The coalescence stability of protein-stabilized emulsions estimated by analytical photo-centrifugation

Abstract

Various protein solutions were studied in order to quantify the emulsifying activity of proteins, and to explore oil-water interfacial tension, oil particle size analysis, and oil phase separation behaviors in protein-stabilized oil-in-water (O/W) emulsions. Three proteins, bovine serum albumin (BSA), β-lactoglobulin (β-lg), and β-casein (β-ca), were employed to disperse hexadecane in various pH and ionic strength solutions in a wide range of oil-water ratios. It was confirmed that the volume mean oil droplet diameter, d(43), changed depending on the oil content, the pH, the ionic strength, and the used protein. In a dilute protein solution (0.01 %) at pH 7, droplet size increased with oil content in so-called surfactant-poor regimes (e.g., above 5%, 10%, and 20% oil content for BSA, β-lg, and β-ca emulsion, respectively) but remained constant at ca. 10 mm, 6 mm, and 20 mm, respectively, in lower oil content surfactant-rich regimes. In surfactant-poor regimes, the most important factor determining the oil drop size was the threshold amount of protein adsorption onto the oil-water interface. In surfactant-rich regimes, on the other hand, it is suggested that drop size may be governed mainly by the mechanical strength of protein films covering the oil drops during emulsification, and this was quantified by the critical osmotic pressure, P(CR). In this study, the P(CR) was measured conveniently in the oil phase separation experiments for protein-stabilized emulsions using analytical photo-centrifugal apparatus. The correlation between the P(CR) and oil droplet size prepared by emulsification at different pH and ionic strength media is discussed.