| Bacillus firmus strain PC024, isolated from Chinese shrimp (Fenneropenaeus chinensis) rearing environment, has demonstrated its ability to increase shrimp resistance to white spot syndrome virus (WSSV) infection and to ferment dietary soybean meal. The purpose of this current study is to optimize the processing conditions for the fermentation of soybean meal using B. firmus. Considering single factor trials, results indicated that the optimum bacterial concentration for inoculation, water ratio, fermentation time and temperature were 2×106 CFU/g, 1:0.8, 90 h, and 37℃, respectively. However, on the basis of univariate tests, the response surface methodology (RSM) was applied to optimize four fermentation factors when they were combined, including temperature, time, water ratio, and amount of bacteria required for inoculation. The optimum fermentation conditions were as follow: fermentation temperature, 39.0℃; fermentation time, 100 h 18 min; water ratio, 1:0.96; and amount of bacteria for inoculation, 3.84×106 CFU/g. Under the optimized conditions, the bacterial concentration reached 1.23×1010 CFU/g after fermentation and it was 5.13% deviated from the predicted value using the RSM model. The quality of soybean meal improved significantly after fermentation. Firstly, the yield of the fermented soybean meal reached (93.89±0.01)%. Secondly, after fermentation, the soluble protein content in the soybean meal increased from (39.16±0.01)% to (58.80±4.54)%, and crude protein content in the soybean meal increased from 50.71% to 55.03%. Finally, the total amount of amino acids (15 types) had 132.30% increase compared to the original content; the most significant increases in the proportion amino acids were arginine (168.60%), lysine (157.20%), serine (152.50%), threonine (139.04%) and glycine (138.40%). It showed that the protein molecules were effectively degraded in SDS-PAGE. This study provides theoretical support for the application of probiotics in shrimp plant-basis diet and disease control in aquatic animals. |
