Competitive Adsorption of Dihydroxy and Trihydroxy Bile Salts with Whey Protein and Casein in Oil-in-Water Emulsions

Abstract

The competitive adsorption between whey protein concentrate (WPC) or sodium caseinate (SCN) and four bile salts, sodium cholate (NaC), dexocycholate (NaDC), taurocholate (NaTC), and glycodeoxycholate (NaGDC), has been studied in protein stabilized oil-in-water emulsions. The bile salts that contain a conjugated amino acid (NaTC and NaGDC) were considerably more efficient at displacing both WPC and SCN proteins from the emulsion droplet interface, even though they are known to have a hydrophobicity lower than that of NaC and NaDC. This is explained in terms of a steric resistance to adsorption from the conjugated amino acids in NaTC and NaGDC. This leads to their adopting an adsorbed conformation at the oil-water interface that penetrates less into the oil phase, causing greater disruption of the adsorbed layer, and hence leads to greater displacement of protein from the interface. Complementary computer simulations of the adsorption of the four bile salts at the decane-water interface support the hypothesis that the NaTC and NaGDC adopt flatter conformations that stick out further into the aqueous phase, which arises from a lower free energy of adsorption. The surface coverage as a function of bulk concentration for the four bile salts has also been measured. These have been found to have a form that fits closely the Langmuir-Freundlich isotherm. The results for NaC suggest that it adsorbs as individual molecules and forms a saturated monolayer over much of the concentration range used in the displacement experiments, since it is below its critical micelle concentration in this range. For the other three bile salts, on the other hand, the primary adsorbing species appears to be the micelle form, since the surface coverage is above that of a saturated monolayer for much of the concentration range studied.