II. Post-harvest losses in perishable crops
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1.1 The problem
"There is a war going on that began millions of years ago. Although the many generations of soldiers have not changed a great deal in appearance during this time, the tactics and weapons have grown more sophisticated. Each army has won a share of the battles, but the consummate victory has eluded both. Neither side can afford to give up' for nothing less than the sustenance of life is at stake. The war I refer to is, of course, the war between humankind and certain species of insects, weeds, pathogens, nematodes, rodents, and other peats that daily compete for our crops, gnaw at our dwellings, infest our domestic animals, or destroy our health" (Kuhr, 1979).
The above statement vividly describes the problem mankind has always had to preserve 0a food supply after he has produced it. The loss of foods in the post-harvest system is not new; it has always boon a problem for mankind. In these days of rapidly enlarging populations in the poorest countries whore food is already short, there is an increasing urgency to do a better job of conserving mankind 'e food supply in order to alleviate hunger and malnutrition. Some far-sighted individuals have been drawing attention to the problem of post-harvest losses for many years.
The United Nations General Assembly resolution of September 1975 focused world-wide attention on the problem of post-harvest food losses and called for concerted action to reduce these losses in the following words: "The further reduction of post-harvest food losses in developing countries should be undertaken as a matter of priority' with a view to reaching at least a 50% reduction by 1985. All countries and competent international organizations should co-operate financially and technically in the effort to achieve this objective".
As a result of this resolution a number of national and international donor agencies expanded existing programmes and initiated new programmes that were directed to the problem of reducing post-harvest losses. Most of those activities have been directed toward reducing losses in cereal grains, oilseeds and grain legumes.
A report prepared by the United States National Academy of Science in 1978 on the problem of post-harvest food loaves in developing countries pointed out the need for giving consideration to losses in food products other then the cereals, particularly roots and tubers, fruits and vegetables. Largely as a result of this report, the donor organizations are beginning to consider intervention programmes that can reduce losses in horticultural crops.
Post-harvest losses of fruit a and vegetables are more serious in devoloping oountries then those in well developed oountries. An additional constraint to improving this situation is that in moat developing countries the number of scientists concerned with post-harvest food losses is significantly lower than those involved in production research. In the early days of horticulture in wolf developed countries, heavy losses occurred in much the same manner as they do today in developing countries. Increasing industrialization in technologically advanced nations gradually brought about improvements in crop handling. Elaborate harvesting equipment replaced the crude harvesting tools. Collection centres were strategically established in major producing areas. Containers were remodelled to add more protection to the produce. Commercial storage plants were installed and grade standards adopted. Engineers and economists became more and more aware of raw material behaviour. Concomitant advances in Refrigeration Technology in the developed countries have made possible establishment of cold chains for the entire post-harvest and handling operations. At the institutional level post-harvest research was initiated. Pilot packing houses were installed, coupled with the development of intensive training programmes. the improvement of product quality and reduction in post-harvest losses became the main concern of producers, middlemen, marketing specialists and consumers. Today, enormous volumes of quality horticultural crops produced in technologically advanced countries are made available to millions of people through improved post-harvest handling. Thus, historically and by necessity, post-harvest technology is part of the normal development processes in agriculture.
These handling procedures are not fully recognized in less developed countries. Here agriculture may be characterized as disjointed. Production is not linked with marketing. With perishable crops like fruits and vegetables, storage, packaging, transport and handling technologies are practically non-oxiatant, Hence, considerable amount a of, produce are loaf. Thus, as more fresh fruits and vegatables are needed to supply the growing population in less developed countries, as more produce is transported to non-producing areas, and as more commodities are stored longer to obtain a year-round supply, post-harvest lose prevention technology measures become paramount. It is distressing to note that so much time is being devoted to the culture of the plant, so much money spent on irrigation, fertilization and crop protection measures only to be wasted about a week after harvest. It is, therefore, important that post-harvest procedures be given as much attention as production practices the stages from planting until the product a roach the consuming public must be a mutual undertaking between the growers and those who will handle the products after harvest.
