The fat in milk is present as globules varying from less than 1 μm to more than 10 μm in size. The larger globules will tend to rise, following Stokes' Law, and create a cream layer. This effect may be avoided by reducing the size of the fat globules, for instance by subjecting the milk to high shear. The milk thus remains homogeneous and the machine generally used for this purpose is known as an homogenizer.
The most commonly used type of homogenizer in the dairy industry employs high pressure generated by a triplex piston pump to provide the force needed to drive the milk, cream or other liquid through an adjustable restriction known as the homogenizing valve. The pressures used for dairy products are commonly up to 30 MPa, though some new applications may use pressures up to 400 MPa. The term ‘high pressure homogenizer’ has been widely used with the term ‘valve homogenizer’ also applied to those operating at up to 30 MPa while the term ‘ultra-high-pressure homogenizers’ has been used for those capable of operating in the higher range, typically from 100 MPa.