What is the purpose of whisking in egg yolk off heat in the Mornay sauce preparation?

Whisking the egg yolk off heat during Mornay sauce preparation is crucial to prevent curdling. When egg yolks are heated, they can coagulate, leading to a grainy texture if they get too hot too quickly. By whisking off heat, you maintain better control over the temperature, ensuring the eggs warm gently without cooking too fast. This technique allows for the smooth incorporation of the yolks into the sauce, resulting in a creamy and velvety consistency.

Creating a custard, incorporating air, and adding richness are not the primary purposes of whisking the yolk off heat in this specific context. While whisking can incorporate air and contributes to richness when emulsifying, the key concern in Mornay sauce making is maintaining the emulsification and smooth texture, thereby preventing the risk of turning the sauce grainy through curdling.