Structure-property relations of β-lactoglobulin/κ-carrageenan mixtures in aqueous foam

There is a great interest in mixed protein/polysaccharide layers at air-water interfaces due to their ability to stabilize aqueous foams. Properties of β-lactoglobulin (BLG)/κ-carrageenan mixtures at different κ-carrageenan concentrations were studied using foam analysis and molecular-level characterizations at the air-water interface. Because air-water interfaces are a major element in aqueous foams, they were investigated with a combination of interface-sensitive methods such as sum-frequency generation (SFG) as well as surface tensiometry. Using SFG spectroscopy we provide new information on the changes in the interfacial charging state and composition when the BLG/κ-carrageenan mixing ratio changes. κ-carrageenan is only weakly surface active as it does not lower the surface tension to a large extend. However, BLG/κ-carrageenan mixtures showed a synergistic effect because the formation of complexes at the air-water interface and in the bulk solution can favorably influence foam stability. BLG (7.5 µM) was mixed with different κ-carrageenan concentrations from 0 to 1 mM while the bulk pH was adjusted to pH values which are below (4.5), slightly above (5.6) and well above (7.0) the isoelectric point of BLG. At pH 4.5, the positive ζ-potential of BLG was drastically reduced to negative values and large aggregates in the bulk formed that showed a reduced surface activity and caused also a reduced net charging state at the air-water interface. The interfacial and bulk properties at pH 4.5, resulted in a poor stability of foams that were likely only stabilized by the weak electrostatic disjoining forces in the foam lamella. At pH 5.6 and 7, cooperative effects of BLG and κ-carrageenan dominate and, in particular, at pH 5.6 give rise to an increase in BLG surface coverage when the κ-carrageenan concentration increases. This resulted in a substantially improved foam stability.