Screening fermentation parameters of the milk-clotting enzyme produced by newly isolated Bacillus amyloliquefaciens D4 from the Tibetan Plateau in China

Abstract

A bacterium producing an extracellular milk-clotting enzyme was isolated from yak grazing soil in the Tianzhu Tibetan autonomous county on the Tibetan Plateau and identified as Bacillus amyloliquefaciens. We used single-factor testing to study the optimum physical conditions and nutritional parameters for production of the milk-clotting enzyme, while the temperature and pH stability of the enzyme were also studied. The optimum conditions for production of the milk-clotting enzyme were: temperature, 37°C; inoculum size, 4% (1.8 × 107 CFU/mL); agitation speed, 170 rpm; and initial pH of the medium, 6.2. The maximum milk-clotting activity and milk-clotting activity per proteolysis activity were found using wheat bran juice at a concentration 18 g/100 mL, 4% (w/v) sucrose and 2% (w/v) skim milk powder. These optimized conditions resulted in a 2.68-fold increase in production of the milk-clotting enzyme. The crude enzyme exhibited good temperature and pH stability; 80% of the milk-clotting activity was retained after incubation at 40°C for 30 min and more than 70% of the activity was retained between pH 5.5 and 7.5. This B. amyloliquefaciens milk-clotting enzyme has potential as a calf rennet substitute.