On Food and Cooking

small amount of yeast and some sugar or fresh
wort, or some actively fermenting wort (this
is called Kräusening). Inside the closed cask
or tank, the liquid traps and absorbs the
carbon dioxide produced. Undesirable odors
can be forced out of the beer by opening the
container briefly and allowing some gas to
escape. These traditional techniques are
sometimes replaced by simply pumping pure
carbon dioxide into the beer — carbonating it.
Some hops or hop extract may also be added
at this point to augment aroma, bitterness, or
both. A few days of cooling and the use of a
“fining” agent — isinglass (fish gelatin), clay,
and vegetable gums are common —
precipitate suspended proteins and tannins
that might later form a haze when the beer is
chilled for drinking; this is called “cold
stabilization.” The beer is then centrifuged to
remove any remaining yeast and precipitate,
filtered, packaged, and usually pasteurized.