Abstract
β-Lactoglobulin (BLG) adsorption layers at air-water interfaces were studied in situ with vibrational sum-frequency generation (SFG), tensiometry, surface dilatational rheology and ellipsometry as a function of bulk Ca(2+) concentration. The relation between the interfacial molecular structure of adsorbed BLG and the interactions with the supporting electrolyte is additionally addressed on higher length scales along the foam hierarchy - from the ubiquitous air-water interface through thin foam films to macroscopic foam. For concentrations <1 mM, a strong decrease in SFG intensity from O-H stretching bands and a slight increase in layer thickness and surface pressure are observed. A further increase in Ca(2+) concentrations above 1 mM causes an apparent change in the polarity of aromatic C-H stretching vibrations from interfacial BLG which we associate to a charge reversal at the interface. Foam film measurements show formation of common black films at Ca(2+) concentrations above 1 mM due to considerable decrease of the stabilizing electrostatic disjoining pressure. These observations also correlate with a minimum in macroscopic foam stability. For concentrations >30 mM Ca(2+), micrographs of foam films show clear signatures of aggregates which tend to increase the stability of foam films. Here, the interfacial layers have a higher surface dilatational elasticity. In fact, macroscopic foams formed from BLG dilutions with high Ca(2+) concentrations where aggregates and interfacial layers with higher elasticity are found, showed the highest stability with much smaller bubble sizes.
Highlights
Air–water interfaces are ubiquitous in aqueous foams and for that reason both molecular structure and molecular interactions within the interfacial plane are a major driving force that influences bubble coalescence, Ostwald ripening and foam drainage.[1]
2.1 Sample preparation b-Lactoglobulin was isolated as described previously[16] and kindly provided by the group of Ulrich Kulozik (TU Munchen, Germany). 15 mM BLG dilutions were prepared by dissolving the protein powder in ultrapure water (18.2 MO cm; total oxidizable carbon o5 ppb) which resulted in a pH of B6.7 in the absence of CaCl2
The spectra are dominated by two narrow bands at 2878 and 2936 cmÀ1 which can be attributed to symmetric CH3 stretching vibrations and the CH3 Fermi resonance, respectively
Summary
Air–water interfaces are ubiquitous in aqueous foams and for that reason both molecular structure and molecular interactions within the interfacial plane are a major driving force that influences bubble coalescence, Ostwald ripening and foam drainage.[1] In each of these cases the interface can act as a potential barrier preventing or promoting the latter. For foams in dairy products the interactions of BLG, a major whey protein, and Ca2+ ions is important due to the high concentration of Ca (B33 mM)[10] in bovine milk which is naturally rich of calcium. While calcium binding to whey proteins is important, the latter does play a major role in the cell metabolism[11] where Ca2+ can have many functions such as co-factors for enzymes or as promoter for signal transduction.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
