Just as lemon curdles milky tea, and cream left out overnight begins to smell sour, cream soups are vulnerable to factors that cause curdling and sourness. Adjusting how you cook cream soup, along with how you store leftovers, keep the soups from being ruined while cooking, or from going bad before their time.
Feeling the Heat
Cream and milk can curdle under prolonged high heat. To counteract these tendencies, add ingredients to the stockpot in the right order. When starch is present in the soup before the dairy ingredient is added, it offers some measure of protection against curdling. Because of this, a floury roux or pureed root vegetable base should be added first when the recipe calls for these ingredients. In addition, pouring cool -- not cold -- milk or cream into the soup, and doing this after the soup has finished boiling, lessens the contrast between the hot soup and the dairy liquid.
Low Fat, High Risk
Switching to a low-fat milk instead of cream or whole milk makes cream soups lower in fat, but the downside to this ingredient adjustment is that lower-fat dairy liquids are more apt to curdle. Cream doesn't curdle as readily at high temperatures because of its protective fat content. To lower the curdle risk, add cornstarch to low-fat "cream" soups. The starch make the milk less likely to turn sour and lumpy during heating, while also thickening the soup.
Some cream soup ingredients contain acidic ingredients, which have the potential of curdling the dairy ingredients. Cream of tomato soup is one example of this potential cross-reaction, because the tomatoes are acidic. Similarly, soups that use vinegar or lemon to add zip to an otherwise bland soup might cause clabbering instead of flavor. Take advantage of baking soda's pH-neutralizing effects by mixing a pinch of it into chopped tomatoes or other acidic ingredients before adding them to your milky soup mixture.
Souring on Leftovers
If your cream soup starts to smell sour or curdle within a day or two after it was made, improper storage may be to blame. Bacteria already in the dairy ingredient will multiply rapidly if the environment is too warm, or when exposed to dirty containers. The larger your stock pot, the longer it will take cream soup to cool after cooking, which invites bacterial growth. Ladle leftover soup into several smaller containers so that the cream soup cools more rapidly and is ready for refrigeration sooner. Clean storage containers are also crucial for keeping out the germs that make cream soups go bad quickly.