Effect of pH and Protein Flexibility on the Structure of Protein Foams: A Multi-Scale Approach

Aqueous protein-stabilized foams are omnipresent in daily life, e.g., within food or in industrial applications. In this study, we apply a multiscale approach and investigate proteins adsorbed at the air/water interface, as well as foam films and macroscopic foams stabilized by four different proteins: β-lactoglobulin (BLG), bovine serum albumin (BSA), casein (CN), and lupine protein isolate (LPI). Protein adsorption at the air/water interface was investigated using Brewster angle microscopy (BAM) and X-ray reflectivity (XRR). Adding to individual thin film studies with a thin film pressure balance (TFPB), we employed small-angle scattering (SANS), which is able to investigate Newton black films (NBFs) within macroscopic foams. The film is thinnest near the isoelectric point (IEP), while adsorption layers at the air/water interface are thickest at the IEP. Importantly, we report for the first time the properties of NBFs within protein foams, enabling a direct comparison across length scales with individual foam films.