We get more calls about curdled food this time of year than any other time. Callers are frustrated when their homemade tomato soup curdles. It can be annoying when making tomato soup or scalloped potatoes to have the product look curdled and lumpy. It certainly is not an appetizing way to serve a meal.
You should know that the protein in milk is likely to clump together or curdle, when exposed to acid or salt. A number of things can help you avoid this situation. When making cream of tomato soup, try adding the tomato to the milk rather than the opposite. Remember to have both the milk and tomato hot, and thicken either the tomato juice or milk before they are combined. Do serve the soup promptly.
If you are baking scalloped potatoes, avoiding high oven temperatures and long cooking time will make the milk less likely to curdle. Parboiling the potatoes shortens the cooking time and the likelihood of curdling. Using evaporated milk further aids the product.
If ham and scalloped potatoes are baked together, curdling will occur. Ham contains curing salts, which make the milk protein extremely unstable and causes them to curdle easily.
Think through the recipe and directions before you start cooking; you should be able to avoid curdling in your dish.