This is an excellent ant-oxidant which helps prevent wines from oxidising during storage. Commercial winemakers also add large amounts to wines with a high acid level to induce a malo-lactic fermentation.
When wines are stored in bottles standing upright, the cork can shrink slightly during storage. This will let air get into the bottle as the level of sulphite drops. Adding ascorbic acid when bottling will help prevent the wine from being oxidised. The ascorbic acid works by removing the free oxygen present in the wine at bottling time. It will work better if Campden tablets or sulphite are added at the same time. It is advisable to use ascorbic acid with light coloured wines.
Only small amounts are needed – 3g per gallon (1/8 teaspoon) added to the wine before bottling. You can add up to 6g (1/4 teaspoon) only if sulphite has also been added.