What the standard requires
The federal standard opens with a one-sentence definition, and the egg is part of it.
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Mayonnaise is the emulsified semisolid food prepared from vegetable oil(s), one or both of the acidifying ingredients specified in paragraph (b) of this section, and one or more of the egg yolk-containing ingredients specified in paragraph (c) of this section.
Paragraph (c) names the eggs the standard accepts: liquid, frozen or dried egg yolks, and liquid, frozen or dried whole eggs. Vinegar, or lemon or lime juice, supplies the acid. Salad dressing has a standard of its own, in 21 CFR 169.150, and that standard also requires egg yolk-containing ingredients.
What replaces the egg
Egg yolk holds the oil and water together, and it is the emulsifier that egg-free versions have to replace. A 2024 study in Foods made vegan mayonnaise from defatted soybean flour and explains what the yolk does. The paper writes egg yolk as EY:
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EY functions as an emulsifier in mayonnaise, containing low-density lipoprotein, high-density lipoprotein, phospholipids, and unbound proteins, which help in forming granular micro-oil particles and in preventing coagulation, thereby imparting the desired texture
Just Mayo, from Just Food Company, says on its own product page that it is made with pea protein:
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Made with pea protein, our mayo is plant-based, egg-free, and made with no soy ingredients or artificial flavors.
The original Vegenaise has no egg in its ingredients, and it uses soy protein. The maker's list reads:
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Expeller-Pressed Canola and Safflower Oils, Filtered Water, Brown Rice Syrup, Apple Cider Vinegar, Soy Protein, Sea Salt, Mustard Flour, Lemon Juice Concentrate.
Aquafaba, the cooking liquid from chickpeas, has been tested too. A 2025 study in Molecules stabilised an egg-free mayonnaise with aquafaba and gum tragacanth, and the paper describes aquafaba like this:
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Aquafaba, the viscous liquid from cooking chickpeas, exhibits emulsifying, foaming, and thickening properties
The 2015 FDA warning letter
In August 2015 the FDA sent Hampton Creek Foods a warning letter about Just Mayo and Just Mayo Sriracha. The letter said, from the labels, that neither product contained eggs, and it explained why the name mattered:
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The term “mayo” has long been used and understood as shorthand or slang for mayonnaise.
The letter then set the egg requirement against the labels:
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According to the standard of identity for mayonnaise, egg is a required ingredient (21 CFR 169.140(c)); however, based on the ingredient information on the labels, these products do not contain eggs.
The letter also said the standard does not permit modified food starch, pea protein or beta-carotene, and that the egg picture on the labels could mislead consumers. FDA called the two products misbranded. The FDA page for the letter also links a close-out letter dated 18 December 2015.
Words to look for
Under the standard, every ingredient has to be declared on the label, so a conventional mayonnaise names its egg in the ingredient list. US food labels also have to name egg, which is one of the major food allergens. The FDA puts the rule like this:
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The law requires that food labels identify the food source of all major food allergens used to make the food.
Egg-free products such as Just Mayo and Vegenaise say so on their product pages, with words such as egg-free and plant-based. Check the ingredient list before you trust the claims on the pack. The FDA's 2015 challenge was to eggless products that used the word mayo.