Abstract
Recent work has evaluated the kinetics of free and total SO2 loss in wine following aerial exposure, but little work exists on the relation of free and total SO2 during the oxidation conditions expected during wine storage. We report on changes in free and total SO2 in three wines (Chardonnay, Merlot, Cabernet Sauvignon) stored for up to 400 d at either 19 or 31 °C in different bag-in-box packages. The rate of total SO2 loss varied up to 7-fold for a given wine across storage conditions, e.g. 0.13–0.94 mg/L SO2 loss per day for the Chardonnay. Total SO2 loss rates were linear even after free SO2 was undetectable and only strongly bound SO2 remained. This indicates that bound SO2 adduct hydrolysis is fast as compared to the rate of SO2 consumption in these packaged wines, likely because the SO2 consumption rate depends on the oxygen ingress rate. Total vs. 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 = 1.33-1.49; Merlot = 1.91-2.10; Cabernet Sauvignon = 1.83-2.00). In contrast to the rate of total SO2 loss, this ratio varied negligibly with storage conditions, suggesting that there was little formation of new SO2 binders 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 1st derivative of [Bound]/[Free] vs. [Free] plots (“Burroughs plots”). As free SO2 decreased to <10 mg/L, the apparent Kd values decreased to values between 1 × 10−4 M – 10−5 M, comparable to values reported for malodorous aldehydes.
- Received November 2019.
- Revision received March 2020.
- Accepted April 2020.
- Published online April 2020
- ©2020 by the American Society for Enology and Viticulture
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