Current recommendations for beer pasteurisation are based on the study in 1951 by Del Vecchio and coworkers. In this work, 14 beer spoilage bacteria were screened for their ability to grow or survive in ale and stout together with the determination of their thermo tolerance at 60°C. Using a capillary tube method, the D‐value (decimal reduction time) and z‐value (temperature resistance coefficient) of the three thermo tolerant bacteria (Acetobacter pasteurianus, Lactobacillus brevis and Lactobacillus hilgardii) were determined. Validation of pasteurisation at a range of pasteurisation units (PU) in packaged product were performed in a tunnel pasteuriser. This study showed that eight of the 14 microorganisms were able to grow in both beer styles, whilst different thermo tolerances were observed amongst the spoilage bacteria. Effective pasteurisation of the selected microorganisms was achieved at significantly lower PU values than those recommended by the European Brewery Convention Manual of Good Practice. In package pasteurisation conducted at 1.6 PU resulted in greater than an 8‐log reduction in viable cell numbers, resulting in ‘commercial sterility’. Although this study demonstrated that successful pasteurisation was achieved for vegetative cells at significantly lower PU values than those recommended, further studies are required to demonstrate the optimal level of pasteurisation for spore forming bacteria and for yeast. © 2018 The Institute of Brewing & Distilling
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 effect of isomerised hop extract on the heat resistance of Saccharomyces cerevisiae BRYC 501 ascospores and Lactobacillus brevis BSO 566 was investigated. Heat resistance of yeast ascospores and L. brevis was determined in premium lager (4.5% ABV) and alcohol-free lager (0% ABV) at their initial bitterness and after adjustment with isomerised hop extract to 25 and 50 IBU. Results showed that D 60 of yeast ascospores in alcohol-free lager was reduced by >30% at 25 IBU and >50% at 50 IBU compared to lager with no added hop extract. In premium lager, D 60 was reduced by >10% at 25 IBU and >30% at 50 IBU. The addition of isomerised hop extract also had a significant impact on the z-values, which increased with higher concentrations of isomerised hop acids in both premium and alcohol-free lager. Slightly higher z-values were observed in premium lager (z = 4.0, 4.5 and 5.0°C for IBU = 6.3, 25 and 50 respectively) than in alcohol-free lager (z = 3.6, 4.0 and 4.2°C for IBU = 8.6, 25 and 50). The addition of isomerised hop extract also reduced the heat resistance of Lactobacillus brevis BSO 566. The findings in this study suggest that less bitter (low IBU) lagers should be pasteurised with higher pasteurisation units (PUs) than more bitter lagers (high IBU) which can be pasteurised at lower PUs.
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