Fatty acid alterations in oils and fats during heating and frying

Oils and fats degrade during the frying process and many reactions with numerous fatty acid alteration products have been examined. The geometrical isomerisation of double bonds leads to the formation of trans fatty acids. At frying temperatures also conjugated double bond systems are detected. The reaction of oxygen with unsaturated fatty acids results in hydroperoxides, which immediately degrade in further radical reactions at frying temperature. A set of oxygenated fatty acids has been detected including epoxy-, keto-and hydroxyl fatty acids. Another route leads to beta-scission at the carbonyl-or the alkyl side of the oxygen bearing carbon atom in the fatty acid chain. In this case short chain fatty acids, aldehydic, keto, and hydroxyl acids appear together with volatile compounds. Also the formation of cyclic and furan fatty acids was detected. As a reaction between fatty acids also dimeric and polymerised fatty acids can be observed. Taking into account the different amounts of these fatty acid degradation products the physiological relevance has to be discussed. Due to high concentrations of dimeric and polymerised molecules these substances can lower significantly the digestibility of fried foods, while oxidised fatty acid monomers are readily absorbed and raise concern about their effect on lipid metabolism. These two different effects of altered TAGs and fatty acids have to be considered separately.