Oil globule microstructure of protein/polysaccharide or protein/protein bilayer emulsions at various pH

Abstract

The microstructure of oil droplets of bi-layer emulsions was studied as a function of pH (i.e. 7, 5, and 3) using scanning electron microscopy. The bi-layer emulsions consisted of a primary emulsion: 5 wt% soybean oil (SBO) in a 1% protein (nonfat dry milk) aqueous solution. The secondary layer was ι-carrageenan, high- (HMp), low (LMp)-methoxyl pectin, or gelatin. The secondary emulsions consisted of 2.5% SBO, 0.5% protein, and 0.2% polysaccharide or protein. Gelatin secondary emulsions were stable at pH 7 with defined droplets and became unstable at pH 5 and 3. The destabilization mechanisms for these emulsions at pH 5 and 3 were different as observed with the SEM: at pH 5 there is complete aggregation of protein due to their proximity to the isoelectric point; and at pH 3 the droplets are perfectly separated, suggesting that at this pH, when the net charge is positive, the destabilization is mainly due to depletion flocculation. HMp secondary emulsions shift from being stable (individual droplets) at pH 3 to being unstable at pH 7 where an extensive webbing is observed between the droplets at this pH value. The ι-carrageenan secondary emulsions are stable at each pH and the individual droplet microstructure is minimally altered as the pH changes. LMp secondary emulsions shift from being stable at pH 7 with individual droplets observed in the SEM micrographs to being unstable at pH 3 where extensive webbing is observed in the SEM micrographs.