Effects of Lactobacillus plantarum, Bacillus subtilis, and a compound protease on the nutritional value and anti-nutritional factors of fermented soybean meal

This study evaluated the effects of Lactobacillus plantarum , Bacillus subtilis , and a compound protease on the nutritional value, protein fractions, amino acid composition, anti-nutritional factors, and small intestinal degradability of soybean meal (SBM) during anaerobic fermentation. SBM was fermented for 7 days at 40% moisture under four treatments: no additive (CON), microbial inoculation (ML; Lactobacillus plantarum (4.5 × 10 7 CFU/g) + Bacillus subtilis (4.5 × 10 7 CFU/g)), protease addition (LE; 1% fresh weight), and combined microbial–protease treatment (MLLE; Lactobacillus plantarum (4.5 × 10 7 CFU/g) + Bacillus subtilis (4.5 × 10 7 CFU/g) + protease (1% fresh weight)). During fermentation, crude protein (CP) increased while neutral detergent fiber (NDF) and acid detergent fiber (ADF) decreased, with MLLE showing the greatest reduction. ML, LE, and MLLE significantly increased the non-protein nitrogen (PA) and rapidly degradable protein (PB1) fractions, with the greatest increases observed in LE, while LE and MLLE markedly reduced proteins >42 kDa. Amino acid dynamics differed among treatments: ML showed a ‘rise–fall’ pattern; in LE, amino acid levels increased steadily and stabilized by day 5, whereas in MLLE, they remained relatively stable. LE effectively reduced proteinaceous anti-nutritional factors (glycinin, β-conglycinin), ML decreased oligosaccharides (raffinose, stachyose), and MLLE reduced all types of anti-nutritional factors. Small intestinal degradability was significantly improved by all treatments, with MLLE achieving the greatest improvement. Improvements plateaued by day 3. In conclusion, anaerobic fermentation of SBM with Lactobacillus plantarum , Bacillus subtilis , and compound protease is an effective approach to enhance protein quality, reduce anti-nutritional factors, and improve intestinal degradability. Considering efficiency and cost-effectiveness, a fermentation duration of 3 days appears optimal.