Examination of the sulfur dioxide-ascorbic acid anti-oxidant system in a model white wine matrix

The effectiveness of sulfur dioxide in inhibiting the ascorbic acid induced oxidation of (+)-catechin in a matrix simulating white wine was investigated. At a sulfur dioxide:ascorbic acid mole ratio of 0.8:1.0, sulfur dioxide extended the delay prior to the onset of (+)-catechin browning as measured by the increase in absorbance at 440 nm from 2 to 4 days post reaction initiation. The lengthening of the pre-browning period was correlated with an increase in the time required to oxidise ascorbic acid completely, the sequence of events being loss of sulfur dioxide (day 3), loss of ascorbic acid (day 4) and on-set of (+)-catechin browning (day 4). Increasing the sulfur dioxide to ascorbic acid mole ratio to 3.0:1.0 inhibited the onset of (+)-catechin browning over the 14-day reaction period. This inhibition was achieved at considerable cost to the sulfur dioxide concentration, with the ratio of sulfur dioxide consumed to ascorbic acid oxidised being 1.7 compared with the expected fraction of 1.0. Reasons for the enhanced consumption of sulfur dioxide and the implications of this study for maintenance of quality in white wine production are discussed.