What is used to help set egg white around the yolk when poaching an egg?

When poaching an egg, vinegar is commonly used to help set the egg whites around the yolk for several reasons. The acetic acid in vinegar lowers the pH of the cooking water, which enhances the coagulation of the protein in the egg whites. This means that the proteins denature and bond together more quickly when they come into contact with the heat, creating a more cohesive and firm structure around the yolk. As a result, the egg holds its shape better during the poaching process and prevents the whites from spreading too much in the water.

The other alternatives do not provide the same chemical reaction necessary for setting egg whites effectively. Salt, for instance, would not have the same effect as vinegar in terms of protein coagulation, and both olive oil and butter are fats that would impede the process of coagulating the egg whites. Instead of helping to set the whites, they would float on top of the water and would not contribute to the intended texture of a perfectly poached egg.