Coalescence of protein-stabilised emulsions studied with microfluidics

Abstract

Emulsion droplet formation occurs in milliseconds to seconds when emulsifier adsorption is often not yet completed, therewith allowing coalescence to take place. Because of these short time-scales, it is difficult to quantify adsorption and coalescence during processing. A microfluidic device can be used to measure coalescence shortly after droplet formation in laminar flow, and this device was used to assess coalescence of oil-in-water emulsions stabilised with dairy proteins (β-lactoglobulin, whey protein isolate, and oxidised whey protein isolate). Different microfluidic designs were used to vary the protein adsorption time prior to droplet collision.Coalescence stability depended on protein concentration (0.0002–0.02 wt %) and adsorption time (11–173 ms): emulsion droplets were stable at low protein concentrations (as low as 0.005 wt % β-lactoglobulin), as long as the time allocated for protein adsorption was sufficient (in this case 31 ms). Protein type was also important for coalescence stability: emulsions stabilised with whey protein isolate were less stable than those with β-lactoglobulin, and coalescence stability further decreased upon protein oxidation. Regarding the effect of pH (3.0–8.0), coalescence stability was lowest around the protein's isoelectric point.With the coalescence channel we demonstrated the importance of adsorption time and interface composition for coalescence stability at low protein concentrations. Coalescence can be measured at small time-scales and in high detail since coalescence measurements are decoupled from droplet formation. The microfluidic coalescence channel can be used as an analytical tool to gain better understanding of fluid interface stabilisation during emulsification, and to develop emulsion formulations ab initio.