If you're not into baking, all sorts of weird images probably come to mind when you hear the term “egg wash.” If you are, however, familiar with baking breads, rolls, pie crusts, and other pastries, you likely already know that an egg wash is a mixture of beaten eggs and a liquid that are brushed onto the surface of said items before baking in order to give the finished crust extra brownness, crispness and/or shine. The liquids involved are generally milk or water.
Before we get into how to use an egg wash, let's take a minute to discover how it works. Eggs and milk both contain proteins and amino acids, as well as certain sugars, that react with one another in the presence of high temperatures – the kind generated by your oven, for instance.
I promised myself I would not use terms like reactive carbonyl group, nucleophilic amino group, or even increased nucleophilicity when trying to describe what happens when these acids and sugars heat up. A French chemist (naturally) named Louis-Camille Maillard did all that back in the 19-teens when he discovered the process of non-enzymatic browning that bears his name, the Maillard reaction. Suffice it to say that the Maillard reaction is what gives things like roasted meats and toasted breads their nice brown color. (Don't confuse this with caramelization. Similar process, different chemicals involved.)
Egg washes can be simple or more complex depending upon the desired results. Whole eggs can be used or you can separate yolks and whites. You can achieve different degrees of color and shine by tossing in a little water, milk, and/or salt. A whole egg and a tablespoon or two of water makes for a nice, golden amber colored surface. A whole egg and a pinch of salt makes for a highly shiny surface. A whole egg and one or two tablespoons of milk yield a medium shine. Use just the yolk and one or two tablespoons of water and you get a shiny surface with a nice golden amber color. Egg yolks and a little cream make a shiny surface with a darker brown color. An egg white by itself yields not only a shiny surface, but one with a light brown color and a crispy crust.
Egg washes help seal in flavors and also provide a degree of flavor themselves. Eggs and milk are usually used on sweets while eggs and water are applied to more savory preparations. And egg washes can be employed to help seal doughs for things like calzones and egg rolls so that the fillings don't spill out during baking.
A few important things about egg washes to bear in mind: For the best results, always use the freshest eggs possible. Always make sure the egg is completely beaten; a chunky egg wash will result in a chunky surface texture. Using a whisk instead of a fork will result in a smoother mix. And any additives – water, milk, salt – should be whisked in to fully incorporate them. Egg washes should be applied smoothly and evenly using a pastry brush. And they should be applied with a light touch; remember, you're putting on a mere glaze, not a coat of paint.
Finally, don't try to be frugal and save the leftover egg wash. Toss it and make some fresh stuff if you need it. The sixteen cents or so that you'll spend on another egg is cheaper than a visit to the doctor for treatment of a food-borne illness.