Why do we add vinegar when we are poaching an egg?
Eggs are made up of two main components—the yolk and the egg white. An egg white is comprised almost exclusively of protein, called ovalbumin, and water. When you cook an egg, the ovalbumin starts to denature, meaning it loses it shape and begins to form a white solid through coagulation.
But there is more than one way to make an egg white denature.
One is to make the solution (the water) more acidic—and vinegar, which is quite an acidic entity, can do just that. So by adding vinegar, we get a double effect of heating, combined with increased acidity to help the egg white coagulate and form a solid white.
The image on the left shows an egg being poached in normal water, and on the one on the right shows an egg being poached in water that has vinegar added. The acidic version has solidified faster and the egg has kept it shape better.