Study On The Methods For Screening Of Fine Brewer's Yeast Strain

The brewer's yeast strain is the base and key of the beer brewage production, so there is important sense to screen excellent beer yeast strains. The major methods for the improvement of beer yeast are mutation breeding, hybridization, protoplast fusion and genetic engineering. In the study, physical and chemical mutagenesis, protoplast fusion technique were used to screen fine brewer's yeast strain.Saccharomyces cerevisiae XB05 was mutated by semiconductor laser and nitrous acid, respectively. The mono-colonies growing on wort plate agar containing diacetyl 135μg/mL were isolated. Six fine strains JW1-1, JW1-2, JW1-3, NW7-41, NW7-42 and NW7-45 were obtained after selection. The results of tests carried out in the 500L and 120t fermentation tanks indicated that the mutant JW1-3 was of strong ability of diacetyl removal. The contents of diacetyl and total higher alcohols in the fermented liquid were relatively low and these characteristics were stable. The sensory evaluation of the beer produced by the mutant JW1-3 was very nice.The strain JW1-3-18 was mutated by nitrous acid. The mono-colonies growing on wort plate agar containing diacetyl 150μg/mL or 165μg/mL were isolated. Four fine strains JWN1-3, JWN1-4, JWN1-6 and JWN1-7 were obtained after selection. Under the conditions of 500L fermentor with 400L 11°Bx wort at 12℃for 10 days, the fermented degree of mutant JWN1-6 was 69.8%, the content of diacetyl, acetaldehyde,total higher alcohols and total esters in the fermented liquid were 0.0254mg/L, 3.03mg/L ,76.82mg/L and 12.98 mg/L, respectively. The sensory evaluation of the beer produced by the mutant JWN1-6 was harmony.Anti-sulfulmeturon-methy mutants which were induced by semiconductor laser or spontaneous mutation were isolated. The relatively fine strain JMW120-6 was obtained after screening.The protoplast fusants were constructed with the protoplast of NW7-45 and heat-inactivated JMW120-6. Three fine strains R9-1,R3-4 and R9-23 were obtained after selection. The protoplast fusants were constructed with the protoplast of UV-inactivated JW1-1 and heat-inactivated NW7-45. Two fine strains DR9-2 and DR9-24 were obtained after selection. Under the conditions of 500L fermentor with 400L 11°Bx wort at 12℃for 11 days, the fermented degree of strain DR9-2 was 69.5%, the content of diacetyl, acetaldehyde,total higher alcohols and total esters in the fermented liquid were 0.0124mg/L, 7.70mg/L, 59.01mg/L and 25.29 mg/L, respectively. The sensory evaluation of the beer produced by the strains DR9-2 was relatively harmony.The cell volume, biomass and content of strains R9-1,R3-4,R9-23,DR9-2 and R9-24 were measured in comparison with their parental strains and the results suggested that strains R9-1,R3-4,R9-23,DR9-2 and R9-24 were fusants.