Fruit a and vegetables, root a and tubers (horticultural crops) are quite different in nature from cereal grains and oilseeds the differences are summarized in Table 1. The causes of spoilage, the rate at which spoilage occurs, the degree of spoilage, and the actions needed to reduce spoilage are substantially different than for the cereals. Because of these differences it is necessary to design a different set of intervention programmes to reduce post-harvest losses in horticultural products,
Table 1 Comparison of Horiticultural Crops
|Cereals and Oilseeds
|Low moisture content, typically 10% to 20%
|High moisture contort, typically 70% to 95%
|Small unit size, typically less than 1 gram
|Large unit size, typically 5 e to 5 kg
|Very low respiration rate
with very small generation of heat.
Heat production is typically 0.05 megajoule/ton/day for dry grain
|High to very high
Heat production is typically 0.5 to 10 megajoule/ton/day at 0°C to 5 to 70 megajoule/ton/day at 20°C
|Soft texture, easily bruised
|Stable, natural shelf life is one to several years
|Periahable, natural shelf life is a few days to several months
|Losses usually caused by molds, insects and rodents sprouting, and bruising
|Losses usually caused by rotting (bacteria, fungi), senescence
|Losses in LDCs usually 10% to 20%
|Losses in LDCs usually 15% to 50%
In the warm humid tropics, fresh fruit a and vegetables have an extremely, low level of natural protection against the climate peat a and bio-chemical and physiological deterioration. It is also in the warm humid tropics that human diseases are moat numerous and widespread and consequently where the need for the nutritional value of fresh fruits and vegetables la moat essential. On the other hand, dried grains and legumes, and to a certain extent, root crops (yams), have a fair degree of natural protection against the various deteriorative elements, even though considerable post-harvest losses do occur.
1.2 Importance of perishable crops
Production data for the root crops are given in Table 2 (Appendix 1 - Page 1). In terms of importance as assessed by volume of production within the tropics these are cassava, potatoes, yams, sweet potatoes, taro and others. It is estimated that some 174 million tone of roots and tubers are produced annually of which cassava constitutes about 60%. Cassava is almost entirely produced and consumed in developing countries although in recent years there has developed a substantial trade in dehydrated cassava chips which are exported from developing countries to European countries as a low-goat animal feed ingredient, The European Economic Community imported 2.3 million tons of dried cassava chips in 1975. Next in volume of production are potatoes, yams and sweet potatoes. These make contributions respectively about 14, 11 and 10%. At the end of the scale taro contributes a lowly 2.2% while miscellaneous tubers account for 2.5%.
Table 3 (Appendix 1 - Page 2) lists the major vegetables and Table 4 (Appendix 1 - Page 3) lists the major fruits that were produced in developed countries, developing countries, and centrally planned economies in 1973. The total world production of roots tubers, vegetables and fruits is enormous and approaches that of the total production of the staple cereal grains as can be seen from the following figure e
|Production 1978 (million tone)
|% of Cereal Production
|Roots & Tubers
|Vegetables & Melons
|Total horticultural production
(Data from FAO Production Yearbook 1978)
The total annual production of horticultural crops in developing countries is 620 million tone which is 85% of the cereal production in these countries. This surpasses the relative world average whore horticultural crop production is only 70% of the cereal production. These high production figures indicate the important role that horticultural crops play in the economy of developing countries although it must be remembered that these crops are far higher in water content than are the cereals.
The ten leading horticultural crops in developing countries in forma of total production are the following
|1978 Production (million tons)
Horticultural crops are essential for a nutritionally balanced diet. Fruits and vegetables are the major source of vitamins A and C, a good source of calcium and iron, and they supply part of the requirements for a number of other minor nutrients. Roots, tubers, bananas and plantains are important sources of calories and also supply a number of minor nutrients, and some protein.
