Reader Eric writes:
What-all is a whole egg doing in a quick bread, say a coffee cake, chemically and thermodynamically? I have a decent sense of what the GF flour will and won’t do (can’t develop gluten that isn’t there, wet dough somehow feels a little more like batter) but eggs have so much going on.
Reader Eric, you’re sure right that eggs are multifunction players. Indeed when you step back and consider everything they deliver in the kitchen — from nutrition and color to textural effects, thickening, leavening, the list goes on — one cannot help but be awed. How can one little chicken-made package deliver so much? So your question is a profound one. I don’t think I know enough to give you a comprehensive answer. However if I were to reduce egg functionality in cakes to the very basics, I would say: structure and emulsification.
Starting with structure, it’s the protein in eggs, particularly the proteins in egg whites, that help hold a cake up. The have a delightful way of coagulating — at the relatively low temperature of 140 degrees Fahrenheit — in such a way that they reinforce the gel that’s created when moistened flour granules heat up and begin to shed their starches molecules. The egg proteins insinuate themselves into that network of starches, then clench up, fixing the gel in place more or less permanently. Which is, how do you say, very nice.
The yolk is responsible for the egg’s other main function: emulsifying the batter. Now, “emulsion” is a word that gets thrown around a lot in culinary circles, but all it really means is a smooth mixture of two liquids that don’t dissolve in one another. In the food world those liquids are usually oil (or melted fat) and water. Emulsifiers facilitate the mixing process by embedding themselves on the surface of droplets, which has the effect of lowering the droplets’ surface tension. The result being that when you beat the mixture the droplets get smaller, and so the mixture gets smoother and more even.
Why is that important in a cake batter? Because a tender cake is a good cake. And tenderness is the direct result of a fine structure. And how do you get a fine structure? Simply by ensuring that all the fat, water, sugar, and starch granules are distributed as evenly in the batter as possible. To achieve that you need lots of teeny tiny bits. Teeny tiny flour granules, teeny tiny droplets of sugar syrup, and above all teeny tiny fat droplets. And you get those via a well emulsified batter.
Baked up, this fine, smooth emulsion delivers a cake layer that is at once tender AND strong. Which seems like an oxymoron until you consider that a coarse crumb, like a biscuit, is like a mass of balloons, whereas a fine crumb, like a piece of high ratio cake, is like a roll of bubble wrap. Both are very light, but put much pressure on the balloons and they collapse. Put pressure on the bubbles wrap and it twists and flexes. It’s not a perfect analogy, but I think you take my meaning. Anyway, thanks for a great question, Eric!