Leavening in the Kitchen: Baking Soda and Baking Powder

Baking soda and baking powder are used in all sorts of baked goods including cookies, cakes, and muffins. They can be used independently or in conjunction. And although they both contain the word “baking” and produce carbon dioxide to help leaven our bakes, there are differences that are crucial to understanding how they work in a recipe.

Baking soda and baking powder produce gas.

The purpose of both baking soda and baking powder is to produce carbon dioxide gas. In the oven, this gas expands to give our baked goods volume and tenderness. If you haven’t already, I suggest reading through the introduction to leavening to learn more about the roles of leavening gases in baking. This demonstration I shared on Instagram may also help you understand the differences between baking soda and baking powder as you read through the rest of this post.

Baking soda is one chemical compound.

Baking soda is the common name for the chemical sodium bicarbonate (NaHCO3), just as table salt is the common name for sodium chloride (NaCl). Baking soda contains sodium, hydrogen, carbon, and oxygen atoms arranged as shown below. The sodium is a positive ion, and the bicarbonate is a negative ion. Because negative and positive charges attract, much as the north and south poles of a magnet attract, the two ions stick together to form a salt. (For more on ions and salts, check out “Proteins in the Kitchen.”) A box of baking soda is full of sodium bicarbonate crystals, just as a canister of table salt is full of sodium chloride crystals.

Baking soda is another name for the chemical compound sodium bicarbonate (NaHCO3).

In water, baking soda dissolves just like table salt does. Because water molecules have positive and negative ends, they can attract the positive sodium and negative bicarbonate ions and pull them apart. (Again, more on this in “Proteins in the Kitchen“!) So, when we combine baking soda with water, we end up with a bunch of sodium and bicarbonate ions dissolved in the water.

In water, baking soda dissolves into sodium and bicarbonate ions.

Baking soda produces carbon dioxide.

If we heat this solution directly, the bicarbonate breaks down. The heat provides energy for the atoms in the bicarbonate to rearrange, and one of the molecules we end up with is carbon dioxide—a leavening gas! But this typically isn’t how we use baking soda. It doesn’t provide much gas, and it also leaves behind a compound with an unpleasant soapy taste. And by the time the batter is hot enough for this reaction to occur, the gluten and starches would be rigid and unable to expand, so the carbon dioxide wouldn’t help with any leavening.

Baking soda needs to react with acid to produce carbon dioxide.

But there is a way to create carbon dioxide before the structure of a batter or dough sets. You know how baking soda fizzes when you pour vinegar on it? That’s a chemical reaction creating carbon dioxide. Any acidic ingredient will undergo the same reaction. Some common acidic ingredients include cultured dairy products such as yogurt, sour cream, and buttermilk; sweeteners like brown sugar, honey, and molasses; vinegar, lemon juice, natural cocoa powder, fruit, and fruit juices.

At the molecular level, these acids contain a positive ion, often hydrogen, partnered with a negative ion. Just like baking soda or table salt, these ions separate when the acid dissolves in water. When we dissolve both an acid and baking soda in water, we end up with a soup of ions. And because ions are attracted to other ions of the opposite charge, the acid and the baking soda can essentially trade partners: the positive sodium ions of the baking soda can partner with the negative ions of the acid, and the negative bicarbonate ions of the baking soda can partner with the positive hydrogen ions of the acid. In the drawing below, I’ve colored the baking soda ions orange and the acid ions blue so it’s easier to keep track.

The second pairing is the crucial one. When the positive hydrogen ion from the acid pairs up with the negative bicarbonate ion from baking soda, they form a molecule called carbonic acid (H2CO3), which immediately breaks down into carbon dioxide and water. All of this occurs at room temperature, which means that we’ll have plenty of carbon dioxide to help us leaven when the batter or dough hits the oven. The other ion pair remains in the baked good as a neutral salt. (In this case, I mean “neutral” as in “neither acidic nor alkaline.” Water is another compound with neutral pH.) These salts produce the flavor of chemically leavened baked goods such as biscuits, scones, and Irish soda bread.

Baking soda and acid dissolve in water to form ions. When these ions trade partners, they form a neutral salt and carbonic acid. The carbonic acid immediately breaks down into water and carbon dioxide, which is used for leavening.

Large amounts of baking soda can produce undesirable flavors.

The taste of the neutral salts in these baked goods is subtle because there aren’t many of them. If we were to add more baking soda, we would have more ions trading partners, and we would create more neutral salts. In large amounts, their taste is undesirable, so we want to be careful not to use too much baking soda. In a recipe, we also want to provide enough acid for all of the baking soda to react. If there aren’t enough acid ions, the baking soda won’t be able to trade partners. Unreacted baking soda also leaves an undesirable taste.

