The froth produced on the top of beer when dispensed with agitation. Composed of a mixture of protein, polysaccharides and iso humulones it forms a layer of liquid around gas bubbles released from the beer. This layer drains slowly so retaining the bubble shape. Antifoaming materials such as lipids reduce the integrity of the layer and collapse the foam. High levels of protein and bitterness in beer will increase foam levels and foam stability.