Influence of fat crystallization in W/O emulsions on the water droplet size determination by NMR diffusometry.

HYPOTHESIS: It is expected that low resolution (LR) NMR diffusometry enables (more) accurate water droplet size determination for solid-fat based water-in-oil (W/O) emulsions with (sub)-micron size water droplets in comparison to liquid-oil based W/O emulsions due to hindered extra-droplet water diffusion. EXPERIMENTS: W/O emulsions with a volume-weighed mean diameter of about 1 µm and a solid fat content (SFC) ranging from 0% to 74% were produced. The aqueous phase contained the ionic marker tetraphenylphosphonium chloride (TPPCl). The water droplet size was estimated using LR and high resolution (HR) NMR diffusometry. FINDINGS: HR-NMR diffusometry showed that the diffusion behavior of water and TPPCl was different, indicating water diffusion beyond the droplet's interfacial boundaries. From a certain SFC onwards, a slower echo decay was observed for the water molecules, thus decreasing the overestimation of the water droplet size in (sub)micron W/O emulsions. For those emulsions, the solid fat matrix is believed to hinder extra-droplet water diffusion, which is most likely to be related to the increased tortuosity of the diffusive path in the porous fat crystal network. Using LR-NMR, it can be verified whether the water echo attenuation is mono-exponential or bi-exponential by increasing the gradient pulse duration for the maximum gradient strength, which is more convenient for routine analysis compared to HR-NMR.

Increasing water solubility with decreasing droplet size limits the use of water NMR diffusometry in submicron W/O-emulsion droplet size analysis.

HYPOTHESIS: Water droplet size analysis of water-in-oil emulsions using water NMR diffusometry yielded values that were, from a certain shear intensity onwards, independent from the shear which was used during production. It was assumed that the constant water droplet size, obtained for samples prepared at higher shear, were only apparent droplet diameters. Considering the well-known increased solubility of the dispersed phase in the continuous phase at smaller droplet sizes, it is hypothesized that water diffusion in the oil phase was responsible for the fact that apparent rather than real sizes were obtained. EXPERIMENTS: W/O-emulsions, prepared with a varying shear intensity, were characterized using dynamic light scattering, light microscopy, T2-relaxometry and PFG-NMR diffusometry. The latter measurements were conducted on both a low- and a high-resolution device and was based on either water (LR- and HR-NMR) or a water-soluble marker (HR-NMR). FINDINGS: Low-resolution PFG-NMR is incapable of accurately determining the droplet size of W/O-emulsions containing (sub)micron sized droplets. On the other hand, using high-resolution PFG-NMR diffusometry and the addition of an oil insoluble marker to the water phase, the application window could be extended towards smaller droplet sizes. Finally, it was shown that T2-relaxometry was capable of detecting differences in droplet size between (sub)micron sized W/O-emulsions.