Physical Properties And Structure

Optimal utilization of cereal grains requires knowledge of their structure and composition. The practical implications of kernel structure and composition are numerous (30-32). They relate to the various stages of grain production, harvest, storage, marketing, and use.

Some physical properties of cereal grains are listed in Table 2 (33). Table 3 summarizes approximate grain size and the proportions of the principal parts comprising the mature kernels of different cereals (34).

Kernel Structure

The cereal grain is a one-seeded fruit (called a caryopsis), in which the fruit coat adheres to the seed at maturity. As the fruit ripens, the pericarp (fruit wall) becomes firmly attached to the wall of the seed proper. The pericarp, seed coats, nucellus, and aleurone cells form the bran. The embryo comprises only a small part of the seed. The bulk of the seed is taken up by the endosperm, which is a food reservoir for the germinating plant.

The floral envelopes (modified leaves known as lemma and palea), or chaffy parts, within which the caryopsis develops, persist to maturity in the grass family. If the chaffy structures envelop the caryopsis so closely that they remain attached to it when the grain is threshed (as with rice and most varieties of oats and barley), the grain is considered to be covered. However, if the caryopsis readily separates from the floral envelopes when the grain is threshed, as with common wheats, rye, hull-less barleys, and the common varieties of corn, these grains are considered to be naked.

Wheat. The structure of the wheat kernel is shown in Figure 1 (35). The dorsal or back side of the wheat grain is rounded, while the ventral side has a deep groove or crease along the entire longitudinal axis. At the apex or small end (stigniatic end) of the grain is a cluster of short, fine hairs known as brush hairs. The pericarp, or dry fruit coat, consists of four layers: the epidermis, hypodermis, cross cells, and tube cells. The remaining tissues of the grain are the inner bran (seed coat and nucellar tissue), endosperm, and embryo (germ). The aleurone layer consists of large rectangular, thick-walled, starch-free cells. Botanically, the aleurone is the outer layer of the endosperm, but as it tends to remain attached to the outer layers during wheat milling. It is shown in the diagram as the innermost bran layer.

The embryo (germ) consists of the plumule and radical, which are connected by the mesocotyl. The outer layer of the scutellum, the epithelium, may function as both a secretory or an absorptive organ. In a well-filled wheat kernel, the germ comprises about 2 to 3% of the kernel, the bran 13 to 17%, and the endosperm the remainder. The inner bran layers (aleurone) are high in protein, whereas

Table 2. Some Physical Properties of Cereal Grains

Name

Length (mm)

Width (mm)

Grain mass (mg)

Bulk density (kg/m3)

Unit density (kg/m3)

Rye

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