"The main protein (ovalbumin), in eggs, is a tightly wound molecule and when it is shaken or beaten, it unravels. Think of shaking a big box full of slinkies and then trying to sort them out. That box will probably remain a stable mess for a while. When this happens in a cocktail shaker, the egg proteins do the same thing, they get all tangled up and this forms bubbles and foam.

Many of the drinks that use egg whites tend to be acidic, like sours, because the acid in the drink stabilizes the egg protein. This inhibits the proteins them from binding with each other, which makes for smaller bubbles and a better foam."

http://www.artofdrink.com/2007/08/eg...-cocktails.php

And I concur with the previous poster about the general ickiness and repulsiveness of sour mixes.