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Interactive video lesson plan for: Why Does Milk Curdle When It Goes Bad?

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Most foods fall apart when they go bad - so why does milk clump together? The answer is bacterial poop, and Lauren can explain.

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So, we’ve all seen things go bad. That banana you really did intend to eat, last Tuesday’s leftovers, Anakin Skywalker, your relationships.

When most food goes bad, you’ll see mold slowly eating holes in it, or maybe a film of slime as bacteria break the food down. But milk doesn’t fall apart when it goes bad: It clumps together.

The simplest answer to why this happens is that bacteria have really interesting poop.

Actually, poop in general is pretty fascinating. By studying excrement, you can learn a lot about an animal -- human animals included -- or even an entire community.

But the bacteria we’re interested in today are of the order Lactobacillales, commonly called lactic acid bacteria because they excrete lactic acid.

Some species in this order (Lactococcus lactis, for example) hang out in soil, grasses, and vegetables all over the world. When milk-producing animals eat that produce, the bacteria pass into their milk. Even pasteurization, which is the process where stuff like milk is gently heated to kill off bacteria, can leave trace numbers of them.

These critters love to eat lactose, the natural sugar in milk. They break it down during digestion, releasing lactic acid as a byproduct. (It’s technically a type of fermentation of the milk sugar.)

Milk is an emulsion, which is the science term for a mixture of things that don’t usually mix. The things, in this case, are water and fats.

Under normal circumstances, fats and water repel each other. But milk also contains complex protein chains called caseins that are made up of both hydrophilic (or “water-loving”) and lipophilic (“fat-loving”) particles.

When presented with both water and fats, caseins grab bits of fat and cluster up into globules called micelles, with the fat on the inside and the hydrophilic bits on the outside.

The hydrophilic particles grab onto electrons in the water, meaning that each micelle winds up having a negative charge. And since negatively charged particles repel each other, the globules suspend themselves throughout the water in order to keep their distance. Thus, milk is an emulsion!

But that changes when you add lactic acid into the mix. Acids are sour-tasting compounds that react with water, releasing positively charged hydrogen ions.

So when bacteria multiply in your milk and produce a bunch of lactic acid, it reacts with the water there, releasing a bunch hydrogen ions. These positively charged particles latch onto the negatively charged micelles and neutralize them.

With no force keeping the micelles apart, they clump together. So you wind up with sour and lumpy -- aka curdled -- milk.

And, OK, that’s pretty gross and unfortunate if you were hoping to have some milk & cereal or put some cream in your coffee. But chemically speaking, this process is awesome!

A controlled version of it is the first step in making yogurt, cheese, and sour cream. We also put lactic acid bacteria to use in fermenting sugars to create lots of foods, like chocolate, kimchi, pickles, miso, sourdough bread, and cured meats and sausages.











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