Baking a Cake
Ben - This week, we're discussing the science of food so what better way than to explore the scientific basis for the perfect cake. I'm joined by Amy Chesterton, a PhD student at Cambridge University. So Amy, other than the very obvious reason of tastiness, why would a scientist care about cake?
Amy - Well I work in a research group and we're interested in powders and pastes generally, especially those relevant to the food and pharmaceutical companies. Cake is particularly scientifically interesting, firstly because the structure is caused by mixing in many ingredients and secondly, because of the changes that occurred during baking.
Ben - Now I see that we have the ingredients out here already. We have eggs, flour, and we have a mixture of butter and sugar. That takes a while to beat so we've already started it, but why is that important?
Amy - Well the mixing of the fat and the sugar is important because the most important ingredient is air. So we're beating the fat and the sugar together to incorporate lots and lots of bubbles.
Ben - Okay, so as I've said before we're giving it a good go, let's just finish it off properly. So I'm obviously putting lots of bubbles into this. What's the next thing that needs to go in?
Amy - Well fat and sugar by itself doesn't make a cake because on heating, the fat will melt and the bubbles will escape to the atmosphere. So now I'm adding the egg. What we want to do is cover all of our fat covered bubbles in egg because the heat on baking will cause the eggs to solidify, the proteins to denature, and that will keep the air in the cake the cake structure up itself.
Ben - So mixing now isn't really to add any extra bubbles. We've already got our bubbles in there. Now we're just making sure that these bubbles are evenly coated with egg. But if the bubbles are already in there from the fat and sugar, and now, we're coating them with the egg that will give it structure, what's the flour for?
Amy - Well the flour also is an important structural aid together with the protein from the egg scaffolding. The starch within the flour will swirl and with the heat, it will gelatinise - create a gel - and together, create a very firm, nice, tender structure.
Ben - Okay, well let's get the flour in there. Still a little bit lumpy at the moment - We need to make sure we mix that in properly. I'll let you put that in. How much of each ingredient did we actually start with?
Amy - Well we're following the most basic cake recipe that's equal quantities of our four ingredients - fat, sugar, flour, and egg. It's the recipe that the Victoria sponge is based on.
Ben - Now we're using an electric hand whisk which may be perceived as cheating, but really, it's very important to make sure that you get that air in there. What sort of flour are you using, because I know that some flour will actually help give extra bubbles.
Amy - We're using self-raising flour so there's added baking powder in there. On contact with the wet ingredients, that will release carbon dioxide and increase the size of our bubbles.
Ben - You said increase the size of our bubbles. Surely if we're creating carbon dioxide, that's going to give us new extra bubbles?
Amy - Well actually, the creaming process is so important because the number of bubbles in our cake batter will be the number of bubbles in our final cake. The carbon dioxide released by the baking powder can only increase the size of the bubbles and that's because the surface tension is too high for new bubbles to be created.
Ben - We've got quite a nice, smooth paste here. Now we are going to put this into little muffin tins and pop it in the oven. What temperature does it need to be on and do you think we'll get to eat them before the show is over?
Amy - Well yeah, we're going to heat at 180 degrees and because we're making cupcakes, they should be ready by the end of the show.
Ben - Excellent! So we're going to start spooning the mixture into our cupcakes now and then we'll pop them in the oven, and later on in the show, we'll come back to see how our cake is doing.
Ben - My kitchen is now filling with smell of cake. It's smelling delicious in here and the mixture that we poured in earlier that was fat, sugar, egg, flour, and importantly air is now starting to rise. Amy, what's actually happening inside the oven?
Amy - Well at the moment, we're at the early stages of baking, so heat is being transferred from the oven to the cake mixture and transforming it into something else. We can see the cakes physically rising now and that's because the tiny bubbles that we incorporated earlier are starting to grow by three mechanisms. We've got thermal expansion, because air expands when it's hot. We've got water vapour which is starting to be produced, and also, carbon dioxide from the baking powder.
