Thermal desorption studies of corn oil at frying temperatures: thermal scission vs autoxidation

Thermal degradation occurs in all oils during frying, limiting useful fry life as well as quality and shelf life of fried foods. To elucidate the reactions involved, thermal degradation processes in corn oil heated to elevated temperatures were studied using thermal desorption techniques to detect released volatiles. Corn oil was loaded into glass Purge & Trap tubes containing a Celite support to increase oil surface area, and heated to 100, 120, 150, 180, and 235°C under nitrogen or air for up to 2 hours. Volatiles were flushed from the tubes and collected on Tenax-Carboxen thermal desorption traps at short intervals to limit the number of products present and provide a map of product changes over time, then desorbed into a Gas Chromatography-Mass Spectrometry system. Under air at 100-120°C, oxidation was slow; major shifts in mechanism occurred between 120 and 150°C and again at 235°C, with exponential increase in both rates of degradation and numbers of different products at each temperature. Above 150°C, complex product mixtures containing primarily C-4 to C-12 alkanes and alkenes, with low levels of oxidation products formed within minutes under nitrogen. Major scission points were carbons adjacent to the last double bond, yielding pentane from linoleic acid and octane and 1-decene from oleic acid. Under air, aldehydes, and alcohols of the same chain length plus 2-pentyl furan and ketones were released in much higher quantities. Simple oxidation products formed early in heating; as heating time increased, product mixtures became quite complex and included many cyclization and rearrangement products. Results support radicals from thermal scissions as major initiators of thermal degradation processes in oils. In air, these radicals form terminal peroxyl radicals and hydroperoxides which then decompose to oxidation products of the same chain length, dimerize, or initiate autoxidation chains by abstracting radicals at C13 of linoleic acid; decomposition of the C13-OOH releases pentane and hexanal. Thermal scissions are especially important for industrial frying conducted under limited oxygen while autoxidation is the dominant degradation affecting quality in food service operations, where oils are heated in air and may be used for many days.M.S.Includes bibliographical referencesby Xiaotian Qi