Beer head (also head or Kräusen) is the frothy foam on top of beer which is produced by bubbles of gas, typically carbon dioxide, rising to the surface. A head is produced during fermentation, and consists of wort protein, yeast and hop residue. The foam is attributed to yeast eating sugars and building up CO2 in the process. The fermentation may take place in the brewery or it may be a secondary fermentation taking place in the cask or bottle. If the beer is no longer fermenting, due to filtering or pasteurising, then a head cannot naturally occur, so a head is produced by force carbon...