Effect of surface-active stabilizers on the surface properties of coconut milk emulsions

Recently we have shown how surface-active stabilizers (≤1 wt% sodium dodecyl sulfate, polyoxyethylene sorbitan monolaurate, whey protein isolate, and sodium caseinate) added either before or after homogenization affect the stability and properties of coconut milk emulsions [Tangsuphoom, N., & Coupland, J. N. (2008). Effect of surface-active stabilizers on the microstructure and stability of coconut milk emulsions. Food Hydrocolloids, 22(7), 1233-1242]. In this work, we propose a mechanism to explain these observations based on changes in surface composition and properties. Coconut milk has a high surface protein load (∼7 mg m^-2), of which the major component is cocosin. Small-molecule surfactants (>0.25%) added to the homogenized coconut milk displaced the coconut proteins from the surface resulting in a change in ζ-potential. Dairy proteins added to the coconut milk after homogenization did not accumulate at the droplet surface. Addition of small-molecule surfactants to the coconut milk prior to homogenization completely displaced coconut proteins from the oil-water interface. Homogenization of coconut milk with proteins resulted in a decrease in total protein surface concentration (to about 2 mg m^-2) and a significant replacement of coconut proteins by the dairy proteins. The change in ζ-potential also reflected the change in protein and surface composition as the values moved close to the value of coconut oil emulsions stabilized solely with the corresponding added stabilizer.