Antioxidant effect of plant extracts of Labiatae family

Abstract

The Labiatae family is universally considered as an important source of natural antioxidant. Methanolic extracts of 8 species of Labiatae were purified and their antioxidant activities were determined by (i) Active Oxygen Method (AOM), (ii) Rancimat and (iii) Schaal oven methods in soybean oil and by AOM in hydrogenated vegetable oil. The effects of extracts concentration and also the combination of extracts and citric acid or α-T on soybean oil stability were studied. Rosemary extract was found to be very effective in stabilizing soybean and hydrogenated vegetable oils by all three methods of evaluation. However, results from extracts with intermediate antioxidant activity i.e. thyme. balm, serpollet, marjoram and T. pubescens differed from one method to the other and also type of substrate or oil. Spearmint and N. bracteata did not show any antioxidant activity in the tested oils at high temperatures. All plant extracts had negative synergism with α-T, but some showed positive effect with citric acid. The stability of soybean oil increased with increase in extracts concentration.