Free Sulfite in Beers-Kinetic Studies. Lucien Chapon, Sylvette Chapon, and Nestor Djeuga, Laboratoire de Chimie Biologique II, U.E.R. Alimentation & Nutrition, Université de Nancy I, Nancy, France. J. Am. Soc. Brew. Chem. 40:0031, 1982.

The first part of the presentation is a theoretical approach to the different factors affecting the equilibrium between free and bound sulfite. It emphasizes the effect of dilution of the sample, concentration of reagents, time of reaction, and interfering substances that can alter the results of an experimental determination of free SO₂. The second part describes a micromethod for the direct determination of free sulfite in beers with hydrogen peroxide as an oxidizing agent. H₂O₂ instantaneously oxidizes the free sulfite, although it does not appreciably react with the usual components of the medium. Hydrogen peroxide is titrated according to a colorimetric method based on the release of methylene blue by oxidation of the leucodye in the presence of peroxidase. The sample volume is usually 50-100 µl. This method complies with the theoretical requirements. The third part is devoted to the applications of this method to the kinetics of fixation of SO₂ by beers and to the protection afforded by SO₂ as an antioxidant. Model solutions confirm that the oxidation of SO₂ by O₂ occurs chiefly through coupling with the oxidation of ascorbic acid. SO₂ largely prevents the peroxidatic phase, actually protecting the oxidizable polyphenols against oxidation. At least a fraction of bound SO₂ shares this property with free SO₂. A reliable microtitration method for bound SO₂ is urgently needed.

Keywords: Ascorbic acid, Beer, Coupled oxidation, Free sulfite, Microtitration