If you’ve ever shopped for milk, you’ve no doubt noticed what our questioner has: While regular milk expires within about a week or sooner, organic milk lasts much longer—as long as a month.



So what is it about organic milk that makes it stay fresh so long?



Actually, it turns out that it has nothing to do with the milk being organic. All "organic" means is that the farm the milk comes from does not use antibiotics to fight infections in cows or hormones to stimulate more milk production.



Organic milk lasts longer because producers use a different process to preserve it. According to the Northeast Organic Dairy Producers Alliance, the milk needs to stay fresh longer because organic products often have to travel farther to reach store shelves since it is not produced throughout the country.



The process that gives the milk a longer shelf life is called ultrahigh temperature (UHT) processing or treatment, in which milk is heated to 280 degrees Fahrenheit (138 degrees Celsius) for two to four seconds, killing any bacteria in it.



Compare that to pasteurization, the standard preservation process. There are two types of pasteurization: "low temperature, long time," in which milk is heated to 145 degrees F (63 degrees C) for at least 30 minutes*, or the more common "high temperature, short time," in which milk is heated to roughly 160 degrees F (71 degrees C) for at least 15 seconds.



The different temperatures hint at why UHT-treated milk lasts longer: Pasteurization doesn’t kill all bacteria in the milk, just enough so that you don't get a disease with your milk mustache. UHT, on the other hand, kills everything.



Retailers typically give pasteurized milk an expiration date of four to six days. Ahead of that, however, was up to six days of processing and shipping, so total shelf life after pasteurization is probably up to two weeks. Milk that undergoes UHT doesn’t need to be refrigerated and can sit on the shelf for up to six months.



Regular milk can undergo UHT, too. The process is used for the room-temperature Parmalat milk found outside the refrigerator case and for most milk sold in Europe.



So why isn’t all milk produced using UHT?



One reason is that UHT-treated milk tastes different. UHT sweetens the flavor of milk by burning some of its sugars (caramelization). A lot of Americans find this offensive—just as they are leery of buying nonrefrigerated milk. Europeans, however, don’t seem to mind.



UHT also destroys some of the milk’s vitamin content—not a significant amount—and affects some proteins, making it unusable for cheese.



There are, of course, lots of reasons people buy organic milk. But if it's the long shelf life you're after, I would recommend you buy nonorganic UHT milk and avoid being charged double.

*Correction (6/6/08): This sentence originally said "milk is heated to 145 degrees F (63 degrees C) for at least 30 seconds." (The error occurred during editing by the staff of ScientificAmerican.com and is not the fault of the expert.)

