Milling
Milling is a unit operation that uses forces such as shear, compression, friction, collision, or impact to reduce solid particles to smaller ones. Milling processes can be used to make flour or to extract gluten and starch from grains and cereals (wet milling).
Milling is the process of dehulling and grinding grains such as oats, wheat, rice and corn into smaller pieces or flours to improve palatability, reduce cooking time and create food products. Its central purpose is to separate the endosperm from the other kernel portions, since most flours are made from the endosperm after removal of the bran and germ.
The grain kernel has three parts, and the proportions govern what a mill can yield. The bran is the outermost layer, making up about 14.5 percent of total kernel weight; the endosperm makes up the majority of the kernel weight at 83 percent; and the germ, the embryo of the wheat plant, is the smallest part at 2.5 percent of total weight. Flour yield, or percentage extraction rate, is the number of parts flour by weight produced per 100 parts wheat milled.
The milling process runs in three stages. Cleaning and conditioning removes all impurities from the grain and prepares it for milling. Crushing or breaking then breaks down the grain in stages to release its constituent parts. Reduction uses rollings and siftings to refine the flour and separate it into different categories, referred to as streams. The machinery follows the same logic: a roller flour mill grinds wheat between two cast rolls (break rolls) that rotate against each other; a purifier separates clean farina, or coarse endosperm, from fine bran or compound particles to feed the primary reduction rolls; and square plansifters and rotary sieves are generally used for screening and grading after grinding, and can also be used as checking sifters.
Two consequences matter for product planning. Milling reduces the nutritional value of flour by removing a large proportion of the minerals and vitamins found in whole-wheat kernels, so flour is enriched with nutrients to compensate for the loss during milling. The by-products from the wheat milling process are known as wheatfeed, comprising bran, coarse residue from break grinds, fine feed and purifier particles, and reduction grinding; compound animal feeds contain bran and fine wheat feed.