Physical Basis of Partial Coalescence

Partial coalescence is initiated when a solid fat crystal from one droplet penetrates into the liquid oil portion of another droplet (Boode 1992; Boode and Walstra 1993a,b; Boode et al. 1993; Walstra 1996a). Normally, the crystal would be surrounded by the aqueous phase, but when it penetrates into another droplet, it is surrounded by liquid oil. This causes the droplets to remain aggregated because it is energetically more favorable for a fat crystal to be surrounded by oil molecules than by water molecules (i.e., the fat crystal is wetted better by liquid oil than by water). Over time, the droplets merge more closely together because this reduces the surface area of oil exposed to water (Figure 7.21).

Partial coalescence may occur immediately after two droplets come into contact with each other, or it may occur after the droplets have been in contact for an extended period (Boode 1992). It is affected by many of the same factors which influence normal coalescence, including contact time, collision frequency, droplet separation, colloidal and hydrodynamic interactions, interfacial tension, and membrane viscoelasticity (Section 7.5.1). Nevertheless, there are also a number of additional factors which are unique to partial coalescence, the most important being the fact that the oil phase is crystalline (Boode and Walstra 1993a,b; Walstra 1996a).

The solid fat content (^SFC) is the percentage of fat that is crystalline at a particular temperature, varying from 0% for a completely liquid oil to 100% for a completely solid fat. Partial coalescence only occurs in emulsions that contain partly crystalline droplets, because a solid fat crystal from one droplet must penetrate into the liquid oil region of another droplet (Boode and Walstra 1993a,b). If the droplets were completely liquid, they would undergo normal coalescence, and if they were completely solid, they would undergo flocculation rather than partial coalescence because the droplets are not able to merge together. Increasing the ^SFC from 0% causes an initial increase in the partial coalescence rate until a maximum

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