Background and aimsMicrowave heating has become a common practice in regular home cooking. Vegetable oils, containing unsaturated fatty acids are susceptible to be oxidised during this procedure. Lipid peroxides may have toxic potential on human health. Antioxidants present in vegetable oils can protect and may delay the peroxidation process. Therefore we have considered of interest to study the variation of total antioxidant capacity and -tocopherol in relation to lipid peroxides and conjugated dienes formation during microwave oils heating comparing five commercial cooking oils currently available on Romanian market.

Materials and methods

Sample processingSunflower, corn, soybean,palm and a mixed oil(containing sunflower, grape, flaxseed and rice oil) were purchased from the local supermarket. To simulate conventional times used in microwave home cooking, different exposure times were tested, namely, 5, 10 and 15 min. For all samples, and for each exposure time, 50 mL oil aliquotes were individually heated in Erlenmeyer dishes in a domestic microwave oven (Maxwell) at maximum potency (1200 W). After cooling, the samples were transferred to Falcon tubes and stored under refrigeration until analysis. TROLOX Equivalent Antioxidant Capacity (TEAC), lipid peroxides as thiobarbituric reactive substances (TBARS), oxidation index as conjugated dienes and vitamin E as -tocopherol were determined on oils samples before and after 5, 10 and 15 minutes microwave heating at 2450 KHz. Total antioxidant activity (TEAC) was determined based on 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid (ABTS) assay developed by Miller and Rice-Evans (1997), with modifications (Re R. 1999). The results were expressed as mol eq. Trolox/l oil.TBARS were determined according to the method developed b Esterbauer (1990) with slight modifications for oils. The results were expressed as M MDA eq/ l oil. Oxidation index, defined as the absorption of conjugated dienes, was followed spectrophotometrically at 234 nm according to Shimada (1991). The oil sample was diluted (1:50, v:v) with hexane. An extinction coefficient of 29,000 mol/L was utilized to quantify the concentration of conjugated dienes formed during oxidation. Vitamin E was determined through the formation of a phosphomolybdenum complex according to Shaguy et al. (1996). Absorbance at 695 nm was measured against the appropriate blank. A calibration curve was constructed using -tocopheril acetate (0,5-5 mM) as standard.

ConclusionsCorn and soybean oils were having the highest initial TEAC values (9.92 and 9.56 mM TROLOX Eq/l) while the commercial palm oil was having the lowest one(6,04mM TROLOX Eq/l). TEAC values were decreased for all the vegetable oils until the end of the study period. There is only a weak negative correlation (-0.324), considered nonsignificant, between the initial TEAC values and the level of TBARS after 15 minutes of heating.Vitamin E is lost along with the oxidation of unsaturated fatty acids during heating, and the degradation follows an obvious trend with temperature and time of exposure. Conjugated dienes showed a remarcable increase along with the heating process.

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