Rancidity

Rancidity is the unpleasant change in fats and oils caused by oxidation, giving them a sour smell and taste. Antioxidants are added to slow it down. Malay: ketengikan · Chinese: 酸败.

Oils and fats react slowly with oxygen in the air, especially at their carbon-to-carbon double bonds. This oxidation breaks the molecules into smaller compounds that smell and taste sour or “off”. The food is then said to have gone rancid, and it is no longer pleasant to eat. Warmth, light and the presence of oxygen all speed rancidity up, which is why fried snacks and cooking oil turn rancid faster when left open in a warm kitchen. Because unsaturated oils have more double bonds, they tend to turn rancid more readily than saturated fats.

Example. A packet of biscuits left open for weeks develops a stale, sour smell; that is rancidity. To prevent it, manufacturers add an antioxidant such as BHA or vitamin C, seal the packet to keep out air, and sometimes flush it with nitrogen so there is less oxygen inside.

Confusion to avoid. Do not confuse rancidity with food going bad because of microbes. Rancidity is caused by oxidation and is slowed by an antioxidant; spoilage by bacteria and moulds is slowed by a preservative instead. The two problems are solved by two different kinds of additive, so keep them apart.

Rancidity is the reason antioxidants are studied in the food additives section, so it usually appears in the same SPM Chemistry question, linking the chemistry of oxidation to everyday food preservation. A complete answer names oxidation as the cause, describes the sour smell and taste as the effect, and gives the antioxidant as the way to slow it down. If you can also say that unsaturated oils, with their many double bonds, go rancid faster than saturated fats, you tie rancidity back to the structure of oils and show the examiner a joined-up understanding rather than an isolated fact.