Effect of continuous usage on the natural antioxidants of vegetable oils during deep-fat frying

Edible frying oils are important source of vitamins and natural antioxidants. They provide protection against diseases such as cancer and cardiovascular diseases. Using oils for prolonged periods, may cause its quality to deteriorate. The effect of continuous usage of vegetable oil used for frying different food types on the antioxidant content was determined. Vegetable oils were obtained from roadside food vendors, fast food outlets, controlled frying in the laboratory, and unused branded and unbranded oils. The oils were analysed for total phenolic acids (PO), flavonoids (FL), Vitamins A and E. Oil samples collected from the roadside food fryers had higher PO and FL concentrations, but lower vitamins A and E than those from commercial fast food outlets. In the controlled experiment, PO, FL, vitamin A and vitamin E concentrations were significantly lower (p < 0.05) in samples used for frying beans, tuber products and pastries but oils used for frying meat products had significantly higher (p < 0.05) total phenol and flavonoid contents. The study showed that changes in antioxidant levels of vegetable oils was not only a function of usage but was greatly influenced by the nature of food with which it was fried. The negative impact of consuming foods and food products produced with reused oils both at home and from commercial food outlets calls for great concern and awareness.