Читать книгу Dry Beans and Pulses Production, Processing, and Nutrition - Группа авторов - Страница 63

The cotyledon comprises the greatest portion of the bean in terms of both weight and volume and contributes a valuable component to the texture and nutritive value of the bean as food. The cotyledon portion, which is responsible for the embryonic leaf tissue during germination, makes up 90.5% of the total bean on a dry‐weight basis. Dry cotyledons have been reported to contain 39.3% starch, 27.5% protein, 1.65% lipids, and 3.5% ash (Powrie et al. 1960). Botanically, the cotyledons of dry beans are a segment of the embryo and are thus differentiated from the endosperm of common cereal grains. As the seed matures, these stored energy reserves increase and upon germination are mobilized and utilized for initial seedling growth.

Relatively small spherical protein bodies (approximately 5 μm) form a matrix that supports embedded starch granules of varied size and oblong shapes (Uebersax et al. 1989), as shown in Figure 3.6a, with an expanded view of this matrix presented in Figure 3.6b. Zimmermann et al. (1967) demonstrated partitioning of nutrients within the cotyledons with greater levels of protein and trypsin inhibitor present in the outer layers compared to the inner layers of the tissue.

Processed texture and nutrient availability of beans are influenced by the dimensions and arrangement of the cotyledonary cells. The outermost cells are an epidermal layer with an inner and outer portion. The innermost cells are elongated, and the outer layer cells are cubical. The next layer is the hypodermis, which has larger elliptically shaped cells. Both the epidermal and hypodermis layers appear granular, which is characteristic of protein.

The remaining and largest portion of the cotyledon parenchyma cells are bound by a distinct cell wall and middle lamella with a few vascular bundles. The parenchyma cells have thick walls that give rigidity to the cotyledon. Within each parenchyma cell, starch granules are imbedded within a protein matrix. The secondary walls found only in mature parenchyma cells are very thick and contain pits that facilitate the diffusion of water during soaking. The middle lamella is composed mainly of pectic substances that serve to hold cells together while giving rigidity and strength to the total tissue. Pectic substances (complex polygalacturonic acid residues that possess various degrees of methyl side groups) actively cross‐link with divalent cations to form cohesive structures that significantly affect the texture of the plant tissues (Gooneratne et al. 1994; Njoroge et al. 2015). This commonly observed mechanism in beans is discussed in Chapter 5.

Lee et al. (1983) produced fractionated navy beans with a 90% cotyledon yield, which was further milled and air classified to yield a fine protein component (32% w/w; containing > 40% protein and 31% starch) and a coarse starch fraction (55% w/w; containing > 67% starch and 16% protein). These results were indicative of the general distribution of protein and starch within the cotyledon portion of the beans.

Fig. 3.6. SEMs showing structural components of dry navy bean: (a) seed cotyledon cells and (b) Starch granules embedded with a protein matrix (CW = cell wall, M = middle lamella, P = protein bodies, S = starch granules, P = protein).

Source: Original images by author, M.A. Uebersax.