Why does milk turn into yogurt?

Milk becomes yogurt when helpful bacteria eat lactose, the sugar in milk, and make lactic acid. That acid changes the milk proteins so they clump into a gentle network instead of floating freely. The result is thicker, spoonable, and tangy because bacteria have reshaped the milk.

Different yogurt styles come from different cultures, temperatures, and straining. Greek yogurt gets thicker when watery whey is removed, while other styles use bacteria that make a sharper taste. The live cultures are still present in many yogurts when the tub reaches your fridge.