Salt-dependent interaction behavior of β-Lactoglobulin molecules in relation to their surface and foaming properties

This multiscale study aimed at assessing the transferability of bulk phenomena to surface and foaming properties in terms of the establishment of structure-function relationships using the example of β-Lactoglobulin (β-Lg). The impact of salt type and concentration (i.e., NaCl and CaCl₂, 0–300 mM) on the interaction behavior of β-Lg molecules in bulk solution (i.e., 0.1%, pH 6.8) firstly was analyzed by means of angular and concentration-dependent static light scattering. Subsequently, surface (i.e., adsorption behavior and surface dilatational properties) and foaming properties (i.e., foam formation and decay behavior including bubble structural characteristics) were investigated. According to the second osmotic virial coefficient, protein-solvent interactions prevailed in presence of NaCl, whereas CaCl₂ promoted protein-protein interactions. These differences particularly appeared for salt concentrations < 200 mM. Surface as well as foaming properties of β-Lg were found to respond to changes in salt type and concentration and to reflect salt-specific effects. In particular, β-Lg adsorption at the air/water interface and foam formation were enhanced, whereby the influence of CaCl₂ was superior to NaCl. This observation was ascribed to the reduction of net surface charge with increasing salt concentration as well as the promotion of protein-protein interactions at the air/water interface. Overall, certain transfer of salt type- and concentration-dependent effects from bulk solution to surface functionality was established, though no straight correlation between bulk, surface and foaming characteristics was detected.