Is it mutar to use an egg white left overnight? e.g. if I separate an egg to use the yolk in a recipe, can I leave the egg white in a cup in the refrigerator overnight (or few days) for later use?
Lechatchilah you should add salt or sugar to the egg white if left over night.
Gemorah Nidah 17a, Teshuvas Shevet Halevi 3:69