Production of Free Sulfur Dioxide by Wine Yeasts

Sulfites (SO2) are well known in the winemaking industry for their anti-oxidative and anti-microbial properties. By maintaining proper free SO2 levels, wines are protected from spoilage bacteria and yeast as well as oxidation which are capable of producing negative sensory effects.  The levels of necessary free SO2 vary depending on the winemaking stage (e.g. malolactic fermentation vs. aging). This project investigated the endogenous free SO2 produced during primary fermentation by a variety of yeast strains within both red and white wines.

A total of 10 commonly used yeast strains were tested; five from Red Star and five from Lalvin. Each strain was tested individually in both a red wine (Noiret) and a white wine (Catawba), for a total of 20 wines produced. The following variables were controlled: initial SO2 levels, sugar levels, yeast nutrient levels, and environmental conditions.

Of the 20 wines produced, each was analyzed for free SO2, pH and residual sugar levels.  The experimental findings showed that one yeast strain, Red Star Côte des Blancs, produced 6.4 ppm free SO2 by aeration oxidation (AO) and 4 ppm by the Accuvin Quick SO2 Test Kit (AQT) in the red wine.  The same yeast produced 3.6 ppm free SO2 by AO and 4 ppm by AQT in the white wine.  The average end pH’s of the red and white wines were 3.63 and 2.82 respectively.  The residual sugar levels measured 0% residual for all yeasts except the Côte des Blancs, producing 0.1% residual sugars in both red and white wines.

Based on the findings of this project, endogenous free SO2 contributions made by yeast are inadequate to protect wines from premature oxidation throughout fermentation and aging, thus less focus should be placed within this area. In order to maintain appropriate SO2 levels, exogenous SO2 should be utilized.