Ultrasonic modification on fermentation characteristics of Bacillus varieties: Impact on protease activity, peptide content and its correlation coefficient

This study aimed to investigate the regulative effects of low-intensity ultrasound on the biomass, protease activity, and peptide content of eight Bacillus strains upon casein as substrate. Protease expression types (constitutive and inducible) were evaluated among eight Bacillus strains based on casein and its hydrolysates media. During ultrasound-assisted fermentation (UAF) of casein with Bacillus varieties, B. amyloliquefaciens 20029 displayed the highest increment in protease activity (37.33%) and peptide content (21.63%). Under the optimal ultrasonic parameters (frequency of 40 kHz, power density of 30 W/L and duration of 15 min after cultivation for 4 h), the protease activity and peptide content from casein fermented with B. amyloliquefaciens 20029 increased by 50.00% and 45.00%, respectively, compared with the control. According to Person's correlation analysis, peptide content had a highly negative correlation (r = -0.77) with protease activity based on eight Bacillus strains, and the r value became -0.75 after extra-adding B. amyloliquefaciens 20029 based on UAF. The practical fermentation of soybean meal and corn gluten meal with B. amyloliquefaciens 20029 further confirmed that ultrasound effectively improved peptide contents by its inducible protease expression. This finding provided a vital reference for screening fermentation strains and substrate proteins during UAF.

Automated summary

This study investigated the effects of low-intensity ultrasound on the fermentation of casein by eight Bacillus strains. The results showed that ultrasonic-assisted fermentation increased protease activity and peptide content, with B. amyloliquefaciens 20029 displaying the highest increments. Ultrasound was found to enhance peptide content by inducing protease expression, providing valuable insights for selecting fermentation strains and substrate proteins in ultrasound-assisted fermentation.