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 (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). So, in a nutshell, 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.
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.
Baking soda’s effect on colour and flavour: the Maillard reaction
The more 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.
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.” 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’s effect on texture
What’s my point, besides possibly boring you to death?
Reference: How Baking Works, 3rd edition. Paula Figoni. John Wiley & Sons, Inc. New Jersey. 2011.