Authors:

Abstract

Dietary free radicals carry significant health risks. Antioxidant capacity and total phenolic content of seven edible oil types [coconut oil-I (loosely packed-wholesale), coconut oil-II (pure, sold in bottles), olive oil, soya oil, sunflower oil, palm oil and mee (Madhuca longifolia) oil] were measured at unheated, heated and reheated stages, to determine the effects of repeated heating on the antioxidant activity. Free radical scavenging assay - 2,2–azinobis-3–ethylbenzothiazoline–6-sulphonate, expressed as Trolox equivalent antioxidant capacity (TEAC) per gram of oil and Folin-Ciocalteu method were used to determine the antioxidant capacity and the total polyphenolic content respectively. Statistical significance was tested using ANOVA Tukey’s post-hoc test. At an unheated stage, mee oil had the highest antioxidant capacity (220.71±5.45 μg/g TEAC) followed by olive oil (22.10±2.77μg/g TEAC) and coconut oil-I (13.02±2.77μg/g TEAC). Other unheated oils showed negligible antioxidant capacities. First heating significantly reduced (p<0.05) the antioxidant capacity in all types of oils in which mee oil (124.43±4.81μg/g TEAC) showed the least reduction. This was followed by coconut oil-I (7.57±2.10 TEAC μg/g). After the second heating, all the oils except mee-oil (86.89±2.77μg/g TEAC) showed negligible antioxidant capacities. Olive oil showed the highest percentage reduction (290%) in antioxidant capacity from un-heated to the re-heated stage. At all the tested stages, mee-oil had the highest phenolic content (12610.6±28.8, 8471.70±68.0 and 7345.91±78.6 Gallic Acid Equivalent/100 g of oil respectively). All the oil types showed a significant change in phenolic content from unheated to re-heated stage (p<0.05). It can be concluded that heating and reheating of tested oil types cause significant reduction in antioxidant capacity. Of them, mee oil and coconut oil-I were comparatively safe to be re-used in cooking in terms of antioxidant capacity.

Keywords: Antioxidant capacity, Total phenolic content, Unheated, Heated, Re-heated oil