Fat emulsions for parenteral nutrition. I: Evaluation of microscopic and laser light scattering methods for the determination of the physical stability.

Droplet size distribution was studied as a measure of the physical stability of fat emulsions. Test emulsions with increasing fractions of larger particles were prepared and investigated by conventional light microscopy and laser light scattering techniques (Photon Correlation Spectroscopy (PCS), Laser Diffractometer). Light microscopy proved to be most sensitive for detecting a few single large particles. All the techniques placed the test emulsions in the same order of increasing content of larger droplets. The application of a two time window analysis for PCS markedly improved the ability to differentiate between emulsions which were very similar in size distribution. In contrast to the applied semi-quantitative microscopic method, the laser techniques characterise the emulsion in absolute figures. Additional zeta potential measurements by Laser Doppler Anemometry (LDA) can quantify reduced electrostatic repulsion as a possible cause for increased formation of larger droplets.