Saccharomyces cerevisiae and S. pastorianus species and strain differentiation by direct analysis in real time time-of-flight mass spectrometry.

Abstract

Seventeen different dried yeast strains, including twelve strains of Saccharomyces cerevisiae and five strains of S. pastorianus, were analyzed using direct analysis in real time (DART) time-of-flight mass spectrometry. The resulting mass spectra were used for rapid species and strain differentiation based upon small-molecule metabolomic profiles. Yeast strains purchased from local shops were suspended in a 1:1 water-methanol solution. Solutions were sampled by dipping the sealed end of a melting point capillary into each vial. Six replicates were measured in positive-ion and negative-ion mode for each strain using an automated linear rail with the DART source operated with helium gas and a gas heater temperature of 350°C. Averaged and centroided mass spectra were exported for analysis with chemometric software. Negative-ion DART mass spectra exhibited less chemical background and more distinctive components than positive-ion DART mass spectra. An on-line search of the Yeast Metabolome Database provided candidate metabolites for selection as features for chemometric analysis. Negative-ion DART mass spectra could distinguish both species and all strains. The DART analysis was also able to identify potential metabolomic differences between top-fermenting and bottom-fermenting yeast, between beer and baking yeast, and between red wine and champagne yeast. All strains could be distinguished by their negative-ion DART mass spectra with 97.7% validation accuracy. Clear differences were observed between dry and liquid forms and Saccharomyces strains with different applications to baking or beverage fermentation. Possible differences in metabolite profiles were suggested, but not confirmed, by accurate mass data.