Research Update: Impact of Pre-Fermentation Cold Soak Conditions on Microbial Populations and Consequences for Wine Aroma

Wine aroma is one of the most important components of wine quality and can be impacted by grape variety, viticultural practices and winemaking procedures. One particular practice that is employed during Pinot noir production to impact wine aroma is cold soaking. In this process, grapes are held at cold temperatures to prevent growth of Saccharomyces cerevisiae and delay the beginning of alcoholic fermentation.

Research conducted by Dr. James Osborne, associate professor at Oregon State University, demonstrated that yeast varieties that are naturally present during the cold soak can influence wine aroma and flavor. In addition, yeast populations on the grapes differed from year to year with higher populations being present in wetter years. Dr. Osborne’s current research project funded by the Oregon Wine Board builds off his earlier results to investigate how cold soak conditions could be manipulated to encourage or discourage growth of certain yeasts and influence wine aroma. James is now researching ways in which a winemaker may manage a cold soak (temperature, SO2, yeast diversity) to impact yeast populations and production of volatile aromas.

In this experiment, Pinot noir wines were produced from grapes cold soaked for six days at two different temperatures (6 or 10°C) with the addition of 0, 50, or 100 mg/L SO2. Seven non-Saccharomyces yeast species commonly isolated from grapes were inoculated and their populations monitored throughout the cold soak. Wine was also produced from grapes that did not undergo cold soak. The finished wine will be assessed for volatile aroma compounds as well as color and phenolic content. Future experiments will explore how the initial grape yeast population and diversity influences yeast ecology during a cold soak by conducting fermentations that mimic high initial yeast populations and diversity often found on grapes in wetter years vs. low initial yeast populations and diversity typically found on grapes in hotter years.

The results from this research project will further understanding of the microbial ecology of a cold soak and how a winemaker may manipulate the parameters of a cold soak in a given year to either minimize spoilage issues such as ethyl acetate and volatile acidity or encourage formation of desirable volatile aromas.