Abstract
Soybean (Glycine max L. Merr.) is a versatile food source with protein, polyunsaturated fats,
fiber, vitamins, minerals and other essential nutrients for humans. The purpose of this study
is to compare the protein digestibility among raw, cooked and fermented soybeans (Natto)
inoculated with Bacillus subtilis var. natto using simulated in vitro gastrointestinal digestion
technique (IVD). Amino acid composition and antioxidant activity change during IVD of Natto
were also investigated. It was found that trichloroacetic (TCA) soluble peptide yield increased
around 46 and 62% in Natto and cooked soybean after IVD, respectively. The soluble protein
fractions of cooked soybean and Natto during IVD were digested from large protein fraction
into smaller protein sub-fraction observed by sodium dodecyl sulfate-polyacrylamide gel
electrophoresis (SDS-PAGE). After IVD, all essential amino acid especially Arg was found to
remarkable increase for 6-fold and 49-fold in cooked soybean and Natto, respectively (P<0.05).
Interestingly, antioxidant properties increased when IVD times were increased (P<0.05).
The increment of antioxidant activities was 2-4 fold changing between before and after IVD.
This can be supported by an increment in antioxidant amino acid composition (Trp, Tyr, Met,
Cys, His, Phe and Pro) in IVD digested fractions especially in Natto. The highest antioxidant
activity was found in fraction after IVD of Natto. These results indicated that Natto had better
digestibility and antioxidant activity compared to raw and cooked soybeans.
