Oxidation
A chemical reaction between oxygen and the components of a food, responsible for fats going rancid and fruit turning brown, which degrades both taste and nutritional value.
Oxidation refers to the reaction between oxygen in the air and the molecules in a food. It explains everyday phenomena: a cut apple that turns brown, an oil that goes rancid, meat that turns grey, a fruit juice that loses its taste within a few hours.
Unsaturated fatty acids are the most sensitive to it, and the more so the more double bonds they have. Oils rich in omega-3, such as flaxseed or walnut oil, are therefore the most fragile: keep them cool and away from light, use them quickly after opening and do not heat them.
Stability under heat does not follow an oil’s health reputation. This is the counter-intuitive point: an oil that is excellent raw can be a poor choice for cooking. Olive oil and oils rich in monounsaturated fatty acids stand up to cooking far better than highly polyunsaturated oils, which are often presented as superior.
Three factors speed up the process: light, heat and contact with air, hence the opaque bottles and small containers. Certain compounds slow the reaction: vitamin E naturally present in oils, vitamin C added to juices, and polyphenols. It is the same mechanism, on the scale of the body, as oxidative stress.