A study was made of the sporulating behaviour of selected strains of Saccharomyces cerevislae and Saccharomyces carlsbergensia and the fermentation properties of these 'parent' yeasts and the strains derived from them by hybridisation of mating isolates. The brewing strains, in general, sporulated less readily than the baking, distilling and other strains. The 'lager' yeasts, strains of Sacch. carlsbergensis, had poorer overall sporulating ability than the 'ale' yeasts which were all strains of Sacch. cerevisiae. Most of the strains of Sacch. cerevisiae were shown to be heterozygous for mating type, and the proportions of strains of each mating type produced revealed that none of these brewing yeasts was simply diploid, but probably either triploid or aneuploid. Two of the strains of Sacch. carlsbergensis were apparently homozygous for mating type genes, and these strains were capable of hybridisation with strongly fertile mating strains derived from other yeasts. The hybrid strains, which had been produced from compatible crosses between appropriate mating strains did not all have properties intermediate between those of their parent yeasts. Examination of the inheritance patterns among hybrid strains for their fermentation characteristics, namely flocculence, head-forming ability, duration of lag phase of growth, and attenuative ability, indicated that all of these properties are probably controlled by multiple gene systems, some of which may act in an additive way. In some of the crosses, the genes conferring flocculence exhibit classical dominance but, among hybrids derived from one brewing strain of Sacch. cerevisiae, non-flocculence was shown to be the dominant trait. More detailed respirometric experiments, using selected hybrids as well as their parent yeasts and component mating strains, demonstrated that polymeric gene systems control the fermentation of the saccharides present in brewers' ale wort, and that the rate at which a yeast can ferment those sugars is related to its protein content, phase of growth and the composition of the wort. Analytical results for beers produced by selected hybrid strains and control yeasts, in production-scale trials, revealed differences in total fusel oil contents, but these did not correlate with orgranoleptic evaluations.
|Date of Award||1974|