Reactions of iron and organic acids relevant to the light-induced spoilage of white wine studied in model solutions

Previous studies in model wine solutions containing tartaric acid and iron showed that during exposure to light, tartaric acid was degraded to glyoxylic acid. This compound can contribute to the spoilage of white wines by reacting with flavan-3-ols to form yellow xanthylium cation pigments and reversibly binding sulfur dioxide, the main wine preservative. The production of glyoxylic acid was attributed to the photodegradation of iron(III) tartrate by ligand-to-metal charge-transfer (LMCT). This work also provided indirect evidence that iron(III) tartrate photodegradation leads to the accelerated consumption of dissolved oxygen. In this project, the combined influence of sulfur dioxide, caffeic acid, pH and temperature on the photochemical (= 300 nm) production of glyoxylic acid in a model wine system containing tartaric acid and iron was assessed. The total amount of glyoxylic acid produced was dependent on the sulfur dioxide concentration, the pH and the temperature, but independent of the presence of caffeic acid (100 mg/L). Increasing the sulfur dioxide concentration (0 ' 40 mg/L) significantly decreased the total amount of glyoxylic acid produced, but did not inhibit its production. Subsequently, model wine solutions containing the individual organic acids, tartaric acid, malic acid, succinic acid, citric acid and lactic acid, in addition to iron(II), were saturated with oxygen and exposed to light from a xenon lamp (= 300 nm). The organic acid photodegradation products identified included a number of carbonyl compounds. Tartaric acid was degraded to 2,3-dioxopropanoic acid and glyoxylic acid, malic acid and succinic acid were degraded to 3-oxopropanoic acid, citric acid was degraded to 1,3-acetonedicarboxylic acid and acetoacetic acid, and lactic acid was degraded to acetaldehyde. Finally, model wine solutions containing the individual organic acids or all the organic acids combined, in addition to iron(III), were stored in sealed clear glass wine bottles and irradiated using fluorescent lamps commonly used in retail stores. For the individual organic acid solutions, little or no dissolved oxygen was consumed in the dark controls, whereas in the irradiated solutions, dissolved oxygen was consumed at a rate that depended on the organic acid. For the combined organic acid solutions, no dissolved oxygen was consumed in the dark controls, while in the irradiated solutions, dissolved oxygen was consumed at a relatively rapid rate. Furthermore, in the irradiated tartaric acid, malic acid, succinic acid and citric acid solutions, the organic acids were partially degraded to the same carbonyl compounds previously identified in the oxygen-saturated solutions irradiated using the xenon lamp. After addition of sulfur dioxide and equilibration in darkness, the pre-irradiated tartaric acid, malic acid, citric acid and combined organic acid solutions exhibited a greater ability to bind sulfur dioxide compared to the dark controls. After addition of the flavan-3-ol (')-epicatechin and incubation in darkness, the pre-irradiated tartaric acid and combined organic acid solutions exhibited greater yellow/brown coloration compared to the dark controls, largely due to the faster production of glyoxylic acid-derived xanthylium cations. In these solutions, the degree of coloration decreased as the concentrations of malic acid, succinic acid, citric acid and lactic acid increased. Overall, this thesis provides insight into the photodegradation of iron(III) carboxylate complexes under wine-like conditions and the secondary reactions that could contribute to wine spoilage.