For breading, you can use yogurt thinned with water, mustard, a pourable batter made from flour and water or milk, or a gel made from ground flaxseed instead of egg. The egg’s job here is to make the breadcrumbs stick to the food, and all of these substitutes do the same job. The color, crispness, and flavor of the crust vary a little depending on the substitute, but the order stays the same: flour, binder, breadcrumbs.
Breading has three layers, and the egg is the binder in the middle. The flour absorbs moisture from the food’s surface and prepares a dry base; the egg sticks to this base and holds the breadcrumbs in place. When heated, it coagulates and locks the two layers together. If you do not use egg, its replacement also needs to be sticky enough to adhere to the flour and thick enough to hold the breadcrumbs. The substitutes below meet these two conditions, and the order of the coating layers stays the same with all of them.
Yogurt is the easiest substitute to find. Strained yogurt or full-fat yogurt thinned with a little water coats the floured food with a thin layer and holds the breadcrumbs well. Yogurt’s mild tang adds a delicate flavor to the crust, and the milk sugar in yogurt makes the crust brown a little darker. Yogurt-coated pieces color faster in oil, so do not let the oil temperature reach the upper end of the range. Adding a pinch of salt and spices to the yogurt gives the coating extra flavor.
Mustard is a good egg substitute, especially for meat or chicken schnitzel. Spread a thin layer of mustard over the surface of the food and press the breadcrumbs directly into it; in this case, you can sometimes skip the flour step. The sharpness of the mustard softens considerably during frying, leaving a mild spice flavor. A batter made with flour and water is the most neutral substitute: combine one measure of flour with one and a half measures of cold water or milk to make a cream-like mixture. This batter holds the breadcrumbs firmly and adds no flavor to the crust.
Flaxseed gel is a plant-based substitute. Mix 1 tablespoon of ground flaxseed with 3 tablespoons of water and let it stand for 10 minutes; the mixture turns into a slippery gel resembling egg white. The gel is sticky and holds the breadcrumbs well, but it adds a slight nutty flavor and dark flecks to the crust. Milk can also be used on its own, but it is not very sticky; a coating dipped in milk will be thinner and may fall off in places while turning thicker pieces.
Whichever substitute you use, two rules apply. The first is drying: pat the food dry with a paper towel before coating it in flour, or the binder will slide off. The second is resting: refrigerate the coated pieces for 15–30 minutes so the layers can settle together. Egg-free breading can be a little more fragile than egg-based breading; do not turn the pieces at all during the first 2 minutes in the oil. If using chicken, judge doneness by the internal temperature, not by the color of the coating: the thickest part should reach 74°C / 165°F.