Combined application of gallate ester and α-tocopherol in oil-in-water emulsion: Their distribution and antioxidant efficiency

Gallic acid or its alkyl ester was added in combination with α-tocopherol in the oil-in-water (O/W) emulsion to investigate their interaction in emulsions. Results showed that all the tested gallate esters (propyl, octyl and dodecyl gallate) showed synergistic antioxidant activities with α-tocopherol, and propyl gallate with the shortest alkyl chain length possessed the highest synergistic ability. The interaction efficiency was consistent with the order of their apparent static quenching constants obtained from fluorescence quenching studies. This suggested that the antioxidant synergy could be due to their molecular interactions. With further confirmation by antioxidant decomposition studies, the mechanism for such antioxidant synergy in O/W emulsions was proposed to begin with the initial quick oxidation of α-tocopherol into α-tocopherol quinine, resulting in its oriented exposure to the interfacial membrane of O/W emulsion and, subsequently, being reduced by propyl gallate esters in the interfacial layer to regenerate α-tocopherol.