Effects of acidity and emulsifier concentration on the distribution of vitamin C in a model food emulsion

We have analyzed the effects of acidity and emulsifier concentration on the distribution of ascorbic acid or vitamin C (VC) in a model emulsion prepared by mixing octane, acidic (HCl) water and the non-ionic surfactant hexaethyleneglycol monododecyl ether (C12E6). VC is oil insoluble, and only the partition constant between the interfacial and aqueous regions (PWI) is needed to describe its distribution within the emulsion. The PWI values are calculated from the kinetic analyses of the variation of the observed rate constant (k(obs)) determined electrochemically for the reaction between the hydrophobic 4-hexadecylbenzenediazonium (16-ArN2+) ions and VC with the emulsifier concentration. The determined PWI values (PWI = 4-25) are low in comparison with those obtained for more hydrophobic antioxidants like vitamin E, and pH dependent, decreasing upon decreasing the acidity of the medium. These low values are reflected in the distribution of VC so that a large fraction (>70%) is located in the aqueous region at low emulsifier concentrations. At a given pH, the %VC in the aqueous region decreases upon increasing the emulsifier concentration; meanwhile, at a given emulsifier concentration %VC increases upon lowering the acidity. The intrinsic rate constants in the interfacial region k(I) for the reaction between 16-ArN2+ and VC have also been determined at different acidities. Their variation with the pH follows an upward bend curve, suggesting an inverse dependence of k(I) with the acidity of the medium, in keeping with the proposed mechanism for the reaction. The results may be of some interest to the food and pharmaceutical industries because of the extensive use of VC as antioxidant or preservative and because of the health benefits of VC. Copyright (C) 2012 John Wiley & Sons, Ltd.