The grain is conditioned to desired moisture content by the addition of water. The purpose of conditioning is to make the bran and germ pliable, thus preventing them from getting powdered. This may be done by moistening the grain and allowed for 24-72 hrs, depending on the air temperature. The cleaned and conditioned wheat is then ready to be milled by one of the following methods:
(i) Stone-milling: The traditional procedure for milling wheat in India has been the stone grinding (chakki) to obtain whole meal flour (atta). In the stone mill, two circular stones are used, each with its surface corrugated radially, with the distance between the stones being smaller towards the outer edge of the stones. In the operation, the cleaned and conditioned grain enters from the above into an aperture in the center of the top stone.
The bottom stone is stationery at all times whilst the top one revolves, grinding the grain more finely as it is pushed to the outside of the stones. The resulting flour is then sieved before being bagged. The method results in 90-95% extraction rate flour, which retains almost all the nutrients of the grain, while simultaneously eliminating the part of the grain which is most indigestible, like cellulose and phytic acid, which binds and carries away the minerals.
(ii) Roller-milling: The roller-milling is a much more complete method than stone-milling and involves a large amount of specialized equipment. It is concerned with the milling of white flour, where bran and germ are separated from endosperm and flour of any extraction rate can be produced. The process can be broken down into two clear stages:
- Breaking: The cleaned and conditioned grain is passed through a series of breakrolls. These are the grooved rollers, which operate in pairs, rotating in the opposite direction to each other, the top one rotating two and a half times faster than the lower roller. It is usual for the mill to have five sets of break rolls, with each set being more finely set than the previous set of break rolls. Each of the above are passed through purifiers, where by means of air currents any minute particles of bran are removed to ensure purity of the white flour end-product.
- Reduction: The purpose of this stage is to reduce the endosperm to fine flour and to extract the germ. The reduction stage is less complicated than the breaking stage. It consists of a series of reduction rollers, which are smooth and each pair is set more finely than the previous set. After passing through each set of reduction roller, the product is sieved; the coarse particles go to the next set of rollers for finer reduction. The process is repeated until all the semolina, which was fed into the reduction rollers, is reduced to fine white flour, germ and a small amount of branny by-product. The germ is extracted early in the reduction stage, where it is easily sifted off because being of a tough and oily nature, it is flattened on the rolls with little fragmentation taking place.
(iii) Fragmentation-milling (Air classification): This is a relatively new method of milling, by which it is possible to control the protein quality and quantity in the production of a particular flour. This is a refinement of roller-milling in that after producing the white flour, it is then processed a further step and is separated by means of air classification into particles of three broad ranges, lesser the size, higher the protein