Nothing goes better with cookies or cereal than a cold glass of milk. (Or almond milk, if that’s your preference.) Few people in the United States pull cartons of milk from their cupboard. Milk is sold and stored cold, but America is a bit of an outlier in that regard. So what gives? Why do we buy milk chilled while Europe and other parts of the world stock and store it outside the refrigerator?

It comes down to different pasteurization methods. In the U.S. and Canada, milk manufacturers make use of high-temperature short-time pasteurization, or HTST. Able to kill bacteria in large batches, HTST is efficient but results in milk that expires relatively quickly—about seven to 10 days after opening. That’s because the temperature used (about 161°F for 15 seconds) is enough to kill most bacteria, but some will proliferate if the milk hangs around long enough.

In Europe and other parts of the world, another technique called ultra-heat-treated pasteurization, or UHT, is used. Milk is exposed to higher temperatures of 284°F for three seconds, decimating virtually all the bacteria and making it shelf-stable for about six months if left unopened. (Once opened, it has to be refrigerated.) Because it’s “cooked” at high heat and burns off some of the sugar, UHT milk also has a slightly different flavor.  

Pasteurization is named after Louis Pasteur, a French scientist in the 1860s who realized heating beer could kill bacteria. Decades later, German agricultural chemist Franz von Soxhlet applied the principle of high heat to milk, since dairy products had a nasty habit of harboring contaminants that could cause diphtheria or tuberculosis. HTST and UHT methods followed, and Europe picked up on the promise of UHT producing milk that wouldn’t spoil quickly.

Even though companies have tried getting Americans to warm to shelf-stable milk—the Parmalat company tried a marketing campaign with Luciano Pavarotti in the early 1990s—it may simply be too late. The idea of purchasing milk in the middle of a grocery store, unrefrigerated, is something that doesn't fit with U.S. food storage habits. While UHT milk is still sold in the U.S., it’s primarily for portable cartons thrown in lunchboxes or for people who want to have milk on hand in a backpack. For most Americans, however, cold milk is the only milk worth considering.

[h/t Reader’s Digest]