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The expression "post-harvest losses" means a measurable quantitative and qualitative loss in a given product.
These losses can occur during any of the various phases of the post-harvest system.
This definition must also take into account cases of product deterioration. However, rather than actual losses, it would be more accurate to call it restriction in the use of the product. As a matter of fact, grain partially damaged by insects, for example, may no longer be suitable either for human consumption or for sale. Where it was intended for those uses, losses in value have obviously occurred, even if the grain can be salvaged by using it for poultry feed.
From an economic point of view, the sum of the losses in quantity and quality of the products inevitably means losses of money.
In addition to direct economic losses, there are those resulting from poor management of post-harvest systems. They are evidenced by a lack of growth in production and in the income of the farmers.
Post-harvest system and losses
Seeds of poor quality, inadequate farming practices, or insect attacks in the field can provoke losses of products even before their harvest. But we are concerned here only with prevention of losses after the harvest.
From the harvest onward, then, the grain undergoes a series of operations during the course of which quantitative and qualitative losses can occur.
The sequence of these operations and the conditions in which they take place can, furthermore, create physical and biochemical phenomena that will bring about an alteration of the grain at later stages in the post-harvest system.
A late harvest, for example, can bring about losses from attacks by birds and other pests.
Insufficient drying of grain can cause losses from the development of moulds and insects.
Threshing can cause losses from broken grains and encourage the development of insects.
Poor storage conditions can bring about losses caused by the combined action of moulds, insects, rodents and other pests.
Transport conditions or defective packaging of grain can lead to quantitative losses of product.
Finally, in addition to these factors, there are others which can often be partly responsible for post-harvest losses, such as, for example: marketing practices, sectoral policies and other socio-economic aspects.
NATURE AND PRINCIPAL CAUSES OF POST-HARVEST LOSSES
| NATURE | DIRECT CAUSES | INDIRECT CAUSES |
| Premature harvest | Inadequate: | |
| Poor maturation | - capital | |
| Poor threshing | - professionalism | |
| In weight | Insufficient drying | - equipment |
| Insufficient cleaning | - pesticides | |
| Bird attack | - packaging | |
| In quality | Rodent attack | - transport |
| Insect attack | - organization | |
| Micro-organism attack | Constraints: | |
| Biochemical change | -social | |
| Economic | Leakage and waste | -economic |
| Moisture content wrong | -political | |
| for storage | ||
| Inadequate storage and | ||
| transport techniques |
It follows that if the question of post-harvest losses is to be tackled empirically, it is above all necessary to know, in each situation, the nature and number of manipulations the product undergoes, as well as the causes and incidence of losses during each stage in the process.
As an example, the following table gives estimates of the quantitative losses of rice for each stage in the post-harvest system in Southeast Asia.
| STAGE | LOSSES | |
| Min. | Max. | |
| Harvest | 1% | 3 % |
| Handling | 2% | 7% |
| Threshing | 2 % | 6 % |
| Drying | 1% | 5 % |
| Storage | 2 % | 6 % |
| Transport | 2 % | 10 % |
| Total | 10% | 37% |
In many developing countries, overall post-harvest losses of cereals and grain legumes of about 10 to 15 percent are fairly common. In some regions of Africa and Latin America, higher rates are found: up to 50 percent of the quantities harvested.
Up to now, we do not have really reliable data on the true level of post-harvest losses. There are many reasons for this information gap:
A reduction of the physical substance of the product is evidenced by a loss in weight.
Nevertheless, a distinction must be made between loss in weight and loss of product. The decrease of the moisture content brings about a lowering of weight, but this is not a food loss. On the contrary, an increase of weight by absorption of moisture, after rains on a stock in the open air, for example, can cause severe damage and thus considerable losses.
Weight losses are due mainly to the prolonged action of pests (insects, birds, rodents), or to leakage of products (perforated bags, spillage during grain handling, etc.). They can occur at practically any stage of production, but especially during the harvest, storage, and transport or handling of
Weight losses caused by pests are not apparent to the casual glance; an inexperienced buyer can thus be fooled. In order to recognize them, one should take an equal volume of clean and sound cereals, grind both samples, and weigh the flour obtained in each case. It will be observed that the damaged sample produces less flour.
