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The majority of studies so far undertaken in the whole field of post-harvest technology have been concerned with grains, grain legumes and other durable products which are stored dry, usually at moisture contents below around 12-14%. In these products, post-harvest deterioration is largely caused by the attack of external agents such as insects, moulds or rodents and does not arise from endogenous factors. Those investigations that have been undertaken on perishable crops have concentrated on the more typical, high-unit-cost horticultural products such as fruits and vegetables rather than on the low-unit-cost staple foods, Essentially different approaches are, therefore, necessary when dealing with the latter group of crop products and in many cases the traditional technologies, developed in the distant past within subsistence agricultural societies, may be especially appropriate. They were developed within societies subject to serious constraints on the material, and even more the energetic, resources available, and which were further, as indicated in Chapter 4, essentially ecocentric rather than technocentric in their philosophical orientation. There exist possibilities for the injection of modern scientific concepts into these traditional systems, although, most unfortunately, there has often been a tendency among those who have received a modern scientific education to reject traditional technologies as "primitive" and fit only to be displaced by sophisticated modern systems, although the latter may sometimes represent sub-optimal technologies for the situation (Coursey, 1978a; 1982). It is part of the purpose of this report to stimulate interest in these possibilities and their application.
The Essential Features of the Perishable Staples and of their Storage in the Fresh State
The more important of these essential characteristics are given in Table 3.1 in comparison with those of the better-known grains and other durables. This is based on a table prepared at the FAO/UNEP Expert Consultation of the Reduction of Food Losses in Perishable Plant Foods (AGS Bulletin, No. 43, 1981), but has been somewhat expanded by the present authors. Some of these aspects need further discussion, however.
It has already been mentioned that most of the crops being considered belong to the humid or at least the semi-humid zones of the tropics. Thus, in many cases, there is little seasonality of harvesting, and supplies of fresh food are thus available for most or all the year. Societies dependent on these crops can thus practice '"storage avoidance" - the spreading of crop production throughout a large part of the year and/or its processing into stable products immediately after harvest, or as in some cases leaving the crop standing in the ground after the optimal harvest date (a concept originally developed by Booth, 1974; 1982). Elaborate storage techniques were never developed and processing consists merely of detoxification where necessary, or of rendering the food into palatable form - often little more than simple cooking at the household level. This situation is in marked contrast to that of grain-based societies, where harvesting of the staple crop is confined to a limited period and storage for a whole year is needed.
Unlike the grains and similar crops, the perishable staples are all of inherently high moisture content, usually over 50% and often around 60% to 70%. This governs virtually all further considerations that bear on the post-harvest technology of these staple foods, whether in traditional or in sophisticated systems. In particular, consideration of this characteristic in each individual case must influence the fundamental decision: is a particular crop product to be stored, when it needs to be stored at all, in its natural fresh state, or is it to be processed soon after harvest into some more durable form? Processing may sometimes be necessary (e.g. with cassava and some yams) to eliminate toxicity, or to enhance the organoleptic acceptability of the food. In other circumstances, its primary function may be to render the food more easily transportable (for medium to long distance trade in food products among subsistence societies has been much more widespread (Lathrap, 1973; Coursey, 1978b) than is commonly realized). Alternatively, processing, most commonly some form of drying, may represent a means of eliminating the perishability from the fresh crop product by its conversion into a more durable, stable product, although any dried or other processed product will have its own storage problems, often comparable to those of the grains.
The carbohydrate element is normally economically the least highly valued portion of the diet, so all staple foods tend to be of inherently low-unit-value, although there is much variation in preference and, therefore, price between individual staples, depending on culture-historical and organoleptic factors. Nevertheless, carbohydrate foods are generally valued primarily on the basis of their calorific value: the perishables, which consist more than half of water, will normally have a lower unit-weight-value than staples of lower moisture content, such as the grains, though this may be offset by their greater productivity (Table 2.2). The perishable staples are also bulky and awkward to handle and have peeling and other preparation losses of as much as 10% to 30%. As perishables thus tend to be of low-unit-cost even when compared with other staples, the use of sophisticated techniques such as refrigerated or controlled atmosphere storage, often used for high-unit-cost horticultural perishable produce is generally precluded. Attention to simple traditional technology thus becomes especially appropriate.