In addition, horticultural crops add variety, enjoyment and a sence of satisfaction with the diet because of their appealing colours, flavours and textures. For example, it has been said that although onions and garlic are not rich in nutrients, they make a vegetarian diet acceptable because of the savoury flavour they impart to the monotonous starchy diet in a developing country. It is no accident that onions rank third, and garlic ranks tenth in volume of vegetable production in developing countries. (Table 3)
On all of these counts- economics, nutrition, acceptability- horticultural crops play a major role in developing countries, amply justifying the contortion that something should be done to reduce the high losses that presently occur in these commodities.
1.3 Definition of terms
Since the ramifications of food and the food supply spread right through society it is necessary to define exactly what is moans by the term "post-harvest food loss" if we are going to have a manageable problem with known boundaries. The working definition given below is based on that developed by Bourne (1977) and modified by the U.S. National Academy of Sciences (1978) and Harris and Lindblad (1978). For the sake of convenience the definition is divided into three parts.
"POST-HARVEST" bogies at the moment of separation of the edible commodity from the plant that produced it by a deliberate human act with the intention of starting it on its way to the table. The post-harvest period ends when the food comes into the possession of the final consumer.
"FOOD" moans weight of whole some edible material that would normally be consumed by humans, measured on a moisture-free bests. Inedible portions such as skins, stalks, leaves, and seeds are not food. Potential foods (e.g., leaf protein) are not foods; they do not become food until they are accepted and consumed by large populations. Feed (intended for consumption by animals) is not food.
The method of measuring the quantity of food in the post-harvest chain should be on the basis of weight expressed on a moisture-free basis. There will be times when information on losses in nutritional unite and economic losses will also be needed but these should not be the prime means of measuring post-harvest food losses.
"LOSS" means any change in the availability, edibility, wholesomeness or quality of the food that prevents it from being consumed by people.
1.4 Causes of losses
There are so many causes for losses in the post-harvest food chain that it helps to classify them into 2 groups and a numb or of sub-groups
A. PRIMARY CAUSES OF LOSS are those causes that directly affect the food. They may be classified into the following groups
Microbiological, mechanical and physiological factors cause moat of the losses in perishable crops.
B. SECONDARY CAUSES OF LOSS are those that load to conditions that encourage a primary cause of loss. They are usually the result of inadequate or non-assistant capital expenditures, technology and quality control. Some examples are:
1.5 Sites of losses
Losses may occur anywhere from the point where the food has been harvested or gathered up to the point of consumption. For the sake of convenience the losses can be broken down into the following sub-headings:
Post-harvest losses in fresh root/tuber crops have their origin in mechanical damage, physiological processes, infection by decay organisms and, occasionally, pest infestation. The losses caused by these processes may occur during all stages of the food supply system from crop maturity, through harvesting, transportation and storage. The degree of lose associated with these factors is determined by the plant material involved, the prevailing environmental conditions and management of the food supply system. The major causes of lose in roots and tubers and the sites where they occur are summarized below:
1.6 Magnitude of losses
Reliable statistics on losses are few. It is possible to find individual cases with losses ranging from 0% to 100%. The extent of losses is highly variable depending on a number of conditions. Stable foods such as cereal grains can be stored in good condition for several years, whereas perishable foods such as fruit a and vegetables, spoil quickly unless given special treatment such as canning and freezing. The longer the time the food is stored the more opportunity there is for losses of all kinds to occur. Perishable crops generally suffer from higher losses than the cereals.
A number of figures for the extent of loss is quoted in scientific literature and by the communications media, but much of this information is unreliable because the amount of loss has been estimated and has not been obtained by actual measurements. There is often the temptation to cite "worst case" figures to dramatize the problem.
Another problem is that even some of the figures that have been obtained by careful measurements are manipulated for various reasons. In some cases there is the temptation to exaggerate the figures of loss, particularly if there is a prospect that high figures of loss will prompt aid from donors. In other cases there is a temptation to minimize loss figures in order to prevent the embarrassment of acknowledging the magnitude of losses, or for political, financial or trading reasons.
