Milk curdles when it comes into contact with acids such as lemon juice and vinegar. Adding these ingredients to milk is a safe way to make curds and cheese.
Curdled milk is often a sign of spoilage. Bacteria that are naturally present in the milk eat lactose and create lactic acid in the process. Acids cause the fats and liquids in the milk to separate, which is why it curdles.
However, milk can curdle without going bad. All that is needed is a bit of acid in place of lactic acid. A tablespoon or two of either lemon juice or vinegar in a small pot of hot milk will cause the fat to immediately separate into curds. These curds make great cottage cheese and ricotta. Cooks can also drain the whey from the curds and use the curds in recipes such as poutine, a Canadian dish.