Coalescence of oil droplets stabilised with β-lactoglobulin and/or phospholipids. A microfluidic study dedicated to the impact of temperature and phospholipid type

Abstract

In scientific emulsification work, the focus is often on a single component and how it behaves at time scales that are at least seconds but often much longer. In practice, emulsions would be produced using mixtures, with droplet formation times in the millisecond range and below. In the current paper, we use a relevant combination of components, β-lactoglobulin (β-LG) and various phospholipids (PL), and investigate them through a microfluidic device that can be used to monitor droplet stability at sub-second time scales. In contrast to conventional emulsion methods, which only permit post hoc analysis, the microfluidic setup provides direct insight into the emulsification process immediately after droplet formation, enabling evaluation of the interplay between interfacial adsorption and droplet coalescence. Droplet stability was greatly enhanced when using a combination of β-lactoglobulin and phospholipids, compared to droplets stabilised by one of the components. For example, for β-LG + 0.005 wt% PL stabilised emulsions the extent of coalescence remained low (number of coalescence events Ncoal < 1) for adsorption times of 31 ms for β-LG concentrations of 0.001 wt% or higher. In comparison, purely β-LG stabilised emulsions were unstable under the same conditions, even at the highest β-LG concentration tested (0.01 wt% β-LG, Ncoal = 2.4) as was the case for the purely PL stabilised emulsions (Ncoal ≫1). There is clearly a synergistic effect that leads to enhanced droplet stability against coalescence. This was even more pronounced at high temperatures. For instance, at 50 °C β-LG + PC 18:1 emulsions were stable, even at β-LG concentration as low as 0.0001 wt% (Ncoal < 1). At elevated temperatures, only minor effects on coalescence stability related to the PL's molecular structure were observed. The insights obtained with microfluidics differed from those obtained for emulsions, where displacement was noted amongst other factors. Apparently, the time scales used in the current work were too short for this to occur, as this is expected to be a phenomenon occurring over at least minutes. From the experiments carried out here, clues can be derived for formulating stable emulsions and choosing suitable production processes. Amongst others, the protein and phospholipid should be carefully chosen, depending on the temperatures used during/after emulsification.