What is the common ingredient in both hollandaise and mayonnaise?

Both hollandaise and mayonnaise are emulsified sauces that primarily rely on egg yolks to achieve their creamy texture and stability. The egg yolks serve as an emulsifier, which means they help blend together the oil or fat with the other liquid components of the sauce. In hollandaise, egg yolks are combined with melted butter and typically lemon juice or vinegar, while mayonnaise is made by combining egg yolks with oil, vinegar, or lemon juice.

The process of emulsification is essential in both sauces, allowing for a homogeneous mixture that would otherwise separate. The richness and flavor profile of both sauces are heavily influenced by the egg yolks, making them an integral and common ingredient.