In short
Rancidity is the spoiling of fats and oils when they are oxidised by oxygen in the air, which gives food an unpleasant smell and taste.
Fats and oils react slowly with oxygen. The products include small molecules with a bad smell. It is faster in the warm, in light and in moist air. Foods can be protected by keeping them in airtight containers, in the cold, and by adding antioxidants. Packing chips in nitrogen instead of air also prevents rancidity.
Butter left in an open dish for several weeks develops a sharp smell. This is oxidation of its fats, an example of a slow chemical change.
Also searched as: rancidity equation.
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