Out of eggs? 1/4 cup of seltzer can replace 1 egg in yellow cake, and in testing, it produced the best texture and flavor of all the substitutes tried

Egg prices soared in the early 2020s, thanks to the outbreaks of avian flu and increased demand, prompting home bakers to search for cheaper alternatives. One of the options was more successful than most people would guess: plain seltzer water. Kate McCarty, a food systems professional with the University of Maine Cooperative Extension, compared side-by-side a number of egg substitutes in a yellow cake recipe. Seltzer gave the best texture and flavor of anything she tried, cost about a nickel a serving by her estimate, and didn't need a special trip to a specialty store.

Why eggs are so difficult to replace

Eggs do a lot of quiet work in cake batter. They help trap air bubbles, bind ingredients together and add moisture and richness. A 2021 study in the journal Foods found that fat-carrying compounds in egg yolk help stabilize air bubbles as sponge cake batter bakes. That kind of stability is what helps to keep a cake from being dense or collapsing once it comes out of the oven. Take out the eggs, and a batter needs some other way to trap and hold that air.

The science behind the fizz

This is where seltzer comes in. Plain carbonated water is simply water that has been dissolved with carbon dioxide gas under pressure. This gas, when it gets into the batter and meets the heat of the oven, expands and pushes outward. The Kirk-Othmer Encyclopedia of Chemical Technology explains that chemical leavening works by releasing carbon dioxide gas through a reaction between an acid and a bicarbonate, which leavens the dough or batter and is trapped there. Seltzer works on the same basic principle except the carbon dioxide is already dissolved in the liquid, so no chemical reaction is needed to release it.

How seltzer stacks up against the competition

McCarty tested four substitutes in the same yellow cake recipe, which normally calls for two eggs. Applesauce was used at a quarter cup per egg and cost about 25 cents per serving. It did hold back the rise, and it changed the flavor and texture. It was a better fit for quick breads than cakes. Flax seed meal, mixed with water, cost about 6 cents per serving and added fiber, protein and omega-3 fats without changing the flavor, although the seeds were still visible in the finished batter. Seltzer, swapped in at a quarter cup per egg, beat out other options on both texture and flavor, and was the least expensive at about 5 cents per serving. Its main fault was color. Cakes made with seltzer were noticeably lighter than cakes made with real eggs, which cost about 44 cents a serving in the same test.

Other alternatives worth knowing

Seltzer and the three substitutes tested head-to-head aren't the only options. Mashed banana, peanut butter, and chia seeds are also used as egg substitutes in baking, but they each change the flavor of the finished product more than seltzer does. UMaine Extension's guide notes that tofu is typically the go-to egg substitute in cooking rather than baking, though some vegan baking recipes elsewhere do call for silken tofu in dense, moist bakes like brownies. Instead of eggs, you can use a thin batter made of chickpea flour; it works much like a flour-based omelet.

How to use it in your own kitchen

The swap itself is simple. Per UMaine Extension's guidance, for each egg the recipe calls for, add a quarter cup of plain, unflavored seltzer, and mix it in at the same point in the recipe as you would the eggs. It works best in recipes where eggs are mostly for lift and moisture, such as a plain yellow or white cake, not in recipes where eggs are for rich flavor or a custard-like set, such as pound cake or flan. An unopened bottle of seltzer keeps for many months at peak quality when stored in a cool, dark place, and remains safe to drink well beyond that, making it nice to have on hand for those times when eggs are expensive or you run out of a carton mid-recipe.

Should you try it?

Seltzer won't work in every recipe that calls for an egg, and no substitute will behave just like an egg. But for a plain yellow cake, it gets close enough that most people wouldn't notice the difference and it costs a fraction of the price of eggs. Next time eggs are scarce or expensive, a bottle of seltzer already sitting in the fridge could be a convenient fix in the kitchen.