What do they do? The book says three things!
1. Add to wine or beer will kill most bacteria and inhibit the growth of mould
2.Campden tablets are also used towards the end of the fermentation process to halt the ferment before all the available sugars are converted by the yeast, hence controlling the amount of residual sweetness in the final product. This balancing between sweet, dry and tart flavours is part of the artistry of wine and cider making.
3. Will get rid of Chlorine in tap water- one tablet to about 20 pints and left overnight will do the job!
They disperse in 24 hours so if you treat wine you should leave it at least 24 hours before you drink it!