Wooohooo!! Now THAT’s the kind of super-nerdy question that really kicks a Joe week off right! I thank you for it, reader Denny! The answer is no, you can’t make a custard with beaten egg and water in the way that you can with, say, beaten egg and milk. All that happens when you heat it is, well, egg drop soup. The question is: why?
The answer is because egg protein molecules repel each other. They’re negatively charged and as a result they naturally push away from each other like two same-sided magnets. In order for a gel network to form, those proteins need to attract — or at least not repel — one another as they uncoil. They’re then free to bond with each other side-to-side to create a molecular mesh. That mesh reduces the flow of the water and the result is thickening.
The next logical question is: how is milk any different from water? The answer is: it contains large amounts of minerals — positively charged ions of magnesium, zinc and potassium that become attracted to the proteins, collect on them, and neutralize their negative charges. At which point the proteins can bond. Cool, eh? Salt also works to neutralize egg proteins because sodium ions are positively charged. So add a little salt to your egg-and-water mixture, Denny, and you should have the gel you seek!
Notice that the lemon meringue filling recipe below calls for salt. It too is a mostly-water custard. Lemon juice and egg yolks both have minerals, but I’m guessing probably not enough to create a completely stable gel. The salt probably gives it a little added insurance.
Mm that felt good. Thanks again, Denny!