The occurrence of mitochondrial mutations in brewing yeast (i.e., petites) has been noted since before the mid-1900’s. Relatively recent work has reported on the occurrence of these mutants in large-scale cylindrical-conical fermenters. It has been generally accepted that elevated temperatures and ethanol levels in the centre of the fermenter cone contribute to the formation of petites. High concentrations of petites present in fermentations are believed to cause process and flavour abnormalities.
In this study, we artificially increased petite levels in our pitched yeast and monitored both fermentation behaviour (i.e., changes in gravity and yeast in suspension) and flavour volatiles. As the concentration of petites in the pitching yeast increased up to 20% the levels of some flavour metabolites (i.e., propan-1-ol, isobutanol, butanedione, iso-amyl-acetate, ethyl-butyrate and pentanedione) increased correspondingly. Conversely, increasing petite levels caused decreases in iso-butyl acetate, 2-methy-butanol, 3-methy-butanol. While the estimation of flavour thresholds is a difficult science it appears that addition of high petite levels did not cause the compounds listed to exceed published beer thresholds.
With respect fermentation attenuation, higher petite levels resulted in lower yeast in suspension values and as one might expect, slower rates of density decline.
While these results should be considered preliminary, they do emphasise the importance of managing pitching yeast to minimise beer process and flavour abnormalities.
Alex Speers is the Professor and Director of the International Centre of Brewing and Distilling at Heriot Watt University in Edinburgh. Previously he was a Professor in the Food Science program at Dalhousie University. Born in Creston, BC, Canada he gained a B.Sc. (Agr.), M.Sc. and Ph.D. at UBC in Vancouver. In the past, Alex has been employed in the Quality Assurance Departments of both Labatt and Molson Breweries. His research interests include various aspects of the brewing and distilling process, including fermentability, yeast flocculation, fermentation modeling, extract calculations, and the properties of (and problems created by) barley malt. He has organized, presented or judged at brewing events in, America, Australia, Canada, China, Ireland, New Zealand, Europe and the UK. Dr. Speers has spent sabbaticals at CUB/Fosters and the Columbia Brewing Company in Creston, BC. He was a past Chair of Editorial Board of the MBAA Tech. Quarterly. Alex belongs to several professional societies and is a member of the editorial boards of the J. ASBC, JIB and the TQ. He has published or presented more than 200 papers, is a Fellow of the Institute of Brewing and Distilling and a Chartered Scientist. In 2011 he was awarded the W.J. Eva award by the Canadian Institute of Food Science and Technology.