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2.1 Definition of the post-harvest system

"Properly considered," Spurgeon says, "the post-harvest system should be thought of as encompassing the delivery of a crop from the time and place of harvest to the time and place of consumption, with minimum loss, maximum efficiency and maximum return for all involved" (The Hidden Harvest, 1976).

The term "system" denotes a dynamic, complex aggregate of logically interconnected functions or operations within a particular sphere of activity. The term "chain" or "pipeline" highlights the functional succession of various operations but tends to ignore their complex interaction.

In considering the system or the agro-food chain as a whole, harvesting can be seen as the hinge, or as a ridge between the pre-harvest slope, corresponding to production activity and the post-harvest slope, extending from harvesting to consumption. These ideas are illustrated in the following diagrams, which give Bourne's graphic representation of the food pipeline (1977), Spurgeon's flow diagram (1977) and Sigaut's diagram of the food web (1979).

Diagram 1. The Food Pipeline
(Source: Bourne, 1977, mimeo)


Diagram 2. Stages of a whole post-harvest system
(Source: Spurgeon, 1977)

transport and distribution
cleaning, classification, dehulling, pounding, grinding,
packaging, soaking, winnowing, drying, sieving,
whitening, milling
mixing, cooking, frying
moulding, cutting, extrusion
quality control: standard recipes
weighing, labelling, sealing
publicity, selling,
recipes elaboration:
traditional dishes
new dishes
product evaluation, consumer education


Diagram 3. Social conditions of production, distribution and consumption
(Source: Sigaut, 1979)


Picture 1. Winnowing wheat, Afghanistan, FAO, 1994


Picture 2. Extension worker demonstrating a mechanical thresher to farmers
Madagascar, FAO, 1994


The post-harvest system encompasses a sequence of activities and operations that can be divided into two groups:

Main elements of the post-harvest system

Harvesting. The time of harvesting is determined by the degree of maturity. With cereals and pulses, a distinction should be made between maturity of stalks (straw), ears or seedpods and seeds, for all that affects successive operations, particularly storage and preservation.

Pre-harvest drying, mainly for cereals and pulses. Extended pre-harvest field drying ensures good preservation but also heightens the risk of loss due to attack (birds, rodents, insects) and moulds encouraged by weather conditions, not to mention theft. On the other hand, harvesting before maturity entails the risk of loss through moulds and the decay of some of the seeds.

Transport. Much care is needed in transporting a really mature harvest, in order to prevent detached grain from falling on the road before reaching the storage or threshing place. Collection and initial transport of the harvest thus depend on the place and conditions where it is to be stored, especially with a view to threshing.

Post-harvest drying. The length of time needed for full drying of ears and grains depends considerably on weather and atmospheric conditions. In structures for lengthy drying such as cribs, or even unroofed threshing floors or terraces, the harvest is exposed to wandering livestock and the depredations of birds, rodents or small ruminants. Apart from the actual wastage, the droppings left by these marauders often result in higher losses than what they actually eat. On the other hand, if grain is not dry enough, it is vulnerable to mould and can rot during storage.

Moreover, if grain is too dry it becomes brittle and can crack after threshing, during hulling or milling. This applies especially to rice if milling takes place a long time (two to three months) after the grain has matured, when it can cause heavy losses. During winnowing, broken grain can be removed with the husks and is also more susceptible to certain insects (e.g. flour beetles and weevils). Lastly, if grain is too dry, this means a loss of weight and hence a loss of money at the time of sale.

Threshing. If a harvest is threshed before it is dry enough, this operation will most probably be incomplete. Furthermore, if grain is threshed when it is too damp and then immediately heaped up or stored (in a granary or bags), it will be much more susceptible to attack from micro-organisms, thus limiting its preservation.

Storage. Facilities, hygiene and monitoring must all be adequate for effective, long-term storage. In closed structures (granaries, warehouses, hermetic bins), control of cleanliness, temperature and humidity is particularly important. Damage caused by pests (insects, rodents) and moulds can lead to deterioration of facilities (e.g. mites in wooden posts) and result in losses in quality and food value as well as quantity.

Processing. Excessive hulling or threshing can also result in grain losses, particularly in the case of rice (hulling) which can suffer cracks and lesions. The grain is then not only worth less, but also becomes vulnerable to insects such as the rice moth (Corcyra cephalonica).

Marketing. Marketing is the final and decisive element in the post-harvest system, although it can occur at various points in the agro-food chain, particularly at some stage in processing. Moreover, it cannot be separated from transport, which is an essential link in the system.

