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Posted 01/26/2021 in Agricuture by Wallace Mukoka



Post-harvest losses occur between harvest and the moment of human consumption. They include on-farm losses, such as when grain is threshed, winnowed, and dried, as well as losses along the chain during transportation, storage, and processing. Important in many developing countries, are on-farm losses during storage, when the grain is being stored for auto-consumption or while the farmer awaits a selling opportunity or a rise in prices.

What provoke the losses of crops

On-farm causes of loss

There are factors affecting post-harvest losses, from the soil in which the crop is grown to the handling of produce when it reaches the shop. Pre-harvest production practices may affect post-harvest returns. Plants need a continuous supply of water. Damage can be caused by too much rain or irrigation, which can lead to decay; by too little water; and by irregular water supply, which can, for example, lead to growth cracks. Too much fertilizer can harm the development and post-harvest condition of produce. Good crop husbandry is important for reducing losses. Weeds compete with crops for nutrients and soil moisture. Decaying plant residues in the field are also a major loss factor.

Seeds of poor quality, inadequate farming practices, or insect attacks in the field can provoke losses of crops even before their harvest. But we are concerned here only with prevention of losses after the harvest.

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 crops. 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.

Losses in weight

A reduction of the physical substance of the crop is evidenced by a loss in weight. There is a different between loss in weight and loss of crop. The decrease of the moisture content brings about a lowering of weight, but this is not a food loss. On the other hand, an increase of weight by absorption of moisture, after rains on a stock in the open air, for example, can cause severe damage and can be called losses.

Weight losses are due mainly to the prolonged action of pests (insects, birds, rodents), or to leakage of crops (perforated bags, spillage during grain handling). They can occur at practically any stage of production, but especially during the harvest, storage, and transport or handling of crops.

Losses in quality

The quality of crops vary widely and involve the exterior aspect, shape and size, as much as the smell and taste. A clean wholesome crop 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. Losses in quality are therefore evidenced by a decrease in the market value of the crop.

Implicit in this definition are the criteria for evaluating the quality of a given batch of grain; these include:

moisture content: suitable for the storage or further handling of the product;

colour: homogeneous and appropriate to the type of product under consideration;

odour: it must not hint that any biochemical change is going on;

cleanness: the number of impurities must conform to established standards of quality;

infestation: the absence of insects or other living organisms must be ascertained

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), degree of infestation by insects or micro-organisms.

 Losses due to change in food qualities

These losses, which are especially important 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), 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.

Assessing losses

There are no reliable methods for evaluating post-harvest losses of fresh produce although techniques for this have been improving. Any assessment can only refer to a particular value chain on a particular occasion and, even then, it is difficult to account for quality loss or to differentiate between unavoidable moisture loss and losses due to poor post-harvest handling and other factors described above.

Author, Wallace Mukoka

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