title_body: What is the background of brewing coffee with eggs?
upvoted_answer: So I did a bit of research, and found a number of regional claims to 'egg coffee', including  Swedish, Norwegian, Vietnamese and the American Mid-West. Of course I also found an article on Putting Weird Things In Coffee, which should be taken with a pinch of salt1.
  There seem to be a variety of approaches, but they seem to fall into three main camps:
  
  Add just the white and the shell
  Add just the yolk and the shell
  Add the whole egg, including the shell
  
  Across the different techniques, adding the shell was the only real unifying theme! It's said to reduce bitterness (I imagine because the shell is essentially calcium-carbonate, which is basic). The technique is to just crush up the shell, mix it and the egg contents with the coffee and pour over boiling water.
  The grounds will bind with the egg protein and sink in the cup. They can then be strained off.
  The only other recipe I found is specifically for Vietnamese egg coffee, which I found here:
  
  Ingredients
  
  1 egg
  3 teaspoons of Vietnamese coffee powder
  2 teaspoons of sweetened condensed milk
  Boiling water
  
  Directions
  Brew a small cup of Vietnamese coffee. Crack
  an egg and discard the whites. Put the yolk and the sweetened
  condensed milk in a small, deep bowl and whisk vigorously until you
  end up with a frothy, fluffy mixture like the one above. Add a
  tablespoon of the brewed coffee and whisk it in. In a clear coffee cup
  (we’re going for aesthetics here), pour in your brewed coffee, then
  add the fluffy egg mixture on top. Presto. Egg coffee.
  
  
  1: No, not like that.