This method can also be useful for avoiding potential weight frauds, since it is easy to augment weight by dampening the grain or by adding foreign bodies like pebbles, earth or sweepings.
In order to avoid any confusion, weight loss of dry matter should be stipulated.
Criteria of quality vary widely and involve the exterior aspect, shape and size, as much as the smell and taste. In this regard, the cultural considerations with which diets and eating habits are imbued cannot be overlooked.
A clean wholesome product is of primary importance in marketing. By taking a handful of grain from a bag, for example, a tradesman can quickly see if it gives off a floury dust and can therefore deduce whether or not it comes from an infestation by insects. Likewise, a bad smell can lead him to suspect rodent attacks, which can be confirmed by the presence of rat or mouse excrement or hairs.
Losses in quality are thus evidenced by a decrease in the market value of the product.
These losses are quantifiable only on condition that criteria or standards of quality have been previously established.
On the basis of objective criteria, the quality of the products can be evaluated by fairly complicated tests, measurements and laboratory analyses.
Many of the criteria adopted are based on evaluation of standards related to the physical condition of the grain and to its food, nutritive and germinative values.
In various countries, quality ratings are based on the general principal according to which grain must be "wholesome, sound, of market quality and odourless".
Implicit in this definition are the chief criteria for evaluating the quality of a given batch of grain; these include:
Generally, multiple criteria combine to define the quality of the products, and they also take into account cultural aspects related to community eating habits. In Senegal, for example, broken rice is highly prized by consumers; for this reason, the degree of breakage, as a standard of rice quality, obviously has less importance than in other contexts.
Losses in quality are mainly the result of mechanical constraints undergone by the product, the action of pests (insects, rodents) and micro-organisms (moulds), or the chemical changes produced within the grains under the effect of environmental conditions (temperature, humidity, duration of storage).
These losses can occur at any stage of production, especially during
Losses due to physical condition
These depend on the physical condition of the grain during a given stage of the post-harvest system.
The physical characteristics generally considered in evaluating the incidence of such losses are: shape and size of the grains, percentage of moisture, presence of impurities (foreign grains, grains that have germinated, are broken, deteriorated or damaged; pebbles, earth, plant residues, fragments of glass or metal, animal hairs or excrement, etc.), degree of infestation by insects or micro-organisms.
Losses due to change in food qualities
These losses, which are especially critical when the grains are intended for human consumption, result from alteration of the organoleptic features (aspect, taste, smell), from the degree of innocuity of the product (absence of toxic products such as toxins, pesticide residues, etc.), and from alteration in its content of vitamins, proteins, fats, glucides and other important nutrients.
Losses due to change in germinative properties
If marketable seeds are desired, the grain must not show altered germinative properties. These can be defined by the rate and percentage of germination, the vigour (stress resistance), the growth-rate of the seedlings and the absence of anomalies in the plants thus obtained.
The alteration of these properties brings about production losses by decreasing the capability of the grain to germinate.
A reduction in the quantities or qualities of grain means a corresponding commercial loss that is evidenced as a loss of money.
But beyond these direct economic losses, an evaluation of losses should also take account of some factors within the post-harvest system that can hamper the growth of production and of income. These include production systems, work schedules and methods, infrastructure, organization models, credit mechanisms etc. Some examples will illustrate this point.
For example, adoption of mechanized or semi-mechanized systems for some operations (harvesting, threshing, drying, etc.) can cut working time while, at the same time, permitting an increase in production by reducing the labour required and exploiting the land to better advantage.
Commercially, if the transport system is inadequate, the farmer may find it impossible to sell his products within the required time-limits and in the places where market prices are the most attractive. The fact of having to forgo a potential profit is beyond a doubt a loss of money.
If a farmer is not able to store products in complete security in existing storage buildings, he may be obliged to sell his production immediately after the harvest, thus becoming unable to profit by market prices when they are at their best. Once again, missing a profit is an economic loss for the farmer.
The consequences of such situations often go beyond individual losses of money: they affect production and the entire national economy.