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.
Rob asks via the feedback form on my web site:
I just read your post on how sugar and water are BFFs, but here’s something that’s been on my mind lately – some cooks tell you that when you combine sugar and eggs, you shouldn’t leave it alone too long, or the sugar will “cook” the eggs.
This makes even less sense to me than how sugar keeps water, water. Can you elaborate in a later post?
Most of the time when we talk about cooking food, we are suggesting the use of heat to make radical changes in the chemical structure of the food. One of those changes is to denature proteins, or to take them from their natural state to some sort of new form.
An example of denaturing protein without heat is applying an acid to fish or poultry, which will cause the protein to turn from the somewhat translucent color it started with to the same sort of color it would had you cooked it, though without any browning from the Maillard reactions.
So let’s talk eggs. In particular, I’m going to talk egg yolk, because I most frequently mix sugar with just the yolk when I’m making a custard. In an egg yolk, you have a lot of proteins floating around in a big pool of water. And some other things, but water is the important bit right now. The proteins are folded up right now, because that’s how they normally are, and it gives them room to float without bumping into other proteins. If they bump into other proteins, they would join together, which ruins the relaxed nature of the swim.
When you cook an egg, you cause the proteins to stretch out so that they’re much longer than they were when they were folded over. This means they take up a lot more space in the swimming pool, which causes them to bump into other proteins and link up.
Paula I. Figoni writes about what happens when sugar is left in the yolk in How Baking Works. Essentially, the sugar and the water get together, as they are known to do. They gossip about the moral implications of emulsions and the exciting lives of amino acids and whatever else young molecules like to gossip about.
The more water that gets tied up with the sugar, though, the smaller the swimming pool for the protein becomes. After a while, they don’t even have to stretch out to bump into each other, it just happens. Because the protein is still folded, instead of making a nice regular mesh that normally happens with a cooked egg, you just get a hard clump of eggy protein. The last thing you want, especially with a custard, is a hard clump of eggy protein.
Hence, don’t wait around once you’ve poured the sugar into the yolk. It’s an easy way to ensure that whatever custard you’re making will be smooth and tasty.