Synchrotron micro-CT for studying coarsening in milk protein-stabilized foams in situ

Motivation: Foam coarsening, i.e., the ripening of bubbles, constitutes a principal aging processes that links the microscopic, chemical composition of a foam with its macroscopic stability and texture. While the effects of chemical composition on macroscopic appearance are well studied, the actual three-dimensional bubble ripening remained quasi-inaccessible due to a lack of experimental techniques. Approach: Using high-speed synchrotron micro computed tomography, the influence of pH on the coarsening of milk protein foams is studied in situ, individually tracking the ripening of thousands of bubbles in order to characterize the inter-bubble gas diffusion by fitting theoretical models on foam coarsening. Findings: In all experiments the inter-bubble gas diffusion of milk protein-stabilized foams depend on pH. Despite having much different initial size distributions the coarsening kinetics of the foams are highly reproducible. A strong correlation between kinetic parameters describing coarsening and foam stability is observed, whereby faster coarsening occurs for less stable foams. Moreover, specific cases of high stability despite high diffusion rates could be observed for beta-lactoglobulin. These observations correlate with known changes in the macro-molecular configurations of the protein which result in different surface coverage of the liquid lamellae. These findings are first evidence of a likely microscopic-mesoscopic relationship in liquid foam chemistry.