The prooxidant role of free fatty acids was studied in soybean oil-in-water emulsions. Addition of oleic acid (0-5.0% of oil) to the emulsions increased lipid hydroperoxides and headspace hexanal formation and increased the negative charge of the emulsion droplet with increasing oleic acid concentration. Methyl oleate (1.0% of oil) did not increase oxidation rates. The ability of oleic acid to promote lipid oxidation in oil-in-water emulsions decreased with decreasing pH with dramatic reduction in oxidation observed when the pH was low enough so that the oleic acid was not able to increase the negative charge of the emulsion droplet. Ethylenediaminetetraacetic acid (EDTA, 200 microm) strongly inhibited lipid oxidation in emulsions with oleic acid, indicating that transition metals were responsible for accelerating oxidation. Oleic acid hydroperoxides did not increase oxidation rates, suggesting that hydroperoxides on free fatty acids are not strong prooxidants in oil-in-water emulsion. These results suggest that the prooxidant activity of free fatty acids in oil-in-water emulsions is due to their ability to attact prooxidant metals to the emulsion droplet surface.