Relating foam and interfacial rheological properties of β-lactoglobulin solutions

Abstract

We have determined bulk rheology of β-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 G₀, the apparent yield stress τ_y, and the critical strain γ_c,foam defining the cessation of the linear viscoelastic response. These quantities were determined at gas volume fractions ϕ between 82% and 96%. Surface viscoelasticity was characterized in shear and dilation, corresponding shear and dilational moduli G^′_i, E′ as well as the critical stress τ_c,surface and strain γ_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 G₀ and τ_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. G₀ increases linearly with G^′_i and even stronger with E′, τ_y varies proportional to τ_c,surface and γ_c,foam scales linearly with γ_c,surface. Furthermore, deviations from these simple scaling laws with significantly higher reduced G₀ and τ_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 ϕ is unusually weak and we attribute these findings to protein aggregation and structure formation across the lamellae than the dominating bulk rheology.