Campden tablets (potassium or sodium metabisulfite) are a sulfur-based product that is used primarily to sterilize wine, cider and in beer making to kill bacteria and to inhibit the growth of most wild yeast: this product is also used to eliminate both free chlorine and the more stable form, chloramine, from water solutions (e.g., drinking water from municipal sources). Campden tablets allow the amateur brewer to easily measure small quantities of sodium metabisulfite, so it can be used to protect against wild yeast and bacteria without affecting flavour. Untreated cider most frequently suffers from acetobacter contamination causing vinegar spoilage. Yeasts are resistant to the tablets but the acetobacter are easily killed off, hence treatment is important in cider production.
Campden tablets are also used as an anti-oxidizing agent when transferring wine between containers. The sodium metabisulfite in the Campden tablets will trap oxygen that enters the wine, preventing it from doing any harm.
It is a common misconception that Campden tablets can be used to halt the ferment process in wine before all the available sugars are converted by the yeast, hence controlling the amount of residual sweetness in the final product. This however is not true. In order to halt fermentation, enough Campden tablets would have to be added to render the wine undrinkable. Alternatively, when used in conjunction with potassium sorbate, the yeast population will be greatly reduced and prevented from reproducing. Without the addition of potassium sorbate the yeast population will only be stunned and eventually repopulate if provided with enough fermentable sugars.