In vitro digestion of emulsions: diffusion and particle size distribution using diffusing wave spectroscopy and diffusion using nuclear magnetic resonance

Abstract

Diffusing wave spectroscopy (DWS) is one of the few techniques enabling the investigation of structures and dynamics in turbid systems that is in the multiple light scattering domain. This makes it an important technique to study colloidal dispersions such as foam, gel or emulsion. In this article, DWS in both back- and forward-multiple scattering was used to monitor the in vitro digestion of turbid undiluted emulsions. Eight formulations were tested using two triglycerides, two emulsifiers and two emulsifier concentrations. The main goal of the study was to interpret the DWS data and compare the results to those from other techniques. We first extended the cumulants/moments fit method used for single scattering to obtain a particle size distribution (PSD) by DWS for multiple scattering. In the case of unimodal distributions, this compares well to PSD obtained from single scattering by dynamic light scattering (DLS). A second interpretation based on the multiple forward-scattering allowed the time-resolved diffusion coefficient to be measured. This was compared to the diffusion monitored by nuclear magnetic resonance (NMR) of turbid undiluted emulsions. Both techniques report similar diffusion coefficients, although NMR measures a true molecular diffusion in different environments whereas DWS measures the diffusion of supramolecular objects in the aqueous phase. These techniques are thus complementary, NMR resolving the kinetics of lipolysis, and DWS resolving the structural transitions, found to be first from a droplet to a vesicle and then from a vesicle to a micelle. In this study, the main formulation parameter influencing the digestion was found to be the type of triglyceride.