Interfacial structures of whey protein isolate (WPI) and lactoferrin on hydrophobic surfaces in a model system monitored by quartz crystal microbalance with dissipation (QCM-D) and their formation on nanoemulsions

Abstract

In this work, the interactions between two oppositely charged proteins in solution, namely whey protein isolate (WPI) and lactoferrin, were investigated for uses in nanoemulsions. ζ-potential and turbidity measurements indicated that these two proteins interact strongly via electrostatic attraction at pH 6. Quartz crystal microbalance with dissipation (QCM-D) was employed to study the adsorption behaviour and the formation of different interfacial structures of the two proteins onto a model hydrophobic surface that mimics the interface of an oil droplet in emulsions. Sequential addition of WPI and lactoferrin (or vice versa) formed thin and rigid protein bi-layers (8–10 nm). However, a viscous and thick surface layer (101 nm) was formed by the protein complex using a mixture of WPI and lactoferrin. High pressure homogenization and solvent evaporation were then used to replicate the QCM-D results on real nanoemulsion systems. Sequential adsorption of WPI and lactoferrin created bi-layered nanoemulsions with stable and relatively small droplets (90 nm in diameter). In contrast, the sequential adsorption of lactoferrin and WPI as well as the use of a mixture of WPI and lactoferrin (mixed layer) resulted in large aggregates with poor stability. This study provided insights into the stability of nanoemulsions influenced by structural features of interfacial layers based on two oppositely charged protein molecules.