Dr. James Osborne, associate professor and extension enologist in the Department of Food Science and Technology at Oregon State University, has prepared the update below. This project receives funding from the Oregon Wine Board as part of its viticulture and enology research grant program.
Cold soaking is a technique used during red winemaking to alter the aroma, flavor and phenolic content of a wine. However, cold soaking does not come without risk as this process can result in the production of spoilage compounds such as acetic acid and ethyl acetate if excessive growth of spoilage yeast such as Hanseniaspora uvarum (Kloeckera apiculata) occurs.
H. uvarum growth is typically suppressed by the addition of sulfur dioxide (SO2) to grapes during processing. However, high concentrations of SO2 will also suppress the growth of other non-Saccharomyces yeast present on grapes that can contribute positively during cold soak. Research in my laboratory has been investigating an alternative technique to prevent spoilage during cold soak by the addition of a high population of a beneficial non-Saccharomyces yeast at the start of a cold soak.
A number of commercially available non-Saccharomyces yeast cultures were tested for their ability to suppress the growth of H. uvarum and reduce acetic acid production during a simulated cold soak in a model grape juice. All of the non-Saccharomyces yeast cultures inhibited the growth of H. uvarum growth and reduced production of acetic acid ranging from 29 to 52%. One of these yeasts, Metschnikowia fructicola, was then tested against a large number of H. uvarum strains. Reduction in acetic acid and growth of all H. uvarum strains occurred when grown in mixed culture with M. fructicola during a simulated cold soak.
Findings from the model system were further validated during Pinot noir winemaking experiments. Cultures of H. uvarum and/or M. fructicola were added to grapes prior to a six-day cold soak at 8oC. The addition of M. fructicola reduced H. uvarum growth during cold soak and significantly decreased the amount of acetic acid and ethyl acetate produced. For example, when only H. uvarum was present during cold soak, over 30 mg/L of ethyl acetate was produced, while if M. fructicola was added, only 6 mg/L of ethyl acetate was produced.
These results suggest that the inoculation of a non-Saccharomyces yeast may provide an additional strategy, aside from the addition of SO2, to reduce the development of volatile acidity during cold soak. This option may be of particular use to winemakers who wish to promote the growth of beneficial non-Saccharomyces yeast and/or conduct uninoculated alcoholic fermentations by limiting the use of SO2 during cold soak.