Heat

• Cream soups usually separate and curdle because they've been cooked on high heat. To avoid this problem, cook any vegetables in broth to tenderize them first and wait to add milk until the last minute. After you've added dairy products, avoid cooking the soup on high heat. Instead, heat the soup gently on low just until it's hot, but don't allow it to simmer or boil. Canned, evaporated milk tends to be more stable, and you can substitute it for fresh milk or half-and-half. Whole cream won't curdle if you don't mind a few extra calories.

Acid

• A less-common reason cream soups might separate and sour is the addition of an acid in the soup. Lemon juice, vinegar, pickled products and even some tomatoes all contain acid, which can cause dairy products to separate. The acids coat the proteins and essentially cook them, creating an end result that's akin to scrambled eggs. Avoid adding lemon juice or vinegar to cream soups. When making tomato soup, use canned tomatoes, or roast fresh tomatoes in the oven prior to pureeing them, which not only creates a more stable product, but improves the flavor. Another trick is to combine 1 tablespoon cornstarch with the milk or cream prior to adding it to the soup. The starch prevents the acid from sticking to the proteins in the milk and keeps it from curdling. Because cream is mostly fat, it is less likely to curdle than milk or half-and-half.

Quantity

• Most cream soups have a base of vegetables and broth. Heavy cream or milk is added at the end of cooking to give a creamy texture to the dish. In most cases, you'll need to add only a small amount of cream -- usually less than a cup. The more cream or milk you use, the more likely you'll have problems, so stick to small amounts.

Alternatives

• The traditional method of using a Bechamel sauce as the base for cream soups eliminates the fear of curdling. To make a Bechamel or white sauce, melt butter in a pan and add an equal amount of flour. Stir the flour into the butter, whisking it constantly. Once the flour has browned slightly, you can add milk. As the milk heats, stir it constantly until it thickens. The flour stabilizes it so it doesn't curdle, and you can safely add any vegetables or meat. Another option is to use pureed vegetables as a thickener and leave out the cream altogether. If you saute them first in a little butter, they'll take on a rich, satisfying flavor with fewer calories and no fear of curdling.