Rancidity in food happens when certain fats break down and create smelly compounds. These compounds form when the fats start to decompose. The main way this happens is when a certain type of fat breaks apart into smaller parts called hydroperoxides. These hydroperoxides then split into two smaller pieces, creating what we call an alkoxyl radical and a hydroxyl radical.
This alkoxyl radical is very active and can react in various ways. It can combine with other fats, react within the same fat molecule, or break nearby bonds. This last reaction, where it breaks nearby bonds, is especially bad because it makes the fat break down into small, smelly compounds that cause the food to taste and smell bad.
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