I know that sulfites are used in wines to prevent oxidation, but how exactly does this happen, which reactions take place and where do the products go?
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3 Answers
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Sulphites work as anti-oxidants in wine because they are very easy to oxidise to sulphates. Schematically:
$$\ce{SO_3^{2-} + \text{oxidant}\to SO_4^{2-}}$$
This ease with which the sulphite ion is oxidised to the sulphate ion means that sulphite added to wine tends to 'mop up' any oxidisers (like air oxygen) that might get into the wine, rather than the alcohol (and maybe other substances in the wine) becoming oxidised.
The sulphite method has the added advantage that it, as well as its sulphate reaction product, is quite harmless to humans.
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In addition to @Gert 's answer, Sulphites also denature (damage so they are ineffective) enzymes (released when the fruit is crushed) which accelerate oxidation.
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We spray metabisulfite on the equipment that is used to crush and press and transfer and ferment the grape juice. It is used to kill any naturally present yeast or enzymes that may make distasteful wine.
