V ~— —— _

FIGURE 1.6 Comparison of emulsions that can be described by normal and log-normal droplet size distributions.

If the log-normal curve shown in Figure 1.6 was plotted as droplet frequency versus log diameter, it would be symmetrical about ln dg.

It should be stressed that the particle size distribution of many food emulsions cannot be adequately described by the simple models given above. Bimodal distributions, which are characterized by two peaks (Figure 1.7), are often encountered in food emulsions (e.g., when extensive droplet flocculation occurs or when there is insufficient emulsifier present in an emulsion to stabilize all of the droplets formed during homogenization). For these systems, it is often better to present the data as the full particle size distribution; otherwise, considerable errors may occur if an inappropriate model is used.

1.3.3. Interfacial Properties

The droplet interface consists of a narrow region (usually a few nanometers thick) which surrounds each emulsion droplet and contains a mixture of oil, water, and emulsifier mol-

FIGURE 1.7 Example of a bimodal distribution resulting from the heat-induced flocculation of droplets in a 20 wt% corn oil-in-water emulsion stabilized by whey protein isolate. (From Demetriades, K., Coupland, J.N., and McClements, D.J., Journal of Food Science, 62, 462, 1997b. With permission.)

ecules (Hunter 1986, 1989). The interfacial region only makes up a significant fraction of the total volume of an emulsion when the droplet size is less than about 1 |im (Table 1.3). Even so, it plays a major role in determining many of the most important bulk physicochemical and organoleptic properties of food emulsions. For this reason, food scientists are particularly interested in elucidating the factors which determine the composition, structure, thickness, rheology, and charge of the interfacial region. The composition and structure of the interfacial region are determined by the type and concentration of surface-active species present, as well as by the events which occur both during and after emulsion formation (Chapter 6). The thickness and rheology of the interfacial region influence the stability of emulsions to gravitational separation, coalescence, and flocculation and determine the rate at which molecules leave or enter the droplets. The major factors which determine the characteristics of the interfacial region are discussed in Chapter 5, along with experimental techniques to characterize its properties.

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