Hi Ruth, mayonaise is not one of our recommended egg substitutes, but it may work fine if you're making a cake and substitute for just one egg. As noted in this article,  not all cake recipes are going to work well with egg substitutes, so you'll want to be careful: "It's also difficult to substitute in cakes that rely on eggs for most of their leavening and structure, including sponge cakes, hot milk cakes, chiffon cakes, and angel food cakes. If the recipe calls for you to beat eggs to ribbon stage and/or fold in stiff egg whites, it's best to look for another recipe rather than try to replace the eggs."

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