High-throughput direct monitoring of microbial resources for oenology by direct injection mass spectrometry

Abstract

Microorganisms have been widely used in oenology since prehistoric times. Their metabolism significantly impacts many wine properties and is particularly essential for the production of flavor compounds, thereby affecting perceived wine quality. Volatile organic compound (VOC) concentration depends not only on growth of single microorganism strain but also on their interaction with each other and the food matrix. For this reason, a rapid and non-invasive screening of the microbial volatilome is of utmost relevance for the evaluation, selection, and optimization of microbial strains in oenology. Our research group has developed and successfully applied a setup based on Proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) coupled to a multipurpose GC sampler for high-throughput and non-invasive monitoring of biotechnological processes. PTR-ToF-MS belongs to direct injection mass spectrometry, and as such, it offers online, real-time evaluation of VOCs with high sensitivity. This contribution describes two case studies of the volatilome of microorganisms relevant to oenology. The first case study involved six different S. cerevisiae strains (four meiotic segregants of a natural strain and two laboratory strains) inoculated onto a solid YPD medium and monitored for 11 days every 4 hours. In this case, more than 300 peaks were extracted from the average spectra associated with each time point, 70 of which were tentatively identified. Univariate and multivariate analyses were performed on the data matrix (3640 measurements x 70 peaks), highlighting the volatilome evolution and strain-specific features. This serves as a paradigmatic example of the method’s potential in terms of its high-throughput capability, sensitivity, reproducibility, and stability. The second case study explored different single and multiple inoculations of diverse oenological yeasts in both commercial grape juice and fresh must. The experiment highlighted variability in the overall volatile profile associated with (i) the different yeast species, (ii) the different yeast combinations, and (iii) the different fermenting matrices. The results of these two case studies demonstrate the potential of PTR-ToF-MS for monitoring experimental variables associated with alcoholic fermentation in wine, thereby opening new opportunities for managing this crucial phase and improving the quality of the final products and optimizing the processes.