Читать книгу Dry Beans and Pulses Production, Processing, and Nutrition - Группа авторов - Страница 62

The intact seed coat (botanically termed the testa) has an important function in protecting the legume from damage due to water absorption and microbial contamination. This is particularly important during harvest and storage. The seed coat consists of 7.7% of the total dry weight in the mature bean with a protein content of 5% (dry‐wt basis) (Powrie et al. 1960; Kigel et al. 2015). Slow darkening or nondarkening seed coats are more desired in the market because they are perceived to be fresh, high‐quality beans (Erfatpour et al. 2021).

The major components in the seed coat structure of legumes include the waxy cuticle layer, the palisade cell layer, the hourglass cells, and the thick cell‐walled parenchyma. These structures are presented in Figure 3.5. These structures have been identified using scanning electron microscopy (SEM) in white beans, navy, pinto, and adzuki beans (Sefa‐Dedeh and Stanley 1979b; Swanson et al. 1985), cowpeas (Sefa‐Dedeh and Stanley 1979a), and broad/faba beans (McEwen et al. 1974).

The waxy cuticle layer is the outermost portion of the seed coat. Although the cuticle does allow some polar and nonpolar compounds to permeate the seed coat, its primary function is preventing water penetration due to its hydrophobic layers that form a waxy barrier (Bukovac et al. 1981). The palisade layer has been reported by many researchers to appear with a linear lucida, or light line, that gives the appearance of two layers of cells (Sefa‐Dedeh and Stanley 1979b). The cell layers immediately beneath the palisade layer are termed hourglass cells. Sefa‐Dedeh and Stanley (1979b) described this layer as the amorphous second layer in the palisade layer.

Fig. 3.5. 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.