I love cake. I love eating it and I love baking it. And one of the most fun things about baking cake, as opposed to other kinds of cooking, is that baking is chemistry. In savoury cooking, the skill is in picking the right ingredients that will be tasty together - an art. But in baking cake, you obviously want good flavours, but it’s meaningless if you can’t get the right combination of flour, sugar, egg, fat, and raising agent. You have to set up the right chemical reactions between your ingredients to get a good cake.
But what are those chemical reactions? What, chemically speaking, converts a bowl of slop into a lovely soft spongy yumminess?
To answer this question, I’m going to take the simplest cake recipe I know, Delia’s All-In-One Sponge, from her Complete Cookery Course. It’s very simple: 4 ounce (or 110 gram for those new-fangled, metric types) each of self-raising flour, butter and caster sugar, two eggs and a teaspoon of baking powder. Mix it up and bake at 170°C (gas mark 3) for half an hour. You can flavour it with vanilla, cocoa, coffee, lemon or anything else you fancy really. Delia’s recipe is a very basic sponge recipe, but it contains everything you need to produce all the chemical reactions that are necessary in baking.
So let’s look at what each of these ingredients is chemically. Flour is starch and protein (self-raising has baking powder mixed in). Sugar is, well, sugar (normally sucrose) and butter is about 80% fat with water. Eggs contain everything to make a living chick, so have quite a lot in them, but for baking purposes we can think of them as protein and water. Baking powder is bicarbonate of soda mixed with a weak solid acid, such as potassium hydrogen tartrate (cream of tartar).
The most important thing about a sponge cake is its sponginess - it is full of air. Loads of different parts of the cake recipe contribute to this. Most important is the bicarbonate of soda, which, when heated with hydrogen ions from the acid in the baking powder produces water, sodium ions, and carbon dioxide. If you use baking soda without added acid, it instead breaks down into sodium carbonate, which tastes weird and soapy, then reacts with the acid in your stomach and makes you fart. Recipes that use baking soda will have acid in them in another way. The carbon dioxide gas causes the cake to rise. In bread-baking, yeast is used to produce carbon dioxide instead.
Other ingredients contribute to the rise of the cake too. When butter and sugar are mixed together, air is incorporated into the mixture. The shape of the sugar crystals is actually important for this, including the size and the sharpness of the edges, as this affects how the sugar goes into the butter. So it’s important to always use the type of sugar the recipe suggests and be careful when making a favourite recipe with a new brand of sugar. Water in the cake-mix evaporates and the steam helps the cake to rise too.
All of this rising is pointless if the shape can’t be maintained though. This is what the flour is for, specifically gluten. Gluten is a protein that crosslinks with the starch to form a big stretchy network of carbohydrate with the helpful quality that it maintains the shape it is stretched into. So, if the gluten in a hot cake-mix is filled with bubbles of carbon dioxide, air, and steam, it will maintain that bubbly texture when all of those gases have gone.
Another major feature of a good cake is a nice brown crust. This comes about as a result of the Maillard reactions, and requires sugar and proteins (egg). Amino acids act as catalysts to caramelise sugar. Firstly, a carbonyl group (a carbon double bonded to oxygen) on the sugar reacts with an amine group (NH2) from an amino acid. This forms an unstable compound that rearranges to give a ketosamine, a sugar with a ketone group (a carbon to oxygen double bond in the middle of a carbon chain) and an amine group. These ketosamines then undergo a number of reactions, which involve them losing their amine group and replacing it with a hydroxide group (OH) next to a carbon to carbon double bond. This resultant chemical is an unsaturated aldehyde or ketone, and is better known as caramel.
So now we have a nice brown airy cake, ideal for scoffing whilst perusing a good science magazine.