Incidence and Characterization of Petites Isolated from Lager Brewing Yeast Saccharomyces cerevisiae Populations

J. Am. Soc. Brew. Chem. 70(4):268-274, 2012 Repeated reuse or "serial repitching" of yeast between successive fermentations exposes brewing yeast cells to a number of biological, chemical, and physical stresses. It is generally accepted that brewery yeast biomass recycling leads to an increased incidence of petite mutations. Once petite levels reach a significant proportion, this can lead to aberrant fermentation profiles and impaired product quality. Analysis of traditionally cold cropped yeast has shown that petite frequency appears to be a function of generation number. In this paper we demonstrate that utilizing warm cropping reduces the occurrence of petites and that petite frequency does not increase with generation number. In addition, RFLP analysis of genomic and mtDNA of independently isolated petite mutants from yeast slurries that have performed 1, 3, 9, and 10 fermentations demonstrated a conserved banding pattern that differs from that exhibited by the wild type strain. A Rho petite strain was also isolated from cropped yeast slurries demonstrating for the first time that such petites do occur spontaneously.