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).
SEED COAT PIGMENTATION AND COLOR
Commercial classes are also characterized by their seed coat color. Seed coat color is defined by the pigmentation underlying the testa and may be distributed throughout the testa as a solid or mottled pattern. The appearance will range from glossy (shiny) to a matte (dull) finish. The glossy finish is associated with the presence of a lipid layer on the surface of the testa. The P locus is known as the ground factor for all seed coat color genotypes (Bassett 2007).
Pigments found within the seed coat are typically phenolic compounds (e.g., phenolic acids, condensed polyphenols, or tannins and flavonoids, particularly anthocyanins) that impart a distinctive color and are reactive to soak water chemistry and pH (Singh et al. 2017). The general descriptive classification of phenolic compounds is presented in Figure 3.7. This broad class of phenolic compounds influences the reactivity of various cross‐linking reactions between proteins and thus is associated with decreased water transmission rates and water‐holding capacity of the seed. These polyphenolic compounds, mainly tannins found in the seed coat, are present in black, red, and brown beans and produce desirable agronomic effects (abiotic stress tolerance and the prevention of in‐pod sprouting). Further, these compounds are potentially associated with a wide range of plant biochemical and metabolic functions including (1) resistance to disease, (2) wound‐healing response, and (3) insect and bird resistance.
However, the polyphenolic compounds (tannins) found in bean seed coats adversely impact nutritional bioavailability (Salunkhe et al. 1990; Hart et al. 2019; Rousseau et al. 2020). These compounds are reactive and will bind with soluble proteins and reduce their bioavailability and interfere with protein digestibility (Elias et al. 1979; Aw and Swanson 1985). Seed coat tannins leached into the soak and cook water are particularly important in subsistence feeding programs because of the detrimental effect they have on overall nutrient bioavailability. This negative impact is dramatic if the tannin‐rich bean broth is used as a component of weaning foods.
Fig. 3.7. General classification of phenolic compounds.
Source: Adapted from Luthria and Pastor‐Corrales (2006).
In recent years, interest in the antioxidant capacity of plant foods and specifically the phenolic content of dry beans (Wu et al. 2004; Xu and Chang 2009; Giusti et al. 2019) has intensified. There is increasing evidence that flavonoids consumed in native foods convey health benefits in human diets through their antioxidant activity (Frankel et al. 1993; Hertog et al. 1993). Condensed and hydrolyzable tannins of high molecular weight also have been shown to be effective antioxidants with even greater activity than simple phenolics (e.g., flavonoid monomers) (Hagerman et al. 1998). Although exceptionally high levels of antioxidant capacity have been reported in raw dry beans (Wu et al. 2004), further work on the retention of these properties in prepared (cooked or canned) beans is warranted.
The genetics of black bean pigmentation has been studied by various researchers and summarized by Hosfield (2001). Feenstra (1960) isolated 18 different compounds from 12 experimental lines. These pigment compounds were identified as anthocyanins, flavonol glycosides, and leucoanthocyanidins. The secondary plant metabolites known as flavonoids are water‐soluble phenolic compounds that possess the basic structural C15 skeleton of flavones. Beninger et al. (1998) reported that the pigments responsible for the wide variation in the color of bean seed coats are flavonoids.
Water‐soluble pigments are readily leached from the seed coat during hydration, blanching, and thermal processing. Retention of pigment is generally a desirable characteristic that imparts attractive appearance and appeal. Black beans are particularly susceptible to pigment leaching