Whether you're making fettuccine Alfredo, a cream soup or even a dessert sauce, if the milk curdles, you're in trouble. With just a few exceptions, curdled milk is never a desired outcome. Although it's safe to eat, it negatively alters the appearance, texture and taste of a dish. Fortunately, curdled milk can be prevented and even fixed.
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Curds and Whey
Although you can safely eat milk that's curdled during cooking, you might not want to. When milk or other dairy products curdle during cooking, the whey separates from the liquid, leaving a watery appearance and grainy texture. The flavor may be slightly sour and the dish definitely won't have the creamy mouthfeel associated with a milk sauce. Among the appearance, texture and taste, a curdled milk sauce is hardy appetizing.
A Cure for the Common Curdle
Before you reconcile yourself to eating curdled milk, try fixing it first. To temper a curdled sauce, heat a bit of uncooked milk in a saucepan. Slowly whisk in the curdled milk. Often, this simple step will fix a broken sauce. You can also try adding an ice cube to the curdled milk, which may cool it enough to fix the sauce. Or combine a little cornstarch with cold water or milk to make a slurry. Whisk the slurry into the curdled milk and heat it to a low simmer, stirring constantly. The starch in the cornstarch absorbs the separated moisture, fixing the sauce while thickening the milk.
Why Milk Curdles
The main reason dairy products curdle during cooking is because they were exposed to high heat. Dairy products with a high fat content, such as heavy cream, are unlikely to curdle even if you boil them, but low-fat milk is particularly finicky about heat. The proteins in the milk cook, causing them to separate from the whey. Acidic ingredients, such as vinegar, lemon juice or wine, can also cause curdling, although this problem is less common.
Curdle No More
Although you can safely eat curdled milk or can often repair a broken sauce, your best bet is to prevent curdling in the first place. Use whole milk, half and half or cream when possible instead of skim milk. If you use skim milk, never boil it. Better yet, place a candy thermometer on your pan and heat the milk no hotter than 180 degrees Fahrenheit. Add a bit of cornstarch dissolved in cold milk or water to prevent curdling and wait to add vinegar, lemon juice or wine until the end of cooking.