To oxidise is to undergo or cause a chemical reaction with oxygen, classically the loss of electrons from a substance. In food, oxidation is overwhelmingly a story of deterioration: unsaturated fats oxidise into rancid, painty off-flavours; cut apples and avocados brown as polyphenol oxidase acts; wine flattens and vinegars; and vitamins C and E degrade. Controlling oxidation is therefore central to shelf life, through airtight packaging, modified atmospheres, antioxidants, and cold storage. But oxidation is also deliberately harnessed: the browning of tea leaves during fermentation, the development of flavour in aged cheese and cured meats, and the Maillard reactions of roasting all depend on oxidative chemistry. Food science is largely the art of encouraging the useful oxidations while suppressing the destructive ones.