Structural characterization of milk protein-stabilized foams: A multiscale approach

Protein-stabilized foams have been subject to wide-ranging investigations within the last several decades. In this context, improvement of the general understanding of the impact of protein structural attributes, in combination with environmental and/or processing conditions on protein’s behavior at the air/water interface or macroscopic foam properties, was and still is the primary objective of scientific and industrial research. Vast knowledge has already been acquired and diverse explanatory approaches have been proposed to describe the mechanisms underlying protein-based foaming. However, still no general consensus prevails. On the one hand, this is due to the high structural complexity of proteins and foams per se. On the other hand, instability of the addressed systems as well as analytical limitations render the direct clarification of surface phenomena challenging. Moreover, the majority of conducted studies only focused on finding a correlation either between bulk and surface properties or between surface and foaming properties. Hardly ever, investigations encompassed relations between bulk (i.e., molecular and structural properties), surface and foaming characteristics. Thus, the aim of this work was a multiscale characterization of selected milk proteins as a function of milieu conditions. Performed analyses included solution properties (e.g., particle size, surface hydrophobicity, surface charge), interfacial characteristics (e.g., adsorption kinetics, surface dilatational properties) as well as bubble structural attributes (e.g., bubble size distribution and count), foam formation and decay performance. Summing up the results, protein’s ability to promptly and pronouncedly interact at the air/water-interface is the key factor in terms of the formation of voluminous and time stable foam structures, which was also reflected in interfacial properties. Thereby, rather unfavorable molecule characteristics like a large particle size could be compensated by factors like surface hydrophobicity and surface charge. Overall, it was shown that a correlation of the obtained results across the different examined levels (solution/interface/bubbles/foam) allows to generate a comprehensive knowledge on structure-functionality relationships.