The oxidative deterioration of food items proceeds either through the peroxidation of unsaturated components in fats and oils or through the catalytic conversion of phenolic groups into quinones and their coupling and polymerisation.

The latter reactions are obvious and may best be observed in cut apples, they are mediated by phenolase enzymes and generally called enzymatic browning reactions. They are delayed by low pH, ocurr at relatively high water activity and will not be studied here. The former, who will be addressed below, may happen under a variety of circumstances, and may generally be said to be favoured by light, transition metal ions, high temperature and low water activity.

As oxidations will exhibit some dependency on oxygen availability. Some of these reactions may be catalysed by enzymes, but many are not. They strongly affect the aroma and odour of foods, both agreeable and disagreable (rancid) notes being produced, depending on the circumstances. The low odour thresholds of the intermediate polarity products which are the result of these reactions render them detectable shortly after their onset.

One may distinguish various types of lipid oxidation, depending on the nature of the agent(s) responsible for its onset and rate. Thus autoxidation and lipoxygenase catalysis are both efective in yielding hydroperoxides.

Induced changes in food aroma are continually assesssed by consumers as objectionable, for example, as rancid, fishy, metallic or cardboard-like, or as an undefined aged, stale or warmed up flavour. On the other hand, the fact that some volatile compounds, at a level below their off-flavour threshold values, contribute to the pleasant aroma of many fruits and vegetables and to rounding-off the aroma of many fat- or oil-containing food should not be neglected.