Thermal desorption studies of volatiles released during heating of food oils

Lipid degradation occurs extensively during deep fat frying, and controlling it is a great challenge, at least in part because the reaction mechanisms responsible remain controversial. With current trends towards increasing unsaturated fats in frying oils, a more complete understanding of these pathways becomes critical. To gain more detailed information about reactions involved in thermal decomposition kinetics and products of frying oils, corn oil/ high oleic sunflower oil blends (fresh, stripped, and steady-state) were heated at 180°C for three hours in an OxipresTM oxygen bomb under 2 bars air pressure. Upon release of pressure, the headspace was vented through a Tenax-Carboxen thermal desorption trap to trap volatiles which were then identified and quantitated by gas chromatography-mass spectrometry. Volatile levels and distributions were integrated with oxygen consumption and non-volatile product data from another study to assess relative contributions of thermal scission and autoxidation reaction mechanisms in overall degradation. Effects of catalytic factors on oil degradation were evaluated by adding metals, phospholipids, fatty acids, and water to the oil blends during heating. Fresh, stripped, and steady-state oils all generated comparable volatile products, but with differing concentrations and distributions. The main peaks coeluting isopentane/ pentane octane/ hexanal > pentanal/heptanes. Peak analyses revealed homologous series of alkanes, aldehydes, alkenes, ketones, cycloalkanes, and furans were also present. Levels of products generally decreased with chain length. No 2,4-decadienal was detected. This product pattern and the kinetics of evolution of different products provides strong support for thermal scission as the dominant degradation mechanism that occurs first to generate scission radicals that yield alkanes directly or oxidize to hydroperoxides, aldehydes, and carboxylic acids. Secondary reactions then initiate autoxidation chains. Factors known to have strong effects on lipid oxidation at room temperature appear to influence product distribution rather than degradation kinetics in heated oils. Integration of volatiles and non-volatiles data with oxygen consumption suggests there are pathways active in thermal degradation that are not being accounted for in current analyses, and these need to be elucidated to fully understand how various factors influence oil degradation as well as to learn how to improve frying oil stabilization.