Optimization of Temperature and Sulfur Dioxide Levels for Non- Saccharomyces Yeast Survivability and Metabolism
Heather Carbon,* Jonathan Brumley, and Charles
Edwards
*Washington State University, School of Food Science, Pullman,
WA, 99164-6376 (heather.carbon@wsu.edu)
Changes in viticultural practices involving extended ripening periods to achieve phenolic maturity have resulted in increased sugar concentrations in grape musts, with subsequent higher ethanol yields in wines. Non-Saccharomyces yeasts, which are found on grapes or in wineries, have been studied as a means to reduce ethanol yields in wines when sequentially inoculated with Saccharomyces cerevisiae. These yeasts consume glucose and fructose prior to inoculation of S. cerevisiae to reduce final ethanol content without negatively affecting wine quality. However, little work has been performed to optimize fermentation conditions, including temperature and sulfur dioxide tolerance, for non-Saccharomyces yeasts. Therefore, the objective of this research was to determine maximum SO2 tolerance and optimal temperature levels for previously-isolated Metchnikowia pulcherrima and Meyerozyma guilliermondii strains from Washington vineyards. In this study, bottles containing 300 mL of sterile synthetic grape juice medium (pH 3.70, 300 mg/L YAN, 240 g/L fermentable sugars) were inoculated with either Mt. pulcherrima P01A016, My. guilliermondii P40D002, or S. cerevisiae D254 (control) at 106 CFU/mL. The bottles were subjected to 0.0, 0.2, 0.4, 0.6, or 0.8 mg/L molecular SO2. Additionally, each bottle was held at 10, 15, 20, 25, or 28°C for 10 days, in triplicate, and sampled daily for population and sugar use. All ferments with >0.4 mg/L molecular SO2 had inhibited growth of the non-Saccharomyces yeasts regardless of temperature. However, non-Saccharomyces culturability and sugar use at 0.0, 0.2, or 0.4 mg/L molecular SO2 were temperature-influenced. Overall, Mt. pulcherrima and My. guilliermondii showed greater sugar use in ferments containing 0.4 mg/L molecular SO2 at temperatures between 20 and 25°C. Continued work regarding the application of optimal conditions to pilot-scale grape fermentations using sequential inoculations of non-Saccharomyces yeasts prior to S. cerevisiae will be conducted.
Funding Support: Washington State Grape and Wine Research Program; American Society of Enology and Viticulture Scholarship Program
