There are some tried and true options that I've seen used from time to time, but determining which is best to use depends on the outcome you'd like to get. Three options: flax seeds, silken tofu, and mashed banana are tested here in the same recipe - scoochmaroo's awesome chocolate chip cookie recipe.
Step 1: Options, Options
In addition to the examples below, there are loads of other things that folks use to replace eggs for whatever reason, including commercial egg replacers.
Perfect for replacing eggs in sauces2 or adding to frosting for cakes, silken tofu is also great in baked goods. It can make baked goods a little bit heavy but will keep them nice and moist, so it's better in brownies rather than angel food cake.
- Use 1/4 cup silken tofu to replace each egg in a recipe
- Great for: cakes, cookies, brownies, sauces
- How: The high protein content in both eggs and tofu make them ideal thickeners, in addition to the fact that they also emulsify well into other ingredients so you're not left with weird lumps and bumps. Upon being heated, protein thickeners like eggs or tofu unwind and then (if at high enough concentrations in the liquid they've been added to) when they begin to cool they set up and make the mixture more solidified.
Adding a slightly nutty flavor, flax seed is wonderful for heavy or dense baked goods.
- Use 1 teaspoon of ground flax seeds and 1/4 cup water to replace each egg in a recipe (grind flax seeds and combine with water - let sit for a minute or so before adding to a recipe)
- Great for: waffles, pancakes, bread, cookies (particularly oatmeal cookies)
- How: For the same reason the flax can aid in digestion, the stickiness and slimey-ness created when ground flax and a liquid are combined make them an ideal egg replacement. Some people even use flax slime for hair gel! The slime is the result of the oils of the flax seeds combining with moisture, and can even happen if the seeds are kept in too humid of an environment. The slime also makes flax an idea replacement for gluten (when combined with several eggs).
Mashed bananas, applesauce, or other fruits can be used in muffins and other treats. I personally prefer to use bananas in things like cookies and applesauce in cakes or things where you're aiming for a more pudding-ish outcome. The end result will be rich, moist, and quite dense (so if you want to lighten things up a little, add a touch of extra baking soda). You may want to add a little extra baking time to help balance out the extra moisture that fruit can add.
- Use 1/4 cup of mashed banana (usually about a half banana) or applesauce to replace each egg in a recipe
- Great for: quick breads (like banana bread!), brownies, cakes
- How: Mashed fruits provide a fair amount of moisture to recipes (as eggs would), and also serve to activate the gluten in flour and bind doughs, etc. together. Unlike eggs, they don't have a ton of "stickiness" of their own and so it can be difficult to get some doughs to hold together using only fruit.
Vinegar and Baking Soda
While not a great option in cookies, sauces, or savory dishes, a vinegar and baking soda combo works very well in cake recipes to keep cakes fluffy and light.
- Use 1 teaspoon baking soda and 1 tablespoon of vinegar to replace each egg in a recipe
- Great for: cakes
- How: More like whipped eggs, the combination of vinegar and baking soda results in a reaction that will really blow the roof off your batter. For the same reason you might use baking soda and vinegar to clear a drain, the combination of acetic acid (what makes vinegar vinegar) and sodium bicarbonate (baking soda) results in the formation of carbonic acid, the bi-product of which is a release of carbon dioxide gas (the bubbles that are produced).
1In addition to silken tofu, firm and extra-firm tofu can be used when making egg-free versions of dishes like quiche or egg salad, or even yummy tofu scrambles!
2I don't know if I could ever get by without the joy that is Hollandaise sauce (which can be made in the microwave!), but this is an awesome alternative (and it would be great with some eggplant bacon on the side).