| Brewer's yeast is the key factor that guarantes the quality of beer.So the selection of the fine strains is significant.The sulfometuron methyl(SM)-resistant brewer's yeast spontaneous mutants were isolated on the the plate agar containing SM. Saccharomyces aerevisiae strains were mutated by argon-laser,ethylmethane sulfonate(EMS) and nitrous acid,respectively.The mutants were separated on the plate agar containing diacetyl 175 mg·L-1 or 0.01%2-deoxigen glucose(2-dG.) The fine strains with high fermentation degree,suitable content of total higher alcohols, low content of diacetyl and acetaldehyde in the fermented liquid,as well as stability heredity were screened.The activities of invertases and diacetyl reductases of fine mutants and their parent strains were determined.The mutants and their parent strains were analyzed by Randomly Amplified Polymorphic DNA(RAPD).Sulfometuron methyl(SM)-resistant spontaneous mutants were isolated from the brewer's yeast strains kept in our laboratory and several fine strains were obtained after screening.The fermentation of SM-resistant mutants ZT14-9-1 and ZT21-9-2 in 500L fermentor suggested that their fermentation characteristics were according with the demands of the industrial production.The sensory evaluation of the beer produced by them was better than that of producing strain.Compared the DNA sequences of the ILV2 gene of SM-resistant mutants ZT21-9-2,ZT14-9-1 and their parent strains,it was found that there were seven and nine bases mutations respectively and the common characters of mutation of three guanines(G),two thymines(T),and one cytosine(C) in the ILV2 gene of ZT21-9-2 and ZT14-9-1.Anti-glucose repression mutants AG14-1-7 with great fermentation characteristics was screened after argon-laser mutagenesis.The fermentation of mutants AG14-1-7 in 500L fermentor for 11 days suggested that the fermentation degree of mutant AG14-1-7 was 69.6%,the contents of diacetyl,acetaldehyde,total high alcohols and acetate in the fermented liquid were respectively 0.0230 mg·L-1, 1.37 mg·L-1,63.28 mg·L-1 and 84.5 mg·L-1.The beer produced by strain AG14-1-7 was of the property ofβ-hops oil fragrant smell and soft mouth feel. Several fine mutant strains were obtained after mutating Saccharomyces aerevisiae strains BX24 and JW1-3 by EMS.The fermentation of diacetyl -resistant fine mutants ED24-4-2 in 500L fermentor for 11 days suggested that the fermentation degree of mutant ED24-4-2 was 69.7%,the contents of diacetyl,acetaldehyde,total high alcohols and acetate in the fermented liquid were respectively 0.0290 mg·L-1, 3.13 mg·L-1,72.93 mg·L-1 and 87.75 mg·L-1.The sensory evaluation of the beer produced by the mutant ED24-4-2 was nice.The mutant ED24-4-2 is of potentially applicability to beer brewing.The strain JY2-2 was mutated by nitrous acid.Several fine mutant strains were obtained after screening and the fermentation character of anti-glucose repression mutant NG22-5 was the best among the mutants.The fermentation of mutants NG22-5 in 500mL flask with 300mL 11.5°Bx wort at 12℃for 8 days showed that the fermentation degree was 67.3%,the content of diacetyl,acetaldehyde and total higher alcohol in the fermented liquid were 0.0579 mg·L-15.61 mg·L-1 and 76.66 mg·L-1, 17.40%,14.48%and 15.17%lower than that of the original strain JY2-2 respectively. In order to inspect further the characters of beer brewing of mutant strain NG22-5,the test expanding fermented scale will be performed.The analysis results of activities of invertases and diacetyl reductases of fine mutants indicated that the enzymatic activities of the mutants were stronger than that of their parent strains.The genetic similarity between strain JWN1-4 and AG14-1-7, BX24 and ED24-4-2,JY2-2 and NG22-5 analyzed by RAPD were respectively 0.777, 0.793,0.769,it was proved that the there were chosely genetic relationships between mutant strains and their parent strains,but also remarkable differences at DNA level.
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