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At harvest time, the cut portions of the plant may contain too much green plant matter, while the grain may not have reached a uniform degree of maturity and may have too high a moisture content.
"Pre-drying" is the stage of the post-harvest system during which the harvested product is dried in order to undergo the next operation of threshing or shelling under the best possible conditions.
In discussing harvest problems, we have mentioned some operations aimed at further pre-drying of products.
It will be helpful, however, briefly to describe the most commonly used pre-drying methods, to which we shall return in the chapter on drying.
Indeed, when distance from artificial drying installations justifies it, and if weather permits, pre-drying operations can be prolonged until they become actual drying operations; that is, until the degree of humidity has been so reduced that the product will keep well.
For maize, the use of cribs permits not only pre-drying, or complete drying of the grains, but also storage of the ears.
The duration of pre-drying is difficult to establish, because it is influenced by several factors.
The weather, moisture content at harvest time, the threshing or shelling systems and machines used, the proximity of artificial drying installations, storage methods: these are only a few of the factors that must be taken into account when establishing the duration and the kind of pre-drying operations.
One of the simplest and most common methods, especially in favourable weather, consists of postponing the harvest.
Once maturity has been reached, the products are allowed to pre-dry standing in the field, before harvesting them.
As for maize, for instance, this method sometimes requires several weeks in the field after physiological maturity.
When used for sorghum, sunflowers, and especially maize, this method is risky and has great disadvantages:
Pre-drying in piles
This method of pre-drying, or drying, consists of placing the newly-harvested product piles, in the field or on an ad hoc drying-floor. Prolonged exposure to the air (in sun or shade) reduces the moisture content of the grains.
The shape and size of these piles can vary according to the product to be pre-dried.
For example, panicles of rice, once harvested, are gathered into sheaves. These sheaves of rice can be put into large buildings to dry in the shade.
If weather permits and if sufficient covered spaces are not available, sheaves of rice can also be allowed to pre-dry in the field.
To accomplish this, wooden drying racks can be built and sheaves of rice propped against them, taking care to shade the panicles from direct sunlight by using straw or other plant matter.
Another system used for pre-drying rice and also groundnuts consists of building aerated "stacks".
The groundnut shells must, however, be placed inside the stacks, so as to be protected from direct sunlight that would cause too-sudden high-temperature dessication.
Pre-drying of groundnuts and sunflower seeds can also be accomplished using wooden drying racks on which to prop the groundnut plants or sunflower heads.
As for bean plants, after they are pulled up they are exposed to the sun and aired in piles in the field.
For sorghum, the ears are put into bunches or stacks, then left in the sun on drying-floors or on suitable trays with screened bottoms.
Redampening of shells and grains during pre-drying must be prevented by protecting the piles from sudden rains or nocturnal humidity and dew.
At the same time, in order to cut down the risks of infestation (moulds, etc.), direct contact of the shells and grains with the ground must be avoided.
Pre-drying in cribs
For maize, as well as for sunflowers, the disadvantages of standing-crop pre-drying can be alleviated by harvesting at maturity and placing the ears (or sunflower heads) to pre-dry in the naturally-ventilated cells called cribs.
By permitting the natural circulation of air among the ears, cribs ensure a slow and progressive reduction in the moisture content of the grains, but too long a stay in cribs may bring about severe losses from insect infestation.
Cribs, which are used both for drying and storage, can be used in damp climates, but more efficiently in dry climates.
They are relatively inexpensive, since they can be built from local materials using traditional techniques.
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