Electrostatic interaction between whey proteins and low methoxy pectin studied by quartz crystal microbalance with dissipation monitoring

Electrostatic interaction between proteins and polysaccharides has attracted considerable attention in the design of fluid interfaces with improved performance, e.g. by sequential adsorption. Here, a real-time quartz crystal microbalance with dissipation monitoring (QCM-D) was used to investigate the sequential adsorption of whey proteins and low methoxy pectin (LMP) as a function of pH. A gold sensor was hydmphobically modified to mimic the oil-water interface. At neutral pH, whey proteins adsorbed onto the hydrophobic surface and formed a viscoelastic film, where most of the adsorption is irreversible. The protein film reversibly became more rigid around its isoelectric point (IEP, pH 5.0) due to structural rearrangements. Interfacial complexation of LMP and the pre-adsorbed WPI occurred over a wide pH range (3.0-6.5), but the adsorbed amount of LMP was pH-dependent. Adsorption of LMP increased the viscoelasticity of the pre-adsorbed WPI layer by electrostatic complexation. Especially, some trapped liquid could be released from the interfacial structure due to the relatively strong interaction (e.g. at pH 4.0). After switching to pH 7.0, the adsorbed LMP was fully detached from the protein layer, suggesting its pH-response behavior. The results indicated that QCM-D is ideally suited to investigate the effect of changing environmental conditions (such as pH) on the characteristics of the adsorbed layer, and to study the interaction between different components by sequential adsorption.