Size determinations of colloidal fat emulsions: a comparative study.

Size and size distributions of colloidal dispersions are of crucial importance for their performance and safety. In the present study, commercially available fat emulsions (Lipofundin N, Lipofundin MCT and Lipidem) were analyzed by photon correlation spectroscopy, laser diffraction with adequate submicron instrumentation (Mastersizer 2000, Coulter LS 13 230) and by asymmetrical flow field-flow fractionation combined with multi-angle laser light scattering. The separation capacity of the field-flow fractionation system and accuracy of size determination by multi-angle laser light scattering was checked with mixtures of monodisperse polystyrene nanospheres. In addition, the ultrastructure of Lipofundin N and Lipofundin MCT was investigated by cryo-electron microscopy. All different particle sizing methods gave different mean sizes and size distributions but overall, results were in reasonable agreement. By all methods, a larger mean droplet size (between 350 and 400 nm) as well as a broader distribution was measured for Lipofundin N compared to Lipofundin MCT and Lipidem (mean droplet size between about 280 and 320 nm). Size distributions of Lipofundin MCT and Lipidem were very similar but a slightly smaller size was indicated by all methods for Lipidem. Sub-micron resolution was best in the Coulter LS but the fraction of larger particles in the upper nm-range was presumably underestimated. The emulsions could be analyzed in a highly reproducible manner by asymmetrical flow field-flow fractionation coupled with multi-angle laser light scattering and this method appears very promising particularly with regard to the separation and/or collection of sample fractions being homogeneous in size.