Separating eggs into yolks and whites isn’t difficult, but you do need to be careful.
If you get some egg white in with the yolks, no problem, but if you get yolk in the white, the whites won’t froth up properly when beaten.
Why? Because the fat in egg yolk can hinder the proteins in egg white from stretching and bonding with one another. (The yolk fat bonds with the protein instead.)
If it’s only a drop of yolk, you may still be able to froth the whites, but it will take much longer, with more beating, and it may ultimately fail, so you won’t have the delicate foam you want for pie-top meringues and other uses.
I do the eggs one at a time, clearing the bowl after each one, so that if I mess up one separation it won’t affect the previous ones.
I use three bowls: a small bowl (1) to catch the white I’m separating, another small bowl (2) for the yolks, and a larger mixing bowl (3) in which to pour the successfully separated whites.
— Hold the egg in one hand and give it a firm tap on the side of the bowl. Separate the halves, keeping the yolk in the shell and allowing the white to fall into bowl #1. Be careful that no yolk falls into bowl no. 1 with the white. If the yolk breaks, gets out of the shell and falls into bowl no. 1 with the white, save that egg in the refrigerator for scrambled eggs, wash out bowl no. 1, and start again.
— After the white has all run out of the shell and into bowl #1, place the yolk into bowl no. 2. Transfer the successfully-separated white from bowl no. 1 to bowl no. 3, a large mixing bowl. Take another egg and repeat the process using bowl no. 1.
As a girl, I once tried to make a sponge cake for my sister’s birthday when my mom was in the hospital. There was just a tiny bit of yolk in the whites, but I figured it wouldn’t matter. I beat and beat the whites, but somehow they never fluffed up like my mother’s did.
My cake, alas, resembled a kitchen sponge rather than a cake, and after a token bite we all agreed it was time for ice cream. And I learned the importance of separating eggs properly!
— You also have to be careful when folding in the beaten whites and flour. “Folding” refers to a slow, sweeping motion that gently incorporates the ingredients so as to maintain the fluffiness of the whites.
You don’t want to stir or beat the mixture, or all the air will go out of it and it won’t rise properly.
I use a rubber spatula and make a vertical “cut” down the middle of the mixture, and then sweep along the bottom of the bowl and then across the top, turning the bowl about a quarter turn each time. As you bring the scraper up, you’re bringing the ingredients from the bottom of the bowl to the top and “folding” them into the others.