Why It Separates

• Sour cream, as well as many other dairy products, curdles when exposed to high heat. The heat causes whey proteins in the sour cream to unwind and bind with casein proteins. When this happens, the proteins separate from the liquid, forming a rubbery, watery mess -- essentially creating a dairy version of scrambled eggs.

Off the Heat

• To prevent sour cream from curdling, add it to soup only after you've removed the soup from the heat. Let the soup cool slightly before slowly stirring the sour cream into it. Even a slight reduction in temperature can mean the difference between a curdled mess and a thick, creamy soup.

Keep the Fat

• To prevent curdling, skip the low-fat varieties of sour cream. Full-fat dairy products are less prone to curdling because the fats in them coat proteins so they don't bind together. Cream, for example, rarely curdles, while skim milk is notoriously difficult to cook with. Low-fat or nonfat sour cream is more likely to curdle than a full-fat product. Stick with regular sour cream, which has 18 to 20 percent butterfat or use creme fraiche instead. Creme fraiche has 30 to 40 percent butterfat and rarely curdles when heated.

Smart Solutions

• Don't despair if your soup has already curdled. You can probably fix it with a few simple tricks. First remove the soup from the heat and place it over a bowl filled with ice water. Sometimes, simply chilling the soup quickly is enough to restore it. Try gently heating some sour cream in a saucepan, but don't let it simmer. Slowly whisk it into the broken soup to restore it. Mix a bit of cornstarch with cold water to make a slurry. Stir the slurry into the soup and bring it to a simmer. The starch expands as it heats to absorb moisture and will thicken the soup, fixing the curdled sour cream.