Ascorbic Acid

Often called "Vitamin C" by laymen, for a short time ascorbic acid was thought to be a viable substitute for sulfur dioxide (SO2) in wine; i.e. it was thought that ascorbic acid would protect wine against oxidation as well as SO2 does. Research has demonstrated this belief to be false. In an oxidative environment, ascorbic acid leads to rapid browning of catechin, a component of wine. The co-presence of SO2 delays the browning, but the delay is prolonged without ascorbic acid present. In other words, the wine ages better with sufficient SO2 present and without any ascorbic acid.

From Jack Keller's Glossary