Heat alone does not cause milk to curdle, but adding even a small amount of acid can result in curdled milk. The lower the fat content in the milk, the more likely it is to curdle.
To be safe, all low-fat dairy products should be heated to no more than 180 degrees Fahrenheit. If a recipe calls for lemon juice or vinegar to be added to milk, it must be cool to prevent curdling. If heating is required, the addition of a starch, like flour or cornstarch, stabilizes the milk, preventing curdling. If unheated milk (to which no acid had been added) curdles, it is spoiled and should be discarded.