Chris Smart (1), Karin Pawlowsky (1), Chris Rice (1); (1) Campden BRI, Nutfield, England

Yeast, Fermentation, and Microbiology

Contamination of beer continues to represent an important problem for the brewing industry. Traditional beers are susceptible to contamination by several types of spoilage microorganisms, including wild yeasts, but do not support the germination and subsequent growth of spores or pathogenic microbes. However, with the emerging popularity of novel low- and non-alcoholic beverages, and additionally sweetened or beer-mix beverages, the potential for microbial survival and growth is increased. To overcome these issues, pasteurization is widely used in the brewing industry; however, the effectiveness of pasteurization varies depending on the treatment time and temperature, beverage composition and type of organisms present. Thermo-tolerant bacteria and yeasts are able, particularly when in spore form, to withstand standard heat-treatment regimes The brewing industry currently calculates its pasteurization regimes based on long-established microbiological parameters: D value (the time required for a one-log decrease in cell numbers), Z value (the change in temperature required for a 10-fold change in the D value), and pasteurization units (PU). Despite much research into the growth and death kinetics of spoilage organisms in the food industry, the brewing industry has published little research on determining the survival of organisms in traditional beers and even less on novel malt-based products. Much of the published guidelines on pasteurization levels for the industry is based on a single publication by Del Vecchio et al. in 1951 that investigated the death kinetics of several poorly characterized organisms using one model beer that, logically, cannot hold true for other microorganisms and beverages with varying compositions. In this study, the potential for the ascospores of the thermo-tolerant wild yeast Zygosaccharomyces bailii to germinate and grow within four malt- and beer-based products was evaluated. After two weeks Z. bailii showed growth in all products. Subsequently, a novel laboratory-scale method, involving the heat treatment of Z. bailii ascospores within thin-walled capillary tubes, was used to model the death kinetics of the organism in a selection of malt-based beverages. The method was designed to replicate the pasteurization units applied to a product in a standard tunnel pasteurization regime. The method was successful in demonstrating a reliable death rate, with the D value of Z. bailii varying according to the product type. An increase in the treatment duration at each temperature negatively impacted ascospore viability in all products tested and increased treatment temperatures negatively correlated with decreased D values. This study was the first to investigate ascospore viability in malt-based drinks after heat treatment. We conclude that there is a need within the brewing industry to conduct further studies to investigate the thermal death kinetics of a range of thermo-tolerant microorganisms in novel beer and malt-based drinks to ensure microbiological stability in these products.

Chris is head of the Brewing Services Department at Campden BRI and is based at the Nutfield site. The department’s activities cover sensory science, NPD, microbiology, process engineering, and training in a wide range of brewing-related technical areas. Chris also has responsibility for the extensive pilot-malting/brewing facilities and the majority of the research conducted within the Brewing Division. Chris has a B.S. degree in biotechnology from the University Reading, a Ph.D. degree in biological sciences (yeast biotransformations) from the University of Warwick, and an MBA from the University of Nottingham. His background includes over 20 years within the food and beverage industries, where he has held a number of positions with organizations such as the Institute of Food Research, Reading Scientific Services Limited, Cadbury Schweppes, Leatherhead Food Research and, more recently, Campden BRI. Chris’ passion for brewing started with an industrial placement at Bass Brewers in Burton while studying for his first degree.