SEM showing structural components of dry navy bean seed coat: C−Cuticle layer, PAL−Pallisade cells, HG−Hourglass cells, PRC−Parenchyma cells.
Source: Ruengsakulrach (1990).
The parenchyma layer cells have thick walls and are readily distinguishable after hydration, as they appear spongy and exhibit noticeable swelling. Once the parenchyma cells are hydrated, increased rates of water imbibition occur. The seed coat possesses high levels of structural carbohydrates (e.g., cellulose and hemicelluose) that contribute the vast majority of total dietary fiber of beans. Aguilera et al. (1982) reported the production of bean seed coat (hull) flours by cracking and air aspiration of seat coats from navy beans. The mass balance for this separation yielded a 7–13% seed coat fraction. Dietary fiber ranged from 31% to 50% with a mean of 40%. Crude fiber (primarily cellulose and lignin) was approximately 60% of total dietary fiber. The ash content of this flour fraction ranged from 6% to 7%. These data demonstrate the high level of fiber components within the seed coat; variability was likely due to nonuniformity in fractionation because of some contamination of cotyledon residue.
The seed coat affects water absorption; however, the precise mechanism is unknown. Research on soybeans shows that the water absorption rate depends on the calcium content, seed coat surface, micropyle structure, and initial moisture content (Saio 1976; Hsu 1983). In studying the structural components, Sefa‐Dedeh and Stanley (1979a) found seed coat thickness, seed volume, and hilum size, along with protein content, to be the primary factors involved in water uptake. Thinner seed coats appear to absorb water more rapidly during initial soaking (0–6 hours). Siah et al. (2014) investigated the effect of soaking, boiling, and autoclaving (pressure cooking) on the phenolic contents and antioxidant activities of Australian grown broad/faba bean genotypes differing in seed coat color. A significant amount of active compounds was shown to leach into the soaking and cooking medium. Boiling was a better method in retaining active compounds in beans than autoclaving.
Seed coat color varies greatly among legumes due to compositional differences (discussed below). The seed coat must remain intact during storage and handling and must hydrate uniformly to enable swelling of the seed constituents during soaking and cooking procedures. Excessive seed coat rupture or sloughing will result in diminished culinary quality. This is particularly a concern with the sensory detection of tough free skins during maceration and excessive starch leaching during cooking or canning.
Zhong et al. (2018) reviewed biochemical and physicochemical functionalities of seed coats of five globally important pulses: chickpea, field pea, broad/faba bean, lentil, and mung bean. It was reported that high levels of dietary fiber, minerals, and potential health‐promoting phytochemicals in the seed coats indicate their great potential to be used as a natural, nutritious dietary fiber.
Cotyledon
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.
Embryo
The embryo is a relatively small portion of the seed mass (typically < 2%) but has a dramatic influence on seed quality. This biologically active component of mature seeds possesses high levels of enzymatic activity that are readily activated by optimum moisture and temperature conditions. The mobilization and oxidization of lipids high in unsaturated fatty acids produce highly oxidized off‐flavors. High temperatures (> 130°F) that may occur during even mildly adverse storage conditions will result in embryo damage with irreversible loss of seed vitality. It is commonly recognized that precursors of the germination process are initiated during ambient water‐soaking conditions. Thus, traditional overnight cold‐water soaking of beans activates many complex embryonic catabolic enzymes associated with seed spouting (e.g., proteases, amylases, pectinases, cellulases and phytases) and, therefore, enhances the water‐hydrating and water‐holding capacity of the bean.
CHARACTERISTICS OF SEED SIZE AND SHAPE
Commercial classes of dry beans are diverse and distinguished by seed size and shape, and seed coat color. The physical characteristics of dry beans are presented in Table 3.1. Seed size is expressed in various formats, depending on convention and practice. These include: (1) grams per hundred seeds, (2) seed counts per 100 grams, (3) seeds per pound, and (4) standard US sieve size characterization.
Expressing seed size as weight per unit is common among plant breeders and seed specialists;