Why does mayonnaise become firm during preparation?

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Mayonnaise becomes firm during preparation because it is an emulsion formed by the continuous beating of egg yolks and vegetable oil.1

As the oil is added little by little to the yolks, lecithins found in the egg yolk act as an emulsifying agent. These molecules possess both a hydrophobic portion, which dissolves in the oil, and a hydrophilic portion, which dissolves in the aqueous phase. By concentrating at the interfaces between the oil and water, these agents stabilize the mixture, creating a dispersion of small oil droplets that results in a thick, creamy consistency. This process effectively transforms liquid ingredients into a stable, semi-solid sauce.21