High-pressure valve homogenizers are the most commonly used method of producing fine emulsions in the food industry. Like colloid mills, they are more effective at reducing the size of the droplets in a preexisting emulsion than at creating an emulsion directly from two separate liquids (Pandolfe 1991, 1995). A coarse emulsion is usually produced using a highspeed blender and is then fed directly into the input of the high-pressure valve homogenizer. The homogenizer has a pump which pulls the coarse emulsion into a chamber on its backstroke and then forces it through a narrow valve at the end of the chamber on its forward stroke (Figure 6.7). As the coarse emulsion passes through the valve, it experiences a combination of intense shear, cavitational, and turbulent flow conditions which cause the larger droplets to be broken down into smaller ones (Phipps 1985). A variety of different types of valve have been designed for different types of application. Most commercial homogenizers use spring-loaded valves so that the gap through which the emulsion passes can be varied (typically between about 15 and 300 |im). Decreasing the gap size increases the pressure drop across the valve, which causes a greater degree of droplet disruption and smaller droplets to be produced. On the other hand, narrowing the gap increases the energy input required to form an emulsion, thereby increasing manufacturing costs. Experiments have shown that there is an approximately linear relationship between the logarithm of the homogenization pressure (P) and the logarithm of the droplet diameter (d) (i.e., log d<x log P) (Walstra 1983, Phipps 1985). Thus a food manufacturer is able to develop empirical equations which can be used to predict the homogenization pressure required to produce an emulsion with a given droplet size. The throughputs of industrial homogenizers typically vary
between about 100 to 20,000 l h-1, whereas homogenization pressures vary between about 3 and 20 MPa.
Some commercial devices use a "two-stage" homogenization process, in which the emulsion is forced through two consecutive valves. The first valve is at high pressure and is responsible for breaking up the droplets, whereas the second valve is at low pressure and is responsible for disrupting any "flocs" which are formed during the first stage (Phipps 1985).
High-pressure valve homogenizers can be used to produce a wide variety of different food products, although they are most suitable for low- and intermediate-viscosity materials, particularly when a small droplet size is required. If the oil and aqueous phases have been blended prior to homogenization, it is often possible to create an emulsion with submicron particles using a single pass through the homogenizer (Pandolfe 1995). If very fine emulsion droplets are required, it is usually necessary to pass the emulsion through the homogenizer a number of times. Emulsion droplets with diameters as small as 0.1 can be produced using this method. The temperature rise in a high-pressure valve homogenizer is usually fairly small, but it can become appreciable if the emulsion is recirculated or extremely high pressures are used. In these cases, it may be necessary to keep the emulsion cool by using a water-jacketed homogenization chamber.
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