Chemical factors affecting degradation processes of vegetable oils during frying

Food oils degrade rapidly when exposed to high temperatures in frying, but the mechanisms responsible have never been fully elucidated. Thermal degradation is usually described as a combination of hydrolysis and accelerated autoxidation. Thermal scissions of lipid chains have also been identified but remain controversial and largely ignored. This research sought to elucidate the reactions underlying thermal degradation, reconcile roles of thermal scission and autoxidation, and determine where catalytic factors intervene in degradation at frying temperatures. Thermal degradation processes were investigated in high oleic sunflower oil:corn oil blends (60:40 w/w) heated in an OxipresTM oxygen bomb at 180°C for three hours under 2 bars (O2, air, or O2/N2 blends) to determine rates and amounts of oxygen consumed during heating. Degradation products (conjugated dienes, peroxide values, aldehydes, free fatty acids) were measured in samples removed at various time intervals. In a parallel study, volatiles released during heating were trapped and analyzed by GC-MS. Effects of oil state and pre-oxidation were evaluated using stripped, fresh, and steady-state frying versions of the oil blend. Effects of oxygen, metals, water, phospholipids and free fatty acids were tested individually and in combinations to reveal synergistic catalysis of degradation. Results supported thermal scission of lipid acyl chains as the dominant process controlling degradation at high temperatures, generating radicals that are the precursors for a broad range of downstream products. Terminal peroxyl radicals formed by addition of oxygen to thermal scission radicals abstract hydrogens from lipid chains to initiate autoxidation chains as a secondary process. Oxygen and water were the major forces driving secondary reactions. Traditional catalytic factors known to induce lipid oxidation had relatively minor effects alone but in combination with water triggered rapid consumption of O2. Except for oxygen, catalytic factors affected secondary oxidation processes and product distributions rather than initiating oxidation. Mismatch between O2 consumption and levels of oxidation products demonstrated presence of reaction pathways that consumed oxygen but led to products other than those measured. Data was integrated to develop a new reaction scheme for thermal degradation of oils. Application of this new information can be used to improve stabilization of frying oils.Ph. D.Includes bibliographical referencesIncludes vitaby Xin Tia