- Pre-fermentation cold soak is a winemaking technique used to alter the color, mouthfeel, and aroma of red wine. While it is still debated what impact cold soak has on these wine quality parameters, current thoughts are that resident yeast populations found in fresh grape must play a role in altering wine aroma. This study investigated the influence of cold soak temperature, SO2 concentration, and the addition of commercial non-Saccharomyces yeast on the composition of yeast populations during cold soak and the consequences for wine aroma. To do this, yeast populations were monitored using culture-dependent and -independent techniques during the 2016 and 2017 production of Pinot noir wines made under various cold soak conditions. In both years, a ‘no cold soak’ control was conducted to determine the overall impact of cold soaking. In 2016, cold soaks were conducted for 6 days at two temperatures (6 ̊C or 10 ̊C) and three SO2 concentrations (0, 50, or 100 mg/L). In agreement with previous research, this study found that cold soak significantly impacted the accumulation of certain ethyl esters and favored the growth of the cold-tolerant yeast, Hanseniaspora uvarum. However, SO2 concentration had the greatest impact on wine aroma and resulted in a near complete loss in culturable microorganisms throughout the cold soak period. Culture-independent techniques revealed that SO2 addition and lower cold soak temperatures also favored the enrichment of resident S. cerevisiae populations, which might have contributed to aroma differences.
In 2017, the spoilage reducing capabilities of commercially available yeast products were tested under cold soak conditions (8 ̊C for 6 days) in both real and model winemaking systems. Under model conditions, all tested non-Saccharomyces yeast products were found to significantly reduce acetic acid concentrations and H. uvarum growth during the cold soak period. The widespread effectiveness of one of these products, Metschnikowia fructicola Gaïa, was then demonstrated against several H. uvarum strains using the same model system. Finally, the influence of H. uvarum starting population on the effectiveness of M. fructicola Gaïa at reducing ethyl acetate, acetic acid, and H. uvarum growth during cold soak and early stages of alcoholic fermentation was tested using 2017 Pinot grapes. Results confirmed the protective actions of M. fructicola Gaïa during the cold soak period and demonstrated the ability of M. fructicola to reduce ethyl acetate accumulation during cold soak, while also minimally impacting other beneficial wine aromas. Collectively, this study demonstrated for the first time that differences in volatile aroma compounds in wines made from cold soaked grapes are not due to the production of the aroma compounds during cold soaking, but rather during alcoholic fermentation. The exception to this being ethyl acetate, which can be produced in large quantities during this time.