The edible products of the perishable staples are living organs and remain so after being harvested; in most cases they exhibit relatively high rates of metabolic activity. The correspondingly high rate of respiratory activity needed to support this metabolism implies that throughout any storage period, part of the total mass of the organ is continually being converted from starch into carbon dioxide and water which are lost to the atmosphere. Appreciable weight loss from this process is thus inherent in any storage and ventilation is necessary so that adequate supplies of oxygen from the air are available for the respiratory process, and thus life, to be maintained. Similarly, storage life will always eventually be terminated by factors associated with the organ's natural biological function, as part of the plant from which it was derived. Organs of dormancy, such as most root crops, with storage lives measured in weeks or months, contrast with fruits and cassava roots whose inherent storage life is normally quite short, days, or at most weeks (Coursey and Proctor, 1975; Proctor, 1981).
It is thus necessary to consider the concept of inherent storage life, which is of fundamental importance in the understanding of the post-harvest behaviour of perishable staples in the fresh state. A great deal can be done to reduce postharvest loses in these commodities and to extend their storage life but there is always a limit beyond which they cannot effectively be kept. Beyond this limit, the produce is either shrivelled or rotted to destruction, or has been so changed by its endogenous metabolism as to have become totally unacceptable as food. As indicated, this inherent life is related to the essential natural biological function of the plant organ and may be only a few days, in the case of the most highly perishable soft fruit which are very susceptible to fungal attack especially when fully ripe and which quickly pass from ripeness to senescence or some leafy vegetables which easily wilt; while at the other extreme, organs such as the tubers of potatoes or yams can remain in the dormant state for several months before their storage life is finally terminated by sprouting. Between these extremes could be cited the more durable fruit such as citrus or bananas, but even here the life of the fruit as an acceptable item of food will eventually be terminated by the natural onset of senescence.
Perishable staples are of relatively low mechanical strength which is also related to the high water content. Soft fruit or leafy vegetables are conspicuously susceptible to mechanical damage but even such apparently rugged items as potatoes and yams are also extremely liable to mechanical injury (Coursey and Booth, 1977). Much attention needs to be paid to the preservation of the physical integrity of the produce: this is a field where the respect that the subsistence agriculturalist traditionally pays to his staple crops, contributes greatly to the success of his storage techniques.
To minimize post-harvest losses in perishable staples, the first essential is to maintain the physical and physiological integrity of the detached but still living plant organs, as losses arise from assaults on this integrity (Coursey and Booth, 1971, 1972; Coursey and Proctor, 1975). Secondly, the natural life may be prolonged - but only within limits - by the provision of optimal environments or by manipulation of the physiological state of the material. Thirdly, by the selection of material for storage that is entirely sound and also in an appropriate condition for storage. Summarizing, losses are minimized by choosing healthy material and keeping it healthy, but at the same time accepting that the life of all living material will eventually end. The factors that affect the storage life of perishable produce are discussed in more detail in the papers quoted above and by Coursey (1983).
The Magnitude of Post-Harvest Loses in Perishable Staples
Although the question of the magnitude of post-harvest loss in perishable commodities has been the subject of considerable debate during the last decade, little reliable information is yet available. Conservative loss estimates for stored grain and similar "dry" produce, as used by FAO in developing its PFL Programme, were about 10%. With the perishable crops it has been conservatively estimated from the limited data to be found scattered in the literature, together with what might best be described as anecdotal information (Coursey, 1972; Coursey and Booth, 1972; Coursey and Proctor, 1975; Coursey and Booth, 1977) that the total loss is of the order of 25%.
Even in the U.S.A. perishable produce has been described (Brody and Sacharow, 1970) as "the victim of phenomenally high waste because of incredibly poor handling practices", and "the loss rate as a result of multiple handling ... is frightful". An analysis of the situation in the U.S.A. over the previous thirty years (Pentzer, 1976) concluded that although there had been some reduction in post-harvest loss with: some commodities as a result of many years of research, with other commodities losses had actually become more serious, while a recent study (Harvey, 1978) found market losses of fresh produce in New York markets to range from as little as 1. 7% for apples to 22.9% for strawberries: of especial relevance to this report is that even under U.S. conditions the commodity which suffered the greatest market loss, after the very delicate strawberries, was ewe-et potatoes (15.1%).
