Abstract
Single ascospores were isolated from 33 top-fermenting yeast strains from British breweries and from 4 baking yeasts which gave adequate performances as brewing strains. Almost every spore of one brewing yeast (N.C.Y.C. 1085) produced a colony, but the average viability of spores from the other 32 brewing strains was under 5%. The spore viability of the baking strains lay between 30 and 60%. Cultures obtained from viable spores (segregant strains) were analysed for their potential usefulness as haploid breeding stock. Excluding strain 1085 which produced only diploid spore cultures, only 25% of viable spores from brewing yeasts gave rise to apparently haploid segregant strains. The corresponding figure for the 4 baking strains was over 50%. The low yield of haploid cultures from brewing yeast spores is thought to be due principally to the polyploid nature of most brewing strains. Examination of the attenuation rates of segregant strains in small fermentations showed that most were inferior or similar to their parent yeasts. About 10% of segregant strains, however, were capable of attenuating wort more rapidly than their parent strains.
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