Rancidity (oxidation)

Rancidity is the breakdown of the fats in a food, mainly through oxidation on contact with oxygen, light, heat and moisture, and it spoils taste and smell but, more importantly, destroys sensitive vitamins such as A and E and lowers nutritional value. Oxidised fats generate undesirable compounds, notably peroxides and aldehydes, and advanced oxidation may promote oxidative stress in the animal (FEDIAF). To slow the process, manufacturers add antioxidants, either natural such as tocopherols or synthetic, and packaging matters too, with barriers against oxygen and light such as [modified atmosphere packaging](/glossary/modified-atmosphere-packaging); the degree of primary oxidation is tracked internally by the [peroxide value](/glossary/peroxide-value). Once a bag is opened, oxidation speeds up considerably, so it is advisable to reseal it tightly, store it away from heat and respect the [best-before date](/glossary/best-before-date). A practical detail worth knowing: a smell of paint, cardboard or stale oil can signal a rancid food that should no longer be fed, and buying bag sizes matched to consumption within a few weeks helps keep fat freshness acceptable. The marker: rancidity is the main reason omega-3-rich foods need careful protection and timely use, which makes it central to the storage and quality advice across the [Petipedia glossary](/glossary), including for [salmon oil](/glossary/salmon-salmon-oil).

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General documentary information. For an individual animal, a veterinarian's advice takes precedence over any online content.