Many recipes call for egg yolks only, so you fill your refrigerator with a container of egg whites but, by the time you figure out what to do with them, they've gone bad. This is an easy technique for freezing egg whites. And if your problem is leftover egg yolks, there's a freezing hack for those, too.
Have an Ice Cube Tray at the Ready
When separating eggs, have an ice cube tray handy. Either separate the egg over the tray, letting the white fall into one compartment only of the tray. Or you can separate the egg over a small bowl and pour the egg white into the tray.
Pour only one egg white into each compartment. This will work for small to large eggs. Jumbo egg whites might be too large for the compartments, but most recipes call for large eggs anyway.
Pop the Tray in the Freezer
Place the ice cube tray in the freezer and freeze the egg whites until firm. Pop the egg whites out and transfer them to a zip-top freezer bag or freezer-safe container with a tight-fitting lid and return to the freezer for up to one year (the same storage time holds true for egg yolks but the method of freezing them is different).
Thaw and Bring to Room Temperature
Now, whenever you need one, two, three egg whites, or more for an egg-white-only recipe, you have them premeasured and at your disposal. Just thaw as many cubes, representing one egg white each, as needed in the refrigerator.
Egg whites will deliver full volume if they are at room temperature. So, once the egg white is completely thawed in the fridge, let it come to room temperature for about 30 minutes before whipping.
Thinking Outside the Box
Everyone knows egg whites and angel food cake are a match made in heaven, but there's so much more that can be done with them like frittatas, macarons, as an egg wash on baked goods for shine and color, in soufflés, and for truly white-white cakes.
A word of caution—only use thawed raw eggs (unless they are pasteurized eggs) in end-products that are cooked. So using leftover uncooked egg whites or egg yolks (unless pasteurized) in cocktails or nonalcoholic beverages is strongly discouraged.