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.
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.
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.
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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).
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“When your tomatoes are acidic the baking soda, sodium bicarbonate, will foam up a little where it is sprinkled onto the soup. That's the sign of acid being neutralized.”
— Chris_Maurer
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What other wild food science tips and techniques have you picked up? Tell us about them in the comments!
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Caroline Lange
Writing and cooking in Brooklyn.
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6 Comments
Chris_Maurer December 16, 2021
I do this every time I make homemade tomato soup, but you must be sparing in how much you add. I usually add 1/8 teaspoon and would not recommend more than 1/4 teaspoon as the flavor begins to be detectable and it is not pleasant. When your tomatoes are acidic the baking soda, sodium bicarbonate, will foam up a little where it is sprinkled onto the soup. That's the sign of acid being neutralized.
Susan P. May 31, 2022
I agree, Chris. If you're not sparing the tomatoes end up tasting like nothing. Is 1/8 teaspoon enough to prevent the milk from curdling?
Chris_Maurer July 20, 2022
I've never used milk, only cream, but have never had a curdling problem with it.
Annette April 10, 2018
My mother taught me this 45+ years ago. Not only does it stop the soup curdling after adding milk or cream, it also reduces the excessive acidity often found in fresh, canned or bottled tomato products. I add a small pinch to almost all tomato recipes 👏🏼👏🏼👏🏼
Ben N. March 19, 2016
Most tomato soup recipes use canned tomatoes (diced/whole, and/or paste).. how can it be done using fresh tomatoes? Roma?
Caroline L. March 20, 2016
hi ben! roma tomatoes are great for soup. i'd recommend roasting them first though! eliminating the tomatoes' wateriness will intensify their flavor—which is what tomato soup is all about! hope you have success... here's a great recipe if you need one! https://food52.com/recipes/23426-roasted-tomato-whipped-burrata-and-basil-oil-soup
