Sodium sulphite is a white crystalline compound used as a food preservative, in medicine, and in paper-making. In food it acts as an antioxidant and antimicrobial, preventing browning in dried fruit, wine, and processed potatoes, and inhibiting bacterial growth. Like all sulphites it must be declared on labels, since sulphite-sensitive asthmatics can react severely, and legal limits cap its use. In the paper industry it pulps wood; in photography it once fixed images. Its preservative power comes from releasing sulphur dioxide, the active agent. It is effective and cheap, which is why it persists despite the allergen labelling it requires, a reminder that preservation always involves trade-offs.