Distributions of phenolic acid antioxidants between the interfacial and aqueous regions of corn oil emulsions: effects of acidity and emulsifier concentration

The oxidative stability of emulsions depends on acidity because pH may affect, among other things, the concentration of antioxidants (AOs) at the reaction site. Here we investigated the effects of pH on the partition constant, P_W^I, and the distribution of two phenolic acid AOs, gallic (GA) and caffeic (CA), between the aqueous (W) and interfacial (I) regions, in emulsions of 1:9 (vol:vol) corn oil/acidic water and Tween 20. P_W^I values are independent of emulsifier concentration, but change substantially with pH following sigmoidal curves with upper limits of P_W^I ≈280 (GA) and P_W^I ≈590 (CA) at high acidity. The distributions of GA and CA between the aqueous and interfacial regions depend strongly on surfactant volume fraction, Φ_I, so that at pH ∼4.0 and Φ_I = 0.005, %GA_I ≈20 and %CA_I ≈50. These values increase to ca. %GA_I = 60 and %CA_I = 90 at Φ_I = 0.05. Increasing the acidity produces substantial changes in %AO_I. At Φ_I = 0.005, a decrease in pH from ∼4 to ∼3 leads to an increase in %GA_I from ∼20 to ∼60 and in %CA_I from ∼50 to ∼75, respectively.

Practical applications: Our results are based on the pseudophase kinetic model originally developed for association colloids and should be of interest to the food industry and food chemists because phenolic acid antioxidants are widely employed to inhibit lipid oxidation. Depending on food emulsion pH, they may be partially ionized, thus altering their concentrations in the reactive region. We expect other phenolic acids to follow the reported trends because the acidity constants of their carboxylic groups are similar. We demonstrated that the efficiency of series of homologous antioxidants (AOs) can be correlated with their percentage in the interfacial region of the emulsions. Thus, application of the pseudophase kinetic model provides a unique approach to a better understanding of the effects of factors controlling AO distributions and efficiencies and a more rational selection of AOs and emulsifiers in food stabilization (e.g., environmental (acidity, T), emulsifiers, AOs HLBs, nature of the oils, etc.)