A study of the antigenic structure of brewing yeasts together with some observations on the induction and properties of petite mutants.

Principal components analysis, using the reported properties of 250 brewing strains of Saccharomyces cerevisiae held at the National Collection of Yeast Cultures, differentiated five groups of strains, each group consisting of two sub-groups. An antiserum was prepared against one strain from each group: these were then used to carry out micro-immunoelectrophoresis of 43 strains, chosen to be representative of the groups. 19 antigens were recognised. Multi-variate analysis methods (principal components and cluster analysis) were then used to determine whether any associations could be observed between antigenic structure and the brewing properties of the strains. Associations were demonstrated between the antigens designated 6 and 7 and the brewing property of head formation, between antigens 4A, 5 and 10 and deposit formation, between antigen 8 and rate of attenuation and between antigens 5 and 7 and clarification with finings. The validity of these findings was confirmed by antigenic analysis of variant strains differing in brewing properties from the parent strain: in these variant strains a change in a particular brewing property was accompanied by corresponding changes in antigenic structure. It became apparent that variation and mutation were more prevalent in brewing yeast than had been realised; the incidence and effects of mutation to respiratory deficiency (RD) were examined as a particular case of this. In the production of brewers wort on laboratory, pilot and production scale formaldehyde was added to the mash. During subsequent fermentations it was shown that in the case of ale yeast A (AYA) the rate of production of RD mutants from the parent strain was dependent on the initial formaldehyde concentration; formaldehyde added at the beginning of the mashing programme caused the greatest degree of mutation. After mashing, levels of free formaldehyde were low (0.1-0.5 ppm), suggesting the formation of a mutagenic reaction product: this was apparently a heat stable dialysable nitrogen compound of less than 10,000 molecular weight. RD mutants were more sedimentary than the parent and some had faster fermentation times. A further ale yeast (AYB) and a lager yeast (LYA) were examined: LYA showed a high mutation rate even in the absence of formaldehyde