What’s the difference between baking powder and baking soda? It’s subtle, but significant
- Written by The Conversation
There is something special about sharing baked goods with family, friends and colleagues. But I’ll never forget the disappointment of serving my colleagues rhubarb muffins that had failed to rise. They were dense, rubbery and an embarrassment to the reputation of chemists as good cooks (#ChemistsWhoCook feeds on social media are full of delicious food).
The cause of my failure was an imbalance between the acidity of rhubarb and the chemical raising agents I used in baking.
Both baking powder and baking soda can play a role in giving baked goods their bubble-filled texture and taste. They are sold side-by-side in the supermarket, and have similar uses. But what’s the difference between them and how can we use those differences to our advantage?
What’s in the box?
A quick look at the packaging shows the difference between the two products.
Baking soda contains one ingredient: sodium hydrogen carbonate, also known as sodium bicarbonate or simply bicarb. Baking soda is well known for its uses in cleaning, cooking and deodorising.
Baking powder is typically a mixture of three ingredients: baking soda, an acid, and a starch derived from corn, rice or wheat. The starch makes it easier to measure the powder and also prevents the acid and base from reacting prematurely in the pantry. Baking powder is used exclusively for cooking.
The common ingredient in both products is the baking soda. This salt can be purified from natural sources, or can be prepared synthetically.
The acid is the key
Baking soda is a base, which means it can chemically react with acids. This fizzy reaction produces bubbles of carbon dioxide, water and a mix of new salts. Baking soda can also release carbon dioxide gas when it is heated at temperatures above 80°C.
When you mix baking soda into a cake batter, you will see some initial chemical activation by food acids. This causes bubbles to form and the mixture to rise.
The acids come from other ingredients in the mix, such as yogurt, buttermilk, or the rhubarb in my failed muffins. Too much acid, and the majority of the carbon dioxide will be released at this batter stage.
Once you place the mixture in the hot oven, the high temperature will form further carbon dioxide bubbles. This thermal activation forms a new salt, sodium carbonate, which can give a residual taste and “soapy” mouthfeel if there’s too much of it left in the final product.
Mixing baking powder into a cake batter will also result in chemical activation to form bubbles. The baking soda in the mixture will react with the acid included in the baking powder mix, as well as any acidic ingredients in the batter.
The type of acid included in the baking powder can subtly change the way the baking powder behaves. The more soluble the acid in the batter, the faster the carbon dioxide will form bubbles.
Recipes that ask for both baking powder and baking soda are likely looking to do two things: neutralise an abundance of food acid from another ingredient, and provide time-delayed, temperature-activated rising.
Baking soda can also increase the surface browning of food by enhancing the Maillard reaction. This class of reactions results in delicious chemical transformations in roasted coffee, seared steaks, baked bread and more.
Meanwhile an excess of baking soda can change the appearance of foods, for example turning blueberry anthocyanins green in muffins or pancakes.
Can I substitute baking powder and baking soda?
Baking (like chemistry) is a precise science. It’s best not to substitute baking soda for baking powder or vice-versa: they have subtly different chemical effects.
If you really need a substitution, the general rule is that you need three times the baking powder for the equivalent quantity of baking soda (so, if the recipe asks for a teaspoon of baking soda, you’d add three teaspoons of baking powder).
But it’s not a precise conversion: it doesn’t take into account the key role of acid that’s already in the baking powder. This could affect the final acid-base balance in your recipe.
You can compensate by adding an acid such as cream of tartar or citric acid. But it can be difficult to get the relative quantities of acid and base correct. These acids are also likely to promote immediate release of carbon dioxide, with less left to activate in the oven – potentially leading to a dense bake.
You definitely shouldn’t substitute baking powder for baking soda when cleaning. The acid in the baking powder will neutralise any cleaning activity of the sodium bicarbonate, while the starch may leave a sticky, streaky mess.
It’s best to keep both baking powder and baking soda in your pantry for their distinct uses. Be sure to share whatever delicious treats you bake with others, as well as sharing your new knowledge of the bubbly chemistry contained within.
