Heat resistance of yeast ascospores and their utilisation for the validation of pasteurisation processes for beers

Beers and other low pH beverages are often stabilised by pasteurisation. There is a lack of guidance as to how many pasteurisation units are required for effective treatment of novel products so as to avoid over‐pasteurisation. Yeast are common spoilers of such beverages and some species can produce heat resistant ascospores. As ascospores are more heat tolerant than vegetative cells they are ideal marker organisms for validating the effectiveness of beverage pasteurisation processes. In this study, 63 yeast strains were screened for their ability to produce spores with 30 strains showing different spore configurations. The rate of ascospore development during incubation on sporulation medium was also determined. It was found that the heat resistance of the ascospores of different species/strains varied widely with Saccharomyces species producing some of the most heat tolerant spores. Ascospores of Saccharomyces cerevisiae BRYC 501 were the most heat resistant with significantly more (over 6–16 times) heat resistance than heat tolerant lactic acid bacteria. The D‐ and z‐values of Saccharomyces cerevisiae BRYC 501 ascospores were determined in alcoholic and non‐alcoholic versions of two lager beers (American and German). The spores were over 14–18 times more heat tolerant in the non‐alcoholic beers and, accordingly, higher PUs need to be applied. Interestingly, at the same/similar alcohol concentration and pH the yeast ascospores were significantly more heat resistant (1.9–2.5 times) in the American than the German beer which may suggest that bitterness contributed to their heat resistance. © 2021 The Institute of Brewing & Distilling