To prove this, I remade the pudding cakes, modifying the amount of baking soda and adding only an eighth of a teaspoon to the second batch, instead of the full teaspoon called for in the original recipe. The second batch of steamed puddings were completely different: significantly lighter in colour, with a firmer, spongy texture. Interesting.
What is baking soda?
Baking soda is the common name for the chemical sodium bicarbonate (or bicarbonate of soda, as the Brits like to call it). The chemical formula for baking soda is NaHCO3. Sodium bicarbonate is basic (as in alkaline) and reacts under acidic conditions to produce carbon dioxide (CO2), the gas that lifts cake batters and helps your cakes rise (water and salt are also produced).
Sodium bicarbonate is a chemical leavener, unlike yeast. Yeast is a living organism, and when you feed it (like when you give it some water and some sugar), it too will produce gas, but through a different process known as fermentation: the yeast organisms eat sugar, digest it, and then they produce CO2 (along with other compounds like alcohol).
To summarize, your breads rise because yeast in the bread dough eat sugar and produce a gas, while your cakes rise because baking soda reacts with acids to produce a gas. This is also the principle behind baking powder, another leavening agent.
Now, why does more or less baking soda make the two batches of cake look so different? There’s more chemistry to discuss here, like the Maillard reaction.
What does baking soda do?
Baking soda causes baked goods to brown via the Maillard reaction
As you bake a cake or bread, you obviously notice a change in colour as the baked good turns golden brown, but you might also notice that the sweet flavours transform into something deeper, and not as sweet: French pastry chefs like to call this “golden brown delicious.” The change in colour/flavour from cake batter to cake occurs because of the Maillard reaction: sugars break down/transform into brown coloured polymers and aromatic substances that contribute to the aroma and flavour of baked goods.
The Maillard reaction turns cakes from pale and very sweet to golden brown delicious. Baking soda and Maillard browning give Boston brown bread its signature taste and look.
What happens if you add too much baking soda?
There’s a fine line between the right amount of baking soda and the wrong amount. Make sure to use the right methods to measure your ingredients.
Too much baking soda causes cakes to brown and may leave a weird taste
The Maillard reaction speeds up under basic conditions (like when you add to a recipe a lot of baking soda, which is alkaline, i.e. basic). Because the original marmalade pudding recipe had a large amount of baking soda in it, the resulting cake batter had a higher pH, and the Maillard reaction occurred faster. The puddings browned more quickly while steaming for an hour. More baking soda, more browning.
Unfortunately, with more baking soda, a lot of it reacts, but some of it (the excess) is left behind, unreacted. This lingering baking soda affects the flavour, which seems “sharper” and too much baking soda might cause your cakes and cookies to taste soapy even.
The puddings made with a full teaspoon of baking soda taste less like marmalade and more like something stronger than the expected citrus flavour. That’s the baking soda. The second batch of puddings had only an eighth of a teaspoon of baking soda, therefore the Maillard reaction occurred much slower. The steamed puddings have a more familiar “golden brown delicious” look to them. The taste of the marmalade is clear, and so is the delicious buttery flavour.
At this point, you would think I’m done with the chemistry chat, but I’m not. I have one more thing to point out about baking soda.
Baking soda is a tenderizer and too much baking soda affects texture
I noticed the original pudding cakes had a very tender crumb, and they were spongy and soft. On the other hand, the puddings made with one eighth of the baking soda were still spongy, but much firmer. Once again, we can blame baking soda. Baking soda provides lots of rising power to the pudding cakes, but actually, I noticed that the cakes with less baking soda were more domed, while the cakes with more baking soda were flatter, but with a more bubbled texture on the sides and bottom.
The baking soda raised the pH of the cake batter, thereby weakening the gluten in the flour: the texture and tenderness of the cakes were affected. Weaker gluten means a looser structure, with more spread, bigger air pockets (a more open crumb) and tenderness. So the cakes with more baking soda seemed to rise less, but in fact, that’s because the gluten was weak and couldn’t support the forming gas pockets. An adequate amount of baking soda (1/8 tsp) allowed the little pudding cakes to rise up and stay up, while an excess of baking soda caused the cakes to spread out, instead of up.
How much baking soda to use in cakes and other recipes: rule of thumb
Too much baking soda is clearly not a good thing, creating too many bubbles in cakes, causing cakes to sink, leading to over-browning, and producing an off-flavour that might even be soapy. So how much baking soda is enough? In general, the basic rule for how much baking soda to add to a recipe is 1/4 teaspoon of baking soda for each cup of all-purpose flour (125 grams).
In recipes where that contain large volumes of acidic ingredients, you would need 1/2 teaspoon of baking soda to neutralize 1 cup of a mildly acidic ingredient, like sour cream, buttermilk, or yogurt. For example, this Irish soda bread with raisins is made with 500 mL (2 cups) of buttermilk and 1 teaspoon of baking soda. Of course, this is just a guideline and recipes may vary for other reasons (pan size, presence of “heavy” ingredients like nuts and dried fruit that might require extra leavening, etc.).
Does baking soda expire?
Baking powder does expire and can lose potency over time, given that baking powder contains both baking soda and an acid (or two) that it can react with in presence of humidity. For this reason, it is super important to check baking powder periodically to make sure it is still reactive. On the other hand, baking soda is just sodium bicarbonate, a single compound and even if it’s exposed to a little humidity from the air, it won’t completely break down and lose potency the same way baking powder does. For this reason, baking soda does not expire.
The problem with older containers of baking soda
Though baking soda doesn’t expire, you still have to be weary when baking with a container of baking soda that has been open for months (or even years). It’s not that the baking soda will break down over time and stop working, but open containers of baking soda exposed to humidity will clump. You will have trouble incorporating clumpy baking soda into your cake batters and other baked goods, which can lead to clusters of baking soda in cake batters and cookie doughs. These clusters will lead to tiny patches of baking soda in your baked goods, causing brown spots and larger air pockets in your baked goods.
How to store baking soda?
Given that baking soda will lose clump if exposed to moisture, it’s important to store it in a cool, dry place, in an closed container that has a good seal to make sure moisture doesn’t get in. Baking soda is often sold at the grocery store in a cardboard box that, once opened, doesn’t shut, so you may want to either store the box in a bag with a seal or to transfer the powder to an air-tight container with a proper lid. This way air exposure will be minimal and your baking soda won’t clump.
Janice Lawandi is chemist-turned-baker, working as a recipe developer in Montreal, Quebec, Canada. She studied pastry at Le Cordon Bleu in Ottawa and cooking at l’Académie Culinaire. She has a BSc in Biochemistry from Concordia University and a PhD in Chemistry from McGill University. Visit janicelawandi.com to see my portfolio.