Study On The Screening Of Glucoamylase-producing Strains And Optimization Of Fermentation Medium

Glucoamylase is an exo-acting enzyme, and glucoamylase hydrolyses single glucose units from the non-reducing ends of amylose and amylopectin in a stepwise manner, and produce glucose as the sole end-product from starch and related polymers. Unlike ?-amylase, most glucoamylase are also able to hydrolyze the 1,6-?-linkages at the branching points of amylopectin, although at a lower rate than 1, 4-linkages. The main glucoamylase was isolated from certain varieties of Aspergillus niger and widely applied in starch sugar, monosodium glutamate, citric acid, beer, wine industry.In this paper, a superior strain of A.niger was selected and bred by UV?60Co-radiation and ion beam. And the optimization of medium and fermentation conditions for mutant strain was conducted.The results showed that the mortality was increased with the dosage of UV?60Co-radiation and ion beam. Then such strain of Aspergillus niger (An-1) was used as original strain. And one prominent mutant strain was obtained through UV?60Co-y ray and ion beam combined mutagenesis, which was designated as AnUVyN+7. The crude exzyme activity of the AnUVyN+7 (1100.9 U/mL) exhibited 12.7 times higher as compared to the original An-1 strain (86.1 U/mL). And this high expression of glucoamylase by the mutant was observed to be irrevocable as the mutant showed stable enzyme activity even after seven repeated sub-culturing.Several culture media were tested at different conditions to optimize glucoamylase production with AnUVyN+7. The optimal shake-flask culture turned out to be corn steep liquor 4.5%, bean cake powder 3.5%, and glucose 8.5%, at initial pH 6 with 15% inoculation at 35±0.5 ?. The enzyme secreted by AnUV?N+7 was 20% higher than the initial medium at the end of fermentation.