Lovely blog readers, today is my birthday, and I can’t think of a better way to celebrate that on the blog than with the science of cake. And by eating cake. Obviously.
So, it may not have come across before now, because I hardly ever mention it really, but I’m totally addicted to cake. I don’t just mean eating cake, although really good cake is one of the greatest pleasures in life, I’m also addicted to baking; I remember learning to bake with Mummylase, sitting on the kitchen side and helping her weigh out ingredients, stirring the mixture, licking the bowl at every given opportunity, distributing satisfying (if uneven) dollops of batter into cake cases and peering anxiously into the oven as they magically rose into perfect light spongy bundles of deliciousness. I have often said that the person who first perfected the art of mixing eggs, flour, sugar and butter together and heating them to form a cake is one of my ultimate chemical heroes.
I have always been fascinated by why. Why do the right amounts of the right ingredients make something so wonderful? What are those ingredients doing in there, how do they interact, and why is the result so much yummier than the original ingredients themselves? Baking is really a chemical reaction between ingredients to produce an end product. That’s why it’s so important in baking to weigh everything out so carefully, because this is a chemical reaction, and any imbalance will change the reaction slightly and therefore change the texture/taste/consistency of the cake. I can testify to this because I’m actually terrifyingly imprecise in the kitchen, which is why my baking is so chaotic and occasionally ends in disaster (grilled cheesecake anyone?!). The advantage of baking, though, is that even eating your failures is usually still enjoyable. In a chemistry lab, this is much less likely to be true. Or safe.
But anyway, I want to understand why my ingredients are doing in my birthday cake. First, the sugar. Sugar has a number of jobs in a cake, obviously it provides sweetness, adding to that delicate sponge flavour, but it also helps air get into the mixture. Anyone who’s ever had a sponge-related baking disaster will know what it is to eat a cake with no air. It’s flat and heavy. Not great, to be honest. So we need the air, and it comes from a number of places, one of them being the interaction between fat and sugar. When you cream the butter and sugar together for a cake, air is carried into the mixture and caught on the edges of sugar crystals, where it hangs on tightly.
The other useful little thing that sugar does is lower the caramelisation temperature, the point at which the mixture will change colour in the heat, so you end up with that perfect firm, browned and golden top. I’m making myself hungry here.
Next ingredient then is the fat. Fat has two key functions in a cake; it is a ‘shortener’, it coats the proteins and carbohydrates in flour to prevent the formation of too much gluten. Some gluten is essential in cakes to stop them falling apart altogether, however too much can ruin the texture. A ‘shortened’ cake, one where the fat has stopped the gluten getting carried away, is crumblier than a mixture with higher gluten content, like bread for example.
The second role of fat is to help trap the air bubbles that the sugar brought into the mixture. Thin layers of fat surround the air, sealing it into bubbles, so that it cannot escape and instead must hang around to make your cake light and fluffy. Provided, of course, that the air bubbles don’t get overexcited and pop when they get hot and bothered in the oven. You stop this happening by adding egg to the mixture. Egg proteins are so often important in baking, in cakes they follow the example of the fat, and form thin layers around the air bubbles. These proteins become rigid when heated, so the expanding air in the bubbles has no means of escape. Fat and protein are an effective prison, if you’re an air bubble.
An additional means of getting air into a cake is to use a chemical baking powder, which is usually a mixture of alkali (bicarbonate of soda) and acid (cream of tartar) that reacts with heat and water in the oven to produce carbon dioxide gases that expand into the air bubbles and help the cake rise, for extra-light and fluffy goodness. The water you need for this reaction comes from the egg white.
The last key ingredient of the cake is flour. Flour gives structure to the cake and it does this in two ways. First, flour proteins link together to form gluten. Gluten forms a stretchy web throughout the cake that can expand as the air bubbles and carbon dioxide force the cake to rise, and then set rigid to stop the cake falling in on itself, which is another cake-related baking disaster that I’m definitely familiar with. The second structural aspect of flour is the starch, starch is involved in helping the egg proteins stay firm and strong, and preventing that all-important air from getting away.
A final addition, which always confused me as a child, is salt. Why, I wondered, somewhat protectively, would you put something savoury like salt into my yummy sweet cake? Well salt helps to strengthen the gluten web, and odd though this may seem, it does also add to the flavour. Fine, my sulky inner child says, you can add the salt then.
And there you have it, the chemistry of a perfect light and gorgeous sponge cake. All that remains to be done, really, is get in the kitchen, get royally covered in flour and generally make a big cake. And an even bigger mess.