PARTIAL COALESCENCE IN OIL-IN-WATER EMULSIONS .1. NATURE OF THE AGGREGATION

The mechanism of aggregation in triglyceride oil-in-water (o/w) emulsions in which the droplets contain fat crystals was investigated. As a surfactant either sodium dodecyl sulphate (SDS) or a protein was used. The crystals must be situated at the o/w interface to cause partial coalescence; if the fat crystals are totally wetted by either the oil or the aqueous phase they do not affect emulsion stability. Although a suitable contact angle (crystal, oil, aqueous phase) appears necessary for partial coalescence to occur, it is obviously not a sufficient condition. Additional conditions are a close enough approach of the globules, allowing the protruding crystals to bridge the aqueous film between the globules, and the presence of sufficient solid fat. Crystals may have an approximately radial orientation at the o/w interface if they are kept in a continuous fat network. If, furthermore, the contact angle (as measured in the aqueous phase) is about 90-degrees, the coalescence efficiency a was high (mostly alpha much greater than 10(-6)). If crystals can move around freely in the oil droplet, they can obtain a tangential orientation at the interface irrespective of the contact angle. Such globules are not very sensitive to partial coalescence (alpha almost-equal-to 10(-6) or less). Even if partial coalescence had already occurred, it could in some cases be undone by changing the wetting properties so that the fat crystals become separated from the interface. This reversibility only held if the change occurred within a few hours after aggregation. After that the partially coalesced emulsion droplets had irreversibly merged into one bigger clump.