Pectin degree of esterification influences rheology and digestibility of whey protein isolate-pectin stabilized bilayer oil-in-water nanoemulsions

Gelation in nanoemulsions happens at a lower oil volume fraction (φ) compared to conventional emulsions due to the increasing contribution of interfacial shell-layer thickness. In this study, the shell-layer thickness was increased by depositing a second layer of pectin on whey protein isolate (WPI)-stabilized nanodroplets. Pectin with two different degrees of esterification (DE), 33% and 73%, was used to investigate the effect of interfacial charge on the repulsive shell layer thickness. Excess WPI was removed from the primary 20 wt% canola oil-in-water nanoemulsion stabilized with 4 wt% WPI (pH 7) by ultracentrifugation before pectin addition. At pH 7, the added pectin remained in the continuous phase and led to almost two orders of magnitude increase in emulsion viscosity, but they were flowable. Changing to pH 3 led to electrostatic complexation of pectin with WPI either mostly in the aqueous phase for the emulsions with excess proteins (WTEP) or at the oil droplet surface for the emulsions without excess proteins (WOEP). The addition of pectin transformed the liquid primary emulsions into viscoelastic gels at only 20 wt% oil due to extensive droplet aggregation for the emulsion WTEP. For the emulsion WOEP, gelation was manifested by the close packing of repulsive droplets. The higher pectin charge (DE33) increased the gel strength of bilayer emulsions compared to the lower pectin charge (DE73) due to the charge cloud-induced enhanced interfacial repulsive barrier. The deposition of pectin also reduced lipid digestibility from 37.2% for the monolayer emulsions to 24.0% and 16.1% for DE73 and DE33-coated bilayer emulsions, respectively. Such findings can be used to better understand and modify the rheological and digestion properties of repulsive emulsion gels for application in reduced fat foods.