Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn't, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you're not sure why, send a tweet to The Food Geek to find out what's happening.
This week's question comes from Ben by way of Marijean:
Hi, Ben (and Marijean),
When you bake a cake, you are creating a relatively loose mesh of gluten and other proteins and filling it up with tiny pockets of air. Yes, I know that doesn't sound as appetizing as cake normally sounds, and I apologize. The gluten is formed from water meeting with the flour, and the air bubbles generally come from tiny holes poked into the sugar from the creaming process, depending on the type of cake that you're making. You may get a little help from a chemical leavener such as baking soda or baking powder, but the essence is: mesh of proteins, tiny bubbles.
In effect, you have a big net, and in the holes of the net are a bunch of balloons. As the oven heats up, a few things happen. First, the air inside the balloons heat, which causes the balloons to expand, because hotter air likes to spread out more than colder air. Hot air is full of energy and bounces around a lot, and cold air just sits like a lump.If that were all that happened, though, the cake would deflate as soon as it were removed from the oven and given a chance to cool.
The second thing that happens is that the proteins in the mesh set, which means that the net goes from being flexible and ropey to being very firm and inflexible. This way, when the cake cools and the air settles down, the structure doesn't collapse.
The whole thing works because the air expands at a lower temperature than the proteins setting. The problem that you're experiencing is that the outside is setting much more quickly than the inside. The reason for this is because the oven uses radiant heat to cook. Well, unless you have a convection oven, then it also uses convection to cook, but don't worry about that right now. The radiant heat hits your pan, which is metal or glass, and heats the pan. It also hits the top of the cake, which heats less quickly than the pan because cake isn't really a good conductor, like metal, nor is it particularly transparent, like glass. So the side of the cake that is only cake and not pan stops a lot of the energy from reaching into the other parts of the cake.
To a certain extent, you are somewhat doomed. The shape of a standard round, square, or rectangular cake is not going to cook evenly, because there's no way to transmit heat in an even manner to the center of the cake pan. Bundt pans and tube pans try to mitigate this by having some pan in the center of the cake and thus making it a topologically simpler problem to solve, but let's go with the harder case of the round, square, or rectangular pans.
So, there'll always be some rise in the center, but there are ways that you can mitigate the extra height. First, you can lower the temperature of the over 10-25°. It might be worth checking the oven temperature to ensure that your oven is cooking at the temperature the recipe calls for, as ovens tend not to do a good job at temperature regulation, and presuming you're using a good recipe, they will have picked a good temperature for that kind of cake.
Second, you can use a lightly-colored aluminum pan. These aren't as easy for cake removal purposes as a dark, nonstick pan would be, but the light color of the sides of the pan will reflect heat away from the cake, thus slowing the setting of the interior of the cake.
Ultimately, though, if you're going for a perfectly flat cake-top for decoration purposes, your best bet is to use a serrated knife and slice off the troublesome bump before icing the cake. This will fix all imperfections, and once the icing is on, no-one need know. This is standard procedure for cake decorators, because, as I said, the problem is not one that is easily solved by baking alone.
Of course, I'm not a master baker, so if any of you have any other hints as to how to even out the cake level just with the baking or the preparation of the cake, please leave some wisdom in the comments.