Table 1. Comparison between properties of cereals and roots and tubers regarding their storage capacity
(Source: FAO, 1984, quoted by Knoth, J., 1993)

Non-perishable food crops
Perishable food crops
Harvest manly seasonal, need for storage of long duration Possibility of permanent or semi-permanent production, needs for short-term storage
Preliminary treatment (except threshing) of the crop before storage exceptional Processing in dried products as an alternative of the shortage of fresh products
Products with low level of moisture content (10-15 percent or even less) Products with high level of moisture in general between 50-80 percent
Small "fruits" of less than 1 g Voluminous and heavy fruits from 5 g to 5 kg or even more
Respiratory activity very low of the stored product, heat limited High or even very high respiratory activity of stored products inducing a heat emission in particular in tropical climates
Hard tissues, good protection against injuries Soft tissues, highly vulnerable
Good natural disposition for storage even for several years Products easily perishable, natural disposition for storage between some weeks up to several months (strong influence of the varieties)
Losses during storage mainly due to exogenous factors (moisture, insects or rodents) Losses due partly to endogenous factors (respiration, transpiration, germination) and partly to exogenous factors (rot, insects)

2.2 Post-harvest losses

"Losses are a measurable reduction in foodstuffs and may affect either quantity or quality" (Tyler and Gilman, 1979). They arise from the fact that freshly harvested agricultural produce is a living thing that breathes and undergoes changes during post-harvest handling.

Loss should not be confused with damage, which is the visible sign of deterioration, for example, chewed grain and can only be partial. Damage restricts the use of a product, whereas loss makes its use impossible.

Some basic definitions are needed before moving on to the various types of loss.

Foodstuff. Products, in the present case crops, edible by human beings; more specifically, the part fit for human consumption. In tropical countries, 75 percent of basic food comes from cereals and pulses. The remaining vegetable-based food is often, especially in wet, wooded zones, supplied by roots and tubers, particularly cassava, yam, taro, plantain, potato and sweet potato. In the food chain, quantities of food are usually expressed in terms of weight but this does not mean that organic structure and nutrients can be ignored.

Grains and seeds. Cereals, pulses and oilseeds grown in most climates and latitudes for human consumption. The main cereals are wheat, maize, rice, barley, sorghum, millet, oats and rye; pulses cover the various species of pea, bean, broad bean and lentil; and oilseeds cover soya, groundnut, sesame, rapeseed and sunflower.

Post-harvest. If harvesting covers the period when the various products grown are removed from the field, after maturity, the post-harvest period runs from exit from the field to the time of culinary preparation. For various reasons, but especially to allow the straw and grain to dry fully, harvesting may be delayed sometimes for months, as happens particularly with maize and rice and in these cases, some people prefer to speak of "post-production" in order to indicate the link between harvesting and post-harvest operations.

Food loss. Food loss refers to total modification or decrease of food quantity or quality which makes it unfit for human consumption.

Types of losses

The first distinction in agro-food losses is that between quantity and quality. Quantitative loss is a loss in terms of physical substance, meaning a reduction in weight and volume and can be assessed and measured. Qualitative loss, however, is concerned particularly with the food and reproductive value of products and requires a different kind of evaluation.

It should be noted that we shall not consider losses occurring during the production period and caused by various crop pests (insects, weeds, disease), even though they have a major influence on food preservation conditions and account in part for the nature and size of post-harvest losses.

After describing direct and indirect losses, we shall thus look at weight loss, then quality loss, food loss, seed viability loss and commercial loss. We should not forget the existence of moisture content and the difference between damage and loss.

Moisture content. In bio-chemical terms, organic products are composed of dry matter and water. The moisture content is the amount of free water within a given product and is expressed either as a decimal proportion or a percentage. For example, with cereals, a 13 percent moisture content is considered a guarantee of satisfactory grain preservation. In agriculture, moisture content or humidity rate is usually indicated as a proportion of the moist product; i.e. the moisture content is the proportion of the weight of moisture to the total weight of dry matter and moisture.

Damage. Damage is a clear deterioration in the product, e.g. broken or pitted grain, which affects more its quality than its quantity and can in the long-term result in a definite loss. Both damage and loss should be quantified in terms of weight and cost.

Direct and indirect losses. Direct losses occur when the disappearance of a foodstuff is caused by leakage (for example, spillage from bags) or consumption by pests (insects, rodents, birds), whereas indirect losses occur when a reduction in quality leads to the consumer's refusal to purchase.

Picture 3: Rural village in Siavonga district. Salvation Army food store infested by Larger Grain Borer, Zambia, FAO, 1997


Weight loss. While weight loss is easy to observe and measure, it does not necessarily mean food loss, since it can result simply from a reduction in moisture content. Moisture loss during drying is therefore not a food loss. On the other hand, an abnormal increase in weight through moisture absorption following rainfall on stocks left in the open can cause serious damage resulting in loss.

