Principles of Cereal Science and Technology, Third Edition
Every cereal scientist should understand the structure of the industrially important cereal grains, as they are of utmost importance for many aspects of cereal technology, e.g., for milling of common wheat or durum wheat, for processing of maize (corn) or rice, or for barley malting. Insight into the three-dimensional architecture of cereal tissues and the compartmentalization of the various cereal constituents is, in these contexts, of prime importance. In this chapter, we deal with the structures of the most important cereals used for food and/or feed purposes (i.e., wheat, maize, and rice), as well as with those of barley, rye, triticale, oats, sorghum, and pearl millet.
In general, members of the grass family (Gramineae), which include the cereal grains, produce dry, one-seeded fruits. This type of fruit is commonly called a “kernel” or “grain.” However, strictly speaking, it is a caryopsis. The wheat caryopsis (or for that matter, the grain, see Fig. 1.1) consists of a fruit coat (or pericarp) and a seed. The fruit coat adheres tightly to the seed coat, which surrounds the remainder of the seed. The seed itself consists of the embryo (or germ), the endosperm, the nucellar epidermis, and the seed coat. The nucellar epidermis and the seed coat enclose the endosperm.
In general, all cereal grains have these same parts in approximately the same relationship to each other. Their caryopses develop within floral envelopes, which are actually modified leaves. These are called the “chaffy parts” or “glumes.” In rice and most cultivars of barley and oats, the floral envelopes cover the caryopsis so closely and completely that they remain attached to the caryopsis when the grain is threshed and constitute the hull of those grains. In wheat, rye, maize, grain sorghum, and pearl millet, the grain and hull separate readily during threshing, and the grains are said to be “naked” (i.e., to have an uncovered caryopsis).
The chemical constituents of cereal grains are often separated from each other by cell walls or other barriers. Such compartmentalization, along with the relatively low water activity, is largely responsible for the stability of the grain during storage. The grains themselves often contain both degrading enzymes and the substrates of these enzymes. Certainly, if the two come in contact and a proper water activity threshold is passed (such as in germination), degradation processes can easily start. However, if enzyme and substrate are protected from coming in contact with each other, the system is stable.