The sign of v determines whether the droplet moves upward (+) or downward (-). To a first approximation, the stability of a food emulsion to creaming can be estimated using Equation 7.9. For example, an oil droplet (p2 = 910 kg m-3) with a radius of 1 |im suspended in water (n1 = 1 mPa s, p1 = 1000 kg m-3) will cream at a rate of about 17 mm/day. Thus one would not expect an emulsion containing droplets of this size to have a particularly long shelf life. As a useful rule of thumb, an emulsion in which the creaming rate is less than about 1 mm/ day can be considered to be stable toward creaming (Dickinson 1992).
In the rest of this section, we shall mainly consider creaming, rather than sedimentation, because it is more common in food systems. Nevertheless, the same physical principles are important in both cases, and the methods of controlling them are similar. In the initial stages of creaming, the droplets move upward and a droplet-depleted layer is observed at the bottom of the container (Figure 7.4). When the droplets reach the top of the emulsion, they cannot move upward any further, and so they pack together to form a "creamed layer." The
thickness of the creamed layer depends on the effectiveness of the droplet packing. Droplets may pack tightly or loosely depending on their polydispersity and the nature of the interactions between them. Tightly packed droplets tend to form a thin creamed layer, whereas loosely packed droplets form a thick creamed layer. Many of the factors which determine the packing of droplets in a creamed layer also determine the structure of flocs (see Section 7.4). The droplets in a creamed emulsion can often be redispersed by mild agitation, provided they are not too strongly attracted to each other or that coalescence has not occurred.
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