Baking soda in toffee?

A kitchen mystery, solved

November 25, 2009

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.

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.

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 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).

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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.

Based in Portland, Oregon, Hank Sawtelle has engineering, legal, and culinary degrees.

There are 13 comments on this item
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1. by dgreenwood on Nov 25, 2009 at 2:50 PM PST

Thank you so much for your detailed explanation. I haven’t ever used baking soda, and agree that it probably tastes better without. Hope your holiday is filled with good things to eat.

2. by zegg on Dec 1, 2009 at 12:40 PM PST

I remember being aged about 11, at a friend’s house, parents were out, and we decided to make toffee. On adding the baking soda, the whole thing foamed out of the pan and onto my hand. I still have the scar from the burn (but we cleaned up so the parents never found out).

3. by Marji Schoeneman on Dec 6, 2009 at 9:26 AM PST

I have been making some peanut brittle, the flaky, soft style with baking soda. However, I’ve made it about 6 times, and sometimes it comes out more compact and not as soft. I can’t tell what I am doing different on these batches so I can avoid the change. After reading this, I am thinking maybe I should use a little brown sugar, or add a little cream of tartar with the soda: Any advanced advice from anyone would be very appreciated.

4. by MRW on Feb 24, 2010 at 6:45 AM PST

Nice article. I’m a chemist and enjoy reading about the chemistry of food.

You made one mistake, though. You seem to be saying that reacting any base with any acid will release carbon dioxide. This isn’t true. Carbon dioxide is released when an acid is mixed with a carbonate (such as baking soda) but not when acid is mixed with other bases.

5. by Hank Sawtelle on Feb 24, 2010 at 7:20 AM PST

Thanks for the correction, MRW. Of course there are acid-base reactions that don’t make CO2. It was an incorrect generalization.

6. by Sarah Melamed on Mar 21, 2010 at 10:53 PM PDT

Baking soda is also used when the water source is very hard.In cases like this beans do not soften even after hours of cooking. Solution- a bit of baking powder or use filtered water in the soaking or cooking liquid.

7. by kim alexander on Aug 9, 2010 at 8:51 PM PDT

Thanks for the baking soda insight when making toffee. I have tried twice today and both batches turned out too brown (and slightly overcooked in taste) although my batch was removed just before the 300 degree mark. I’m wanting a toffee result that is still crunchy but lighter in taste and color. Will adding the bakind soda AND cream of tartar in the final stage (as you suggested) result in this kind of texture? I even removed the second batch at around 285 degrees and it is inconsistent in texture and, also, too dark and too bitter!! Any suggestions? Thanks Hank! Kim

8. by wastewater treatment on Feb 23, 2011 at 3:19 AM PST

Great instructional post. Love the way it was written. You don’t find that many step-by-step walk throughs out there, not as this one. Thanks for taking the time to put this together.

9. by Melissa on Apr 24, 2012 at 9:33 PM PDT

Yes, thank you for your info. I am in a food science class for my dietetics degree and one of the questions for the lab on non-crystalline candies (peanut brittle) was “What is the source of the acid that reacts with the baking soda in the peanut brittle?” I was about to put down that it was the vanilla extract, but after reading your article I have better information! Thanks!

10. by Dawn Cramer on Oct 10, 2012 at 9:53 AM PDT

I was told to always use a wooden spoon and to keep it boiling until it turns the color of a brown paper sack. I have never had to use a candy themometer. Works everytime. Love Toffee

11. by Dawn Cramer on Oct 10, 2012 at 9:53 AM PDT

I was told to always use a wooden spoon and to keep it boiling until it turns the color of a brown paper sack. I have never had to use a candy themometer. Works everytime. Love Toffee

12. by Kelly C on Dec 6, 2012 at 1:50 PM PST

This was terribly helpful, thank you! And full of wit to boot!

13. by anonymous on Oct 3, 2014 at 7:23 AM PDT

Nice article! In your quest to learn where the acid is that reacts with all that baking soda... you may be interested to learn that baking soda actually undergoes thermal degradation at these temperatures. No acid required.

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Hank Sawtelle has engineering, legal, and culinary degrees, and an intense curiosity about food and cooking. Follow Hank’s blog, Sous Vide Jones.

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