Weight loss can be caused by leakage, during transport for example, if sacks have holes or are insecurely attached. It is often the result of prolonged infestation and consumption by insects, rodents and birds or poor packaging. Weight loss from pests is not immediately apparent and may deceive an inexperienced purchaser. It can be checked by taking an equivalent amount of clean, healthy cereal, milling the two samples and weighing the flour from each. The poorer sample will produce less flour. This method can also be used to check whether the weight is really correct, for it is easy to increase it by moistening the grain or adding foreign bodies such as pebbles, earth or waste material.

Quality loss. Quality criteria cover a wide range and are concerned both with external features, shape and size and with odour and taste. The cultural factors that can influence diets and food habits must also be borne in mind.

The cleanliness and healthy condition of a product are primary concerns for the market and correspond to what is referred to as a "sound, legal and merchantable" product in commercial law. For example, if a trader takes a handful of grain from a sack, he can quickly see if it releases dust and work out if this is the result of insect infestation. Similarly, a bad smell can arouse suspicions that rodents have been at the grain, verifiable by the presence of rat or mouse droppings and hair. Many other objects can be mixed in with a foodstuff and reduce its value: bad grain, scraps of straw or other plant residue, soil, pebbles, bits of glass, etc. All such objects are hard to remove but some of them represent a greater risk of contamination than others: the soluble excreta of pests, oils, pesticides, pathogenic organisms spread by rodents and toxins from fungi and moulds.

It goes without saying that the presence of foreign bodies, which can distort the weight of a batch being sold, also affects the quality and thus the market value of a product.

Food loss. While food loss clearly results from a loss in quantity, it also results, but more insidiously, from a loss in quality and edibility, making it unfit for human consumption. Staple foods contain not only essential nutrients but also important vitamins. For instance, grain hearts are rich in protein and vitamins; however, they are also the preferred target of rodents and insects.

The various nutritive parts of products are thus the prey of different families of parasites. Weevils feed especially on the endosperm, the inside of the seed, which is rich in carbohydrates, while many parasites attack the cereal cover, which is rich in vitamins. Vitamin content is also affected by humidity during storage and by mould infection.

Seed viability loss. Seed set aside for sowing, like any product used for reproduction, is preserved with great care in order to maintain its full germinative potential. As noted above, the protein-rich grain heart can be a favourite target of certain pests. Atmospheric conditions also play a part, as they can weaken the seed's productive potential; variations in light, temperature and humidity, leading to excessive respiration, are particularly responsible here.

Commercial loss. Commercial loss is the translation of the various types of loss listed above into economic and monetary terms. Although the price of a foodstuff is usually based on weight, many other factors play a part. This applies especially to the qualitative elements emphasized above, starting with cleanliness and purity, which will be all the more sought after if supplies are abundant in the market-place.

Reference to abundance brings us to the key economic factor of the supply-and-demand situation at a given moment.

A producer's ability to take advantage of a time when the scarcity of a commodity pushes up the price, is an element in good marketing management of his production, based on information and foresight. This assumes good organization, in which structural and not only market-situational factors are taken into account (for example, having enough storage facilities available and in good condition to be able to hold on to the harvest until the price rises) as well as keeping abreast of long-term developments, such as socio-cultural changes, which affect the future.

Although this kind of market-oriented attitude depends on an individual's skills, it also has some collective aspects that deserve development. For the present, we would note that the ability to capitalize on a product depends on a number of technical and economic factors, in which the quality of the merchandise plays an increasingly important role but so also do the human qualities of the producer and hence, the training that readies him to take his place as an economic player in the life of his country.

Irreducible losses and compensation. While loss of weight during drying is normal and measurable, there are other, said "irreducible" losses, which arise basically from respiration of the product and mechanical rubbing of grain against itself, as well as the breakage inevitable with certain machines. It should therefore always be remembered that losses, whether pre-harvest or post-harvest (i.e. in production, distribution, storage or marketing) cannot be materially reduced to zero and that they have to be compensated for through extra production. The production increase rate must be progressively higher than that of losses, if such compensation is to be adequate, so that compensation for a 20 percent loss will require 25 percent more production, for a 40 percent loss, 66 percent more and for a 60 percent loss, 150 percent more.

Table 2. Crop production increases required to offset post-harvest losses
(Source: Bourne, 1977, mimeo)

Post-harvest loss

Consumable grain

Production required
to give 100 tonnes after
post-harvest loss

Increased production
needed to offset
post-harvest loss









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