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Homogenized milk is any sort of milk that has been mechanically treated to ensure that it has a smooth, even consistency. The homogenization process typically involves high temperatures, agitation, and filtration, all aimed at breaking down milk's naturally occurring fat molecules. Once broken, these molecules stay suspended in the milk and resist separation. The process makes fat filtration much easier for manufacturers, and lengthens milk's shelf life.
Why Milk Is Homogenized
Milk is a combination of fats, proteins, and water. When raw milk is left to stand for any length of time, the fat molecules typically float to the top. This creates a layer of cream that many farmers and raw milk aficionados use as a measure of the milk's quality: the thicker the cream, the better the milk. Many find the separation distasteful, however, if not an impediment to actually drinking the liquid.
Homogenization allows milk manufacturers to combine the cream and milk so that it does not separate. The process is purely mechanical, and involves no additives or chemical treatments. The main goal behind milk homogenization is to reduce the size of the fat molecules in milk because smaller molecules tend to stay suspended in the body of the liquid. Only large globules float to the top.
The process typically starts with agitation. Milk is placed in a large drum or barrel that is spun at high speeds. Warm fat molecules disintegrate more easily than cold ones, so heat is often applied as well. The turbulence caused by the agitation starts breaking down the fat.
Next, the milk is pressed through narrow sieves or filters. This forces the fats to break apart even more to fit through the microscopic holes. Modern homogenization techniques can reduce fat molecules by a factor of nearly 500.
History and Early Iterations
The first homogenized milk was made by Frenchman Auguste Gaulin. His machine, which was a three-piston thruster outfitted with tiny filtration tubes, was patented in 1899. Modern agitation and thrusting mechanisms have improved upon this model, and today's tools are able to achieve much smaller fat molecules than Gaulin could have imagined. Still, the basic idea remains the same.
Customer preference is usually only one of the reasons dairy farmers and manufacturers homogenize their milk. On larger farms, this process allows milk from many different herds to be blended together more easily. Simply combining milk from two cows or goats into a single container doesn't always produce a uniform result. Milks with different chemical make-ups often do not blend well, and the liquid may separate and doesn't always taste the same. When homogenized, however, even very different batches can form one unified whole.
Homogenized milk also has a longer shelf life because the cream cannot rise to the top and clump together; this allows it to be transported over greater distances. Large-scale dairies often find this to be an advantage, as it means they can do business with buyers in more places. Consumers, too, often appreciate longer-lasting milk. Homogenized milk will often last for a week or more once opened, whereas separated milk must usually be consumed within a few days.
The filtration part of the process also makes it very easy for dairy farmers to remove a certain percentage of the fat. In whole milk, all of the sifted fats are added back in. For 2%, 1%, and nonfat versions, different percentages of fat are removed and discarded, or else used for other applications like making ice cream or butter. It is possible to get the same results by skimming off measured percentages of separated cream, though the homogenization process makes the calculations much more efficient and precise.
Relationship to Pasteurization
Most milk products sold in grocery stores in the United States are both pasteurized and homogenized. Though these terms tend to go together, they represent very different processes. In pasteurization, milk is heated to very high temperatures, then rapidly cooled in order to kill off microbial growth. Pasteurization tends to alter the taste of milk, but is considered by many to be essential in ensuring that the milk is safe for people to drink.
Homogenization has nothing to do with safety, but is usually driven by aesthetics and taste preferences. It is entirely possible to have milk that has been homogenized but not pasteurized, or pasteurized but not homogenized. If both processes are to be performed, however, homogenization typically comes last, since the heat of the pasteurization tends to make the fat breakdown easier.
Governments do not usually require milk to be homogenized, in part because it is a strictly non-chemical process. Milk is so frequently treated in this way that some governments have intervened in the labeling process, however. In the United States, for instance, the government's definition of "milk" assumes that it has been subjected to this process. This means that manufacturers do not have to say that their milk is homogenized — but they do need to say something if it is not.
Homogenized milk is generally considered safe, and has long been thought to be more easily digestible than natural cream-on-top milk. Still, there remain some experts who question whether forcing milk fats to separate might have negative effects on people's health.
One of the biggest challenges to homogenized milk concerns heart disease and arterial plaque buildup. Some medical researchers believe that the smaller, agitated milk fat molecules that result from homogenization may bind more easily to the walls of the heart's arteries, clogging them and potentially leading to heart disease and other ailments. Although this theory has gained a lot of attention, an equally large body of research would seem to refute it, and there does not seem to be enough information available to draw a universal conclusion.