Accelerated Proteolysis of Soy Proteins During Fermentation of Thua-Nao Inoculated With Bacillus Subtilis

Thua-nao is a traditional Thai-fermented soy product. It was prepared by the conventional method and also with starter inoculation. Inoculated soybean exhibited a higher rate of fermentation than did the natural fermentation as indicated by greater rate of pH increase, higher extent of proteolysis, ammonia-nitrogen content and nitrogen solubility. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis patterns indicated the extensive degradation of soy proteins during fermentation. Activity staining of Thua-nao extract suggested that proteolysis of soybean was mediated by various proteinases. Active proteinases with estimated molecular weight (MW) of 29,000, 27,000 and 19,000 Da were present during natural fermentation of soybeans. However, proteinases with MW of 40,000 and 29,000 Da were predominant in inoculated soybeans and like the activity bands observed with spent culture broth of Bacillus subtilis BIOTEC 7123. The results suggested that proteinases released by the dominant species in the inoculum, especially B. subtilis, play an important role in proteolysis of soy proteins during fermentation. Inoculation with B. subtilis resulted in an increased proteolysis, which is useful for subsequent reactions leading to development of Thua-nao characteristics.