Properties of a Milk-Clotting Microbial Enzyme

Abstract

The clotting and proteolytic properties of a milk-clotting enzyme (microbial rennet) isolated from a culture of Bacillus cereus were investigated and compared to the corresponding properties of calf rennet. The clotting activity of microbial rennet was found to be less sensitive to pH changes of the substrate than calf rennet. The maximum speed of clotting was obtained at 40–45C and at 75–80C with calf and microbial rennet, respectively. Both calf and microbial rennet showed a high degree of inactivation after heating for three minutes above 55 and 65C, respectively. Specific viscosity measurements indicated the production by both enzymes of casein degradation products of different shape and size. Nitrogen distribution, gel electrophoresis, and agar-gel plates containing casein were used to determine the proteolytic action of the two enzymes. Microbial rennet was more proteolytic than calf rennet and exhibited a specific proteolytic action on κ-casein similar to that obtained with calf rennet. Other casein fractions were degraded continuously and nonspecifically, but β-casein was the fraction most susceptible to hydrolysis.