Final answer:
Egg yolk acts as an emulsifier in Hollandaise sauce, with lecithins in the yolk stabilizing the mixture of oil and water by having hydrophilic and hydrophobic ends.
Step-by-step explanation:
The ingredient that acts as an emulsifier in Hollandaise sauce is egg yolk. Emulsifiers are substances that help stabilize emulsions, which are mixtures of two liquids that normally wouldn't mix together, like oil and water. Mayonnaise, for example, is an emulsion of oil in vinegar, made stable by the addition of egg yolk, which contains lecithins. These lecithins are key components of egg yolk that have both hydrophilic (water-attracting) and hydrophobic (water-repelling) ends, making them ideal to act as a bridge between the oil and water components in an emulsion, similar to the way bile salts and phospholipids work in the body to emulsify fats for digestion.