Antioxidant activity of carnosic acid and methyl carnosate in bulk oils and oil-in-water emulsions

This study was aimed at evaluating the antioxidant activities of carnosic acid and its methyl ester, methyl carnosate, in inhibiting the formation and decomposition of hydroperoxides in bulk and emulsified corn oil triglycerides at 60 degrees C. In both lipid systems, methyl carnosate was a better antioxidant than carnosic acid, and both were more active than alpha-tocopherol. The difference in antioxidant activity between methyl carnosate and carnosic acid was greater in emulsion than in bulk systems. Carnosic acid was less stable than methyl carnosate and alpha-tocopherol in bulk corn oil and 1% Tween 20 micelle solution. The partitioning of carnosic acid into the water phase may explain its low activity in emulsions. The measurement of antioxidant depletion may not be a valid method to evaluate antioxidant activities because the antioxidant activities of carnosic acid and methyl carnosate were not related to their oxidative stability in bulk oil and Tween 20 solution.