Physicochemical Properties of Roasted Soybean Flour Bioconverted by Solid-State Fermentation Using Bacillus subtilis and Lactobacillus plantarum

To produce novel cheese-like fermented soybean, the solid-state fermentation of roasted soybean flour (RSF) was performed using 1.0% inoculum Bacillus subtilis HA and Lactobacillus plantarum, with the initial 60% substrate moisture for 10 hr at $42^{\circ}$, resulting in pH 6.5, 0.82% acidity, 3.5% mucilage, 14.3 unit/g protease activity, 7.6 unit/g fibrinolytic activity, 216 mg% tyrosine content and $1.7{\times}10^{10}$ CFU/g of viable cell counts. After the second lactic acid fermentation with 10~30% skim milk powder, the fermented RSF resulted in an increase in acidity with 1.64~1.99%, tyrosine content with 246~308 mg% and protease activity in the range of 5.2~17.5 unit/g and 0.966 water activity. Viable cell counts as probiotics indicated $1.6{\times}10^8$ CFU/g of B. subtilis and $7.3{\times}10^{10}$ CFU/g of L. plantarum. The firmness of the first fermented RSF with 2,491 $g{\cdot}{\o}mm^{-1}$ greatly decreased to 1,533 $g{\cdot}{\o}mm^{-1}$ in the second fermented RSF, although firmness was slightly increased by adding a higher content of skim milk. The consistency of the second fermented RSF also decreased greatly from 55,640 to 3,264~ 3,998 in the presence of 10~30% skim milk. The effective hydrolysis of soy protein and skim milk protein in the fermented RSF was confirmed. Thus, the second fermented RSF with a sour taste and flavor showed similar textural properties to commercial soft cheese.