When a cow is milked, and as the milk settles, a layer of cream forms at the top of the milk. This used to be the way people would judge the quality of milk. A thicker layer of cream meant better quality milk, and especially when milk was still normally sold in bottles, you could easily see into the bottle to judge the cream layer.

Pasteurization had become standardized for milk in most countries, since heating the milk destroys any bacteria in it, making the milk safer to drink. Homogenized milk was the next step. Pasteurized milk could still easily have milk fat proteins separate from the rest of the milk. For some people this was viewed as a disadvantage, though others argue homogenized milk doesn’t taste as good.

Since milk is an oil and water combination, it doesn’t stay mixed. Homogenized milk is run through tiny tubes, sometimes during the pasteurization process to keep fat and liquid molecules together. Fat molecules are reduced in size and tend to disperse more evenly throughout the milk so that creaming on the top of milk doesn’t occur. You can also use the homogenization process to reduce overall milkfat in milk. 2% milk is stripped of some of the fat molecules to produce lower fat milk

Homogenized milk also helps deal with a side effect of pasteurization. When milk is heated, the white cells and bacteria collect on the bottom of the milk, forming a thick, and many think, disagreeable layer. When milk has been fed thorough a small tube or orifice, this bottom layer gets mixed through the milk. The process of making homogenized milk, which gained the most popularity in the 1950s, has resulted in longer lasting milk, and the ability to ship milk greater distances.

Not everyone is pleased with homogenized milk, and sometimes you can look for unhomogenized versions. These still may be pasteurized, but many come in raw milk versions, which can prove unsafe to drink. Though it must be said that many people drink raw milk with no ill effects, there is occasional bacterial contamination of raw milk that poses a potential health risk.

On rare occasions you can find pasteurized but not homogenized milk. If you’re trying to whip cream, try to look for it at specialty stores, as it tends to whip much more quickly than does homogenized cream. You may find the occasional dairy that produces unhomogenized milk, but do remember this should not be treated in the same way as the homogenized liquid. First, you may want to give it a shake when you use it to break up the cream molecules. Second, it usually will expire more quickly, and may be more apt to absorb unpleasant tastes from your refrigerator.