Production of a commercial class of common beans is dependent on the development of adapted cultivars (i.e., commercially cultivated varieties, frequently abbreviated “cv”). This is a dynamic process (see Chapter 2) and must be maintained to ensure well‐adapted cultivars (desired agronomic traits possessing flavorful cooking quality). Typically, a market class will have a number of predominate cultivars that are suitable for growers and processors. These cultivars are comingled upon receipt at the elevator; thus, each must meet minimal market and processing standards. The numerous processing differences among cultivars are well documented (see Chapter 2). A comprehensive list of dry bean cultivars released in the United States during the past six decades identified nearly 300 distinctive named cultivars or plant introductions (Sutton and Coyne 2010). This exceptional array of dry beans, each possessing specially selected traits, is a result of both public sector (university‐based and USDA programs) and private‐sector bean‐breeding programs. Extensive field trials and canning tests are conducted to provide marketers and processors with data to assure uniformity of quality and performance of new cultivar releases. It is important to recognize that any list of bean cultivars is transitory and will be continually updated as emerging needs and developments occur (Adams 1978; Urrea and Valentin‐Cruzado 2020).
The diversity of commercial market classes has increased to meet market and consumer interests, including an array of distinctive bean classes to address specific regional or ethnic needs. These include: (1) Mayocoba, a large‐seed yellow bean; (2) Azuki, a small bright red bean used to produce an paste (a highly consumed starch‐based confection) in Japan; (3) Tebo, a large‐seeded white bean (size between navy and great northern types) used to make an paste in Japan; (4) Soldier bean, a large‐seeded white bean with a red accent on the longitudinal axis transecting the hilum region; (5) Swedish brown, a large solid‐colored brown bean; (6) Flor de Mayo, a small multi‐colored Mexican bean; and (7) Anasazi, noted as the Native American bean of the ancients in the southwestern United States.
Further, there is great interest in the revival of so‐called heirloom beans in the United States. Interest in such exotic and often highly differentiated beans (size, shape, and particularly color) is gaining momentum among gardeners and food hobbyists. Several companies sell heirloom bean seeds in the United States in response to this trend. A selection of heirloom beans include: Jacob’s cattle, appaloosa, runner cannellini, vallarta, tepary, Santa Maria pinquito, ojo de cabra (goat’s eye), flageolet, Christmas lima, black calypso, sangre de toro, and vaquero.
Most commercial market classes of beans are currently available through production under certified organic programs. These beans are produced in relatively limited quantities and command a premium price due to market forces and the need for their identity to be preserved during distribution. It is noted that to date, no genetically modified (GM) beans have been produced in North America. This is partly due to (1) the limitations in genetic transformation technologies, (2) the inherent high costs of transformation research, and (3) a strong interest to maintain open trade channels as nongenetically transformed food. In Brazil, transformation has been reported in pinto beans (cv. Olathe), which enabled incorporation of bean golden yellow mosaic (BGYM) virus resistance (Bonfim et al. 2007).
PHYSIOLOGY OF COMMON BEAN SEED
The physical and chemical properties of dry beans are determining factors associated with subsequent final product quality. The seed structure of the dry bean comprises a seed coat and an embryonic cotyledon (Guilhen et al. 2016; Heshmat et al. 2021). The structural features of the seed tissues (seed coat, cotyledon, embryonic axis) and the cellular and subcellular components (palisade, hourglass, and parenchyma cells; cell walls; middle lamella constituents; and other organelles) greatly influence hydration, cooking, and processing performance of dry beans.
Structural and anatomical features of bean seed
The macro structure of the seed highlighting the primary tissues is presented in Figure 3.4. The seeds of leguminous plants differ greatly in general appearance (color, size, shape) and in several less apparent attributes such as seed coat thickness and permeability of the hilum. However, all possess a similar seed structure comprising a seed coat, cotyledons, and embryonic parts. The seed coat or testa is the outermost layer of the seed and serves as a protector of the embryonic structure. The cotyledons comprise the largest mass of the seed and serve primarily as energy storage (starch and protein) tissue. The embryonic structures are varied in size, relatively high in lipid content and provide the germinating loci for the seed (Moïse et al. 2005; Bassett et al. 2020).
Two external anatomical features include the hilum and micropyle, both of which have a role in water absorption and seed gas exchange. The hilum, commonly referred to as the “stem scar,” is a large oval scar where the seed and stalk were previously joined within the pod (Kigel et al. 2015). The micropyle is a minute opening in the seed coat that served as a junction where the pollen tube entered the valve. The remaining portion of the seed is the embryonic structure that includes two cotyledons, the epicotyl or embryonic stem tip, the hypocotyl or embryonic stem, and the radicle or embryonic root. This portion of the seed is responsible for germination and is extremely vulnerable to damage during adverse handling and storage.
Fig. 3.4. Schematic of a dry bean seed.
Source: Adapted from Georges (1982).
Seed coat
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.