Another precaution that needs to be taken in assessing overall losses is to ensure that the arithmetic of calculating loss figures is correct. The extent of losses can be unwittingly exaggerated unless the arithmetical calculations are correctly performed. Some examples of misleading arithmetical calculations are discussed by Bourne (1977).
The pattern of losses varies widely from country to country. There is a marked contrast between the site of major losses in developed countries and developing countries. In a typical developed country losses may be fairly high during harvesting because the agricultural machinery that is used to harvest the crops leaves some of the commodity in the field and mechanically damages some of it. Considerable quantities of foods a may be discarded at the point of harvest because they are of the wrong size, shape or colour. These are planned losses. In developing countries harvest losses are usually lower because most of the crop is hand picked. The amount of material rejected in developing countries is less because the expectation of quality and uniformity is generally lower than for developed countries.
In developed countries losses are generally small during processing, storage and handling because of the efficiency of the equipment, good quality storage facilities, and close control of critical variables by a highly knowledgeable cadre of managers. In contrast, in developing countries losses in processing, storage and handling tend to be rather high because of poor facilities and frequently inadequate knowledge of methods to care for the food properly.
Table 5 (Appendix 1, Page 4) lists some estimates of losses of selected commodities in developing countries. The tragedy of these enormous losses in developing countries is not only that this is a severe economic loss in regions that are struggling to escape from poverty but also a major loss of important nutrients to populations who are malnourished.
Pantastico (1977) pointed out that post-harvest losses in developing countries often exceed production losses and cites as an example the following figures for losses is the Philippines:
|Production (m. tons)
|Production Value ($)
|Loss Value ($)
|1, 640, 541
|248, 564, 310
1.7 Loss assessment
There appears to be no established generally accepted methodology for determining post-production lose in root/tuber crops. The National Academy of Sciences (USA) publication (1978) on post-harvest losses makes the following differentiation between assessment, measurement and estimation of losses:
Assessment is a rough quantitative approximation of food loss or the characterization of the relative points of lose in a particular food supply system. This approach implies a measure of subjectivity resulting from a lack of sufficient information.
Measurement on the other hand is a more precise quantitative observation with less subjectivity. With measurements there is a high expectation of reproducibility without observer bias.
Estimation is the interpretation of a number of scientific measurements. Here the process of interpretation depends on the experience and judgement of the observer.
At whatever level of precision post-harvest lose is determined the value will be specific in time and for location. This is due to the fact that lose is a function of the condition of the material, the prevailing environment, the nature and intensity of bio-degenerating organisms and the crop material management. None of these are constant. They are all dynamic factors liable to continuous change. As a consequence, crop loss, however determined will always be variable. This is illustrated by yam storage. Some cultivars of the rotundata/cayenensis/discorea species remain dormant for about 3 months, during which period storage loss is low. When placed in the traditional yam barn the keeping characteristics of these yams are good. It is generally believed that little improvement can be achieved without radical change in storage technology. However where the tubers are infected by nematodes the storage potential is much reduced and high losses occur.
It would be useful to have a standard method for assessing losses for each type of commodity but this is a difficult task due to crop diversity, inherent perishability and the complexity of the marketing and distribution channels.
1.8 Effects of the environment on food losses
The environmental conditions under which foods are stored and processed can have a major effect on the keeping quality of the foods and the amount that is lost. The major environmental influences on the keeping quality of foods are the following:
Temperature. In general, the higher the temperature the shorter the storage life of horticultural products and the greater the amount of lose within a given time, as most factors that destroy the produce or lower its quality occur at a faster rate as the temperature increases. This statement applies to the rate of growth of spoilage microorganisms, the rate of indigenous physiological change and physical processes such as water loss and wilting, Figure 1 show. changes in the quality of lettuce and asparagus during storage at different temperatures, Lettuce stored at 25 C becomes unsaleable within 7 days, while lettuce stored at 10 C will reach the unsaleable condition in approximately 18 days and lettuce stored at 0°C requires 35 days to reach the point of being unsaleable. For asparagus the loss of quality occurs at a more rapid rate than lettuce, reaching the point of being unsaleable in 3 days when stored at 25°C. Figure 1 is a clear demonstration of the rationale for the extensive cold storage facilities that are used for storing horticultural produce.
