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

Abstract

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 (K_d) 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 K_d values decreased to values between 1 × 10^-4 M and 10^-5 M, which are comparable to those reported for malodorous aldehydes.