Adsorption of Gum Arabic, Egg White Protein, and Their Mixtures at the Oil−Water Interface in Limonene Oil-in-Water Emulsions

Abstract

The adsorption behavior of gum Arabic, egg white protein, and their mixtures at the oil-water interface for 20% limonene oil emulsions has been investigated at pH 3.5 and 7.5. It has been shown that protein-rich fractions of gum Arabic adsorb onto limonene oil droplets and that there is no significant molecular mass dependence. The amount adsorbed was determined from differences in the intensities of the gel permeation chromatography elution profiles of the gum before and after preparing emulsions and was found to be approximately 6.5 and approximately 5 mg/m(2) at pH 3.5 and 7.5, respectively. These values are greater than might be expected for monolayer coverage. The amount of protein associated with the gum Arabic adsorbed was about 0.25 mg/m(2), which corresponds to approximately 26% of the total protein present in the gum. In comparison, the amount of egg white protein adsorbed was found to be approximately 1.2 and approximately 0.8 mg/m(2), respectively, at pH 3.5 and 7.5, which are typical values for monolayer coverage. For gum Arabic-egg white protein mixtures (1:0.05 w/w corresponding to approximately 1:1 on a molar basis) at pH 7.5, both species are negatively charged, and there is no interaction between them. On formation of emulsions, they compete with each other for surface sites, and egg white protein molecules are adsorbed preferentially, although not exclusively. At pH 3.5, the molecules have opposite charge and interact and at this ratio form soluble electrostatic complexes, which are adsorbed at the interface during emulsification. The droplet size of emulsions prepared with gum Arabic was significantly less than for egg white protein over the concentration range studied. Gum Arabic-egg white protein mixtures (1: 0.05 w/w) at pH 3.5 produced emulsions with a droplet size similar to gum Arabic alone, while those prepared at pH 7.5 produced emulsions with a significantly larger droplet size.