Abstract:Ten strains，which could produce protease and had obvious hydrolysis circles on defatted milk powder plate，were isolated from Kefir Grians. Three strains KX-1, Kx-3, Kx-7, which ratio of transparent circle diameter to colony diameter was higher than 2.00 on defatted milk plate，were used to be isolated again through time-enzyme activity curve. The strain Kx-7 had the strongest protease activity (reached 146.43 U/mL) when fermented for 30 h. Single factor experiments were used to optimize the medium at first followed with the most important factors selection with Plackett-Burnan (PB) design. The results revealed that three important factors (fructose, casein and Triton X-100) had significant effects on protease production from Kx-7.
