Meike Kliche (1); (1) Technical University of Munich, Freising, Germany
Recent exploration of craft beer brewing focuses on the employment of multiple hop varieties, while the variety of yeasts impacting beer flavor is hardly exploited. Nevertheless, yeasts produce a wide range of secondary metabolites, which influence the beverage’s final aroma greatly. Ale-type beers are produced by employing Saccharomyces cerevisiae strains for fermentation at elevated temperatures, in comparison to S. pastorianus strains, which produce lager-style beers at cooler temperatures. Still, strain differences are expected to be prominent with respect to the formation of volatile compounds, part of which define the flavor. The differentiation and classification of new Saccharomyces isolates is a time-consuming and laborious process of trial-and-error batch brewing, which small- and medium-sized breweries do not have the resources for. In this study a metabolomics approach was chosen to characterize 11 yeast strains along the volatilomes produced under fermentative conditions. The strains, eight of which are well known in the brewing industry and represent a general flavor diversity, were grown and preconditioned in YPD media and fermented under anaerobic conditions at 9°C and 30°C for 5 days and 3 days, respectively. The volatile profile was acquired via gas chromatography and mass spectrometry (GC-MS) after concentrating the volatile organic compounds in the headspace with an SPME fiber (PDMS/DVB). Fifty-four substances were separated on a PEG column, with helium as the carrier gas, over a temperature gradient from 60°C to 210°C. Two mutually exclusive sets of seven and nine compounds were formed during fermentation with top- and bottom-fermenting yeasts, respectively. For example, a strong temperature dependency was observed for the formation of phenylethanol, which is produced in a distinctly higher amount under the “top-fermenting” conditions for all tested strains. Phenylethanol (as sole compound) is associated with a floral odor reminiscent of rose. Since a correlation of MALDI-TOF MS proteomic patterns with sensotypes based on pilot brewing trials was demonstrated by Lauterbach et al. (WBC, 2016), volatilome patterns are expected to correlate with proteomic patterns as well. In our work, we demonstrate that aroma-active compounds vary with strain and fermentation conditions and can be identified in a simple GC-MS model. This approach, therefore, may enable the fast screening of new yeast isolates for differences in their flavor potential preceding the full brewing process. The “sorting” of unknown yeast isolates into “volatilo types” suitable for specific beer styles extends the potential for specialty craft beer brewing with tempting flavors.
Meike Kliche received a B.S. degree in pure chemistry from the Free University of Berlin in 2011 and an M.S. degree in toxicology from the Charité, one of the largest teaching hospitals in Europe, in 2014. She did internships in a laboratory for trace analysis in food and in a working group researching primary cell culture assays as an alternative to animal testing for contact allergenicity. Meike joined the Chair of Technical Microbiology led by Rudi F. Vogel at the Technical University of Munich in 2014, she is working as a doctoral student on the profiling of Saccharomyces brewing yeasts using metabolomic and transcriptomic methods.