Compound mutation of Aspergillus niger J1-1 strain with high milk-clotting activity by ultrasonic and UV and the optimization of fermentation conditons

Aspergillus niger J1-1was used as parent strain to be mutated by ultrasonic and UV treatment.Preliminary screening was based on size of milk-clotting zones and the secondary screening was done on solid fermentation medium.A mutant strain J1-3 with good genetic stability and high milk-clotting activity was screened.The enzyme activity of the mutant strain reached 600U/ml in solid state fermentation,which was 57% higher than that of the parent strain.A decrease of 7.1% in the enzyme yield of the mutant was observed after eight times of subculture.Furthermore,the fermentation conditions of the mutated strain were optimized by single factor design and orthogonal design.The optimal fermentation conditions were as followed: fermentation temperature 2℃,fermentation time 6d,water 20mL and added sample 4.Under these conditions,the enzyme activity was 828U/mL,which was 38% higher than that before optimization.