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Baking soda in toffee?

(article, Hank Sawtelle)

Hopefully you are the right person to ask about this. I am curious about making toffees and brittles, and I've noticed that some recipes have baking soda thrown in at the end. Why is that? And what's the difference when you see a recipe that doesn't contain baking soda?
— Cyndi O., Hoboken, N.J.


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Oh, man, this is a good one. Why don't they ever explain this stuff in recipes? “Just trust us and drop in a random, semi-industrial chemical before serving.”

Baking soda, a.k.a. sodium bicarbonate, is, chemically speaking, a base. Going back to high-school chemistry, bases are the opposite of acids. The pH scale measures how acid or alkaline (basic) a substance is, with 7.0 being neutral. Anything lower is acidic, anything higher is alkaline. So in a broad sense, the reason baking soda is added to foods is to raise the pH.

There are a number of situations where an alkaline pH is desirable in the kitchen, as many chemical reactions are affected by pH. For example, a higher pH promotes certain browning reactions, so baking soda is found in most pretzel recipes to achieve the customary dark-brown color. Without baking soda, pretzels would just be curly beige bread sticks.

[%image reference-image float=right width=400 caption="Why do some toffee recipes contain baking soda?"] 

Vegetable structures (like beans) break down more completely in a higher pH environment, so some hummus recipes call for baking soda in the bean-cooking water. (I don't like it — the complete absence of texture is a little too baby-foody for me.) 

Similarly, an elevated pH can help maintain color when blanching green vegetables like broccoli, but again at the risk of a mushy texture. 

The balance of sour (acid) flavors can also be changed by neutralizing some of the acid with a base such as baking soda. Incidentally, a lot of the stinky molecules in the fridge are acidic, which is why an open box of baking soda left in the fridge can help neutralize odors.

But the most dramatic use of baking soda, and the one that's relevant to candy-making, takes us back past chemistry class all the way to the Science Fair. As the (lazy) kid who made the volcano can tell you, when acids and bases (such as vinegar and baking soda) come into contact, they react, and carbon dioxide is released. 

This makes baking soda useful as a [/articles/features/baking_chemistry "chemical leavener"] in recipes that contain acid, such as chocolate-chip cookies, which include acidic brown sugar and chocolate. (Baking powder, incidentally, combines baking soda with a built-in acid to leaven recipes that don't contain acid.)

Brittles and toffees accumulate small amounts of acid from the browning reactions that occur during cooking. This is one reason why the baking soda is added at the end of cooking. The soda reacts with the acid to make bubbles, and the syrup foams. When the cooked syrup is poured out and begins to harden, many of the tiny bubbles are trapped before they can escape (another reason the baking soda is added at the end). The bubbles lighten the texture of the brittle, making it easier to chew without busting an incisor, canine, or molar (but cavities are still guaranteed). 

I had this awesome theory that the vanilla extract often called for at the end of a brittle recipe was acidic, and therefore contributed to the foaming action of the baking soda. You seriously should have seen how excited I was about this presumed breakthrough. I eagerly mixed vanilla and baking soda in a small bowl, but just ended up with flat, vanilla-flavored mud. Apparently the vanilla is added late to prevent the volatile aromatic compounds from boiling away during cooking. 

However, the foaming effect can be emphasized by adding an acid ingredient to the candy recipe and increasing the amount of baking soda accordingly. According to Harold McGee, 1/2 teaspoon of baking soda will react with 1 teaspoon of lemon juice or 1 1/4 teaspoons of cream of tartar. 

All of these additives affect the flavor of the final product, though, which is why most classic recipes just use sugar, butter, and nuts. Teeth are overrated anyway.

p(bio). Based in Portland, Oregon, Hank Sawtelle has engineering, legal, and culinary degrees.

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