The thermal deterioration of cooking oil during deep-frying with a diacylglycerol (DAG)-rich oil (DAG-OIL) was compared with that for a cooking oil composed of a blend of commercial cooking oils (TAG-OIL) with a comparable FA composition and tocopherol content. Analyses of several indices of deterioration indicated no substantial difference in panisidine values, iodine values, and petroleum ether-insoluble oxidized FA between DAG-OIL and TAG-OIL. The polymerized glyceride (PG) content was lower for DAG-OIL than TAG-OIL. However, the PG value did not reflect the degree of polymerization of the FA chains directly, since both DAG-OIL and TAG-OIL generated polymeric products but of different types. An analysis of the polymerized FA content revealed no significant difference in the degree of polymerization of either of the oils. The total polar compounds included nonaltered DAG as an altered compound, and, as a result, this index was not appropriate for DAG-OIL. DAG-OIL underwent hydrolysis more rapidly than TAG-OIL. This difference was mainly correlated with moisture contained by the oil during frying and with the total molarity of the glycerides. Even though DAG-OIL was used until it became a waste oil, the extent of thermal oxidation was the same as that for TAG-OIL, although some indices showed a different trend from TAG-OIL. Molecular structure had no influence on the thermal deterioration of the frying oil. We conclude that the choice of indices is an important factor when the deterioration of DAG-OIL is evaluated.

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