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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; desired seed size is defined by the commercial market class. Thus, grams per 100 seed count will vary from 48–56 g/100 seed for the large‐seeded kidney bean to 15–16 g/100 seed for the small white classes. Navy bean seeds weighing 17–19 g/100 seeds are common. Alternatively, black beans are generally selected to a size of about 500–550 seeds/100 g.

Expressing seed size as the number of seeds per pound is common within the agricultural sector of the dry bean industry and is typically used to establish seed planting rates required for desired plant densities (plant population per acre). The approximate number of seeds per pound for specific dry bean market classes is provided in Table 3.2.

Table 3.2. Seed size of selected commercial market classes of dry beans (expressed as seed count per pound).

Source: Smoliak et al. (1990), Kandel and Endre (2019).

Commercial market class	Seeds/Pound
Black	2,100–2,500
Cranberry	900–1,000
White kidney	825–1,280
Great northern	1,300–1,600
Light/dark red kidney	800–1,000
Navy	2,200–2,400
Pink	1,300–1,600
Pinto	1,200–1,600
Small red	1,300–2,000
Yellow	1,000–1750

Seed size is influenced by growing conditions, seed maturity, and position within the plant canopy and the individual pod. Variation in seed size has been demonstrated under differential abiotic conditions (low moisture and high temperatures). Exceptionally small seed size diminishes processing performance (e.g., decreased hydration capacity and minimal swelling). Therefore, seeds are commercially screened to remove smaller seed and to increase the homogeneity of the lot. Selectively screening by size increases the hydration yield performance of a bean lot and enhances the consistency of the finished product.

US standard sieves are used to screen beans for grade standard specifications. The United States Department of Agriculture (USDA) standards for dry beans (USDA 2017a) state that “well‐screened” beans “shall mean that the beans are uniform in size and are practically free from such small, shrivelled, underdeveloped beans, splits, broken beans, large beans, and foreign material that can be removed readily…through use of sieves.” Sieves are to be constructed of 0.319‐inch‐thick metal with perforated round holes. Sizes are specified as a 30/64 sieve (0.4687 inch on a 11/64‐inch center); a 28/64 sieve (0.4375 inch on 19/32‐inch center); and a 24/64 sieve (0.0319 inch on a 17/32‐inch center). All rows of perforations are to be staggered. The screening is conducted in commercial operations (see Chapter 4), and these sieve specifications are used during USDA grading procedures to the assess sample uniformity for size.

Commercial market classes have characteristic seed shapes that range from spherical to elongated (e.g., navy beans are generally characterized by a small round seed whereas kidney beans have elongated seed that resemble the human kidney). Similar to seed size, seed shape is under genetic control; however, deviations may occur due to stressed growing conditions (i.e., water availability and temperature profiles throughout the growing season).