Kinetic Method for Determining Antioxidant Distributions in Model Food Emulsions: Distribution Constants of t-Butylhydroquinone in Mixtures of Octane, Water, and a Nonionic Emulsifier

The absence of reliable estimates of distributions of antioxidants in food emulsions hinders the development of a useful method for comparing the efficiencies of antioxidants. Here we describe the application of a pseudophase kinetic model, originally developed for homogeneous microemulsions, to the determination of distribution constants of tert-butylhydroquinone, TBHQ, in a fluid, opaque, model food emulsion composed of the nonionic emulsifier C12E6, octane, and water. This kinetic method should be applicable to a wide variety of charged and uncharged antioxidants in emulsions composed of charged and uncharged emulsifiers. The distribution constants for partitioning of TBHQ between the oil and surfactant film regions, KOI, and the aqueous and surfactant film regions, KWI, were obtained by fitting changes in first-order rate constants, kobs, with emulsifier volume fraction for the reaction of 4-hexadecyl-2,6-dimethylbenzenediazonium ion, 16-ArN2+, with TBHQ. The rate of formation of the reduced arene product hexadecyl-2,6-dimethylbenzene, 16-ArH, was followed by HPLC. About 90% of the TBHQ is in the surfactant film at about 2% volume fraction of C12E6, which suggests that this region may be the primary site of antioxidant activity for neutral phenolic antioxidants. Keywords: Antioxidants; tert-butylhydroquinone; TBHQ; emulsion; nonionic emulsifier; nonionic surfactant; C12E6; HPLC; pseudophase kinetic model; kinetics; distribution constants