I live in North-Norway, and some times I make coffee this traditional way with egg. This type of coffee was served to rich people ca 1850-1940, when one mostly stopped using the method. The main purpose of using an egg is to make a very clear but rich tasting coffee. But we only use the white of the egg, never the yolk. The protein in the egg white binds to coffee grains and it falls to the bottom, so that the liquid itself becomes clear and nice. One can in no way taste some of the egg in the coffee. Just try by yourself