Improvement of nutritive value of soybean meal by microbial hydrolysis with Bacillus subtilis Hs-2 for use as raw material in Nile Tilapia (Oreochromis niloticus) diet

Soybean meal (SBM) is the popular feed ingredient in fish diets due to its good quality protein. However, the SBM has a low content of some amino acids and contains several anti-nutritional factors (ANFs) that interfere with digestion, absorption, and utilization of nutrients in the diet. This study aimed to investigate the appropriate processing conditions for improving the nutritional quality of SBM using microbial hydrolysis with Bacillus subtilis Hs-2 under optimal temperature at 37 °C at different fermentation periods of 24, 36, and 48 h compared to non-fermented SBM as a control. The results showed that Bacillus subtilis Hs-2 could grow exponentially during 0–24 h. After incubation with various fermentation periods, no significant difference (P > 0.05) existed between the SBM hydrolysates at 24, 36, and 48 h. Moreover, crude protein contents increased by 6.43%, 6.86% and 7.66%, respectively, while protease inhibitor activity decreased by 8.40%, 10.66%, and 11.68%, respectively, and protein digestibility increased by 35.36%, 43.29% and 44.82% respectively. The highest total amino acid was found in SBM hydrolysate at 24 h (P < 0.05), whereas the fiber content decreased to the lowest in SBM hydrolysate at 48 h (P < 0.05). The phytic acid content and degree of hydrolysis exhibited no significant difference among all experimental groups (P > 0.05), ranging from 2.20–2.33% and 6.21–6.67%, respectively. Some metabolites such as isoleucine, tryptophan, oxaloacetic acid, succinate, acetylcholine, and cellobiose were increased in SBM hydrolysate at 24 h. For SBM hydrolysate at 24 h, the results in the release of small peptides may act as antimicrobial against the following pathogenic microorganisms: Escherichia coli, Acinetobacter baumannii, Salmonella spp., Vibrio parahaemolyticus, and others. Overall, the results of this study indicated that soybean meal fermented with Bacillus subtilis Hs-2 has excellent potential to improve the nutritional value and could usefully be a fish meal replacement in the Nile Tilapia diet.