Why sour milk doesn't make you sick | Ludwig Explains

Added: 2026-05-19

[00:00:00]Old milk smells horrible. It tastes like a crime. But weirdly, it almost never sends you to the hospital. Why is the most disgusting thing in your fridge also one of the safest? Inside a carton of milk, dozens of microbes are racing to multiply. One group always wins.

[00:00:18]Lactobacillus.

[00:00:20]These bacteria love cold dairy. They eat the sugar in milk called lactose, and they grow faster than almost anyone else in there. As lactobacillus eats lactose, it releases lactic acid. The milk's pH drops from around 6.7 down to about 4.5.

[00:00:38]That acid makes the proteins clump.

[00:00:40]That's the curdling, and that's the smell. Here's the twist. Dangerous bacteria like salmonella, E. coli, and lististeria can't survive that acidity.

[00:00:52]They need a near neutral pH to multiply.

[00:00:55]Lactobacillus has basically poisoned its own room. It's fine. The villains aren't. We've been exploiting this for thousands of years. Yogurt, cheese, sauerkraut, kimchi, sourdough. Every one of them is the same trick. Lactobacillus locks the door behind itself and we get to eat.

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[00:01:21]making the confusing click.