results. One is rapid fermentation at a high
temperature with ale yeasts (strains of
Saccharomyces cerevisiae) that clump
together, trap the carbon dioxide gas that they
produce, and rise to the wort surface. The
other is slow fermentation at a low
temperature with lager yeasts (Saccharomyces
uvarum or carlsbergensis) that remain
submerged in the wort and fall to the bottom
when fermentation is over. These are often
called “top” and “bottom” fermentations.
Top fermentation is usually carried out at
between 64 and 77ºF/18–25ºC and takes two
to seven days, during which the yeasty foam is
skimmed off several times. Because the yeast
layer at the top has a good supply of oxygen
and is inevitably contaminated by other
airborne microbes, including lactic-acid
bacteria, top-fermented beers are often
relatively acidic and strong in flavor. Bottom
fermentation goes on at distinctly lower
temperatures, 43 to 50ºF/6–10ºC, takes six to
barry
(Barry)
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