What you can make with leftover egg whites depends on how many you have. One egg white is enough for breading, an omelet, or brushing dough to give it a glossy finish; a few egg whites can go into Turkish almond and coconut cookies; and as the number of whites grows, desserts built around whipped foam, such as meringue and pavlova, become options. The decision starts with the bowl, not the recipe.
Egg whites are mostly water and protein, with no fat. When whipped, they trap air and set into a firm structure when cooked, adding lightness and hold to a recipe. Leftover whites often remain after a recipe uses the yolks, and the number on hand varies each time. Most recipes that call for egg whites use a specific amount, so choosing a recipe starts with the amount of egg white, not the recipe itself. One egg white weighs around 30 g on average, depending on the size of the egg; weigh the whites for recipes that require precision.
Whipping 1 egg white is not enough to carry one dessert, but it is just right for binding and coating. After meatballs, fish, or vegetables are dredged in flour, dip them in a lightly beaten egg white and coat them in bread crumbs; the white helps the coating stick to the surface and keeps it from falling off during frying. Adding 1 egg white to an omelet makes it lighter. Brushing egg white over cookie or poğaça (Turkish savory roll) dough gives it a glossy surface in the oven; it stays lighter in color than a surface brushed with yolk. Thinly coating nuts with egg white also helps spices stick to them when they are toasted.
With a few egg whites, you can make cookies. Turkish almond cookies and coconut cookies are made with egg whites; in these recipes, the whites act as both binder and leavening agent, and the yolks are not needed. Mix the whites with sugar and ground almonds or coconut, shape the mixture into small mounds on a baking sheet, and bake until crisp on the outside and soft inside. Line the baking sheet with parchment paper because egg whites and sugar stick to the pan. A few egg whites can also be whipped separately and folded into soufflés and cake batter; in that case, add them to the whites separated from whole eggs.
As the number of egg whites grows, you can make desserts built around foam: meringue, pavlova, and meringue cake toppings. For these desserts, the sugar is calculated based on the weight of the egg whites, so weigh them and measure the sugar accordingly. The mixing bowl and whisk must be grease-free and dry; even one drop of yolk in the whites or a trace of grease in the bowl can keep the foam from expanding. One egg white is too little for these recipes: it does not reach the whisk wires sufficiently at the bottom of a large bowl, so the foam does not form.
Egg whites often accumulate from several recipes, and how and for how long they can be held is a storage question. They keep in a covered container in the refrigerator at 4°C / 39°F or below for 2 to 4 days; freeze them for longer storage. When collecting them in one container, write the number on the container because you cannot tell by sight how many whites have been combined. Desserts made with raw egg whites and not cooked are a separate food-safety concern and require pasteurized products; for that reason, set leftover whites aside for a recipe that will be cooked. Do not use egg whites that have become cloudy, turned pinkish, or have a changed odor.