Humidity. There is movement of water vapour between a food and its surrounding atmosphere in the direction towards equilibrium water activity in the food and the atmosphere, A moist food will give up moisture to the air while a dry food will Absorb moisture from the air. Fresh horticultural products have a high moisture content and need to be stored under conditions of high relative humidity in order to prevent moisture loss and wilting, exceptions to this being onions and garlic. Dried or dehydrated products need to be stored under conditions of low relative humidity in order to avoid absorbing moisture to the point where mold growth occurs.
Figure 1 Loss of quality in fresh lettuce and asparagus stored at various temperatures (from Lutz and Hardenburg 1968)
Solar Radiation. The solar radiation that falls upon foods held in direct sunlight increases the temperature above the ambient temperature. The amount of increase in temperature depends on the intensity of the radiation, the size and shape of the food' and the duration of exposure to the direct rays of the sun. The intensity of solar radiation depends upon latitude, altitude, season of the year, time of day, and degree of cloud cover. Under clear skies it is most intense when the sun is most directly overhead. Hence the intensity of radiation is greater in tropical zones than in temperate zones. As discussed above, a high temperature is deleterious to food quality and increases wastage. It is ironic to note that, in the temperate climates where the intensity of solar radiation is moderate, almost all food is kept inside under cover whereas in tropical climates, where solar radiation is much higher, considerable quantities of food can often be found in the direct rays of the sun deteriorating in quality at a rapid rate.
Altitude. Within a given latitude the prevailing temperature is dependent upon the elevation when other factors are equal. There is on the average a drop in temperature of 6.5°C for each Km increase in elevation above sea level. Storing food at high altitudes will therefore tend to increase the storage life and decrease the losses in food provided it is kept out of the direct rays of the sun.
Atmosphere. The normal atmosphere contains by volume, approximately 78% nitrogen, 21% oxygen, 1% argon, 0.03% carbon dioxide' various amounts of water vapour and traces of inert gases. Modifying the atmosphere can improve the shelf life and reduce wastage of certain foods.
One type of controlled atmosphere storage (CA) is refrigerated storage in which the level of oxygen is reduced to about 3% with the carbon dioxide content being raised to 1 to 5%, depending on the commodity. This CA storage may double the storage life over that of regular cold storage for certain varieties of apples and pears by slowing down the natural rate of respiration.
Many fruits, the "climacteric fruits", generate ethylene gas during ripening and the presence of this gas Accelerates the rate of ripening. If the ethylene is removed from the atmosphere surrounding these fruits as it is generated, their storage life may be extended. Experiments have shown that placing such fruits in a fairly gas-tight container with potassium permanganate, which absorbs ethylene gas, can substantially extend the storage life even at ambient temperature.
"Modified atmosphere storage" is another type of controlled atmosphere storage. This term denotes storage of horticultural products in a beneficial atmosphere other than air that is not under as close regulation as in CA storage. Modified atmosphere storage can be obtained in boxes of pears, apples, and cherries that are lined with polyethylene film which acts as a barrier to the escape of carbon dioxide and the ingress of oxygen. Another method of obtaining a modified atmosphere storage is by the addition of dry ice which increases the carbon dioxide in the atmosphere to some extent,
Time. The longer the time the food is stored the greater is the deterioration in quality an' the greater is the chance of damage and loss. Hence storage time is a critical factor in lose of foods, especially those that have a short natural shelf life. The time involved from harvest to consumption of perishable commodities is much shorter than that from planting to harvest. While production time could take about several years for fruit trees, and generally about three to four months for vegetables, the duration of post-harvest handling could be as abort as one day to a few weeks. Any improvement in post-harvest handling treatments would therefore involve less risk and would be more economical than improvements in production.
