If gritty mac and cheese, lumpy pudding or grainy white sauce doesn't sound appealing, avoid the urge to salvage milk that curdled during heating. You shouldn't just pour milk into a pot and bring it to a boil -- especially nonfat and low-fat milk; the less fat, the less stable a heating dairy product. Dump out the curdled milk and start over with an informed approach to prevent this mishap.
Skip the skim, 1 percent and 2 percent milk in cooking. Though healthier, they're no good for boiling. They too easily break or curdle. Besides, they add no richness and little flavor to your dish. Use whole milk or -- better yet -- light or heavy cream. Choose a pot that holds at least three times the volume of liquid you're heating, because dairy products expand quickly when they hit a boil.
Achieving a Boil
Put the milk over medium-high heat and leave the pot uncovered. Frequent gentle stirring or whisking prevents curdling. The lower the fat content, the more often you should stir. Stir whole milk at least every 30 to 45 seconds, stir light cream every minute or so and stir heavy cream at least every 2 minutes. Remove the pot from the heat and turn it down as soon as the liquid hits a boil to prevent overflowing. Reduce the liquid at medium-low heat. Alternatively, mix in about 1/4 teaspoon of flour, cornstarch or another starch thickener to stabilize the milk and prevent curdling; it thickens the liquid too.