Modifications of red wines colouring material during aging, in barrels or in vats, needs the presence of oxygen to induce a production of traces of acetaldehyde from joined oxidation between alcohol and phenolic compounds. Acetaldehyde makes possible a polymerization of anthocyanins ; other phenolics take place in this reaction. As consequence, colour is increased and stabilized in spite of a decrease of anthocyanin level. An excess of sulphur dioxide rnakes these essential reactions lower.
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