Relating foam and interfacial rheological properties of β-lactoglobulin solutions.

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 τy, and the critical strain γc,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, E' as well as the critical stress τc,surface and strain γc,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 τy 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 and even stronger with E', τy varies proportional to τc,surface and γc,foam scales linearly with γc,surface. Furthermore, deviations from these simple scaling laws with significantly higher reduced G0 and τy 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.