Comparative study on the deterioration of oils by microwave and conventional heating

Refined cottonseed oil and hydrogenated palm oil were heated by two methods, conventionally by gas-cooker (deep fat frying) and by microwave energy. Quality assurance methods such as refractive index, color, diene content, acid value, peroxide value, thiobarbituric acid value, iodine value, petroleum ether insoluble oxidized fatty acid content, and degree of polymerization were determined. Exposing the oil samples to various heating times and microwave oven power levels caused some hydrolysis to free fatty acids and accelerated the formation of hydroperoxides and secondary oxidation products. The value from each test increased with both the power settings of the microwave oven and the time of heating. In general, the development of rancidity for refined cottonseed oil heated by microwaves was twice as fast as that produced by conventional heating. Also, the chemical values indicating deterioration for refined cottonseed oil were much higher for refined cottonseed oil than for hydrogenated palm oil in all cases. Gas-liquid chromatographic analysis of fatty acids obtained from the heated oils indicates the occurrence of oxidative degradation and production of short-chain acids.