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"Storage" means the phase of the post-harvest system during which the products are kept in such a way as to guarantee food security other than during periods of agricultural production.
The main objectives of storage can be summed up as follows:
In order to attain these general objectives, it is obviously necessary to adopt measures aimed at preserving the quality and quantity of the stored products over time.
Influences of environmental factors
To conserve the quality of products over long-term storage, degradation processes must be slowed down or even stopped.
Degradation of grains during storage depends principally on a combination of three factors:
During storage, as during other phases of the post-harvest system, the combined effects of these three factors can sometimes cause severe losses.
Temperature and moisture
Temperature and moisture are determining factors in accelerating or delaying the complex phenomena of the biochemical transformation (especially the "breathing" of the grain) that are at the origin of grain degradation.
Furthermore, they have a direct influence on the speed of development of insects and microorganisms (moulds, yeasts and bacteria), and on the premature and unseasonal germination of grain.
In the general diagram of conservation designed by Burges and Burrel, the relationship between temperature and moisture content is established in order to determine the area of influence of certain important degradation phenomena, such as: the development of insects and moulds, and the germination of grain.
Diagram of cereal conservation
It is easy to observe that the higher the temperature, the lower must be the moisture of the grain in order to ensure good conservation of the products.
In view of their influence on the speed of development of these degradation phenomena, the temperature and moisture content of the grain condition the maximal duration of storage.
DURATION OF WAREHOUSING (in days)
| TEMPERATURE | ||||||
| MOISTURE | 5°C | 10°C | 15°C | 20°C | 25°C | 30°C |
| 13% | 180 | 115 | 90 | |||
| 14% | 160 | 100 | 50 | 30 | ||
| 15% | 100 | 50 | 30 | 15 | ||
| 16% | 130 | 50 | 30 | 20 | 8 | |
| 17% | 65 | 35 | 22 | 12 | 5 | |
| 18% | 130 | 40 | 25 | 17 | 8 | 2 |
| 19% | 70 | 30 | 17 | 12 | 5 | 0 |
| 20% | 45 | 22 | 15 | 8 | ||
| 21% | 30 | 17 | 11 | 7 | ||
| 22% | 23 | 3 | 8 | 6 | ||
| 23% | 17 | 10 | 7 | 5 | ||
| 24% | 13 | 8 | 4 | 4 | ||
| 25% | 10 | 8 | 6 | 3 | ||
As an example, the preceding table shows the recommended durations of warehousing, according to the temperature and moisture content of the grain.
The temperature depends not only on climatic conditions but also on the biochemical changes that are produced inside a grain mass, provoking undesirable natural heating of the stored products.
As for the moisture content of the stored grain, it depends on the relative humidity of the air, as shown in the air-grain equilibrium curves.
With a relative air humidity below 65-70 percent, many grain-degradation phenomena are slowed down, if not completely blocked.
In this sense, the "safeguard" moisture content is defined as that corresponding to an equilibrium with the air at 65-70 percent relative humidity.
The following table shows the moisture content recommended for long-term storage in hot regions of various sorts of grain.
| GRAIN | MOISTURE | GRAIN | MOISTURE |
| Paddy | 14.0% | Sunflower | 9.0% |
| Rice | 13.0% | Wheat | 13.0% |
| Maize | 13.0% | Millet | 16.0% |
| Sorghum | 12.5 % | Coffee | 13.0% |
| Beans | 15.0% | Cocoa | 7.0% |
| Groundnut | 7.0 % | Copra | 7.0 % |
Oxygen content
Like grain, micro-organisms and insects are living organisms that need oxygen.
Storage of grain in places that are low in oxygen causes the death of insects, cessation of development of micro-organisms, and blockage, or slowing down, of the biochemical phenomena of grain degradation. This favours the conservation of grain, but may affect its germinating power.
Agents causing deterioration of stored grain
The principal enemies of stored grain are micro-organisms, insects and rodents.
Micro-organisms
Micro-organisms (moulds, yeasts, bacteria) are biological agents present in the soil which, when transported by air or water, can contaminate products before, during and after the harvest.
Their presence and growth cause severe changes in the nutritive value and the organoleptic features of grain (taste, smell, aspect).
Furthermore, they are responsible for the alteration of important germinative properties of seeds (vigour and capacity to germinate) and, in the case of moulds, for the potential formation of dangerous poisons (mycotoxins).
Impurities, and cracked or broken grains, foster the development of micro-organisms.
Furthermore, temperature and humidity have a determining influence on the growth rate of these degradation agents.
It has been observed that micro-organisms develop at temperatures between -8°C and +80°C, when the relative humidity of the air is over 65 percent.
On the contrary, atmospheres that are low in oxygen help check the development of these degradation agents.
Insects
Insect infestations can occur either in the field, before the harvest, or in the places where products are stored.
In some cases, these infestations are difficult to discern with the naked eye, since the damage is provoked by the larvae developing inside the grain.
The insects most likely to infest stored products belong to the following families:
Insects can be responsible for significant losses of product. Furthermore, their biological activity (waste production, respiration, etc.) compromises the quality and commercial value of the stored grain and fosters the development of micro-organisms.
Insects can live and reproduce at temperatures between +15°C and +35ºC.
On the contrary, low humidity slows or even stops their development, and a low supply of oxygen rapidly kills them.
Rodents
Rodents invade and multiply in or near storage places, where they can find an abundance of food.
They cause serious damage not only to stored products but also to packaging and even to storage buildings.
The principal rodents, those most common and likely to attack stored products, belong to the following species:
Prolonged attacks by these pests inevitably results in serious quantitative losses of stored products.
To these losses must be added those arising from the decrease in quality of the foodstuffs, caused by the filth (excrement, secretions) rodents leave behind in the stored products.
This contamination is as important from the marketing standpoint as it is for hygiene and health. Indeed, rodents are often the vectors of serious diseases (rabies, leptospirosis).
There are basically two methods of storage: in bags and in bulk.
Bags can be stored either in the open air or in warehouses; bulk grain is stored in bins or silos of various capacities.
The choice between these methods and the degree of technological sophistication of the storage buildings depend on many technical, economic and socio-cultural considerations.
The traditional storage systems used by small farmers must also be mentioned. With their use of artisanal construction techniques and local materials, these are the systems that prevail in the rural communities of many developing countries.
