This study was conducted to obtain the optimal fermentation process to improve the nutritional value of bean dregs (BD) and soybean meal (SBM) using single-factor experiments and response surface methodology with small peptide content as the index. The optimal fermentation process was as follows: Bacillus subtilis was selected for aerobic fermentation for 12 h, Saccharomyces cerevisiae + Lactobacillus plantarum + Lactobacillus rhamnosus was selected for anaerobic fermentation, ratio of BD and SBM = 1:1, 0.65% acid protease +0.65% papain, 15% wheat middlings, 5% inoculum size, 34 °C fermentation temperature, and 11 d fermentation time. Following fermentation, the small peptide content increased from 7.35% to 39.58%. The free amino acid content and the organic acid content increased from 2.34 mg/g to 41.39 mg/g and 28.76 mg/g to 158.61 mg/g, respectively. The added proteases greatly improved the overall protease activity of the fermentation medium, promoted the fermenting microorganism growth, amylase secretion, and reducing sugar metabolism, increased the organic acid content, made the pH lower, and improved the fermentation efficiency and the quality of fermented BD and SBM.
