Photooxidative Rancidity in Fats and Oils

Photooxidative rancidity occurs when fats and oils deteriorate after exposure to light, particularly ultraviolet (UV) light. This process primarily impacts unsaturated fatty acids, which are more susceptible to oxidation due to their chemical makeup. The double bonds in these fats make them highly reactive when exposed to light, triggering the formation of free radicals and reactive oxygen species (ROS)1. These unstable molecules start a chain reaction that breaks down the lipids, leading to the production of off-flavor compounds such as aldehydes, ketones, and alcohols, which cause unpleasant odors and tastes.

The mechanism of photooxidation begins when light is absorbed by photosensitizing agents naturally found in foods, such as chlorophyll in plant oils or riboflavin (vitamin B2) in dairy products. Once these molecules capture light energy, they transfer it to nearby oxygen, generating singlet oxygen—a particularly reactive form of oxygen. Singlet oxygen then reacts with the unsaturated fatty acids' double bonds, forming lipid hydroperoxides, which are highly unstable and break down into secondary compounds. These oxidation byproducts are responsible for the rancid smells and flavors that spoil food.

Preventing photooxidative rancidity involves limiting the amount of light, particularly UV light, that reaches vulnerable foods. This can be achieved by packaging food in opaque or UV-resistant containers. For example, oils are often stored in dark or UV-filtered glass bottles, and products like nuts or dairy should be kept in shaded environments. Cold storage also slows the oxidation process3. Moreover, the addition of antioxidants like tocopherols (vitamin E) and ascorbic acid (vitamin C) can help neutralize free radicals, thereby delaying the onset of oxidation and extending the food's shelf life.

Proper management of photooxidative rancidity is vital to maintaining the taste, aroma, and nutritional quality of foods high in unsaturated fats, such as vegetable oils, nuts, and dairy, as oxidation can significantly diminish their sensory and nutritional attributes.
Photooxidative Rancidity in Fats and Oils