My friend once made cookies following a recipe I’d used to make chewy, flat cookies. He ended up with beautiful puffy cookies. At first, I was shocked that his substitution of maple syrup for honey could have caused such a huge difference, but then I bit into a cookie and realized what had happened. It tasted like straight-up baking soda. It turns out he’d measured the baking soda in tablespoons, not teaspoons. The baking soda reacted with all the acid in the batter to produce lots of carbon dioxide (hence the puffy cookie), but there was still so much baking soda left over that it overwhelmed the taste. For the sake of science, I tried squeezing some lemon juice onto the cookie before I took a bite. It neutralized the baking soda, and I could taste some of the sweetness that had been hidden. Unfortunately, it wasn’t a practical way to eat cookies and we ended up tossing them.

We couldn’t eat the baking soda cookies, but someone else didn’t seem to mind!

Baking soda works quickly.

In addition to potential flavors we may add with baking soda, we have to consider how quickly it reacts. As we mentioned, the ion partner trade with the acid and subsequent production of carbon dioxide occur at room temperature (or even in cold water). Thus, as soon as baking soda is combined with an acid and with water (and many of our acidic ingredients already contain water), it will start producing carbon dioxide. If we let the batter sit on the counter for too long, the carbon dioxide will simply diffuse out of the batter before it’s done its job. Thus, if a recipe uses baking soda, it’s important to get the batter or dough into the oven quickly, while there’s still plenty of carbon dioxide to help with the leavening. This is especially important with thinner batters, which are worse at holding air.

Baking powder contains three components.

But what if we need some time for the batter to rest before we bake it? Baking powder can help. Baking powder is a mixture. It contains baking soda, at least one type of acid salt, and a starch (often cornstarch). An acid salt is just the powdered form of a compound that will become an acid when it dissolves in water. Cream of tartar is one example. Baking powder essentially pre-mixes an exact amount of acid with baking soda, so when the components of the baking powder dissolve in water, the baking soda has acid to react with to produce carbon dioxide and a neutral salt. This means that we don’t need to worry about adding an acidic ingredient to the recipe because it’s already mixed into the baking powder for us.

The cornstarch helps to extend the shelf life of the baking powder. By absorbing moisture from the air, it prevents the baking soda and acid salt from dissolving and reacting while the baking powder is still in the can. It also “dilutes” the baking soda. In every spoonful of baking powder, very few particles are actually the baking soda that will produce carbon dioxide. Thus, to create the same amount of carbon dioxide, we would need more baking powder than baking soda.

Baking powder is a mixture of sodium bicarbonate (a.k.a. baking soda), acid salts, and starch.

Baking powder contains different acid salts.

But if you’ve ever added baking powder to cold water, you might be confused. Given everything we’ve just discussed, the baking soda and acid salt should dissolve, react, and form carbon dioxide. But the mixture only fizzes a little. It’s nothing compared to the violent bubbling you get when you mix baking soda with vinegar. The baking powder doesn’t even dissolve completely. Only if you combine baking powder with boiling water will you see a lot of bubbles. Why is this?

All commercial baking powders nowadays are double-acting, which means they produce carbon dioxide in two bursts: one when the baking powder gets wet in order to lighten the batter and make it easier to handle, and again when it’s heated in the oven in order to raise the baked good. In other words, at least one step in the baking powder reaction needs heat to occur. This is how baking powder prolongs its production of carbon dioxide. But how does this happen?

I hinted at the answer earlier. In cold water, “the baking powder doesn’t even dissolve completely.” Remember that the baking soda can only react with the acid if the acid salt is dissolved. Only then do we have an ion soup where the ions can trade partners. Baking powder’s magic lies in its acid salts: it often contains several of them. Some of these dissolve at room temperature to produce the first burst of carbon dioxide, and the others only dissolve when the water heats up in the oven. They’re called fast- and slow-acting acids, respectively. Baker’s Corner and Clabber Girl baking powders, for example, contain the fast-acting acid salt monocalcium phosphate (MCP) and the slow-acting one sodium aluminum sulfate (SAS). You can read them off the ingredients label if you take a look below! Some baking powders and boxed cake mixes even coat their acid salts to delay their dissolution even longer. This way, most of the carbon dioxide forms when the batter is in the oven and ready to rise, where it can exert its maximum leavening power.

Both Baker’s Corner (left) and Clabber Girl (right) baking powders contain SAS and MCP.

Hot water dissolves the acid salts.

Why does heat help the acid salts dissolve? Heat actually encourages a lot of chemical processes because it’s a source of energy. (Remember how it helps sugar to dissolve?) As acid salts dissolve, their atoms adopt some unstable configurations. The less stable the configurations, the less likely they are to form, the less likely the acid is to dissolve. The ions that make up the acid salts are like two people who are comfortable in their relationship even though it is unhealthy for them. They each might be able to find a better partner, but they’re hesitant to risk the instability of leaving the current relationship.

The ion pairs in fast-acting acids such as cream of tartar or MCP are more willing to take risks, so these salts dissolve quite readily, even in cold water. The ions in slow-acting acid salts, however, like SAS, need a lot of encouragement. They get this encouragement from heat energy, which supports the individual dissolved ions until they find another ion partner. Ultimately, since the baking powder manufacturers played matchmaker very precisely, all of these acid ions will find new ion partners from the baking soda, and we form the carbon dioxide that leavens our baked goods.

