Milk, even milk that has been pasteurized, contains a bacteria called lactobacillus. The first part of the word, “lacto,” refers to the sugar that these particular bacteria eat—lactose. When these bacteria process lactose, they secrete lactic acid. It’s this acid that makes milk taste sour and leads to curdling.

It actually doesn’t have to be lactic acid that curdles milk; any acid will do. Try taking a small glass of milk and adding lemon juice or vinegar to it. The milk will start to curdle just the same.