An egg wash is simply egg brushed onto food before baking. It is usually beaten, and it may include the yolk or the whites, be thinned with water or other liquids, or be seasoned.
Uses for egg washes include browning baked goods, sticking a crumb coat or a batter onto meat before baking or frying, and holding together pieces of dough. Applied to a crust or a bread, an egg wash can make the cooked surface glossy and golden to dark brown. Thinned egg white is light in color and crispy, while yolks darken the surface as they cook. Eggs can also work as a binding agent. Because the protein in egg coagulates or clumps together as it cooks, it can be used to stick sprinkles or garnishes to cookies before baking or to hold edges of a pastry closed. The moisture and protein form a stronger connection than the edges of the pastry pressed together alone. Pasteurized whole egg or egg whites can also be used, and may be the best option if there are concerns about cooking the dish thoroughly. Egg wash should be applied lightly. If too much is put on, it can drip off and burn or make the food soggy.