SDS/EDTA is used to dissociate flocculated fat droplets (if any) before droplet size measurement. SDS is a strongly surface-active molecule, and also negatively charged. It can replace the emulsifiers initially adsorbed onto the fat globules, providing electrostatic repulsion between droplets. Also, EDTA binds calcium, which can disrupt calcium bonds between fat globules covered with caseins. Therefore, when the measured particle size, for a given sample, is different without and with SDS/EDTA, it means that the fat globules were non-irreversibly flocculated (at least to some extent).