To theoretically predict the rate at which gravitational separation occurs in an emulsion, it is necessary to have information about the densities of the dispersed and continuous phases, the droplet size distribution, and the rheological properties of the continuous phase. The density of the liquids can be measured using a variety of techniques, including density bottles, hydrometers, and oscillating U-tube density meters (Pomeranz and Meloan 1994). The droplet size distribution can be measured by microscopy, light-scattering, electrical pulse counting, or ultrasonic methods (see Chapter 10). The rheological properties of the continuous phase can be characterized using various types of viscometers and dynamic shear rheometers (see Chapter 8). In principle, it is possible to predict the long-term stability of a food emulsion from a knowledge of these physicochemical properties and a suitable mathematical model. In practice, this approach has limited use because the mathematical models are not currently sophisticated enough to take into account the inherent complexity of food emulsions. For this reason, it is often more appropriate to directly measure the gravitational separation of the droplets in an emulsion.
The simplest method of monitoring gravitational separation is to place an emulsion in a transparent test tube, leave it for a certain length of time, and then measure the height of the interface between the droplet-depleted and droplet-rich layers using a ruler. This procedure
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