Do Vegetable Oils Go Bad?

Oxidation of Vegetable Oils

Depending on the storage and preservation conditions, fixed vegetable oils can also deteriorate. We can define deterioration in vegetable oils as oxidation.

What is Oxidation in Vegetable Oils?

The reaction that occurs over time when fatty acids come into contact with oxygen is called oxidation. Compared to saturated fatty acids, unsaturated fatty acids can be easily oxidized. In particular, the increase in the number of double bonds facilitates oxidation. Metals, heat, light, etc. accelerate oxidation.

The ethylene bond (-CH=CH-) in the structure of unsaturated fatty acids can easily be saturated with hydrogen or halogens. The unsaturated fatty acid becomes saturated. Or the double bond can be opened by oxidation and new products can be formed. The formation of pelargonic acid and azelaic acid from oleic acid can be given as an example.

When potassium permanganate (KMnO4) is used as an oxidant and oleic acid is converted to dihydroxy stearic acid at low temperature by adding 2 OH groups to its double bond. If the oxidation progresses and the temperature is increased, the molecule is oxidized more. As a result, dihydroxy stearic acid loses a molecule of water and breaks down from the double bond. These are azelaic acid and pelargonic acid.

Various groups are formed by the oxidation of unsaturated fatty acids with molecular oxygen and the addition of O2 to the double bonds. The compounds formed in this event, known as autoxidation or rancidity, and which create an undesirable taste, appearance and odor in the oil, are groups such as peroxide, epoxide, ketohydroxide. The decomposition of these groups, especially at high temperatures, produces various products, mostly consisting of acids and aldehydes.

The peroxidation of polyunsaturated fatty acids, which occurs in vivo and is called lipid peroxidation, is a chain reaction. During the reaction, free radicals, which contain unpaired electrons and are very reactive, are formed. Free radicals enter into large-scale reactions in the living organism. However, the defense system in the organism prevents or controls the harmful effects of these compounds with its antioxidative effects.

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