Creamy tomato soups are easy and delicious meals, but the natural acidity of tomatoes can curdle the milk you add. To avoid any weird chunks floating around in your soup, you need a little baking soda.
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To achieve the perfect level of creaminess in your tomato soups, Caroline Lang at Food52 suggests you add about half a teaspoon of baking soda. It will neutralise the acid in the tomatoes so they will taste less acidic, and they won't curdle the milk in your soup. A lot of recipes will recommend this, but it's a nice tip to remember if you're coming up with your own recipe or even trying to make the canned stuff a little creamier. If you want to use cream for your tomato soup recipe instead of milk, however, you probably won't need baking soda (or only a pinch of it at least). The fat in the cream helps protect the protein and keeps it from curdling.