Lipoxygenases (linoleic acid oxygen oxidoreductase) are ubiquitous. Present in many vegetable and animal cells. Lipoxygenases catalyse the oxidation of 1-cis-4cis unsaturated fatty acids to the hydroperoxides, as happens during autoxidation, but will not oxidise with oleic acid.

Lipoxygenases are metaloproteins with an Fe atom in its active center. The enzymes are activated by hydroperoxide and the reaction pathway always includes the migration of one double bond in order to yield a conjugated system, which is normaly a hydroperoxide but may also be a peroxy radical. These reactions occur at substrate and pH-dependent rate even at ror below room temperature and in the dark.

Enzyme-catalysed oxidation is initiated even in the absence of hydroperoxides. This means the enzyme alone is able to overcome the energy barrier of this reaction, and therefore it has to be thermally inactivated if the reaction is not to be allowed to proceed.

At least two fundamentally different types of lipoxygenase enzymes exist in plants. Type I lipoxygenase oxidises only free fatty acids and will do so with a high stereo- and regioselectivity giving rise to an optically-active hydroperoxide with a conjugated cis-trans-diene system, with the function at either end of the pentadiene system. It will, for instance, form preferentially either 9- or 13-hydroperoxides (R, or S) from free linoleic acid as the substrate.

Occurrence and properties of various lipoxygenases

Type II lipoxygenase, on the other hand, will yield both 9- and 13-hydroperoxides (as in the noncatalysed autoxidation), and it also reacts with esterified substrates, thus attacking oils and fats without requiring prior release of fatty acids by a lipase enzyme.

This enzyme will also oxidise carotenoids and chlorophyll and thus degrade them to colourless products, a property used in flour "bleaching". This can be performed by the addition of , for instance, a small quantity of uncooked mown soyabean to the dough. This activity may be rationalised if the possibility that this enzyme may release alkylperoxyl radicals, and not only hydroperoxides, into solution, and these attack the pigments.

The type II enzyme present in legumes will when in contact with lipid substrates, yield a mixture of aldehydes identical to that obtained during autoxidation.

High temperature degradation of oils and fats:

When subject to high temperature as during cooking and especially frying operations, reactions of oils with food components can greatly accelerate oxidation. The reactions of oils and fats with other food components under these circumstances will be addressed with the degradation of emulsions.

Deterioration of heated oils, as may be seen in oil used for multiple frying operations, is characterised by development of a dark colour and settling of a polymeric dark mass. During frying operations, high oil temperature and water vapour conjure to make oxygen pressure very low. Deterioration will ensue thanks to partial oxidation, but most low molecular weight compounds formed will be steam volatilised and only those with low vapour pressure will accumulate. These include fatty acids and high molecular weight polymeric dark brown material.

Reactions involved in these transformations include fragmentation, mainly by b-scission after hydroperoxide formation, leading to volatiles responsible both for the initially pleasant notes in frying and later for the unpleasant odour of spent frying oil, as described, and also to cyclization reactions, both intra- and intermolecular, occuring via electrocyclic transition states, such as the Diels-Alder and -Ene reactions.

These reactions and other similar ones may explain the appearance of decomposition products with cyclic structures. Polymers with ether or peroxide bonds may also appear due to reaction of alkoxide or alkylperoxide radicals directly with alkyl or alkoxide radicals, especially in the later stages of oxidation.

The b-scission reaction may leave carbonyl groups attached to the triacylglyceride moiety (core aldehydes). Core aldehydes are non-volatile and extremely reactive species, capable of adding to amine groups from protein material (available from other components of food) and starting processes which eventually lead to policyclic compounds, which may be heterocyclic and will contribute to the light absorption pattern of the polymeric mass. Total solids content is one of the criteria for discarding a cooking oil, especially because this may be determined experimentally with some ease. It has therefore been used in many countries as a legal criterium limiting appropriateness of an oil for frying.

Eventually the foaming from the frying oil increases and smoke is liberated upon heating even at low temperatures. At this stage the use of oil in frying is no longer only a long term health risk but also represents, especially if using an open flame, a dangerous behaviour because of it's increased flamability and the risk of fire.