Pickering emulsion stabilisation by potato protein microgels: effect of pH and subphase

The aim of this study was to understand the effect of pH and oil subphase when stabilising emulsions by plant protein-based microgels versus non-microgelled counterparts. Potato protein microgels (PoPM) were used to stabilise Pickering oil-in-water (O-W) emulsions at pH 3 and 7 and subphase of varying polarity - tetradecane and octanol and compared against conventional emulsions stabilised by non-microgelled potato protein (PoP). Confocal microscopy and static light scattering were used to monitor changes in droplet size and microstructure during storage, whilst interfacial shear rheology was used to assess the viscoelasticity of the adsorbed interfacial protein layers using various subphases. Despite the larger oil droplet sizes and lower interfacial viscoelasticity observed for octanol emulsions, systems stabilised by PoPM appeared more resilient to environmental variations irrespective of pH or subphase. This is most likely due to the ca. 4 x larger size of the adsorbed entities of PoPM, promoting steric hindrance and the higher mechanical strength of the adsorbed PoPM films. Strikingly PoP unfolding (confirmed via dynamic light scattering and small angle X-ray scattering (SAXS)) facilitated similar aggregation of PoP to thermally cross-linked PoPM, with resultant emulsion stability behaviour of PoP-stabilised emulsions resembling PoPM-stabilised ones at pH 3. Overall, this study highlights the importance of understanding how using plant protein microgels may offer benefits to interfacial stabilisation when the polarity of subphase varies, such as stabilising essential oils versus vegetable oils, which has been relatively underexplored in the literature.