Ben - So there's lots of different processes going on physically inside there, including tiny steam engines taking advantage of the expansion of water as it turns into vapour. But what about the chemical changes that are happening, the changes to the proteins we were talking about earlier?
Amy - Well, it depends on the exact temperature at the moment. If we're above 60 degrees or so, we'll start to have starch granules expanding. They'll eventually disrupt and form a gel. The proteins will also denature at a similar temperature so we'll start to get the structure forming.
Ben - So the structures are already forming. It's growing. It smells delicious. Can I get this open and start tasting them now?
Amy - At the moment, no. All of our bubbles are still surrounded predominantly by liquid so, the cake shape and size that we can see is held up by pneumatic supports. If we removed it from the heat, everything would cool down, the bubbles would contract because of the thermal expansion in reverse, and the water vapour would condense, and we would have very poor-looking cakes.
Ben - So, is it safe to open it and have a look or is it best just to leave it alone until they're perfect?
Amy - Again, because we don't want the batter to cool down at all, it's not a good idea to open the door or remove the cakes yet. You'll just have to wait for later.
Ben - So we'll have to wait a little bit longer before we get to taste the cake.
Ben - Well, it's now time to actually take it out of the oven. Here's the moment of truth so we're going to see what the cakes look like. Here they come. Obviously, if you're doing this at home, be very careful that it's a bit hot. Ooh! They look delicious, that waft of smell that came out. I'm just going to poke one - it's beautifully soft, spongy, and it's gone a gorgeous brown on the top. Amy, what's happened now?
Amy - Well at the end of the baking, we got Maillard reactions occurring. That produces a nice brown colour and also the distinct cooked taste of the crust. So the Maillard reaction is actually a reaction between the protein or the amino acids, and the sugars producing hundreds of flavour compounds.
Ben - Now this is the same reaction that we see when you fry chips or when you roast a chicken. So, it's a set of reactions that produce this distinct colours, and that distinct flavour profile that gives you that gorgeous oven-cooked or roasted or fried flavour.
Amy - Yeah, that's right and I'm just cutting into one now to have a look and we can see that on the inside, we've not got such a brown colour. That's because the inside of the cake had quite a lot of moisture right until the end of baking. So the Maillard reactions wouldn't really happen. They occur at higher temperatures than the inside of the cake would've achieved, but the crust which heats up quicker will have dehydrated and Maillard reactions occurred.
Ben - So that's why we get the crispy outer layer. So if we'd left it in there for too long, then eventually would've all got to the right temperature, those reactions would've occurred all the way through and this would've turned into a stiff, crunchy, sort of biscuit rather than a cake.
Amy - Yeah, although the crust would've got too hot at that point and turned black, dark and non-edible.
Ben - Well that would've been an absolutely travesty. Now we've taken it out and it hasn't collapsed. So can we assume that the starch and the egg protein has locked in that structure for us?
Amy - Yes, so it would've solidified towards the end of baking and that actually stops the bubbles from expanding. They can't expand when the cake is solid. So instead, you get the bubbles sort of popping and forming a continuous network, which is why when we open it, we can see this nice open structure, rather than individual small bubbles.
Ben - So although it is a sponge cake, it doesn't look like a bathroom sponge which has those lots of little tiny holes. But why would scientists need to know about how cake actually functions? You said before that these pastes are interesting, but how can we apply the science of cake to other industry, and of course, how can we apply other science to make better cake?
Amy - Well, the structure of our final cake here was completely determined by the batter we made it from, and that's true of products produced industrially in all sorts of sectors. So, it's the relationship between the paste and the final product which is scientifically interesting and something which I'm interested in as a researcher.
Ben - So that might include - obviously in our case, it's cake - but that might include how you mix and manufacture say, a pharmaceutical product, because again you need to make sure that the final product is very well understood, very homogenous, and almost identical from one batch to the next. So it's understanding these processes, be they in drugs or in delicious cake that really is what scientists are looking in to.
Amy - Yes. It's useful for quality control and also for development of new products.
Ben - Well thank you ever so much. That's Amy Chesterton from Cambridge University.