Factors affecting the growth and production of milk-clotting enzyme by Amylomyces rouxii in rice liquid medium

Amylomyces rouxii is one of the main fungi usually coexisting with yeasts in Chinese yeast ball, the starter of chiu-niang, a traditional Chinese fermented product from rice. In the present study, growth and production of milk-clotting enzyme (MCE) in gelatinous rice liquid culture of A. rouxii as influenced by waxy (gelatinous) rice content in the medium (5-20 %), temperature (25-40 degrees C), cultivation time (1-6 days), shaking speeds (0-150 rpm) and metal ions (Na+, K+, Zn2+, Mg2+, Mn2+, Cu2+, Ca2+, Fe3+ and Al3+) were investigated. Results revealed that rice content in the medium, shaking speed, temperature and cultivation time all affected the mycelial propagation and the production of milk-clotting enzyme by A. rouxii in the rice liquid culture. The maximum milk-clotting enzyme activity of ca. 1.22 unit/mL of medium was observed in the 3-day static culture of test organism grown at 30 degrees C in the medium containing 20 % of gelatinous rice, while mycelial propagation increased with the increase of cultivation time and shaking speed. Furthermore, a significant increase (p < 0.05) in the milk-clotting enzyme activity of ca. 1.90 unit/mL of medium, which was about 1.55-fold of the control, was observed when Al3+ was added to the rice liquid medium.