Microfluidic investigation of the coalescence susceptibility of pea protein-stabilised emulsions

Abstract

Proteins are used to stabilise oil-in-water (O/W) emulsions, and plant proteins are gaining interest as functional ingredients due to their higher sustainability potential compared to e.g., dairy proteins. However, their emulsifying properties are not that well understood, and depend on how their production process affects their physicochemical status. In the present work, we use the soluble fraction of commercial pea protein isolate to stabilise O/W emulsion droplets formed in a microfluidic device, and record coalescence stability after droplet formation (11–173 ms) for different protein concentrations (0.1–1 g/L). For the shortest adsorption times (11–65 ms) droplets were unstable, whereas for longer adsorption times differences in coalescence stability could be charted. Metal-catalysed oxidation of pea proteins performed for up to 24-h, prior to emulsion formation and analysis, increased the coalescence stability of the droplets, compared to fresh pea proteins. This may be explained by oxidation-induced protein fragmentation, leading to low molecular weight products. The Langmuir-Blodgett films looked highly heterogeneous for films prepared with fresh or mildly oxidised (3-h) proteins, and was more homogenous for 24-h oxidised proteins. This could be the cause for the observed differences in emulsion coalescence stability, structurally heterogeneous films being more prone to rupture. From this work, it is clear that the emulsifying properties of pea are strongly dependent on their chemical status, and associated structural properties at the molecular and supramolecular levels. The present microfluidic device is an efficient tool to capture such effects, at time scales that are relevant to industrial emulsification.