Studying yeast volatolome with Proton-Transfer-Reaction Time-of-Flight Mass Sepctrtometry

Yeast metabolism plays a key role in the production of flavor compounds in wine thus affecting its final quality and sensory profile. Volatile compound concentration is influenced by the growth characteristics of yeast strains. For this reason a rapid and non-invasive screening of the yeast volatolome is of outmost relevance. Proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) showed promising results in monitoring Saccharomyces cerevisiae volatile production in dough and bread (Makhoul et al. 2014). Sixteen different wine yeast strains of Saccharomyces paradoxus, Saccharomyces cerevisiae and their hybrids were selected for studying volatile organic compound (VOCs) release by a commercial PTR-ToF-MS 8000 instrument (Ionicon Analytik GmbH, Innsbruck, Austria) coupled to a multipurpose head-space automated sampler (Gerstel GmbH, Mulheim am Ruhr, Germany). The samples (five biological replicates of each yeast strain, the substrate used for their growth (solid YPD, Yeast Peptone Dextrose) and lab air) were left for 3 days in dark at 30°C. Then, the headspace of each sample was automatically measured 5 times every 260 minutes for 60 seconds which guaranteed total replacement of headspace by pure air. Due to high ethanol production during yeast growth, an inert gas dilution was applied in order to prevent primary ion depletion and formation of ethanol clusters which might affect the quantification of volatiles. Data processing of PTR-ToF-MS spectra consisted of dead time correction, external calibration and peak extraction (Cappellin et al. 2010). One-way ANOVA (p-value < 0.01) for each yeast strain higher than the substrate reduced the dataset from 349 to 114 mass peaks. Principal component analysis of total yeast aroma profiles showed a clear separation of yeast strains. In addition, we were able to identify significant differences in off-flavor sulfuric compounds produced in higher amount by wild strains (both S. cerevisiae and S. paradoxus). Sulfur-containing VOCs produced during the yeast metabolism reactions are known to play significant role in wine aroma adding both fruity notes and off-flavor (Swiegers J. H et al, 2007). In this work, for the first time, PTR-ToF-MS coupled to a multipurpose headspace automated sampler was applied for a rapid and non-invasive analysis of the yeast colonies. The technique was successful in characterizing different yeast strains and identifying differences in the release of important classes of compounds.