Abstract:[Objective][Methods] Box-Behnken design combing with response surface modeling was used to study the relationships between cell growth of a moderate thermophilic Bacillus subtilis BY25 and its thermostable protease yield under different carbon (glucose) and/or nitrogen (soybean waste powder) sources as well as pH values. [Results] The higher values of the coefficients of determination, lack-of-fit<4 and p value<0.05 were in good agreement with the optimum combination of process parameters, which proved the fitness of the selected model for analyzing the experimental data. Data revealed that both low and medium nitrogen applications greatly contributed to the growth of Bacillus subtilis, which however inhibited the yield of thermostable protease; under high nitrogen application both bacterial growth and protease yield were stimulated. Contour plots displayed the maximal enzyme yield was accompanied with 47.3% and 61.4% maximal biomass under low and medium nitrogen applications, but with 96.3% maximal biomass under high nitrogen application. [Conclusion] The higher nitrogen concentration could increase the specific enzyme productivity, and soybean product waste could contain a better inducer for protease synthesis in Bacillus subtilis.
