Stuart Wilkinson (1), Sue James (2), Chris Powell (1), Katherine Smart (2); (1) University of Nottingham. International Centre for Brewing Science (ICBS), Nottingham, U.K.; (2) SABMiller Plc, U.K.

Yeast, Fermentation, and Microbiology

Ale (Saccharomyces cerevisiae) and lager (S. pastorianus) brewing yeasts have different origins and genealogies. Ale-type strains can be traced back to Roman and Egyptian civilizations, while lager yeasts are believed to represent a more recent hybridization event (occurring ~2-300 years ago) between an S. cerevisiae strain and an “S. bayanus-like” species. Irrespective of their origins, current production yeast strains have evolved over the interim period, largely through artificial selection by the brewer for batches of beer with desirable traits. Yeast used to conduct these fermentations were traditionally selected and transferred to a fresh batch of unfermented beer with the knowledge that this was more likely to lead to a “good” final product. This process has been relatively successful, giving rise to many of the strains used today. However, within the previous 20-30 years there have been major innovations within the industry that have led to increased demands on the current brewing yeasts employed. These center on requirements for increased process efficiency through faster fermentation times (quicker attenuation, shorter VDK rest), the ability to ferment higher gravity worts efficiently, and obtaining yeast populations with improved physiological condition both during and after fermentation (characterized by stress resistance). As such there is growing potential for the use of novel yeast strains that can meet these criteria. Advances in molecular and cell biology, through both genetic modification and classical breeding approaches, as well as for selection of “natural” strains from novel environments have created the possibility to generate libraries of new hybrid yeasts with potential for use in beer fermentations. However, screening large numbers of yeast strains for brewing-specific phenotypes can present a technical problem, as performing traditional assays and small-scale fermentations are simply not viable. As such, rapid screening assays are required in order to short-list potential candidate strains for more in-depth analysis. Here we present the use of a phenotypic microarray (PM) technique as a high-throughput screening tool for evaluation of novel yeast strains. The PM effectively conducts “micro-fermentations” (~100 µL) in 96-well plates, and the system can simultaneously run up to 50 plates (4,800 fermentations) at one time. A sealed anaerobic system was used to screen the metabolic performance of novel yeast strains for a variety of key performance indicators such as nutritional requirements (including carbohydrates and nitrogen), as well as tolerance to stress factors associated with industrial fermentations. This approach was used to conclude if novel strains exhibit improved and desirable phenotypes over current brewing yeast strains. It is anticipated that PM analysis will be used in the future to determine the suitability of novel strains to ferment a variety of wort types and to optimize fermentation efficiency.

Stuart Wilkinson is a postdoctoral research fellow at the University of Nottingham. His current role is evaluation of desirable brewing traits in novel yeast strains. After having obtained a B.S. (Hons.) degree in biology he started his scientific career as an ecotoxicologist specializing in chronic toxicity of biochemicals in aquatic species. A change in career direction then saw him move into the biofuels sector (at the University of Nottingham), working as an analytical chemist for the BBRSC Bioethanol LACE (Lignocellulosic Conversion to Ethanol) research project. He then completed his Ph.D. degree in bioenergy and brewing science at the University of Nottingham, working under the supervision of Dr. David Cook and Prof. Katherine Smart (SABMiller Plc.). His Ph.D. project focused on the conversion of barley-derived spent grains (BSG) to bioethanol and other additional high-value products. Stuart is a keen biologist and a full member of the Royal Society of Biology (MRSB), in addition to being a member of the Institute of Brewing and Distilling (IBD) and the American Society of Brewing Chemists (ASBC). He can be contacted on