**5.4 Breeding targets for food grade soybeans**

Breeding for food-grade soybeans with unique seed composition has focused on a specific nutritional trait of the soybean seed. Examples of such varieties are given according to the fraction from which the targeted trait origins. Food-grade soybean that targets a specific trait such as varieties high in total protein content, high in β-conglycinin, low in lipoxygenase, high in specific amino acids such as lysine, methionine, and threonine, and low in allergenic proteins [13]. High-protein soybeans (>43%) are used for tofu, soymilk, soy sauce, beverages, baked goods, pudding, cheese, and meat analogs. The breeding of food-grade soybeans can be classified into three major categories: the breeding of large-seeded soybeans, the breeding of smallseeded soybeans, and the breeding of soybean with unique seed composition [13].

#### **5.5 Large-seeded soybeans**

By targeting specific traits, soybean breeders try to develop soybeans with good yield and quality [5]. Large-seeded soybeans are bred for tofu, soymilk, miso, edamame, and soynuts [13]. An important factor for the breeding of tofu soybeans is the tofu yield, which is defined as the weight of fresh tofu produced from a unit of harvested soybean. Seed size and seed appearance are also of importance for tofu soybeans. Tofu soybeans are larger than 20 g/100-seeds [13]. It is possible to produce good quality tofu with dark hilum beans but this requires prior dehulling of the beans and careful soymilk filtration [5]. In order to avoid these additional processing steps, soybeans with a yellow cotyledon, yellow seed coat, and clear hilum are preferred. Moreover, a thin but strong seed coat that is free from cracking and discoloration is desirable [13]. Soybean seeds with high protein content exceeding 45% on dry matter basis and improved ratio of 11S/7S is desirable for tofu soybeans as this enhances tofu yield and gelling characteristics, respectively [5]. A high protein/oil ratio provides a higher tofu yield and firmer texture; therefore, low oil content is preferred. Moreover, tofu soybeans should have high water uptake, a low calcium content, and a high germination rate. The carbohydrate content and composition influence the taste of tofu and soymilk [13]. High total sugar content (above 8% on dry matter basis) [5], high sucrose, low raffinose, and low stachyose are highly desirable for tofu and soymilk [13]. Examples of tofu and soymilk varieties: Black Kato, Toyopro, Grande, Proto (from Minnesota), Vinton-81, HP 204, IA1007, IA1008 (from Iowa), and Harovinton [13, 47].

**59**

*Food Grade Soybean Breeding, Current Status and Future Directions*

Vegetable soybean varieties should meet certain requirements such as sweeter seeds with thin seed coat and large seed size (>30 g/100-seeds dry weight) [13]. As the pods are eaten directly, genotypes with sparse gray pubescence with green and thin seed coat are preferred [13]. Moreover, edamame cultivars should possess as less as possible of one-seeded pods as they require greater effort to shell by consumers. Those cultivars with genetically "stay green" and delayed yellowing toward maturity make it possible for growers to have extended harvest period closer to maturity. Vegetable-type soybean should possess important nutritional traits such as high content of sugar (sucrose and maltose) and free amino acids to impart sweet and delicious taste. Sucrose is primarily responsible for the sweetness of vegetable soybeans, where sucrose content is preferably higher than 10% on dry matter basis. Certain free amino acids, such as glutamic acids, are major contributors to the taste

*Natto* beans are small-seeded soybeans typically used for the fermented soy foods popular in Japan. For natto, small to ultrasmall soybeans (smaller than 9 g/100-seeds [2] of maximum of 5.5 mm diameter are preferred for better fermentation. The seeds have preferably a near spherical shape as this reduces the ratio of the tough seed coat to softer cotyledon [2]. Also, clear hilum and thin seed coat are desirable traits for natto soybeans. Natto soybeans are nutritionally characterized by a high carbohydrate content [13]. A high content of soluble sugars (>10%) on a dry weight basis results in a softer natto product, an important requirement for natto [5]. The composition of sugars is important for the effectiveness of fermentation [13]. To obtain a steady and controlled fermentation, low sucrose content with high stachyose and raffinose content is favored [5]. Soybean with moderately high protein content is desirable in order to provide amino acids for fermentation. Oil content must be low, i.e., less than 18% of the dry matter as it enhances water absorption [13]. For a softer natto product, seeds must additionally possess high water absorption capacity during soaking, which is the first step of natto manufacturing. Breeders use standard small-seeded lines, such as the cultivar Vance (known for having a medium ability for water uptake), to compare selected lines for water absorption capacity [5]. Soybeans with medium seed size (10–12 g/100-seeds) and a high germination rate are preferred for bean sprouts. High-protein, high-isoflavone, high-sugar, and lipoxygenase-free soybeans are desirable for soybean sprouts [13]. It is reported that the Asian small-seeded lines exhibited high diversity indices

than the U.S. lines for seed hardness, calcium content, and stone seed rate. In addition, the average genetic diversity of the U.S. small-seeded soybeans (1.48) was lower than that of Asian small-seeded soybeans (1.57), suggesting narrower genetic base in the U.S. lines. Seed uniformity, hardness, protein, and calcium content appeared to be relatively high in diversity index for both the U.S. and the Asian large-seeded lines. The U.S. small-seeded soybeans were desirable for natto production because of their softer texture with higher water absorption capacity and lower stone seed ratio. However, the Asian large-seeded soybeans had a lower stone seed ratio and a higher water absorption capacity. Therefore, using the Asian large-seeded genotypes may potentially improve seed quality for tofu and soymilk [48]. Therefore, the Asian soybean gene pool may serve as valuable genetic source

for increasing protein content of the U.S. food-grade soybeans.

*DOI: http://dx.doi.org/10.5772/intechopen.92069*

*5.5.1 Vegetable soybean*

of vegetable soybeans [13].

**5.6 Small-seeded soybeans**

*5.6.1 Natto*
