metabolism (p. 762).
Nitrogen Makes Creamy Foams In the last
decade, many beers have come to be endowed
with an especially fine, creamy head that used
to be largely limited to stouts. The creamy
head comes from an artificial dose of nitrogen
gas that may be injected into beer at the
brewery, or in the bar or pub by the tap that
delivers beer from the keg, or by a small
device inside an individual beer can. Nitrogen
is less soluble in water than carbon dioxide, so
its bubbles are slower to lose gas to the
surrounding liquid, and slower to coarsen and
deflate. Nitrogen bubbles remain small, and
persist. They also don’t carry the tart
prickliness of carbon dioxide, which becomes
carbonic acid when it dissolves in beer and on
the surface of our tongue.
Foam in the Glass An initially vigorous
pouring action develops the head of foam with
a small, easily controlled portion of the beer.