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

Abstract

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

The impact of isomerised hop extract on the heat resistance of yeast ascospores and Lactobacillus brevis in premium and alcohol‐free lager

“…This suggests that less bitter lagers with low IBU, should be pasteurised with higher PUs than more bitter lagers (medium and high IBU) which can be pasteurised at lower PUs. This was highlighted in our earlier study (6) where an American-style lager (5.6 and 6.9 IBU), which had a significantly lower bitterness than the German lager (25-26 IBU), required higher pasteurisation units. There are no publications investigating the effect of bitterness or hop extracts on the heat resistance of yeast or beer spoilage bacteria and this study is the first report to investigate this topic.…”

“…The impact of hop extracts on the behaviour of microorganisms has been explored by many researchers (1)(2)(3)(4)(5) but there is no published data on the impact of hop extracts on the heat resistance of yeast or Lactobacillus. Our earlier work (6) showed that heat resistance of beer spoilage microorganisms was significantly lower in more bitter, German style lagers, and higher in less bitter, American style lagers. Consequently, the calculated pasteurisation units (PU) required for German style (more bitter) were half those for the American style lagers.…”

“…Saccharomyces cerevisiae BRYC 501 (yeast which produces the most heat resistant ascospores) (6) and Lactobacillus brevis BSO 566 (the most heat resistant bacterium in our previous study) (10) were used in this work and were stored in liquid nitrogen at -196°C (Air Products, UK).…”

“…Lagers were investigated at three bitterness values: initial bitterness (6.3 for premium lager and 8.6 for alcohol-free lager) and supplemented to 25 and 50 IBU. The capillary tubes method was as described previously (6,10) with the test solution (50 μL) inoculated with microorganisms at 10 7 -10 8 CFU/mL pipetted into soda glass capillary tubes G119/02 (Fisher Scientific, UK). The tube ends were heat sealed and then placed in a water bath at the test temperature (58, 60, 62, 64 or 65°C) and held for the required pre-established time.…”

The enhanced susceptibility of alcohol‐free and low alcohol beers to microbiological spoilage: implications for draught dispense

“…Studies on the vulnerability to spoilage of alcohol-free and low alcohol beers have been limited to pasteurisation and the observation that ethanol augments heat in killing microorganisms. Experiments with spoilage yeasts (9,10), yeast spores (9)(10)(11) and bacteria (10,12,13) were shown to be significantly more heat resistant in alcohol-free beer than beers containing alcohol. Consequently, alcohol-free and low alcohol beers in small pack (and large pack) require greater pasteurisation to achieve commercial stability than beers containing alcohol.…”