Editing Milk (section)

===Creaming and homogenization===
[[File:Cow milking machine in action DSC04132.jpg|thumb|A milking machine in action]]
Upon standing for 12 to 24 hours, fresh milk has a tendency to separate into a high-fat cream layer on top of a larger, low-fat milk layer. The cream often is sold as a separate product with its own uses. Today the separation of the cream from the milk usually is accomplished rapidly in [[centrifuge|centrifugal]] [[cream separator]]s. The fat globules rise to the top of a container of milk because fat is less dense than water.<ref name="On Food and Cooking"/>

The smaller the globules, the more other molecular-level forces prevent this from happening. The cream rises in cow's milk much more quickly than a simple model would predict: rather than isolated globules, the fat in the milk tends to form into clusters containing about a million globules, held together by a number of minor whey proteins.<ref name="On Food and Cooking"/> These clusters rise faster than individual globules can. The fat globules in milk from goats, sheep, and water buffalo do not form clusters as readily and are smaller to begin with, resulting in a slower separation of cream from these milks.<ref name="On Food and Cooking"/>

Milk often is [[Homogenization (chemistry)|homogenized]], a treatment that prevents a cream layer from separating out of the milk. The milk is pumped at high pressures through very narrow tubes, breaking up the fat globules through [[turbulence]] and [[cavitation]].<ref>{{cite web |last=Goff |first=Douglas |title=Homogenization of Milk and Milk Products |url=http://www.foodsci.uoguelph.ca/dairyedu/homogenization.html |work=Dairy Science and Technology |publisher=University of Guelph |access-date=February 8, 2011 |year=2010 |archive-url=https://web.archive.org/web/20110524061747/http://www.foodsci.uoguelph.ca/dairyedu/homogenization.html |archive-date=May 24, 2011 |url-status=dead}}</ref> A greater number of smaller particles possess more total [[surface area]] than a smaller number of larger ones, and the original fat globule membranes cannot completely cover them. Casein micelles are attracted to the newly exposed fat surfaces.

Nearly one-third of the micelles in the milk end up participating in this new membrane structure. The casein weighs down the globules and interferes with the clustering that accelerated separation. The exposed fat globules are vulnerable to certain enzymes present in milk, which could break down the fats and produce [[rancidification|rancid]] flavors. To prevent this, the enzymes are inactivated by pasteurizing the milk immediately before or during homogenization.

Homogenized milk tastes blander but feels creamier in the mouth than unhomogenized. It is whiter and more resistant to developing off flavors.<ref name="On Food and Cooking"/> Creamline (or cream-top) milk is unhomogenized. It may or may not have been pasteurized. Milk that has undergone high-pressure homogenization, sometimes labeled as "ultra-homogenized", has a longer [[shelf life]] than milk that has undergone ordinary homogenization at lower pressures.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2002/12/021223084204.htm |title=Research Can Lead To Longer Shelf Life For Dairy Products |publisher=Sciencedaily.com |date=December 23, 2002 |access-date=August 28, 2010 |archive-url=https://web.archive.org/web/20100819113427/http://www.sciencedaily.com/releases/2002/12/021223084204.htm |archive-date=August 19, 2010 |url-status=live}}</ref><!--Homogenized milk may be more digestible than unhomogenized milk.<ref name="MichalskiJanuel">{{cite journal |title=Does homogenization affect the human health properties of cow's milk? |doi=10.1016/j.tifs.2006.02.004 |year=2006 |last1=Michalski |first1=Marie-Caroline |last2=Januel |first2=Caroline |journal=Trends in Food Science & Technology |volume=17 |issue=8 |pages=423–37}}</ref>

Kurt A. Oster, M.D., who worked during the 1960s through the 1980s, suggested a link between homogenized milk and [[atherosclerosis]], due to damage to [[plasmalogen]] resulting from the release of bovine [[xanthine oxidase]] (BXO) from the milk fat globular membrane (MFGM) during homogenization. Oster's hypothesis has been widely criticized, however, and has not been generally accepted by the scientific community. No link has been found between atherosclerosis and milk consumption.<ref name="MichalskiJanuel"/>-->