Gravitational Separation

In general, the droplets in an emulsion have a different density to that of the liquid which surrounds them, and so a net gravitational force acts upon them (Dickinson and Stainsby 1982, Hunter 1989, Dickinson 1992, Walstra 1996a,b). If the droplets have a lower density than the surrounding liquid, they have a tendency to move upward, which is referred to as creaming (Figure 7.3). Conversely, if they have a higher density than the surrounding liquid, they tend to move downward, which is referred to as sedimentation. The densities of most edible oils (in their liquid state) are lower than that of water, and so there is a tendency for oil to accumulate at the top of an emulsion and water at the bottom. Thus, droplets in an oil-in-water emulsion tend to cream, whereas those in a water-in-oil emulsion tend to sediment.

Gravitational separation is usually regarded as having an adverse effect on the quality of food emulsions. A consumer expects to see a product which appears homogeneous, and therefore the separation of an emulsion into an optically opaque droplet-rich layer and a less opaque droplet-depleted layer is undesirable. The textural attributes of a product are also adversely affected by gravitational separation, because the droplet-rich layer tends to be more viscous than expected, whereas the droplet-depleted layer tends to be less viscous. The taste and mouthfeel of a portion of food therefore depend on the location from which it was taken from the creamed emulsion. A sample selected from the top of an oil-in-water emulsion that has undergone creaming will seem too "rich" because of the high fat content, whereas a sample selected from the bottom will seem too "watery" because of the low fat content. Gravitational separation is also a problem because it causes droplets to come into close

FIGURE 7.3 Food emulsions are prone to creaming because of the density difference between the oil and water phases. Inset: the forces acting on an emulsion droplet.

FIGURE 7.3 Food emulsions are prone to creaming because of the density difference between the oil and water phases. Inset: the forces acting on an emulsion droplet.

contact for extended periods, which can lead to enhanced flocculation or coalescence and eventually to oiling off, which is the formation of a layer of pure oil on top of the emulsion. When a food manufacturer is designing an emulsion-based product, it is therefore important to control the rate at which gravitational separation occurs.

Each food product is unique, containing different types of ingredients and experiencing different environmental conditions during its processing, storage, and consumption. As a consequence, the optimum method of controlling gravitational separation varies from product to product. In this section, we consider the most important factors which influence gravitational separation, as well as strategies for controlling it.

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