependence on chemicals, in many cases no viable alternatives exist. A wide range of fungicides are now available that are effective and safe to use. Incipient infections can be eliminated or reduced by application of certain fungicides such as benomyl which have the capacity of diffusing into host tissue and killing the pathogen in situ. Certain fungal pathogens initiate infections of fruits and vegetables during the growing season and these can be controlled best by timely application of fungicides prior to harvest. It is essential to ensure that chemical controls are used properly.
  4. preventing contamination during the washing process
    In the case of tomatoes and possibly other fleshy vegetables when warm product is washed in cold water infection is augmented. Because of the temperature differential air in the tissue contracts and this draws water, often containing soft rot bacteria' into the tomato via wound e and fresh stem scars. If soft rot bacteria are present, water deeper than about 0.3 m increases the risk of infection because of the pressure differentials inside and outside the fruit. If washing is needed' proper, prompt drying is essential in order to prevent rapid growth of spoilage organisms in superficial wounds and lenticels. Sweet potatoes and yams have a longer shelf life when stored unwashed.

Unresolved problems

The actual physiological processes involved in the rapid deterioration of cassava after harvest are act yet fully understood, although much research is in hand. For many other commodities also, knowledge is lacking about changes in physiological processes in the post-harvest period. In particular there is a need to determine the relationships of those changes to the increased susceptibility of perishable products in the pool-harvest period.

Date is lacking with respect to the effect of chemicals, singly or in combination, AS used in the growth period on the storability of commodities. Fertilizer, weed control chemicals and vine killers for potatoes are examples of the chemicals used which may affect storage characteristics or disease susceptibility

The physiological basis for resistance of perishables to post attack needs to be known. Mechanisms of resistance of micro-organisms to different fungicides and bactericides are also little understood.

Biological control systems should be further explored. For example, it is known that, by dipping root stocks into suspensions of an avirulent strain of the grown gall bacterium that produces a very specific antibiotic (bacteriocin) later infection by the grown gall (bacterium) of stone fruits can be prevented. Other similar antagonistic relationships are known to exist: they may offer possibilities for more techniques for biological control of insects. Biological controls are an important feature in the plant protection programmes of the People's Republic of China.

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