Threshing removes the grain from the stem. However, the grain is not usually ready for cooking until the husk (glumes) has been removed.
The exact details of grain processing depend on two main factors; the physical structure of the grain concerned and the nature of the flour product being made from that given grain.
Wheat, sorghum and pearl millet usually thresh naked that is without the glumes. Maize is on a cob covered with a sheath: the sheath must be removed and the grain taken from the cob.
A combine harvester threshes the maize free from both sheath and cob. Barley, oats, rice and most of the small millets thresh with the glumes adhering tightly to the kernel so that even a modern threshing machine or a combine harvester cannot remove the grain from this husk.
The first step in any milling process for this latter group is to remove the glummer. This is done by rubbing the grains one against the other in a mortar or by simple rotary machine, by splitting off the glumes in a disk huller, or by use of rubber rollers rotating at a differential speed.
The objective at this stage of milling is to obtain as high an out-turn as possible of whole grains.
Once free from glumes, all grains may be treated in the same way. Cereals have an outer layer and a gem, known collectively as bran, which is rich in lipids that rapidly oxidize to give off-flavors. The bran also tends to be less easily digested than the endosperm.
Milling of wheat into flour has two principle aims: to separate the bran from the endosperm; and to reduce the endosperm particles to assize sufficiently small to be rapidly hydrated on subsequent processing.
The objective of the second stage of milling is, therefore, to remove the bran. In the case of rice, the grain is required whole, so great care is taken not to break the grain.
In the process of rice polishing, separation of bran from endosperm is achieved simply by abrading the outer bran layers away leaving the intact endosperm.
Most other cereals are eaten as flour or meal (a coarse flour). The more of the bran that is removed, the better the flour will keep.
Grain Processing: Milling
