a Morsel of Science: How Baking Soda and Baking Powder Work
FOOD FOR THOUGHT
If you’ve ever baked a cake, you know that most recipes call for either baking soda or baking powder. But have you ever wondered what these two ingredients actually do? And what’s the difference between them, anyway?
It turns out that any cake fit for a celebration needs one of these ingredients to help it ‘rise’ to the occasion.
I’ve shared the morsel of science I learned in “Tip of the Tongue”; for those looking for more scientific detail (like I was), check out “Building Depth of Flavor”.
Happy snacking!
TIP OF THE TONGUE
Baking soda and baking powder are both examples of chemical leavening agents. Leavening agents are ingredients that help baked goods rise. Not all leavening agents are chemical — some are biological (like the baker’s yeast commonly used for bread), while others are mechanical (like whisking heavy cream into whipped cream). In all three cases, air bubbles are being introduced into the liquid to increase its volume.
Since baking soda and baking powder are chemical leavening agents, they increase the volume of the batter through a chemical reaction. This chemical reaction — which occurs between an acid and a base — produces carbon dioxide. It’s the same type of reaction used for the baking soda volcanoes that you might remember from elementary school.
While there are a few different definitions of acids and bases, for the purposes of this post, an acid is a chemical that gives away a proton (H+), and a base is a chemical that receives a proton (H+). Protons are hydrogen atoms that are missing one electron, and they are responsible for ‘sour’ tastes. Acids and bases are everywhere around us, such as vinegar, which is an acid, and bleach, which is a base. When you combine an acid and a base, they react to form new chemicals, some of which are perfectly safe (like in cakes), and some of which are extremely dangerous (which is why it’s dangerous to combine vinegar and bleach).
The carbon dioxide from this acid-base reaction forms air bubbles that cause the batter to rise and create what bakers call the ‘crumb’ of the cake. The main difference between baking soda and baking powder has to do with the acid component in this chemical reaction. Baking soda is a base, and so the acid in the chemical reaction needs to be added as a separate ingredient. Examples of this include the vinegar added to red velvet cake and the buttermilk added to pancake batter. Baking powder is baking soda that is already pre-mixed with an acid. Recipes that don’t have an acidic ingredient usually specify baking powder, not baking soda, for this reason.
BUILDING DEPTH OF FLAVOR
The chemical name for baking soda is sodium bicarbonate (NaHCO3), which is also what baking soda is called in many parts of the world. The reaction of sodium bicarbonate and an acid produces sodium, water, and carbon dioxide. (NaHCO3 + H+ → Na+ + H2O + CO2)
Have you ever put too much baking soda and thought your cake tasted bitter? That’s because when the acid and base aren’t balanced in the reaction, they produce sodium carbonate (Na2HCO3), water, and carbon dioxide. Sodium carbonate has a strong, unpleasantly bitter flavor.
Baking powder is made up of sodium bicarbonate (a base), an acid, and a filler (usually cornstarch) that keeps the sodium bicarbonate and the acid separate. When baking powder is added to a liquid, the filler dissolves, which allows the acid and base to react.
There are two types of acids that are used in baking powder: slow-acting and fast-acting. Slow-acting acids require a high heat before they can react with a base, which means they don’t really work until the batter is actually in the oven. Fast-acting acids react with bases at lower temperatures, often at room temperature, so they work during the mixing stage. A fast-acting acid you’re probably familiar with is monopotassium tartrate, also known as cream of tartar, which is often used to stabilize whipped cream.
What about ‘double-acting baking powder’? Double-acting baking powder has two acids: one slow-acting and one fast-acting. Together, they allow for carbon dioxide production during both the mixing and baking stages for optimal cake rising. Single-acting baking powder only has a slow-acting acid, so it won’t do anything until the batter is in the oven.
So how does this chemical reaction make a cake rise? Why doesn’t the carbon dioxide just escape like it does in a soda? Leavening a cake occurs through three basic steps: mixing, foaming, and baking. During mixing, air bubbles are introduced into the batter mechanically. This is also how whipped cream and meringues are formed. In the foaming stage, these air bubbles start to fuse together, and the fast-acting leavening agents start working. The carbon dioxide they produce diffuses first into the air bubbles added during the mixing stage, and then whatever doesn’t fit there gets trapped in — or solubilized into — the liquidy batter. This ‘foaming’ is what causes those little bubbles to appear if you let pancake batter sit for a few minutes.
Finally, we get to the baking stage. While heat can turn baking soda into carbon dioxide and sodium carbonate (yuck!) starting around 118℉, the bulk of the carbon dioxide is produced by the reaction between baking soda and slow-acting acids between 122 and 176℉. In the same temperature range, proteins like gluten start to break down — or denature — and starches absorb water from the batter. Together, they form a sort of goopy gel.
Remember the carbon dioxide in the foaming stage that got trapped in the batter? Combined with all the new carbon dioxide being produced by the slow-acting acid, there’s no longer enough space for all this gas. The heat of the oven causes the carbon dioxide to expand, which pushes apart the starches and denatured proteins in the goopy gel. At the same time that the carbon dioxide is expanding, those starches and denatured proteins also start solidifying into a firmer structure, which encases these expanding bubbles and limits how big they can get. The ‘crumb’ of a cake is determined by how much the expanding air bubbles and the solidifying batter balance each other out.
Without enough gluten, which is great at forming these structures, these air bubbles collapse because they’re not stabilized enough. This is why gluten-free alternatives are often a little denser than their traditional counterparts. A soft and fluffy gluten-free baked good gets around this issue by using a few tricks, including a variety of non-gluten proteins, such as gelatin, and stabilizing carbohydrates, such as xanthan gum.
What happens if you don’t use the right amount of baking powder? If you don’t use enough, the carbon dioxide produced will easily diffuse into the air bubbles from the mixing stage, but there won’t be enough to get into the starch granules of the batter. This will leave you with a cake with big, uneven air bubbles throughout the cake. If you use too much, the carbon dioxide will be produced too early and will expand the air bubbles before the starch/protein gel has started to solidify. As a result, the air bubbles will collapse on themselves to form a dense and sunken cake.
SOURCES
Babinčáková M. Leavening agents: The chemistry of baking discovered with a computer-based learning. Journal of Chemical Education. 2020 Mar 10;97(4):1190–4.
Neeharika B, Suneetha WJ, Kumari BA, Tejashree M. Leavening Agents for Food Industry. Int. J. Curr. Microbiol. App. Sci. 2020;9(9):1812–7.
Pop G. Researches regarding the chemical leavening agents’ role in quality of bakery products. Journal of Agroalimentary Processes and Technologies. 2007;13(1):105–12.
Vidal LM, Wittkamp T, Benz JP, Jekle M, Becker T. A dynamic micro-scale dough foaming and baking analysis–Comparison of dough inflation based on different leavening agents. Food Research International. 2023 Feb 1;164:112342.
Asamoah EA, Le-Bail A, Oge A, Queveau D, Rouaud O, Le-Bail P. Impact of Baking Powder and Leavening Acids on Batter and Pound Cake Properties. Foods. 2023 Feb 23;12(5):946.
Karishma Smart, Ph.D. is a science writer and cell biologist who loves talking about food, cooking, and science, not necessarily in that order. She completed her Ph.D. in biomedical sciences, which involved a lot of science and even more thinking about snacks.
