Determining Antioxidant Distributions in Intact Emulsions by Kinetic Methods: Application of Pseudophase Models

Abstract

Accumulation, compartmentalization, and/or encapsulation of active ingredients at the interfaces of surfactant-based colloidal systems (micelles, microemulsions, liposomes, emulsions, etc.) are of great interest to the food industry, material science and bioengineering because many synthetically reversible or irreversible bimolecular reactions occur in the interfacial region formed between the oil and water regions and/or phases. For instance, the observed rate of inhibition between antioxidants and the peroxyl radicals formed during the oxidation of lipid-based emulsions depends on their effective concentration in the interfacial region. Antioxidants in one region of the emulsion transfer rapidly to the other regions so that they are uniformly distributed between the various domains of the emulsion after bulk mixing all components, leading to dynamic equilibrium. Determining the distribution of the antioxidants in the system is difficult because of the physical impossibility of separating regions without disrupting the existing equilibria. Thus, any attempt to determine their distributions and effective concentrations of components are best done on the intact system.

This chapter describes some methods for determining partition constants in binary and multiphasic systems of antioxidants with special emphasis on a recently chemical kinetic method developed to determine the distributions and effective concentrations of antioxidants while permitting components to move freely without the need of creating artificial barriers between compartments to separate domains. Here we will focus on the fate of antioxidants distributions prior to reacting with lipid radicals to demonstrate how their effective concentrations of each reactant in the interfacial region control their efficiency. The methodology is an excellent example of how physical-organic chemistry provides solutions and contributes to a better understanding of food problems such as the antioxidant inhibition of the lipid oxidation reaction.