Colloid mills are widely used in the food industry to homogenize medium- and high-viscosity liquids (Loncin and Merson 1979, Walstra 1983, Fellows 1988). A variety of different designs of colloid mill are commercially available, but they all operate on fairly similar physical principles (Figure 6.6). The liquids to be homogenized are fed into the colloid mill in the form of a coarse emulsion, rather than as separate oil and aqueous phases, because the device is much more efficient at reducing the size of the droplets in a preexisting emulsion (secondary homogenization) than at homogenizing two separate phases (primary homogenization). The coarse emulsion is usually produced directly from the oil and aqueous phases using a high-speed blender. It is then fed into the homogenizer and flows through a narrow gap between two disks: the rotor (a rotating disk) and the stator (a static disk). The rapid rotation of the rotor generates a shear stress in the gap which causes the larger droplets to be broken down into smaller ones. The intensity of the shear stresses can be altered by varying the thickness of the gap between the rotor and stator (from about 50 to 1000 |m), varying the rotation speed (from about 1000 to 20,000 rev min-1), or by using disks that have roughened surfaces or interlocking teeth (Gopal 1968). Droplet disruption can also be enhanced by increasing the length of time the emulsion spends in the colloid mill, either by decreasing the flow rate or by passing the emulsion through the device a number of times. Typically, the flow rate can be varied between about 4 and 20,000 l h-1. It should be noted
that many of the factors which increase the effectiveness of droplet disruption also increase the manufacturing costs (by increasing energy costs or reducing product throughput). Food manufacturers must therefore select the rotation speed, gap thickness, rotor/stator type, and throughput which give the best compromise between droplet size and manufacturing costs. It is usually necessary to have some form of cooling device as part of a colloid mill in order to offset the increase in temperature caused by viscous dissipation losses. Colloid mills are more suitable for homogenizing intermediate- and high-viscosity fluids (such as peanut butter, fish, or meat pastes) than high-pressure valve or ultrasonic homogenizers (Table 6.2). Typically, they can be used to produce emulsions with droplet diameters between about 1 and 5 |m.
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