Viscoelastic Properties of Diglycerol Ester and Protein Adsorbed Films at the Air−Water Interface

Abstract

The dynamic surface pressure (π) and surface viscoelastic properties (surface dilatational modulus, E, its elastic, Ed, and viscous, Ev, components, and loss angle tangent, tan φ) of lipids (diglycerol-monostearate and diglycerol-monolaurate) and proteins (sodium caseinate and β-lactoglobulin) at different concentrations in the aqueous phase were measured using a dynamic drop tensiometer. Temperature, pH, and, ionic strength were maintained constant at 20 °C, 7, and 0.05 M, respectively. The surface dilatational properties depend on the chemistry of the molecule (length of the hydrocarbon chain in lipids and disordered or globular characteristics of the protein). Lipid films are essentially elastic, but protein films are viscoelastic. The values of Ed increase with the emulsifier (lipid or protein) concentration in solution and were higher at the critical micellar concentration (CMC) for lipids or at the adsorption efficiency (AE) for proteins. The values of E reflect not only the amount of emulsifier adsorbed at the interface but also the degree of interaction between adsorbed emulsifier molecules. E increases with the van der Waals interactions between lipid hydrocarbon chains, which are higher for diglycerol-monostearate than for diglycerol-monolaurate. The values of E for caseinate adsorbed films were lower than those for β-lactoglobulin adsorbed films. These differences are due mainly to differences in the looping of amino acid residues for adsorbed films of random coil (caseinate) and globular (β-lactoglobulin) proteins at the air−water interface. The values of E are lower for protein compare to lipid adsorbed films. The role of surface dynamic properties of lipid and protein adsorbed films on foam formation and stabilization is discussed.