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Good head retention is measured in terms of the time required for the head to collapse to half of its initial height. This should be at least a minute in well-brewed and conditioned beers. The beading should also be uniform and tight, leaving lace on the glass as the beer is consumed. Good head retention is promoted by several factors, including isohumulones, high original gravity, alcohol content, dextrins and the levels of high- and medium- molecular weigh proteins. Adequate carbonation is also important. Most of these variables are style-dependent, but the brewer can increase the protein content by adjusting the length and temperature of the protein rest and using adjuncts such as flaked wheat and barley. Fatty acids carried over from the trub and unclean glassware are both detrimental to head stability, since they decrease the surface tension of the foam, causing the bubbles to collapse.
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