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The answer comes down to the way the milk is processed. Most milk is pasteurized, which means it's heated to a temperature high enough to kill illness-causing bacteria. In the 1920s, a UK company developed a way to pasteurize milk continuously, instead of in individual batches, for a cheaper and more efficient process than the ones used previously. High-temperature short-time pasteurization, or HTST, is what the U.S. and Canada (bagged milk notwithstanding) still use today.

But in the 1960s, the packaging company Tetra-Pak came up with a new technique. Ultra-high-temperature or ultra-heat-treated pasteurization (UHT) heats the milk to an even higher temperature than HTST. The result is milk that stays fresh and shelf-stable outside of the fridge for about three months — way beyond the seven-to-ten refrigerated days of HTST. That's what the majority of the world uses, and why you can find milk out on the shelves in many countries.

Milk you don't have to refrigerate that won't expire for months — who doesn't like that? Americans, that's who. The Italian company Parmalat tried selling UHT milk on American shelves in the early 1990s, and failed magnificently.

For one thing, the high temperatures make UHT milk taste a bit more "cooked" than HTST milk. But by and large, the failure was probably cultural. Americans refrigerate a lot of things other countries don't — bread, butter, even eggs. The U.S. puts more ice in their beverages, and drinks their beer and white wine colder. So is it any wonder that an offer of lukewarm milk got a hard pass?

Still, times are a'changing. With the U.S. gobbling up a quarter of the world's energy production, maybe the country could use less refrigeration. Luckily, with the rising popularity of other shelf-stable milks like soy milk and almond milk, Americans are getting more comfortable with buying their milk off the shelf. Walk into an American grocery store today, and you'll find single-serving milks sold at room-temperature from the likes of Horizon Organic and Fairlife. The revolution is coming, and it is unrefrigerated.