Tofu is produced by mixing condensed soymilk with a coagulant. It has a low calorie and high protein and contains various important vitamins and minerals; therefore, consumption of tofu as a healthy and dietetic food has increased worldwide. For systematic application in breeding programs to increase tofu yield, evaluation of tofu characteristics is essential. To test the various characteristics of tofu, typically, a considerable amount of seeds (approximately 100 g, is required. Here, we propose a simple tofu quality test method requiring a small amount of seeds (15 g), which is 75% less than the amount needed for conventional testing. Tofu was manufactured using 20 soybean varieties with a conventional method (i.e., a tofu making machine, TM) and four simple methods: natural filtering (NF), weight pressure (WP), hand pressure (HP), and rotation machine (RM). The correlation between the five methods was assessed with respect to each tofu characteristic, and the HP and RM methods were statistically similar to the conventional methods, whereas the other methods showed differences. Regarding tofu yield, the HP and RM methods showed significantly higher correlation coefficients of 0.79 and 0.78, respectively, and the coefficient of variation between replications was also low. Therefore, the HP and RM methods were the best to easily characterize the tofu yield using a small amount of seeds. Considering the efficiency of the test, the RM method appears to be more effective than the HP method for testing multiple lines. These results will be used for the identification of quantitative trait locus/genes related to tofu quality and marker-assisted selection of breeding new soybean varieties.

In Korea, black soybeans are traditionally consumed after cooking with rice to supplement protein and oil which are lacking in rice. Seed cooking quality including seed traits after cooking with rice, which is important for consumers, were so far not comprehensively considered during the process of breeding. In this study, we first evaluated seed quality after cooking with rice, we tested the correlation between seed cooking quality and seed water absorption ratio, and we attempted to identify the Quantitative trait locus(QTL)/gene using two recombinant inbred line (RIL) populations, i.e., Daepung × Socheong2 and Daepung × Ilpumgeomjeong. Based on phenotype and correlation analyses, the main factors affecting the hardness of soybeans cooked with rice may differ between RIL population. In the Daepung × Socheong2 RIL population, one QTL associated with seed hardness after cooking with rice was identified on chromosome 11, and Glyma.11g049600, encoding peroxidase, is proposed as a candidate gene. In the Daepung × Ilpumgeomjeong RIL population, two QTLs associated with seed hardness after cooking with rice were identified on chromosomes 7 and 19, one QTL related to seed water absorption on chromosome 3, and Glyma.19g092600 encoding pectin methylesterase inhibitor are proposed as candidate genes. This is the first study on soybean cooking quality after cooking with rice, and the locations of four related QTLs were identified. The results will be of use for future development of high-quality black soybean varieties with high consumer preference using molecular breeding methods.