2.9. Flavour degradation by oxidation - rancidity and antioxidants
Fats and oils that are essential components of shortenings, mayonnaises, and cooking oils contribute unique flavours to foods. Fats and oils are chemically referred to as lipids and occur in food mainly as triglycerides. These fats and oils may have a positive or negative effect on the flavour of foods depending on the chemical reactions taking place during processing and storage. Fats and oils are very susceptible to the to oxidation, which produces undesirable chemical compounds and is responsible for the formation of off-flavours which may be detrimental to health.
The oxidation of fats and oils is influenced by light, trace metals, antioxidants, temperature, and fatty acid composition. Even through thermal oxidation of deep-fat frying may provide good flavour, auto-oxidation, which produces volatile flavour compounds, is mainly responsible for the deterioration of flavour quality of oils during storage. These volatile compounds include esters, aldehydes, alcohols, ketones, lactonas, and hydrocarbons. Unsaturated aldehydes and ketones, which have the lowest sensory threshold, are principally responsible for the undesirable oxidized flavour of food.
auto-oxidation is a slow, radical process which proceeds via a chain reaction including induction, propagation and termination steps that are shown below
| initiation RH ® R* + H* |
| propagation R* + O2 ® ROO* ROO* + RH ® ROOH + R* |
| termination ROO* + R* ® ROOR R* + R* ® R-R |
Unsaturated fatty acids undergo oxidation by the above free-radical chain reaction to form hydroperoxides. The hydroperoxides are decomposed to form off-flavour compounds, which are called secondary oxidative products.
To prevent oxidation of fats and oils, antioxidants are widely used in food and cosmetic industries. These compounds (natural or synthetic) minimize oxidation of fats and oils by becoming oxidized themselves or by donating hydrogen to fatty acid free radicals to terminate free-radical chemical chain reactions.
In the last years, the research of natural antioxidants has been increased because of the possible toxicity of some synthetic antioxidants, namely butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), among others. Many herbs and spices possess antioxidant characteristic in fat and oil systems. In 1952, Chipault et al reported 32 natural spices used as antioxidant, in lard, but only rosemary and sage were effective.