Oxidation in food

Oxidation is a process or a chemical reaction that takes place when a substance comes into contact with oxygen or another oxidizing substance. Oxidation reactions occur in food and non-food items.

Oxidation is responsible for the deterioration in the quality of food products, including off-flavors and off-odors. It is affected by processing, packaging and storing methods, as well as product ingredients.

Antioxidants are added to food to prevent them spoiling. Tocopherols, ascorbic acid, carotenoids, flavonoids, amino acids, phospholipids, and sterols are natural antioxidants in foods. Antioxidants delay the onset of oxidation by donating hydrogen atoms to quench free radicals, forming a stable antioxidant radical that is unable to participate in propagation reactions, slowing down oxidation.

Antioxidants inhibit the oxidation of foods by scavenging free radicals, chelating prooxidative metals, quenching singlet oxygen and photosensitizers, and inactivating lipoxygenase.

Foods which contain edible oils will spoil once exposed to oxygen from the air due to oxidation reactions. This chemical change results in a bad flavor and smell from the food. Phenols and the enzyme phenolase are found in the cells of the apple, and when these are exposed to oxygen in the air, for example through slicing, the oxygen causes a reaction.
Oxidation in food

Deterioration in food quality due to lipid oxidation

Lipids play a vital role in the metabolism of cells by providing a source of energy and reserve storage materials. There are many catalytic systems that can oxidize lipids. Among these are light, temperature, enzymes, metals, metalloproteins and microorganisms.

Cations in foods, such as Fe2+ and Cu2+, may induce a diversity of undesirable effects that influence the nutritional quality of foods. Iron actively catalyses lipid oxidation and its presence even in trace amounts has long been recognized as potentially detrimental to the shelf-life of fats, oils, and fatty acids.

Lipid oxidation is one of the major causes of quality deterioration in natural and processed foods. Oxidative deterioration is a large economic concern in the food industry because it affects many quality parameters such as flavour (rancidity), colour, texture, and the nutritive value of foods. Oxidation can occur in both triglycerides and phospholipids of food because lipids are divided into two main classes; polar lipids (phospholipids) and neutral lipids (triglycerides).

Oxidation affects many interactions among food constituents, leading to both desirable and undesirable products. Food lipids are the foods components that are most susceptible to oxidation, therefore oxidation reactions are one of the major sources of deterioration that occurs during manufacturing, storage, distribution and final preparation of foods.
Deterioration in food quality due to lipid oxidation 

Oxidative rancidity

Rancidity can occur in many products or ingredients during storage. It affects taste and odor, and can have an impact on nutritive value. Exposure to light, pro-oxidants and elevated temperature will accelerate the reaction. Rancidity is associated with characteristic off-flavor and odor of the oil.

There are two basic types or causes of rancidity that cause and/or contribute to the degradation of foods: oxidative and hydrolytic. One occurs when oil reacts with oxygen and is called oxidative rancidity. The other cause of rancidity is by a combination of enzymes and moisture.

Oxidative rancidity, one of the major causes of quality deterioration in foods, is caused by the oxidative deterioration of lipids by atmospheric oxygen. It is the reaction of double bonds in unsaturated fatty acids with oxygen.

Lipids oxidize through a complex series of reactions giving rise to a myriad of non-volatile and volatile compounds that are responsible for off-flavors even at concentrations in the parts-per billion range.

One of the most common methods for measuring the oxidative rancidity of vegetable oils is the peroxide value (PV). The PV determines the amount of peroxides formed during early oxidation stages, expressed as millimoles or milliequivalents of peroxide oxygen per one kilogram of oil.
Oxidative rancidity

Lipid oxidation

Lipid oxidation is a major cause of food quality and flavor deterioration. It is important reaction that limits the shelf life of many foods. Oxidation of lipids is primarily dependent on the degree of unsaturation of their fatty acid constituents.

It is also affected by other components present in the food matrix as well as conditions under which the product is stored.

Deterioration of foods by lipid oxidation generally displays an induction period during which very little oxidation occurs. This is followed by a stage when deterioration process rapidly.

Lipid oxidation which is also called autoxidation as it occurs in bulk fats and oils proceeds via a self sustaining free radical mechanism that produces hydroperoxides that under scission to form various secondary products include aldehydes, ketones, organic acids and hydrocarbons.

Significant of lipid oxidation is not limited to the development of off-flavors in food, but it also affects the nutritional value of food as it brings about loss of essential polyunsaturated fatty acids, fat-soluble vitamins, protein value and destruction of pigments when present.
Lipid oxidation