MCP, a fast-acting acid salt, can dissolve in cold water. However, SAS, a slow-acting acid salt, needs heat to dissolve. Note that this figure shows only the dissolution of the acid salts, not their subsequent reaction with baking soda.

Choosing a chemical leavener

Although both baking soda and baking powder rely on the reaction of baking soda with an acid to produce carbon dioxide, there are several distinctions to consider when we decide which chemical leavener to use in a recipe or when we substitute one for the other. And based on which one(s) are used in a recipe, we need to be mindful of how we work with the batter or dough.

Timing the carbon dioxide

As we alluded to earlier, baking soda reacts quickly with any acid in the batter. Thus, when we work with baking soda, we don’t want the batter or dough to sit on the counter for too long before it goes into the oven. Otherwise, the carbon dioxide will diffuse out of the batter and we will lose its leavening power. If we use baking powder, most of the carbon dioxide will not be created until the batter is in the oven, so it’s okay to move a little slower.

Adjusting acidity

When baking soda reacts with an acid, it produces carbon dioxide and a neutral salt. In other words, baking soda is an alkali that neutralizes the acid. Since acidity is so important for other aspects of baking, including structure and color, we might choose the chemical leavener based on the acidity of the other ingredients.

Many muffins and quick breads, for example, don’t contain especially acidic ingredients. If we were to leaven with baking soda only, there might not be enough acid in the batter to react with the baking soda. The unreacted baking soda would not contribute to leavening, and it would leave an unpleasant taste. Even if there were enough acid in the batter, by neutralizing the acid we decrease the overall acidity of the batter. Since proteins from eggs and flour need an acidic environment to set, baking soda could prevent the baked good from solidifying properly.

On the other hand, gingerbread cookies are made with molasses, an acidic ingredient. Since there’s plenty of acid available, the cookies are traditionally leavened with baking soda. Furthermore, since acidity decreases browning, baking soda helps the cookies darken by neutralizing the acid and decreasing the acidity of the dough. Gingerbread cookie dough also takes a while to get into the oven since it needs to be rolled and shaped. The thick cookie dough holds air better than a thin cake batter, but some modern recipes supplement the baking soda with some baking powder to ensure enough leavening power is available in the oven.

Acidity is also important for the color of certain ingredients. Cocoa powder, for example, has a darker color when it’s alkaline and a redder color when it’s acidic. Thus, if we want to make darker-colored chocolate baked goods, we can use baking soda to increase the alkalinity of the batter or dough. Conversely, if we want a redder product, like for red velvet cake, we can use baking powder with a combination of acidic ingredients to increase the acidity of the cake batter.

These two chocolate cakes were made with different cocoa powders and leaveners. The cake on the left contains Dutch process cocoa powder and baking soda. The more alkaline batter made a dark cake. The cake on the right contains nautral cocoa powder and baking powder. The more acidic batter made a browner cake.

Substituting chemical leaveners

As we’ve discussed previously, chemical leaveners must be balanced with the other ingredients in a recipe to produce baked goods with high volume and a tender texture. As always, it’s best to follow a recipe strictly if you’d like results consistent with the recipe developer’s, but if you’re in a bind, you can try to substitute.

Baking powder instead of baking soda

If you’re substituting baking soda with baking powder, keep in mind that baking powder is diluted and has less leavening power per gram or teaspoon. You’ll typically need four times more baking powder than baking soda to get the same leavening power. Furthermore, because baking powder contains its own acid, it won’t react with any acidic ingredients in your batter, and the batter will be more acidic. It might set sooner and brown more. If the recipe contains an obvious source of acid, such as a teaspoon of lemon juice or vinegar, you can omit it, but otherwise it’s hard to change the acidity of the batter without also changing flavor or texture.

Baking soda instead of baking powder

If you’re going the other way, the same concepts apply. You’ll need to decrease the amount of baking powder to about a quarter if you’re using baking soda instead. You’ll also need to add an acid for the baking soda to react with, such as cream of tartar, lemon juice, or vinegar. However, because all of these acids are fast-acting, you’ll have to get the batter into the oven quickly to reduce the risk of losing carbon dioxide, leavening power, and ultimately, volume.

Conclusions

The white powdery forms of baking soda and baking powder are unassuming, but they are crucial ingredients for baked goods including cookies, muffins, biscuits, cakes, and quick breads. They lighten batters and doughs as we mix them, change the pH of a batter, and most importantly, produce carbon dioxide. This carbon dioxide leavens our baked goods to give them high volume and tender texture. In the next post, we’ll discuss another source of carbon dioxide in baking, most often used for bread: yeast.



References

Corriher, S. O. Bakewise; Scribner: New York, 2008.

Figoni, P. How Baking Works, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, 2011.

Vollhardt, N.; Schore, P. Organic Chemistry: Structure and Function, 7th ed.; W.H. Freeman and Company: New York, 2014.

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