The first type of rancidity is oxidative rancidity. This type of rancidity is irreversible. Fats and oils slowly take up oxygen over a period of time resulting in a rancid flavour. The uptake of oxygen is related to the unsaturation of the fat. Hard fats are resistant as they fontain fewer double bonds, whereas fish oils are very susceptible. The reaction is started by the presence of metals, UV light and high temperatures. Oils should be stored in cool dark places. Antioxidants can slow down racidity for example vitamin E.
The second type of rancidity is hydrolytic rancidity. It is common in butter, margarine and cream. Water must be present, it occurs as the result of the hydrolysis of triglyceride molecules to glycerol and free fatty acids. Enzymes accelerate the process such as which split fat. Fats should be heat treated to kill any microorganisms and inactivate enzymes present.