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Put Baking Soda in Your Cream of Tomato Soup! Here's Why

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Yes, all cooking is science; whether you're making waffles or a steak, you're enacting small transformations on a series of ingredients in order to create a very new result. But rarely, it seems, are those scientific transformations as immediate as what happens when you add baking soda to your cream of tomato soup.

How to Make Tomato Soup Without a Recipe
How to Make Tomato Soup Without a Recipe

Many cream of tomato soup recipes call for a small amount of baking soda (often as little as half a teaspoon) to be stirred into the tomato base just before the milk is added. Why is this? asked usclious on our Hotline. Cv came to the rescue, citing the Fannie Farmer Cookbook: Baking soda prevents the milk from curdling in the acidic tomatoes.

Tomatoes are naturally acidic, falling at about 4.6 on the pH scale. Baking soda, on the other hand, is naturally basic (or "alkaline"), about 9 on the pH scale. When you add baking soda to tomato soup (or sauce, or chili), it neutralizes the acid in the tomatoes. Not only will this make the tomatoes taste less acidic (good news if your tomatoes turned out more sour than you expected), but it also means that you can now add milk to your soup without risking curdling it.

The pH scale, and where some common items fall on it.
The pH scale, and where some common items fall on it. Photo by National Oceanic and Atmospheric Administration

So what about creamy tomato soups that don't have baking soda added, like Amanda Hesser's Cream of Roasted Tomato Soup? The recipe uses cream instead of milk, which is crucial: The fat in the cream protects the dairy from curdling.

A little more science: Fat protects the proteins in milk. As Harold McGee explains in ​On Food and Cooking, fat globules attach to the milk protein casein when milk is heated. The more fat there is, the more the milk protein is protected—which prevents curds of casein from forming (a.k.a. prevents curdling).

What other wild food science tips and techniques have you picked up? Tell us about them in the comments!

Tags: Tips & Techniques, Ingredients, Food Science, Hotline