Free, Bound, and Total Sulfur Dioxide (SO2) during Oxidation of Wines

Recent work has evaluated the kinetics of free and total sulfur dioxide (SO2) loss in wine following aerial exposure, but little work exists on the relationship between free and total SO2 during the oxidation conditions expected during wine storage. We report changes in free and total SO2 in three wines (Chardonnay, Merlot, and Cabernet Sauvignon) stored for up to 400 days at 19 or 31°C in different bag-in-box packages. The rate of total SO2 loss varied up to seven-fold for a given wine across storage conditions, e.g., 0.13 to 0.94 mg/L SO2 loss per day for the Chardonnay. Total rates of SO2 loss were linear even after free SO2 was undetectable, and only strongly bound SO2 remained. This finding indicated that bound SO2 adduct hydrolysis was fast, as compared to the rate of SO2 consumption in these packaged wines, likely because the SO2 consumption rate depended on the oxygen ingress rate. Total versus free SO2 plots were linear for free SO2 concentrations greater than ~5 mg/L. The slope (Δtotal/Δfree) of this linear region was wine dependent (Chardonnay = 1.33 to 1.49; Merlot = 1.91 to 2.10; Cabernet Sauvignon = 1.83 to 2.00). In contrast to the rate of total SO2 loss, this ratio varied negligibly with storage conditions, suggesting that minimal formation of new SO2 binders occurred over the course of oxidation. The apparent adduct dissociation equilibrium constant (Kd) of each wine as a function of free SO2 concentration was determined from the first derivative of [Bound]/[Free] versus [Free] plots (“Burroughs plots”). As free SO2 decreased to <10 mg/L, the apparent Kd values decreased to values between 1 × 10-4 M and 10-5 M, which are comparable to those reported for malodorous aldehydes.