Challenging the assumptions around the pasteurisation requirements of beer spoilage bacteria

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 degrees 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. (C) 2018 The Institute of Brewing & Distilling