Acid can curdle cream (or milk). The tendency to curdle increases as the ratio of acid to cream, the temperature of the mixture, or the salt quantity increases. Since three of the star ingredients of cream of tomato soup are cream, acid-rich tomatoes, and salt, the threat of curdling lurks in your kitchen whenever you prepare the soup.
To thwart the curdling tendency, heat the liquefied tomato mixture and cream separately, then slowly add the acidic liquid to the cream near the end of the cooking process. Once mixed, do not heat the cream of tomato soup for very long or at a temperature above 180 degrees F. Add the salt just before serving.
Another pointer is to use the freshest cream possible. The more a cream ages, the more susceptible it becomes to curdling because its lactic acid content increases.
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