What it actually is
It runs as a chain reaction. A radical forms, usually started by a trace of iron or copper, by light or by heat. That radical attacks a fat molecule and creates another radical, which attacks the next. Each step produces the next, which is why oxidation accelerates once it has begun and why the early stages are invisible.
Why it matters
In most fat containing products it is what ends shelf life, not microbiology. The product is entirely safe and completely unsellable. It is also the defect most often missed in development, because a trial judged on day one cannot see it.
What changes it
The degree of unsaturation of the fat, trace metal content, exposure to light and oxygen through the pack, temperature, and whether an antioxidant or a chelating agent is present. Cooked and comminuted products oxidise faster, because the process exposes more surface and releases more iron.
