Differentiation of top- and bottom-fermenting brewing yeasts and insight into their metabolic status by MALDI-TOF MS

Yeast and Fermentation Session
Julia C Usbeck, Technische Universität München, Freising, Germany
Co-author(s): Jürgen Behr and Rudi Vogel, Technische Universität München, Freising, Germany

ABSTRACT: For the production of fermented beverages the most important industrially used yeast species belong to the genus Saccharomyces. Top-fermented ale-type beers are brewed with S. cerevisiae, while bottom-fermented lager beers, which are fermented at much lower temperatures, employ S. pastorianus. This yeast species is a genetic hybrid of S. bayanus and S. cerevisiae. Apart from technological parameters, each specific strain affects processing and the quality of the final product, e.g., flocculation behavior, temperature optima, fermentation speed and rate, the spectrum of secondary metabolites, and hence the aroma profile. These ecotypes are differentiated by time-consuming and laborious biochemical and DNA-based methods to enable a constant beverage quality and characteristics. However, their physiological differences must also be reflected in their enzymatic setting. Matrix assisted laser desorption ionization–time-of-flight mass spectrometry (MALDI-TOF MS) offers a fast and easy method to differentiate yeasts along their peptide mass fingerprints. Therefore, we explored this method according to its differentiating potential for brewing yeasts and their metabolic status. Peptide mass fingerprints of top- and bottom-fermenting Saccharomyces strains were generated by MALDI-TOF MS upon optimized sample preparation and instrument settings and analyzed by a cluster analysis for strain or ecotype level differentiation. Furthermore, we investigated the effect of different culture conditions on selected strains representative for different beer types in relation to specific propagation or fermentation stages in the brewery, e.g., varying sugar concentrations and availability of oxygen. The differentiation of top- and bottom-fermenting brewing yeasts was achieved by >95% of more than 400 samples. Top-fermenting S. cerevisiae strains could further be subdivided into ecotypes according to their application in the production of different beer types, like wheat or alt beer. Differences within S. pastorianus strains were also present, but not as distinctive as for S. cerevisiae. The status of yeast fermentation or respiration could be precisely discriminated, while differences resulting from low and high sugar concentrations were less decisive. These results enable fast classification of unknown strains, improvement of quality control, and pursuit of different physiological states in the yeast culture during the brewing process.

Julia C. Usbeck was born in 1984 in Wuppertal, North Rhine-Westphalia, Germany. In 2009 she finished her studies in food chemistry at the Westfälische Wilhelms-Universität, Münster, followed by a mandatory practical year to accomplish the second state examination. Currently she is working on her Ph.D. thesis on the ability to detect beverage spoiling yeasts using MALDI-TOF mass spectrometry at Technische Universität München under the supervision of Rudi F. Vogel at the Chair of Technische Mikrobiologie in Weihenstephan.