In vitro protein digestibility and biochemical characteristics of soaked, boiled and fermented soybeans.

Protein digestibility of soybean obtained from the main manufacturing steps for natto, such as soaking (soaked soybeans 'S'), boiling (boiled soybeans 'B'), and fermentation (fermented soybeans 'F'), was examined in this study. Biochemical indices for the processed soybeans from each manufacturing step and those digested fractions by simulated in vitro gastrointestinal digestion were also evaluated. The result showed a significant (P < 0.05) increase in the protein digestibility of B (48.71 ± 0.04%) and F (50.21 ± 0.45%) compared to that of S (20.58 ± 0.25%), accompanying the accumulation of small protein sub-fractions and essential amino acids. Besides, antioxidant activity indices of all digested fractions increased around two to fourfold at the end of the simulated digestion. F showed a consistently increasing trend when the digestion stage progressed and maximum values overall at the final digestion stage.  Soybeans from fermentation step showed higher protein digestibility and indispensable amino acids as well as potential bioactivities than those from boiling and soaking step. The results demonstrated that manufacturing steps improved nutritional values of soybean protein, such as bioavailability of amino acids and certain bioactivities.