Fairly often in my cooking a recipe will call for only an egg white or yolk. It feels wasteful to discard the unused half, but I am concerned about the storage of it. How long can I keep just a white or a yolk in the refrigerator?
Eggs are highly perishable once they’re broken, so not long. It depends somewhat on how cold the refrigerator temperature is and how often it’s opened and closed, but I’d say no longer than four days for whites or a couple of days for yolks. And to be on the safe side, use them only in recipes which require cooking. If you’re sure of the quality of your eggs, you may wish to disregard this disclaimer.
Whites have a high water content, and will keep without any special treatment in a well-sealed container. Yolks dry out quickly, so they should have a little water added to cover them. They will start looking a bit funny because the water lightens the surface color, but it’s not harmful. Store both whites and yolks in as small a container as possible to exclude air.
If you have the time, you can also quickly drop any extra yolks into simmering water to poach them. Cooked, they’ll keep a little longer, say up to five days. Use them in any way you’d use hard-boiled egg yolks. They’re good as a garnish on green salads, can be added to egg or tuna salad for extra richness, and you can make mayonnaise with them.
Because I prefer to cook rather than bake, recipes for extra egg whites don’t get called into use much in my kitchen. Most such recipes are for sweet things like meringues and cake frostings. Two exceptions for a single egg white are (1) it can be whisked and used as a glaze for baked goods (such as to help sesame seeds adhere to a loaf of bread), and (2) it can be added to a soufflé. Most soufflé recipes call for an extra white to offset any loss of air in the folding process, so there’s a good reason to make a soufflé (either sweet or savory.) Or you can freeze whites individually in an ice cube tray and accumulate them until you have enough to make something which calls for several whites, like angel food cake.
Extra yolks, on the other hand, can go into custards, mayonnaise, béarnaise sauce, carbonara sauce for pasta, crêpe batter, pancakes, omelets, scrambled eggs, meat loaves or meatballs… lots of things which aren’t sweet. One yolk mixed with a little heavy cream is a rich glaze for scones, or can be used as a liaison, or thickener, for sauces.