Sequential adsorption of whey proteins and low methoxy pectin at the oil-water interface: An interfacial rheology study

Protein-polysaccharide bilayers have attracted a growing interest in the design of emulsion stabilizers with improved performance. An in-depth understanding of their interfacial properties helps rational interfacial engineering. Here, the sequential adsorption of whey protein isolate (WPI) and low methoxy pectin (LMP) at the oil-water interface as a function of pH was investigated with a modified drop tensiometer allowing external phase exchange. Residual WPI in the bulk phase could accumulate at the interface (i.e. pre-formed WPI film at pH 7.0) when the pH was adjusted to be close to its iso-electric point, leading to reduced interfacial tension and increased surface concentration. However, the interfacial tension was hardly influenced by successive pH changes and LMP addition if the excess protein was washed away beforehand. A maximum interfacial viscoelasticity was reached when the pH was close to the isoelectric point of WPI, corresponding to the minimum hydration degree. Whereas the deposition of LMP onto a protein-coated oil-water interface significantly increased the interfacial viscoelasticity, its magnitude was largely dependent on the pH conditions. Besides, a more linear rheological response of the bilayer structures than of the single WPI layer was observed during the large amplitude oscillatory cycles. Moreover, the adsorbed LMP at acidic pH conditions exhibited pH-responsive desorption upon exposure to neutral pH, as evidenced by the restored interfacial viscoelasticity as well as by in situ quartz crystal microbalance with dissipation monitoring (QCM-D) measurements. The more rigid and thicker WPI/LMP bilayers (visualized by cryo-scanning electron microscopy) not only effectively prevented the aggregation of emulsion droplets during storage, but also showed outstanding mechanical properties to prevent droplet coalescence.