In the developing countries of the tropical world, the situation is certainly worse than in the developed world, but even less hard information is available. A number of specific cases of both staples and other perishable produce were reviewed by Coursey and Proctor (1975), Proctor (1981) and Coursey (1983). Apart from the estimates already quoted of a total post-harvest loss in tropical perishables of around 25%, it was suggested by Parpia (1976) that in tropical Africa and India, losses in perishable foods are around 30%. The meeting concerning perishables held in 1977 by the U.S. National Academy of Sciences (N.A.S., 1978) worked, in the absence of any hard factual information, on the Delphi Principle of summarizing the estimates and guesses of a number of professionals of some authority in the field. Some of the figures produced by this process are given in Table 3.2, where they are compared with those in the T.P.I. (now T.D.R.I.) publications quoted. Probably the most useful outcome of these discussions, together with those that took place at the FAO/UNEP Expert Consultation in 1980 (AGS Bulletin, No. 43, 1981) was to etablish that the magnitude of post-harvest losses in fresh perishables is virtually impossible to quantify without reference to a particular commodity and individual situation. It is evident that under different conditions and length of storage and with different commodities, almost any loss figure between 0% and 100% may genuinely be found. Overall, losses of perishable staples are extremely serious and it is probable that the total loss of perishable staples in the developing world is between 10% and 30%, varying according to commodity and location-specific storage conditions. Each local situation needs investigation and analysis on a particular individual basis and broad global figures of loss such as have been quoted are of value mainly in indicating the magnitude of the problem that exists which in turn implies the need for urgent action. These loss figures would be far higher were it not for the widespread practice of storage avoidance with the perishable staples in traditional societies in the tropics.
Processing of Perishable Staples in Traditional Societies
The processing techniques adopted within traditional societies for perishable staple foods are usually extremely simple, in keeping with the ecocentric philosophies of most of these societies. As already indicated, processing is used either to eliminate toxicity, or to convert the more highly perishable food products,- i.e. those with extremely short storage life into more stable products. Only manual operations and manually operated equipment are considered in this publication. Mechanical devices are not considered as traditional but as improved methods which actually may reduce drastically the length of the processing and partially eliminate the tedious work. Traditional processing requires no special devices beyond pans, mats, woven basketry and wooden sticks and is, therefore, a low investment cost method.
Drying techniques are used for the manufacture of relatively stable, low-moisture-content products which may be more convenient for long-term storage or for transportation. As will be seen in Chapter 5, most of the perishable staples are sometimes dried, usually be slicing or chipping, sometimes followed by parboiling, drying and finally pounding or grinding into flour. The essential drying phase of the operation is usually carried out using some form of uncoated heat energy: most commonly sundrying is adopted, the sliced, chipped or occasionally grated material being spread out in the sun, usually on mats, or hard prepared surfaces, to minimize contamination. Little is understood of the nature of the drying process and few investigations have been made, except on cassava chips in the commercial context (Manurung, 1974; Best, 1978) which indicate that air flow may be more important than the degree of insolation. Alternatively, the "waste" heat from household fires lit primarily for cooking or other purposes may be used and in these cases some contribution to preservation may also be made by the insect-repellent effects of the smoke. Only in comparatively rare instances such as the manufacture of farinha or gari from cassava, are substantial amounts of heat energy, derived from wood fires, generated especially for the processing operation: even here, some initial removal of water is undertaken by pressure, e.g. by using the tipiti, which is a more energetically economical method of drying than the application of heat (Lancaster et al., 1982). In general, therefore, traditional processing methods for the manufacture of dried products make only minimal energy demands.
Detoxification, e.g. of cassava and some yams is most commonly undertaken by soaking the product, either whole, sliced or grated, in water. Most commonly, the running water of streams is used, but where this is not available, static water in pots or other containers may be used. In either case little or no energy is required by the processing. Soaking processes are also used, quite irrespective of the need for detoxification, to extract starch from the crop product, for use in various forms as food. Many traditional food processing operations involve what are loosely described as "fermentations" (Hesseltine, 1965; Hesseltine et al., 1967). Little is known of the actual nature of the processes involved, except in the case of the fermentation of cassava to make farinha or gari where a number of micro-organisms are known to be involved, but in other cases the changes that take place may be purely endogenous biochemical ones without the involvement of any exogenous micro-organisms. The function of these "fermentations" may include detoxification, as in the case of cassava; the development of sourness or acidity in the food material, for example under the influence of lactobacilli, which can enhance the storage life even of undried products such as the Polynesian ma-type foods (Cox, 1980a); or simply the development of preferred organoleptic properties. As with the other types of processing described, no significant external inputs of energy are required, although some proportion of the original crop product may be consumed or lost in the course of the "fermentation", in supporting the metabolic processes of the micro-organisms involved.
TABLE 3.2 Estimates of Post-Harvest Losses in Perishable Staples (%)
| Commodity | Early TPI Estimates1 | NASē (1978) Estimates |
| Potatoes | 8, 30 | 5-40 |
| Sweet Potatoes | 35-65, 95 | 55-95 |
| Yams | 5, 15 | 10-60 |
| Cassava | — | 10 |
| Taro | 12-15 | — |
| Plantains | 33 | 35-100 |