Lipid oxidation is a chemical process in which the unsaturated bonds present in fats and oils react with oxygen, leading to the formation of off-flavors, odors, and potentially harmful compounds. This reaction can occur passively at room temperature when fats are exposed to air, but is accelerated by heat, light, and the presence of metal ions.
In food systems, lipid oxidation is a primary factor contributing to the degradation of food quality, affecting flavor, aroma, texture, shelf life, and nutritional value. The process is particularly significant in products containing high levels of unsaturated fatty acids, as the double bonds are more susceptible to oxygen attack.
During food preparation and storage, lipid oxidation may occur under several conditions. Passive oxidation typically takes place during storage, especially in poorly sealed packaging or in the presence of light, causing gradual deterioration, commonly termed as rancidity. Thermal oxidation happens during cooking processes such as frying, roasting, or baking, when elevated temperatures significantly accelerate oxidation rates. Protective measures, such as the use of antioxidants (e.g., tocopherols, BHA, BHT) and packaging that limits oxygen exposure, are employed to mitigate oxidative spoilage in the food industry.
The susceptibility to lipid oxidation varies between animal and vegetable fats, primarily due to their fatty acid composition. Animal fats, which are higher in saturated fatty acids and lower in polyunsaturated fatty acids, are generally more resistant to oxidation. In contrast, vegetable oils, especially those rich in polyunsaturated fatty acids (such as soybean, sunflower, and corn oil), oxidize more rapidly.
The term "rancidity" is used colloquially and scientifically to describe the unpleasant taste and odor resulting from lipid decomposition. While all rancidity is not due to oxidation (some is due to hydrolytic reactions), oxidative rancidity is the most common type encountered in both animal and vegetable fats, impacting sensory and nutritional properties of foods.