Enhanced Milk-Clotting Enzyme Production of Mucor Miehei Mutated by UV and LiCl

Milk clotting enzyme is an important enzyme in cheese industry, to obtain an industrial strain with high milk-clotting enzyme activity, Mucor miehei was used as the origin strain and mutagenized by Ultraviolet (UV) and UV combining lithium chloride(LiCl), respectively. The result shows that lower concentrations of LiCl can increase the survival rate of the spores treated with UV, while higher concentration of LiCl can increase the mortality of the spores treated with UV. The highest positive rate (reached 32.17%) was achieved in the compound mutation, which was higher than that of the UV mutagenesis (21.45%). On the basis of multiple mutation, a mutant UV-LiCl-6 was screened. The milk-clotting activity of UV-LiCl-6 reached 2703.58 SU/mL and was 57.70% higher than that of the parent strain. Results of the pass-generation test indicated that the strain had good genetic stability and may be ideal strains for the production of milk-clotting enzyme. Additionally, it was demonstrated that compound mutagenesis with UV and LiCl is an effective mutation method for the breeding of enzyme-producing strains.