*5.6.1 Natto*

*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.
