Mutagenic breeding of Quambalaria cyanescens strain for the production of milk-clotting enzymes

A strain for producing rennet was screened from red kojic rice and identified as Quambalaria cyanescens. In the current work, various mutagenic treatments, such as UV, diethyl sulfate and their combinations were applied for improving the production of milk-clotting enzymes. The mutagenic conditions were optimized based on the fatality rate of the strain. A significant increasing in the productivity of the fungal rennet produced from the strain Quambalaria cyanescens was achieved and its milk-clotting activity (MCA) was increased from 57 to 117 (SU mL(-1)) through the mutagenic breeding. Further study showed that MCA was greatly inhibited (P<0.0001) by pepstain A, indicating it belongs to an aspartic acid protease, but absence of serine protease, metalloproteinase and cysteine protease. The mutated strain with the highest activity of milk-clotting enzymes showed a stable capacity to produce rennet through the test of its heritability. This is the first report of the breeding study of Quambalaria cyanescens for its capacity to produce microbial rennet.