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

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.

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.

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.