I love custard. There’s nothing more comforting than sponge and custard. Or bananas in custard. Or custard for breakfast. I am very aware of how awesome custard is as a food, but it wasn’t until I was watching the hilarious results of four celebrities trying to make custard slices on The Great Comic Relief Bake Off that I actually starting thinking about how awesome the science of custard is. And about how I wanted to make custard slices, but that’s not so much the point.
Custard is a really cool demonstration of how chemical reactions can change the properties of a mixture, in this case from a thin runny liquid to a deliciously viscous custard, and sometimes even further onwards to a solid.
So how DOES custard thicken? It’s all about the molecular magic of proteins. Custard thickening relies on a crucial chemical property of some proteins; the fact that they coagulate when heated. If you want to see a really obvious example of this, crack an egg onto a hot surface; the proteins in the egg coagulate, they become solid, setting the egg. What’s happening here is that the heat is denaturing the proteins; breaking apart the interactions that support the structure of the protein, leaving you with a long chain of amino acids floating about. This, however, is a problem. You see, there is a divide amongst amino acids; two rival gangs. The Hydrophilic Homeboys simply love water, and the Hydrophobic Hoppers abhor it. When the protein is folded up nicely, all the members of the hydrophobic gang are buried on the inside, hidden away from the scary water, singing like in West Side Story. They’re happy. However, when you denature a protein, the hydrophobic amino acids are no longer hidden. In this situation, they panic. Their only hope is to find another hydrophobic buddy nearby, and another, so that they can all cling together, trying to hide their faces away. They’re so panicky, they don’t care if the amino acid they find is part of their chain or not, and so the chains start to stick together, forming a solid interconnected mass, held together by terrified little hydrophobic amino acids, repulsed by water and trying to hide.
Not all proteins behave like this when heated, however. For example, if you poured milk onto a hot surface, it wouldn’t coagulate into a solid in the same way that an egg white does. This is because the main protein in cow’s milk, casein, is one of those stubborn heat-resistant proteins. We should probably be glad about that actually, or milk would thicken tea, and that would be yucky. Anyway, that’s the reason you need to add egg to milk to make custard, it’s essential to have a decent proportion of heat-coagulating proteins, such as the albumin in egg whites, otherwise you’ll be stuck with runny custard. Albumin is a molecular custard hero.
Now, this is all well and good, but I cannot be the only person here who has ever made custard that isn’t a smooth, thick semi-liquid that I want to eat with a spoon, but instead looks like someone vomited in my pan? Please tell me I’m not the only one? Okay, good. The problem is that if you heat up the proteins too fast, the hydrophobic molecules will clump together very quickly, and separate themselves entirely from the liquid, forming curdled lumps. You need to heat custard slooooowly, without overheating it, to stop the denaturation happening too quickly or going too far.
And there is another way to help with this, to simultaneously slow down coagulation AND make your custard 100% yummier. You just need to add sugar. If there are sugar molecules in the mixture, when the proteins denature the hydrophobic amino acids are less like to bump into each other and cling together in terror, because they’re more likely to bounce off a sugar molecule instead. Overall, it takes more time for the hydrophobic amino acids to gang together and coagulate; the process is steadier and doesn’t result in curdling.
And that’s it; the super scientific magic of molecular interactions turns a mixture of milk, eggs and sugar into a gorgeous thick yummy custard. Just another way in which science is amazing. And tasty.
A Note: This article is about the science of egg-based custard, rather than custard powder custard, although both are equally yummy under the right circumstances, they don’t actually thicken in the same way. I may have a look at custard powder chemistry another time, but right now I really need to go make myself some custard. Writing this has made me hungry.