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Grain cleaning and insecticide treatments
Some of the controls described above are aimed mainly at establishing the condition of the grain at the time of its reception at a storage centre.
These controls permit not only a better definition of the market value of the product but also an identification of the type and urgency of treatments which the grain should undergo before storage, marketing or further processing.
For example, the presence of impurities requires appropriate cleaning of the batch of grain. If the grain is too moist, it must quickly be dried. To counteract insect infestation, immediate action is needed to rid the grain of insects.
Drying operations have been described above. We shall now consider the operations of cleaning and insect-control.
After threshing, grains (or shells, in the case of groundnuts) are contaminated by impurities (earth, small pebbles, plant and insect waste, seed cases, etc.).
These impurities hinder drying operations and make them longer and more costly. After drying, especially by traditional methods such as open-air drying, the grain may still be contaminated by impurities.
These impurities lower the quality of the product and are also a focal point for potential infestation during storage.
"Cleaning" means the phase or phases of the post-harvest system during which the impurities mixed with the grain mass are eliminated.
This operation, which may be accompanied by a sorting of the products according to quality, is indispensable before storage, marketing or further processing of the products.
It is also necessary as an operation preliminary to artificial drying of the products in dryers.
Indeed, it would be not only costly but also superfluous to waste time, effort and money on drying the impurities along with the grain.
Traditional methods
The simplest cleaning method, known as winnowing, consists of tossing the grain into the air and letting the wind carry off the lightest impurities.
Although widespread in farming circles, this cleaning method does not eliminate the heavier impurities (gravel, foreign grains, earth, etc.).
Cleaning devices
If the desired product is to be completely free of impurities and suitable for long-term storage, appropriate cleaning devices must be used, such as: winnowers, pre-cleaners and cleaner-separators.
Winnowers
These machines, some models of which may reach an output of one ton per hour, can significantly contribute to improving product quality and marketing, especially in production areas.
Sometimes operated by hand but more often motorized, winnowers are relatively simple machines that consist mainly of a hopper to receive the grain, a fan and a set of sieves.
The grain unloaded into the hoppers first has its lightest impurities removed by running it through a current of air produced by a fan.
Then, the set of sieves completes the cleaning of the grain at the same time it is sorted according to size.
Pre-cleaners
These motor-driven devices are generally used to pre-clean grain that has been harvested when moist, before it goes to the artificial dryer.
There are several models of pre-cleaner:
Diagram of a clodder: 1 Grain entry; 2 Adjustment; 3 Suction; 4 Impurities exit; 5 Grain exit.
Cleaner-separators
These machines, whose large outputs can be as much as 20 tonnes an hour, are certainly the most effective instruments for cleaning grain, and all storage centres should be equipped with them.
Cleaner-separators are motor-driven and consist mainly of a reception hopper, a fan and set of vibrating sieves; they clean grain by repeated suction of the lightest impurities, followed by siftings of the grain.
The set of sieves, composed of a coarse sieve (clodder) for the bulkiest impurities and a fine sieve (sifter) for the finest impurities, must be chosen with care, taking account of the shape and size of the grains to be cleaned.
The usual recommendation is a set of sieves with oblong perforations for long grains, and, for round grains, a clodder with round perforations and a sifter with oblong perforations.
For optimal use of the machine, it is also important to take care in making the various adjustments (to the flow of grain, the suction, and possibly to the speed of vibration of the sieves).
The effort to protect grain against insects can take two forms:
In both cases the insects must be destroyed without altering the food quality of the product.
But, to accomplish this, some general hygienic measures and steps for treatment of the premises must be observed.
For grain, there are various methods of insect-control: biological, physical, mechanical and chemical. Chemical control is the method most widely used today. It features two broad types of treatment:
Treatment of premises
Before any application of insecticide to storage buildings (warehouse, silo), the entire premises must be thoroughly cleaned.
The range of available insecticide products is broad enough to treat different surfaces according to their characteristics.
Thus, uneven surfaces (bricks, breeze-block, raw wood, etc.) are treated by spraying with powder mixed with water until it is runny; on the other hand, for smooth, non-porous surfaces (metal, polyester) spraying with a stickier concentrate is preferred.
Ambient treatment is designed to destroy flying insects by aerosols in hermetically sealed premises.
This treatment should preferably be carried out in the evening, when flying insects are their most active.
Treatment of grain with contact insecticide
This consists of covering the grain with a film of insecticide that acts on contact with insects, with effects that vary in rapidity and persistence.
These products come in various forms (powders for dusting, powders to be mixed with water, liquid concentrates, or fumigants) that dictate their techniques of application.
For grain that is to be stored in bulk, the insecticide is incorporated directly into the grain by spraying before the silos are filled.
For storage in bags, previously cleaned grain is mixed with powder or sprayed before bagging.
In order to avoid reinfestation of grain stored in bags, further repeated dustings or sprayings are carried out while the bags are being stacked and during the storage period.
The machinery used for dusting grain can range from the simple mechanical duster to motorized dusters; however, with this type of equipment the grain is not treated uniformly, some areas receiving more dust than others.
Spraying can be mechanical (pressure sprayer), pneumatic or thermal, and provides a better distribution of the product over the grain.
In big storage centres, in order to obtain an even more regular distribution and a good coating of insecticide, the grain is fine-sprayed by a compressor equipped with a mist nozzle.
Misting grain before silo storage
Although contact systems of treatment are certainly effective on fully-developed insects, they have little or no effect on the eggs or larvae.
Furthermore, some residues of the product, though not highly toxic, may linger foodstuffs.
Treatment of grain by fumigation
Fumigation is a treatment that rids stored grain of insects by means of a poisonous gas called a fumigant. This substance, produced and concentrated as a gas, is lethal for specific living species.
Unlike contact powders, the fumigant penetrates to the interior of the grain mass and reaches the largely invisible incipient forms (eggs, larvae) developing there.
Fumigants spread throughout the area where they are released; therefore their use requires that the enclosure concerned be totally sealed off.
Thus, when grain stored in bulk is fumigated, the bins must be perfectly airtight.
For grain stored in bags, the usual method is to cover the bags with a tarpaulin whose edges are sealed to the ground or the walls.
The effectiveness of fumigation depends, on the one hand, on the actual concentration of the gas and, on the other, on the length of time during which the grain is fumigated.
Depending on whether methyl bromide or phosphine is used, the duration of fumigation should be 24 to 48 hours for methyl bromide, or a minimum of five days for phosphine.
The latter product is more commonly used, since its application, in the form of pellets spread throughout the grain mass, is the simpler.
It is essential to recognize, however, that fumigants are very poisonous to people and therefore the staff that is to use them must be carefully trained in their application. For all these treatments, it is important to scrupulously observe the recommended protective and safety measures (masks, gloves, hand-washing, hermetic sealing of phosphine containers, etc.).
Furthermore, remember that these treatments are curative, and have no persistence over time: therefore, a combination of the techniques of contact insecticide and fumigation is recommended.
