Evaluation of antioxidant activity of some plant extracts and their heat, pH and storage stability

In the present study, three plant foods, namely, drumstick leaves (Moringa oleifera), mint leaves (Mentha spicata) and carrot tuber (Daucus carota) were extracted with ethanol and analyzed for their antioxidant activity. The antioxidant activity of extracts was evaluated according to the amount of malonaldehyde (MDA) formed by the FeSO4-induced oxidation of linoleic acid and a high PUFA oil (sunflower oil) at 37 degrees C in Trizma-buffer (pH 7.4). At a concentration of 1.5 mg/ml of linoleic acid the extracts from drumstick and carrot had a higher antioxidant activity (83% and 80%) than alpha-tocopherol (72%). In sunflower oil, the extracts from drumstick leaves and mint leaves were found to exhibit a similar activity (46% and 44%). The extract from drumstick exhibited the highest activity in both lipid systems. In addition, the stability of extracts to pH (4 and 9) and temperature (100 degrees C, 15 min) was investigated. The antioxidant activity of the extracts from mint leaves and carrot was higher at pH 9 than pH 4, while that of drumstick extract remained the same under both pH conditions. The extract from carrot was more heat-stable than other extracts. The three extracts stored in the dark at 5 and 25 degrees C after a 15 day period did not show any significant change (p <= 0.05) in their antioxidant activity. These data indicate that selected plant extracts are potential sources of dietary antioxidants. (c) 2005 Elsevier Ltd. All rights reserved.