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"Storage" means thephase of the post-harvest system during which the products arekept in such a way as to guarantee food security other thanduring periods of agricultural production.
The main objectives of storagecan be summed up as follows:
- at the food level, to permit deferred use (on an annual and multi-annual basis) of the agricultural products harvested;
- at the agricultural level, to ensure availability of seeds for the crop cycles to come;
- at the agro-industrial level, to guarantee regular and continuous supplies of raw materials for processing industries;
- at the marketing level, to balance the supply and demand of agricultural products, thereby stabilizing market prices.
In order to attain these generalobjectives, it is obviously necessary to adopt measures aimed atpreserving the quality and quantity of the stored products overtime.
Influencesof environmental factors
To conserve the quality ofproducts over long-term storage, degradation processes must beslowed down or even stopped.
Degradation of grains duringstorage depends principally on a combination of three factors:
- temperature,
- moisture,
- oxygen content.
During storage, as during otherphases of the post-harvest system, the combined effects of thesethree factors can sometimes cause severe losses.
Temperature and moisture
Temperature and moisture aredetermining factors in accelerating or delaying the complexphenomena of the biochemical transformation (especially the"breathing" of the grain) that are at the origin ofgrain degradation.
Furthermore, they have a directinfluence on the speed of development of insects andmicroorganisms (moulds, yeasts and bacteria), and on thepremature and unseasonal germination of grain.
In the general diagram ofconservation designed by Burges and Burrel, the relationshipbetween temperature and moisture content is established in orderto determine the area of influence of certain importantdegradation phenomena, such as: the development of insects andmoulds, and the germination of grain.
Diagram ofcereal conservation
It is easy to observe that thehigher the temperature, the lower must be the moisture of thegrain in order to ensure good conservation of the products.
In view of their influence on thespeed of development of these degradation phenomena, thetemperature and moisture content of the grain condition themaximal 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 precedingtable shows the recommended durations of warehousing, accordingto the temperature and moisture content of the grain.
The temperature depends not onlyon climatic conditions but also on the biochemical changes thatare produced inside a grain mass, provoking undesirable naturalheating of the stored products.
As for the moisture content ofthe stored grain, it depends on the relative humidity of the air,as shown in the air-grain equilibrium curves.
With a relative air humiditybelow 65-70 percent, many grain-degradation phenomena are sloweddown, if not completely blocked.
In this sense, the"safeguard" moisture content is defined as thatcorresponding to an equilibrium with the air at 65-70 percentrelative humidity.
The following table shows the moisture contentrecommended for long-term storage in hot regions of various sortsof 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 andinsects are living organisms that need oxygen.
Storage of grain in places thatare low in oxygen causes the death of insects, cessation ofdevelopment of micro-organisms, and blockage, or slowing down, ofthe biochemical phenomena of grain degradation. This favours theconservation of grain, but may affect its germinating power.
Agents causing deterioration of stored grain
The principal enemies of storedgrain are micro-organisms, insects and rodents.
Micro-organisms
Micro-organisms (moulds, yeasts,bacteria) are biological agents present in the soil which, whentransported by air or water, can contaminate products before,during and after the harvest.
Their presence and growth causesevere changes in the nutritive value and the organolepticfeatures of grain (taste, smell, aspect).
Furthermore, they are responsiblefor 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 brokengrains, foster the development of micro-organisms.
Furthermore, temperature andhumidity have a determining influence on the growth rate of thesedegradation agents.
It has been observed thatmicro-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 thatare low in oxygen help check the development of these degradationagents.
Insects
Insect infestations can occureither in the field, before the harvest, or in the places whereproducts are stored.
In some cases, these infestationsare difficult to discern with the naked eye, since the damage isprovoked by the larvae developing inside the grain.
The insects most likely to infeststored products belong to the following families:
- Coleoptera (damage by larvae and adult insects);
- Lepidoptera (damage only by larvae).
Insects can be responsible forsignificant losses of product. Furthermore, their biologicalactivity (waste production, respiration, etc.) compromises thequality and commercial value of the stored grain and fosters thedevelopment of micro-organisms.
Insects can live and reproduce attemperatures between +15°C and +35ºC.
On the contrary, low humidityslows or even stops their development, and a low supply of oxygenrapidly kills them.
Rodents
Rodents invade and multiply in ornear storage places, where they can find an abundance of food.
They cause serious damage notonly to stored products but also to packaging and even to storagebuildings.
The principal rodents, those mostcommon and likely to attack stored products, belong to thefollowing species:
- black rat, also called roof rat (Rattus rattus),
- brown or Norway rat, also called sewer rat (Rattus norvegicus),
- mouse (Mus musculus).
Prolonged attacks by these pestsinevitably results in serious quantitative losses of storedproducts.
To these losses must be addedthose arising from the decrease in quality of the foodstuffs,caused by the filth (excrement, secretions) rodents leave behindin the stored products.
This contamination is asimportant from the marketing standpoint as it is for hygiene andhealth. Indeed, rodents are often the vectors of serious diseases(rabies, leptospirosis).
There are basically two methodsof storage: in bags and in bulk.
Bags can be stored either in theopen air or in warehouses; bulk grain is stored in bins or silosof various capacities.
The choice between these methodsand the degree of technological sophistication of the storagebuildings depend on many technical, economic and socio-culturalconsiderations.
The traditional storage systemsused by small farmers must also be mentioned. With their use ofartisanal construction techniques and local materials, these arethe systems that prevail in the rural communities of manydeveloping countries.
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