An important characteristic in beers is the ability of the beer to retain a nice foamy [[head]] for a long period of time. Commercial brewers go to great lengths to improve '''head retention''' by a variety of additives. However homebrewers also have access to quite a few ingredients and additives that can help your foam last until the last drop. Note that enhancing head retention is closely related to [[Body|enhancing the body]] of the beer.
Foam is the result of CO2 bubbles rising through the beer.
These bubbles attach themselves to substances in the beer and form a skin around the bubble. Obviously the more CO2, the more bubbles, but the goal of the brewer is not bubbles but stability of the head. As foam collapeses, evaporating bubbles tend to solidify the beer near the surface, allowing more beer to be poured with less foaming after about a few minutes have passed. Head stability depends on the presence of substances with low surface tension in the beer which can form stable elastic bubbles. The two primary contributors to head retention are certain high molecular weight proteins and isohumulones (alpha acids from hops). Therefore beers with more proteins that are highly hopped will have a higher head retention.
==Methods for Improving Head Retention==