Two highly protease-producing isolates, identified as Bacillus subtilis M14 and Bacillus coagulans M53, were selected from sixty isolates of bacteria from different environments for extracellular protease production and used in this investigation. Both strains were studied for their protease activity on seven culture media. The two strains gave maximum protease production on Malik and Mathur medium at 30˚C and pH 7.0 for 5 days. The optimum incubation period for protease production was established after 3-4 days by both two strains. Maximum protease activity and specific activity of protease were obtained through acetone precipitation (6 0% saturation). Optimum pH for protease activity of B. subtilisM14 and B. coagulans M53 was found to be 8 and 7, respectively, while optimum temperature for activity of the two proteases were 35 and 30˚C, respectively. Protease from B.subtilisM14 retained 65% of its activity after 30 min at 65˚C, while the enzyme from B. coagulans M53 retained 45% of its activity after 30 min at the same temperature. Proteases of the two strains were added to Domiati cheese milk at a rate of 500, 1000 and 1500 U/kg. Obtained results showed that, acidity, Shilovich index, tyrosine, tryptophane and soluble nitrogen as ripening indices were increased in cheese treated with both two enzymes than control. The quality of cheese greatly improved by increasing protease concentration reaching the highest scores (1500 U/kg milk) at the end of the ripening period. From the repining indices results it can be concluded that, protease from both strains can be used for improving the quality of Domiati cheese. |
