Antioxidant activity of a rosemary extract and its constituents, carnosic acid, carnosol, and rosmarinic acid, in bulk oil and oil-in-water emulsion

This study was aimed at evaluating the antioxidant activity of a commercial rosemary extract and the active constituents carnosol, carnosic acid, and rosmarinic acid, in inhibiting the formation and decomposition of hydroperoxides in tocopherol-stripped corn oil and in the corresponding corn oil-in-water emulsions. In bulk corn oil, the rosemary extract, carnosic acid, rosmarinic acid, and alpha-tocopherol were significantly more active than carnosol. In contrast, in corn oil-in-water emulsion, the rosemary compounds were less active than in bulk oil, and the rosemary extract, carnosic acid, carnosol, and alpha-tocopherol were more active than rosmarinic acid. Similar results were obtained in corn oil-in-water phosphate buffer emulsion at pH 5, but alpha-tocopherol was less active. Carnosol and carnosic acid were much more active antioxidants in corn oil-in-water emulsions buffered at pH 4 and 5 than at pH 7. The decreased antioxidant activity of the polar hydrophilic rosemary compounds in the emulsion system may be explained by their interfacial partitioning into the water, thus becoming less protective than in the bulk oil. system. The effect of pH may be related to the stability of the rosemary antioxidants.