Rancidification a change in the composition of fats during storage that makes them unsuitable for food, owing to their extremely rank odour and taste. Rancidification is caused by the appearance of certain volatile aldehydes, ketones, and acids of low molecular weight, and nonvolatile peroxide substances. It may be biochemical, caused by the formation of moulds on the fats, or it may be purely chemical, resulting from the oxidation of fatty acid radicals in glycerides upon exposure to atmospheric oxygen.
To prevent rancidification, fats are prepared under conditions that exclude the possibility of contamination by mould spores. They are thoroughly purified, and preservatives (for example, salt) are added to prevent biochemical rancidification. Fats (butter, margarine) should be stored in well-sealed packages at a temperature of 2°–6°C.