Propagation and fermentation trials with high gravity resistant mutant lager yeast strains

Fermentation at higher wort gravities can increase productivity in the brewing industry. Flavour matching and loss of yeast vitality and viability have hampered progress in this area. In this study, protocols to isolate high gravity resistant lager yeast strains have been developed. Several new lager yeast strains isolated by VTT Biotechnology showed improved fermentation performance. Industrial lager yeast A15, mutagenised with ethylmethanesulphonate (death rate 0 ± 15 %), was put through an anaerobic 24°P fermentation, stored in its own beer and fed first with 18% maltose and then 6% maltose/50% ethanol. Samples were plated on YP/maltose. Rare survivors obtained at days 24 (n=38) and 37 (n=11) were screened in small-scale fermentations. Three mutants were selected for pilot-scale fermentation tests. Propagation and fermentation samples were analysed daily. After lagering, filtration and bottling, the final products were subjected to sensory analyses and quality control analyses. All mutant yeast strains exhibited a faster fermentation rate and a better final attenuation compared to the parent strain. In general all mutants performed better in high gravity wort, but the end products had the same flavour profile compared to the parent. The improved behaviour of these (non-GMO) mutants under industrially relevant conditions means they have immediate industrial application. This study was supported by the European Community (contract no. QLK1-CT-2001-01066).