Understanding the emerging beer spoilage yeast Saccharomyces cerevisiae var. diastaticus
Examensarbete för masterexamen
Saccharomyces cerevisiae var. diastaticus has recently become infamous in the brewing industry as a potent beer spoilage organism. This variety of S. cerevisiae is characterised by the production of an extracellular glucoamylase, encoded by the STA gene family and in particular STA1, which allows it to degrade starch and dextrins present in finished beer and thereby prolong the fermentation process. Overfermentation of beer causes excess carbonation, leading to bottle gushing and occasional bursting, as well increased ethanol content, and as such must be avoided by brewers. To further this goal, better understanding of the behaviour of diastaticus is required. Notably, the presence of STA1 in a brewing yeast strain is not always indicative of an overly fermentative phenotype, and indeed there can be large variations between the level of diastatic activity of different diastaticus strains. This project investigated the diastatic activity of a wide array of diastatic S. cerevisiae strains used in the brewing industry, encompassing all currently identified diastatic brewing yeast types. Assaying of the strain collection for growth on starch revealed distinct diastatic phenotypes, which were then further detailed using selected strains by metabolite profiling using HPLC. STA1 expression appears to be favoured by normal growth conditions in many strains, but some highly diastatically active were able to utilise starch for growth even in stressed conditions. Sequencing of a fragment of STA1 from the entire strain collection together with phylogenetic analysis revealed few genetic differences and no genetic basis for variation in diastatic phenotype between diastaticus strains. Experimental contamination of finished beer with a strongly diastatically active strain demonstrated the strong spoilage potential of highly diastatically active S. cerevisiae var. diastaticus, even in refrigerated conditions (8°C). Overall, this project demonstrated the diversity of diastatic phenotypes in S. cerevisiae var. diastaticus and the importance of taking into account those differences for the brewing industry.
Saccharomyces cerevisiae var. diastaticus, glucoamylase, super-attenuation, beer spoilage, STA, STA1