Chiang Mai Journal of Science

 Optimization of silk degumming protease production from isolated Bacillus sp. C4 SS-2013 was performed by batch culture in a stirred tank bioreactor. Central composite design (CCD) was applied to optimize aeration and agitation rates and predicted by response surface methodology (RSM). The maximum protease production (P) of 1,890 U/mL and a specific production rate (q_p) of 21,412 U/g/h were obtained at the agitation and aeration rates of 400 rpm and 2 vvm which corresponded to a K_La value of 182.16 h^-1. These experimental values were coincident with the predicted values and the models were proved to be adequate with the coefficient of determination (R²) of P, q_p, and K_La equal to 0.950, 0.931 and 0.881, respectively. The pH-stat fed-batch culture could increase the silk degumming protease production to 4,437 U/mL (2.35-fold) and q_p to 36,820 U/g/h (1.72-fold). The addition of 10 mM CaCl₂ into the crude protease and storage at 4^OC for 8 weeks could maintain its activity at about 52 % of the initial activity. Both the agitation and aeration rates significantly affected the protease production, specific protease production rate and K_La. The improvement of production and stabilization during storage of silk degumming protease were achieved in our study which has potential for industrial applications.