Energy Input
The size of the droplets in an emulsion can be reduced by increasing the amount of energy supplied during homogenization (as long as there is sufficient emulsifier to cover the surfaces of the droplets formed). The energy input can be increased in a number of different ways depending on the nature of the homogenizer. In a high-speed blender, the energy input can be enhanced by increasing the rotation speed or the length of time that the sample is blended. In a high-pressure valve homogenizer, it can be enhanced by increasing the homogenization pressure or recirculating the emulsion through the device. In a colloid mill, it can be enhanced by using a narrower gap between the stator and rotator, increasing the rotation speed, by using disks with roughened surfaces, or by passing the emulsion through the device a number of times. In an ultrasonic homogenizer, the energy input can be enhanced by increasing the intensity of the ultrasonic wave or by sonicating for a longer time. In a microfluidizer, the energy input can be enhanced by increasing the velocity at which the liquids are brought into contact with each other or by recirculating the emulsion. In a membrane homogenizer, the energy input can be enhanced by increasing the pressure at which the liquid is forced through the membrane. Under a given set of homogenization conditions (energy input, temperature,
composition), there is a certain size below which the emulsion droplets cannot be reduced with repeated homogenization, and therefore homogenizing the system any longer would be inefficient.
Increasing the energy input usually leads to an increase in manufacturing costs, and therefore a food manufacturer must establish the optimum compromise between droplet size, time, and cost. The energy input required to produce an emulsion containing droplets of a given size depends on the energy efficiency of the homogenizer used (Walstra 1983).
Under most circumstances, there is a decrease in droplet size as the energy input is increased. Nevertheless, there may be occasions when increasing the energy actually leads to an increase in droplet size because the effectiveness of the emulsifier is reduced by excessive heating or exposure to high pressures. This could be particularly important for protein-stabilized emulsions, because the molecular structure and functional properties of proteins are particularly sensitive to changes in their environmental conditions. For example, globular proteins, such as P-lactoglobulin, are known to unfold and aggregate when they are heated above a certain temperature, which reduces their ability to stabilize emulsions (Section 4.6).
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