Today, an experiment.  The University of Houston presents this series about the machines that make our civilization run, and the people whose ingenuity created them. 

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I recently performed an experiment quite unwittingly. Heating some milk while visiting France, I brought it to a boil, though only for a second or two. The result wasn’t hot milk for my coffee. Instead, I found curds – the milk solids used to make cheese – and whey – a semi-clear liquid that consists mostly of water. The whey also contains most of the milk’s lactose, which is why people with lactose intolerance frequently have no trouble consuming cheese.

For those of us who live in the United States this comes as a bit of a surprise. In the U.S., bringing milk to a boil yields boiling milk. And the answer all boils down to the way the milk is pasteurized.

We’re all familiar with pasteurization. Heat milk to kill enough bacteria that it remains safe to consume when it arrives at the grocery store from the dairy farm. In fact, this heating process long predates its namesake Louis Pasteur. The real breakthrough came with the experimentation of French chef Nicolas Appert in 1795, where he took to heating all manner of things to help preserve them – fruits, vegetables, jams; you name it. His process was successful and caught on internationally. Of course, Appert had no inkling about germ theory. He just observed that the heating process worked. Experimental evidence leading to the acceptance of germ theory would wait another half century for the work of a fellow Frenchman, the aforementioned Louis Pasteur.

While pasteurization involves the heating of food, two important variables are how hot and how long. In France, milk is typically heated to three-hundred degrees for around three seconds. In the U.S., milk is heated to roughly half that temperature but about five times longer. And while we don’t often think about it, heating brings about a wealth of molecular changes when applied to any food. The result: French milk can (and is) stored unrefrigerated on grocery store shelves for three to six months. And, as we know, it also turns to curds and whey when boiled. When properly refrigerated, U.S. milk has a shelf life of only seven to ten days.

Of course, the process by which milk is pasteurized also changes its flavor, and the lower temperature U.S. process brings about less change. The French are willing to accept this tradeoff of flavor versus practicality – except when they’re not.

Take cheese. In the U.S. virtually all milk derived products must use pasteurized milk as a matter of law. Not so in France. When it comes to making cheese, pasteurized milk is not a legal requirement. Most of the cheese found in grocery stores is pasteurized, but unpasteurized cheese is widely available and sought after at the many fromageries – shops dedicated solely to selling cheese. And it’s understandable why the French make this exception. This is, after all, a country whose love affair with cheese is like any great love affair: consuming, complex, surprising, sometimes difficult, always unforgettable.

I’m Andy Boyd at the University of Houston where we’re interested in the way inventive minds work.

(Theme music)

For a related episode, see CHEESE. 

The image of curds and whey is from the author’s boiling of milk in France.

All other images are from Wikimedia Commons.

This episode first aired on August 14, 2025.