Transitions in structure in oil-in-water emulsions as studied by diffusing wave spectroscopy.

Transitions in structure of sodium caseinate stabilized emulsions were studied using conventional rheometry as well as diffusing wave spectroscopy (DWS). Structural differences were induced by different amounts of stabilizer, and transitions in structure were induced by acidification. Special attention was given to the sol-gel transition. In this study the criterion of the sol-gel transition being frequency independent was verified for emulsions using DWS. It was shown that this sol-gel transition did not correspond to the so-called ergodic-nonergodic transition. Differences, as a function of the pH, were found for emulsions containing different amounts of stabilizer. The emulsion droplets in an emulsion without extra stabilizer formed a continuous network upon acidification, while the droplets in emulsions with an excess of stabilizer formed a network of oil droplets at neutral pH. Upon acidification of the latter one, the initial network of oil droplets fell apart, and eventually a network of sodium caseinate, in which the oil droplets were embedded, was formed. This caused the appearance of two sol-gel transitions. The breaking of the initial network as well as the network formation of sodium caseinate in time was observed by DWS.