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Homogenization is a generic term which refers to processing a solution so that it becomes uniform. It crops up in many industrial and scientific applications, although it is often used specifically to refer to milk, as part of a two stage process which prepares milk for sale. The first step, pasteurization, sterilizes the milk so that it is safer to drink. Homogenization stabilizes it for a smoother mouthfeel and flavor.
Milk is actually a solution of two different materials which do not normally mix, in this case oil and water. When milk which has not been subjected to homogenization is allowed to stand, the creamy fat globules will slowly rise to the top of the milk. Sometimes, this is a desired effect, as is the case when making skimmed milk, extracting the milk fat for use in cream and butter. However, when consumers take milk home, they do not expect it to separate. Therefore, the two different substances in milk must be blended so that they do not separate out.
In order to accomplish homogenization, the milk is forced through a very fine screen at high pressure. The particles of fat break down and combine with the watery portion of the milk, resulting in a uniform liquid which will not separate out, since the fat particles are blended with the water. The resulting fluid is known as an emulsion, since it represents the combining of two normally unmixable substances. In the case of homogenized milk, the emulsion is highly stable and it will not separate.
When milk undergoes homogenization, the taste does change slightly. The fat is more evenly distributed throughout the milk, resulting in a more creamy flavor and texture. Pasteurization has a far larger impact on the flavor of dairy products, but is generally viewed as necessary since it prevents food borne illness. Most commercial milk is pasteurized and homogenized, although it is possible to find milk which has not been put through the homogenization process. Even more rarely, consumers may be able to find entirely raw milk.
Some other examples of emulsions include oil and vinegar dressing, mayonnaise, and butter. In most cases, the emulsion is irreversible, but in some instances, the ingredients may separate, indicating incomplete homogenization. This is especially the case with oil and vinegar dressings, which usually need to be shaken to break down the fat particles before they are used. When emulsions are made improperly, they will tend to separate, which can be an indicator of poor quality for consumers, especially with mayonnaise. Should supposedly homogenized milk begin to separate, it indicates that the homogenization was not done properly, or that the milk has gone bad.