Study On Selected Breeding Of High-Yielding Strains And Mechanism Of Producing Lactase

Systematic study on the screening of lactase-yielding strains which produces the thermostable β -galactosidase, liquid fermentation of mutant strain UCo-3 and mechanism of producing lactase were carried out in this paper. The main results were as following:1. Induced breeding is an effective method of meliorating fungi. The spores of original strain D_2-26 were treated with the cooperation of UV and γ rays. The procedure on induced breeding and the correlation of lethal rate, radiation dose and positive mutant rate were researched. The optimum condition was 15-min UV and 2000 Gy γ rays.A lactase-yielding mutant strain was obtained, which was treated with 15-min UV and following 2000 Gy γ rays. The test on capability of mutant strain producing lactase was carried out and the result indicated that lactase activity of mutant was about 16.27 U/mL each time through 8 times from generation to generation and its inheritance was stable.After primary observation on the colony, foot cell and conidium of mutant strain UCo-3, it was found that the regularly ring-shaped colony of the mutant strain was somewhat different from that of its parents. The center of back colony had the obvious fold. It was named as A. niger UCo-3.2. The research on mechanism of strain UCo-3 producing lactase was investigated. It was found that pectic had an obvious inductive function on producing lactase. The possible reason was that firstly pectic induced to produce poly methylgalacturonase(PMG) and then PMG hydrolyzed pectic to galacturonic acid and other products. Those products not only provided mutant with energy to grow, but also combined wit- h the inductive part of lactase to induce lactase.3. Optimized culture medium in liquid fermentation were: pectic 0.75%, soybean powder 4%, corn liquor 3%, NH₄NO₃ 0.15%, K₂HPO₄ 1%, NP-5 0.3%. Optimized culture condition were: pH 5.5, 30℃, 140r/min, 30 mL/250mL, 10⁴ spore per mil inoculum quantity. After optimum of liquid fermentation, the maximum enzyme activity of UCo-3 was 23.26 U/mL.