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. Recent research has demonstrated that yeast naturally present during the cold soak can impact wine aroma and flavor (Hall et al. 2017). This research builds off these results and investigates how cold soak conditions could be manipulated to encourage or discourage growth of certain yeast and the consequences for wine aroma. Specifically, ways a winemaker may manage a cold soak (temperature, SO2, yeast diversity) were investigated for their impact on yeast populations and production of volatile aromas. Pinot noir wines were produced where the grapes were cold soaked for six days at two different temperatures (6 or 10?C) with the addition of 0, 50, or 100 mg/L SO2. Six 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. Temperature and SO2 concentration impacted the growth of non-Saccharomyces yeast during the six day cold soak in a species specific manner. The highest populations observed were in the cold soak at 10?C when no SO2 addition had been made. Here H. uvarum increasing in population from approx. 10³ cfu/mL to almost 10⁸ cfu/mL by the end of the cold soak. As increasing concentrations of SO2 were added to the grapes prior to cold soak the growth of the non-Saccharomyces yeast including H. uvarum decreased. When 50 mg/L SO2 was added only low populations of H. uvarum, T. delbrueckii, and L. thermotolerans were detected at the end of the 6?C cold soak while at 10?C only H. uvarum was detected. When 100 mg/L SO2 was added there were few culturable yeast present in the cold soaks at either 6 or 10?C. Overall, increasing SO2 was more effective at minimizing H. uvarum growth than decreasing the temperature as there was still significant growth of H. uvarum at 6?C when no SO2 addition was made.

All wines made from grapes that underwent cold soak had significantly higher color and polymeric pigment content than wine made from grapes that did not undergo cold soak with only small differences in color and polymeric pigment content being noted between wines made from grapes cold soaked under different SO2 and temperature conditions. Initial volatile aroma analysis demonstrated significant differences between the concentration of a number of esters in wines made from grapes that were or were not cold soaked. In particular, wines made from grapes cold soaked with no SO2 additions had lower concentrations of certain ethyl esters.