Abstract: In this study, we screened out a strain with high protease activity and evaluated its effect on soybean meal. The strain, coded as JL8, was an aerobic spore-forming Bacillus isolated from pig feed in Jilin province, which could produce (519.1 ± 4.7) U/g protease. This isolate was identified as Bacillus siamensis by physiological and biochemical tests and 16S rDNA sequence analysis. The optimal fermentation time was determined based on the spore quantity of JL8, and soluble reducing sugar content, total protein content, the ratio of soluble to total protein and the degradation degree of allergenic proteins in soybean meal. Then, the microstructure of soybean meal protein was investigated by scanning electron microscopy (SEM) and X-ray diffraction before and after fermentation. Results showed the optimal fermentation time was 24 h at 37 ℃ and at a water content of 50%. Moreover, the SEM images revealed that fermentation significantly changed the microstructure of soybean meal protein, making it looser. The X-ray diffraction patterns showed that the backbone of soybean meal protein was changed and the content of β-sheet was significantly increased by Bacillus fermentation, which may improve the solubility of soybean meal protein.

Key words: Bacillus, protease, fermented soybean meal, scanning electron microscopy, X-ray diffraction