Identifying changes in chemical, interfacial and foam properties of β-lactoglobulin–sodium dodecyl sulphate mixtures

Abstract

Techno-functional properties of proteins, such as foam stability, can be affected by the presence of low-molecular-weight surfactants. In order to understand and control the foam properties of such protein–surfactant mixtures, a thorough characterization of foam and interfacial properties needs to be supplemented by a detailed analysis of the structural changes of the protein and possible complexation with the surfactant. In this study, β-lactoglobulin (BLG) was mixed with sodium dodecyl sulphate (SDS) in different molar ratios (MRs). The foam half-life time of BLG-SDS mixtures decreased from that of pure BLG (315min at MR 0) to 44min at MR 20, which is close to the half-life of SDS at the respective concentration. With a further increase in the MR, the foam stability of the mixture increased, similar to the stability of SDS, to 250min at the highest MR (MR 100). The minimum in the foam stability curve was not reflected in the interfacial properties (Π and Ed). Π decreased and Ed increased continuously with increasing MR from values close to those of protein towards values typically found in pure surfactant solutions.The results show no clear correlation between the interfacial and foaming properties. In addition, it was shown by isothermal titration calorimetry and mass spectrometry that SDS molecules bind to the BLG. This leads to the formation of BLG-SDS complexes. These complexes have large influence on the foam properties in the mixture. The combination of analytical methods that were used give insights about protein complexation and the resulting change of foam properties of the mixture.