Effect of ionic strength on the sequential adsorption of whey proteins and low methoxy pectin on a hydrophobic surface: A QCM-D study

The effect of ionic strength on the sequential adsorption of whey protein isolate (WPI) and low methoxy pectin (LMP) onto a hydrophobic surface was investigated through a real-time QCM-D technique. A gold sensor was hydrophobically modified to simulate the oil-water interface. At neutral pH, whey proteins readily adsorbed onto the hydrophobic surface and formed a viscoelastic, heterogeneous layer. With increasing ionic strength of the continuous phase, the protein layer became more compact and less hydrated. A similar but stronger effect was observed upon switching the pH from 7.0 to 4.5 (i.e. close to the iso-electric point of WPI). The sequential adsorption of LMP onto the pre-formed protein layer was studied at pH 4.5 over a wide range of ionic strengths (from 7 to 500 mM). The enhanced frequency and dissipation shifts indicated the adsorption of LMP, as well as changes in interfacial viscoelasticity. Especially, a non-monotonic ionic strength dependency (i.e. maxima at 20 mM after rinsing) was observed for both frequency and dissipation shifts, which suggested that the added salt facilitated the electrostatic deposition of LMP. However, the much smaller shifts at higher ionic strength (e.g. up to 500 mM) were mainly due to less hydration of the adsorbed LMP layer rather than an appreciably reduced LMP adsorption. Similarly, upon exposure of the pre-formed mixed layer (at pH 4.5 with an ionic strength of 20 mM) to higher ionic strengths (up to 500 mM), most of the LMP remained adsorbed, even at the highest ionic strength considered. The obtained results provide interesting insights in the characteristics of protein-polysaccharide bilayers which are known to be promising emulsion stabilizers.