Relating foam and interfacial rheological properties of β-lactoglobulin solutions

Abstract

We have determined bulk rheology of b-lactoglobulin (BLG) foams and surface viscoelasticity of corresponding protein solutions by varying pH as well as type, valency and concentration of the added salt in a wide range. Foam rheology was characterized by the storage modulus G 0 , the apparent yield stress s y , and the critical strain g c,foam defining the cessation of the linear viscoelastic response. These quantities were determined at gas volume fractions f between 82% and 96%. Surface viscoelasticity was characterized in shear and dilation, corresponding shear and dilational moduli G 0 i , E 0 as well as the critical stress s c,surface and strain g c,surface marking the onset of non-linear response in oscillatory surface shear experiments were determined at fixed frequency. Beyond the widely accepted assumption that G 0 and s y are solely determined by the Laplace pressure within the droplets and the gas volume fraction we have found that both quantities strongly depend on corresponding interfacial properties. G 0 increases linearly with G 0 i and even stronger with E 0 , s y varies proportional to s c,surface and g c,foam scales linearly with g c,surface . Furthermore, deviations from these simple scaling laws with significantly higher reduced G 0 and s y values are observed only for foams at pH 5 and when a trivalent salt was added. Then also the dependence of these quantities on f is unusually weak and we attribute these findings to protein aggregation and structure formation across the lamellae than the dominating bulk rheology.

Rheology of foams and highly concentrated emulsions

“…For instance, for foams stabilized by globular proteins such as β-lactoglobulin, G o is larger than the value predicted by Eq. (2) by up to an order of magnitude [4,15,34]. The enhancement of G o depends on the pH and on the ionic strength.…”

“…The enhancement of G o depends on the pH and on the ionic strength. At a pH corresponding to the isoelectric point (pH = pH iep ), where proteins carry no net charge, the repulsion between molecules is reduced, leading to the formation of aggregates and networks across the films [15,34]. The strong mechanical reinforcement of the foams is attributed to this effect [15,34].…”

Interfacial Particle Dynamics: One and Two Step Yielding in Colloidal Glass

“…In most applications the shear rheology of the particle-laden interface greatly influences the macroscopic performance of multiphase systems, especially the stability of emulsions and foams in dynamic environments 9,10 . The shear-induced deformation of emulsion droplets and foam bubbles, as well as the liquid drainage in the plateau-borders, can be strongly influenced by the degree of particle mobility and reorganization at the interface [11][12][13] . It has also been reported that the arrested coalescence of bubbles/droplets is related to the shear yield strength of the interfacial layer, as the deformation and breakup of the bubble/droplet is limited by the rigidity of the interface 14 .…”

Interfacial properties, film dynamics and bulk rheology: A multi-scale approach to dairy protein foams

“…Stability against disproportionation has also been correlated with the higher interfacial dilatational elasticity E' of proteins [15]. Concerning foam rheology, it has been shown for β-lactoglobulin (β-lg) that a higher dilatational and shear interfacial elasticity can increase the yield stress and storage modulus of a foam [16].…”