Final Report: This is a completed project for the Washington Grape & Wine Research Program
Project Duration: 2018-2021

Title: Microbiology and Chemistry of Washington Wines
Project Duration: 2018-2021
Principal Investigator: C.G. Edwards
Organization: School of Food Science, Washington State University, Pullman, WA 99164-6376
Telephone: 509-335-6612

Section 1. Summary:

One approach to limit spoilage associated with Brettanomyces bruxellensis is application of so-called “hurdle technology” where synergies between two or more antagonistic factors are used to inhibit the target microorganism to a greater extent than relying upon only one factor. To this end, the current project has studied how interactions between molecular SO2, ethanol, and storage temperature affect this spoilage yeast under vinification conditions. While yeast growth was influenced by ethanol and storage temperature, mSO2 greatly impacted the culturability of the yeast. Statistical analyses may indicate significant interactions between these three factors which affect the yeast including formation of off-odors associated with 4-ethylphenyl and 4-ethylguaiacol.

Additional research has illustrated the importance of wine composition on growth of B. bruxellensis. Here, a single strain of the yeast was inoculated into six different red wines (primarily Merlot) which had similar concentrations of mSO2 (none) and ethanol (13.5% v/v) and were all stored at 18°C. While the yeast grew well in some wines, rapid declines in culturabilities were noted for others. Differences in growth do not appear to be due to a lack of nutrients but suggested the presence of unidentified inhibitory factors.

Work involving commercial application of non-Saccharomyces yeasts as a means to reduce ethanol production has continued. Merlot grapes were obtained from a commercial vineyard and fermented under industrial conditions in 2018 (120 kg must per fermenter). Two different non-Saccharomyces yeasts, originally obtained from a regional vineyard, were inoculated and fermentations proceeded under either temperature regime A (maximum 15°C for 72 hr. followed by 25°C) or B (maximum 17.5°C for 72 hr. followed by 25°C). Those musts inoculated with Mt. pulcherrima and vinified under temperature regime B contained the lowest amounts of ethanol, compared to those with only S. cerevisiae present (14.3% vs. 15.0% v/v). Sensory analysis confirmed that wines produced with Mt. pulcherrima (temperature regime B) were deemed to have less ‘dried fruit’ and ‘herbaceous’ (aroma), ‘bitter’ (taste), and, most importantly, less ‘ethanol’ (flavor) and ‘hot/ethanol’ (mouthfeel) in comparison to control fermentations with only S. cerevisae. However, research conducted in 2019 was inconclusive due to hindrance of non-Saccharomyces yeasts through reduced initial must temperatures as a result of harvest conditions. It therefore appears that must temperature is a critical factor affecting ethanol reductions use of Mt. pulcherrima and My. guilliermondii. With optimization of conditions, reducing final ethanol concentrations is achievable without quality loss and possible improvement.

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Enology // Yeast //