Principles of Cereal Science and Technology, Third Edition
Milling transforms cereals into more-palatable, more-desirable food ingredients. Dry milling is the separation of the anatomical parts of the grain as cleanly as possible. Subsequently, some of the parts are reduced in particle size. Milling generally involves recovery of the main tissue (i.e., the starchy endosperm) and the concomitant removal of the material the miller calls “bran” (i.e., the pericarp, the seed coat, the nucellar epidermis, and the aleurone layer). In addition, the germ is usually removed from the endosperm. Because of the relatively high oil content of the germ, its presence increases the risk of the product becoming rancid and thereby less palatable. The bran and germ are relatively rich in protein, dietary fiber, B vitamins, minerals, and fat, and the separated endosperm is therefore lower in these components than the original grain. Thus, while milling increases the palatability of cereal products, it decreases the nutritional value of the main product obtained.
In dry milling, the particle size distribution of the endosperm products obtained is dictated by the end use of the product. In general, it is desirable for rice or barley endosperm to remain in one large piece, while a large grit is desirable from maize and coarse semolina from durum wheat. Wheat and rye, at opposite ends of the size spectrum, are generally milled into fine flour products. It follows that different types of equipment are used for dry milling. However, they consistently aim to produce palatable products with a good shelf life.
While the main part of the present chapter deals with the milling of common wheat, many of the unit operations described below are also used as part of dry roller milling for other cereals such as durum wheat, rye, and maize. The roller milling processes of the latter are therefore discussed only briefly. Finally, for some cereals such as barley, sorghum, and millet, decortication or attrition milling is used. Such processing is described briefly in the present chapter. Milling of rice and oats is discussed in Chapter 10.
Maize and rice are also wet milled by processes in which the goal is to isolate their starches from the endosperm tissue. The term wet milling refers to the fractionation of wet (soaked) endosperm tissue. These processes are covered in Chapter 9.