Biological pressures. Bacteria and fungi are always present in the atmosphere to contaminate food and cause spoilage should conditions favour their growth. However, it should be emphasized that the contamination or inoculation process with bacteria and fungi occurs to an equal extent during the harvesting process. Soil organisms as well as foliage pathogens can be introduced. Bacteria that cause disease in plants are not usually introduced from the air par so except in aerosols. Micro-organisms can multiply very rapidly whenever conditions are favourable for growth. The only foods that are free from micro-organisms are those that have been thermally processed, such as canned goods.
A similar situation occurs with insects. Insects are in the field and can accompany foods as they are brought from the field into storage. Most of the stored products' insects can increase is number by a factor of 10 to 50 times par month under favourable conditions. Stored food insects are ubiquitous, hiding in storage facilities and moving with stored foods when they are moved. Consequently we can assume that foods can become infested with insect pests at any time unless special precautions are taken.
Rodents and sometimes birds can exert biological pressures similar to those of insects and micro-organisms. The great capacity of living organisms for multiplication by geometric increase generates heavy biological pressures upon stored foods.
1.9 Environmental considerations
A number of people who have accepted the idea, of the need for better conservation of food wonder why the United Nations Environment Programme (UNEP) is interested in food losses and control of food losses. There are substantial reasons for UNEP's interest in this area and these will now be discussed.
UNEP is interested in promoting the health and well-being of both people ea. the environment, as well as sustainable development. Reducing food losses should improve nutritional status and human health' especially in those countries here a large proportion of the population is inadequately fed. Decreasing food losses offers an opportunity to reduce the pressure on the land and still deliver the some quantity of food to the table, thus reducing to some extent environmental damage caused by agricultural practices. Hence, UNEP encourages the conservation of food because of its positive environmental effects.
UNEP is interested in efficient and non-wasteful utilization of resources. Although it is difficult to obtain firm figures, it is generally conceded that considerably less energy and other inputs are required to conserve food than to produce an equal quantity of food. For example, it has been estimated that the total energy cost of good grain storage practice is about one percent of the energy cost of producing that grain. Hence' UNEP encourages good conservation practice because of its more efficient use of energy and other inputs.
UNEP's activities include reviewing proposed new initiatives to determine the environmental impact of these initiatives before they are put into effect and to help select from among competing initiatives those that are most desirable from the environment standpoint' to encourage their use, and to do this in the early planning stages of the project. Hence, UNEP is interested in what methods are proposed to be used to reduce post-harvest food losses
The next section of this report lists thirteen methods that are used to reduce losses in horticultural products. The first twelve methods have little adverse effect on health' safety or the environment and are recommended for use on environmental grounds. Of course, it is recognized that additional criteria will be used to make the final selection of methods to be actively promoted.
The thirteenth method of control is by use of post-harvest chemicals which is of great interest to UNEP because of the justifiable concern about the consequences of man-made chemicals that are used and discharged into the environment. The question of post-harvest chemicals is reviewed and the conclusion drawn that they probably cause little harm to human health and little damage to the environment when they are correctly used, but shore may be grave risks to both if they are misused. This leads to the conclusion that postharvest chemicals should only be used in those locations whore shore is adequate training in their proper use backed up by adequate machinery to enforce proper use. The environmental approach in this case is not to oppose the use of chemicals but to point out the need for using them properly and carefully.
The finding that most of the thirteen methods for reducing losses do not aggravate the environment or harm human health is encouraging. It means that intervention programmes can be planned with the knowledge that they cause much positive good and little harm. It is worth the effort of this exercise to establish this finding.
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