Surface Shear Rheology of β-Casein Layers at the Air/Solution Interface:  Formation of a Two-Dimensional Physical Gel

Abstract

A magnetic rod interfacial shear rheometer was used to measure the properties of β-casein adsorbed at the air/solution interface as a function of aging time. Over a wide range of concentrations (1 × 10-6 to 2 × 10-2 wt % β-casein) the initial rheology of the adsorbed surface layer is dominated by a viscous response of the interface. For solutions in the range 5 × 10-3 to 2 × 10-2 wt %, interfacial gelation is observed after ∼15 h of aging, long after the surface tension has stabilized. In particular, although both components of the complex interfacial shear modulus (i.e., the storage and loss moduli) gradually increase with aging time, the ratio of the loss to the storage modulusthe loss tangentdecreases and drops below unity. The frequency dependence of the shear modulus is consistent with sol−gel transitions observed in bulk systems and described within the context of percolation theory.

Surface rheology and interface stability.

“…Those measurements were performed with a bicone fixture (Anton Paar Physica MCR 300 rheometer). Our results for β-casein are less consistent with Bantchev & Schwartz (2003) using an interfacial stress rheometer, though their results are for much lower concentrations than we studied, so we cannot make a direct comparison. In their measurements, they also noted that for short times the viscous modulus was higher than the elastic modulus, but that the elastic component dominated at later times.…”

“…We have focused our efforts on β-lactoglobulin and β-casein as these two proteins have been previously studied with interfacial rheology [Bantchev & Schwartz 2003, Blijdenstein et al 2010]. Time-dependent interfacial moduli are shown in Figure 44.…”

Emulsion Drops with Complex Interfaces: Globular Versus Flexible Proteins

“…[3] Adsorbed proteins modify the properties of liquid interfaces in several ways: (i) they are polyampholytes and decrease the interfacial tension upon adsorption; depending on the pH and the ionic strength, they can also provide charge to the interface; (ii) unlike many traditional surfactants, protein adsorption layers impose local viscoelastic properties onto liquid interfaces. [4][5][6][7][8] Furthermore, adsorption of proteins is effectively irreversible, as demonstrated in subphase exchange experiments: [4,5] only minor desorption and negligible changes in interfacial tension are observed after a protein solution is replaced with protein-free buffer surrounding a protein-covered pendant drop. These effects have been described for a number of proteins of both biological and technical relevance, such as b-casein, [6,7] b-lactoglobulin, [9][10][11][12] bovine serum albumin, [13] or lysozyme.…”

“…[4][5][6][7][8] Furthermore, adsorption of proteins is effectively irreversible, as demonstrated in subphase exchange experiments: [4,5] only minor desorption and negligible changes in interfacial tension are observed after a protein solution is replaced with protein-free buffer surrounding a protein-covered pendant drop. These effects have been described for a number of proteins of both biological and technical relevance, such as b-casein, [6,7] b-lactoglobulin, [9][10][11][12] bovine serum albumin, [13] or lysozyme. [5] While the interfacial chemistry and rheology of adsorption layers formed by these proteins have received increased attention recently, less is known about the role their transport properties play in the context of high This study focuses on the flow behavior of emulsion drops with complex interfaces.…”

“…Developments after this study have been summarized by Dickinson, [2,9] van Vliet et al, [26] Damodaran, [32] Möbius and Miller, [15] and Murray. [22] Interfacial shear rheology can provide information about protein-protein interactions; [7] however, steady shear experiments are also known to cause fracture in rigid protein layers. [33] Therefore, suitable experiments for proteins should be performed either at very small deformations, or the deformation-dependence itself must be taken into account, for example using stress-strain curves [12] or strain amplitude sweeps.…”