Testing and comparing oxidative stability of vegetable oils and fats at frying temperature

Abstract

A new method has been developed to estimate the stabilising activity of synthetic and natural food additives at frying. Non-refined and refined vegetable fats and oils were heated at a temperature of 170°C after adding water-conditioned silica gel for two hours. The degraded products were measured to assess the oil stability at frying temperature. The determination of polymeric triglycerides by size exclusion high-pressure liquid chromatography (HPLC) was carried out for the estimation of the oxidative heat stability of vegetable fats and oils. Tocopherols, various tocopherol esters and phytosterol fractions, phenolic compounds, like quercetin, oryzanol, ferulic acid, squalene, butyl hydroxytoluol (BHT), butyl hydroxyanisol (BHA), and other compounds, like ascorbic acid 6-palmitate and gallates, are added to refined sunflower and rapeseed oil and their efficacy determined. Both linoleic and oleic rich oils gave comparable results for the activity of the various compounds. α-tocopherol, tocopherol esters, and BHA have low effects at frying temperature. Ascorbic acid 6-palmitate and some phytosterol fractions were found to have the greatest antioxidant activity. Corn oil was more stable than soybean oil and rapeseed oil better than olive oil. It was also observed that non-refined oils proved to have a better stability at elevated temperature than refined oils. The results show that the stability of the vegetable oils at frying temperature is a function of more than just the fatty acid composition. There is evidence which supports a co-relationship between the unsaponifiable matter content and oxidative stability. It is believed that a radical peroxidation mechanism predominates at lower temperatures. When a large volume of oil is heated in a fryer and the oxygen supply is poor, non-radical reactions such as elimination (acid catalysed dehydration) or nucleophilic substitution take place.