Petite mutation in aged and oxidatively stressed ale and lager brewing yeast

Aims: To determine the role of oxidative stress and chronological ageing on the propensity of brewing yeast strains to form respiratory deficient 'petites'. Methods and Results: Four industrial yeast strains (two ale and two lager strains) were exposed to oxidative stress in the form of H ₂O₂ (5 mmol l^-1) for two hours. Cell viability and occurrence of petites were determined by the slide culture and TTC-overlay techniques, respectively. Increases in petite frequency were observed but only in those strains sensitive to oxidative stress. Chronological ageing under aerobic conditions led to an increase in petites in strains sensitive to oxidative stress. No such increase was observed under anaerobic conditions. Conclusion: Ageing may contribute to mitochondrial DNA damage and increase the propensity of brewing yeast cells to become respiratory deficient. Tolerant strains may be less likely to generate petites as a result of serial re-pitching. Significance and Impact of the Study: Continuous re-use of brewing yeast is associated with an increase in the frequency of petites within brewery yeast slurries, a phenomenon resulting in reduced fermentative capacity. The cause of petite generation during brewery handling is unknown. We show that endogenous oxidative stress has the potential to generate petites within brewing yeast populations.