Gross interference by sulphur dioxide in standard determinations of wine phenolics

Abstract

Abstract There is a very strong interaction between sulphur dioxide (SO 2 ) and 0 ‐dihydroxy phenols in response to the Folin–Ciocalteu colorimetric assay, currently the established procedure for analysis of total phenols in wines and plant extracts. Because of this previously unsuspected interference, the method is shown to be generally quite invalid in application to white wines; measures of total phenols may be magnified several‐fold by the SO 2 artefact. By use of model solutions, it has been shown that the extent of interference depends primarily on the molar ratio of SO 2 : 0 ‐dihydroxy phenols. The mechanism of the artefact can be interpreted with reference to the chemistry of the photographic process, in which the oxidation‐reduction reactions involved in image development are similarly mediated by the presence of bisulphite in alkaline solution.