A chemical compound used to kill wild yeasts, sterilize equipment and prevent oxidation. It is a naturally occurring substance in wine that in small amounts is a preservative. It can be used both in the vineyard and during wine-making to protect grapes and wine from spoilage. Sulphites are a form of sulphur that occur naturally as a by-product of fermentation. Because a tiny percentage of the population is allergic to sulphur, wine labels must carry the message “contains sulphites” if the wine contains more than 10 parts per million (ppm) sulphites.