Pre-grading can be done before or after drying the kernels and may greatly reduce the final grading work. For large-scale processors pre-grading can be done mechanically, separating mainly the whole from the broken kernels and sometimes separating the different size groups of whole kernels.
The shelled kernel is covered with the testa and to facilitate removal, i.e. to peel in order to produce the blanched kernel, the shelled kernel is dried. This also protects the kernel from pest and fungus attack at this vulnerable stage. The moisture content is approximately 6% before drying and 3% after. It is important that the drying capacity is higher than the shelling capacity in case of periods of heavy rainfall because the drying operation will be lengthened since the kernels will absorb the moisture very quickly.
Sun drying, where the kernels are spread out in thin layers under strong sunshine is possible, however artificial drying becomes necessary for medium or larger-scale producers.
Drying usually takes six hours, at a temperature of around 70°C. A uniform temperature throughout the drier is essential to avoid under drying or scorching. Figure 5 and 6 shows a Practical Action tray dryer, details of which are available on request.
It is in the dried condition the kernels are most vulnerable, being both brittle and susceptible to insect infestation. Therefore, at this stage, they must be handled with care and moved to the next stage of peeling as quickly as possible.
At this stage, the testa is loosely attached to the kernel, although a small amount of kernels may have already lost the testa during the previous operations. Manual peeling is done by gentle rubbing with the fingers. Those parts still attached to the kernel are removed by the use of a bamboo knife. One person can peel about 10-12kg of kernels per day.
The mechanized processes of peeling differ widely. They include air-blasting, suction, a freezing operation and a system of rubber rollers. The operation has a low efficiency due to the difficulty of removing the testa and the amount of breakages can be as high as 30%. Currently research and development is taking place to improve the viability of the mechanization of this operation.
The grading operation is important as it is the last opportunity for quality control on the kernels. With the exception of a few grading aids, all grading is done by hand. Power driven rotary sieves are one mechanical method, another being two outwardly rotating rubber rollers aligned at a diverging angle. For large operations looking towards export markets, it is necessary to grade the kernels to an international level.
Before the kernels are packed it is necessary to ensure that their moisture content rises from 3% up to around 5%. This is to make the kernels less fragile, thus lessening the risk of breakage during transport. In humid climates, the kernels may absorb enough moisture during peeling and grading to make a further rehumidification process unnecessary.
source: www.practicalaction.org, photo from kriyayoga.com