8 - Fermentation
8.2.2 Attenuative Phase
The attenuative phase is marked by a time of vigorous
fermentation when the gravity of the beer drops by 2/3-3/4 of the
original gravity (OG). The majority of the attenuation occurs during
the primary phase, and can last anywhere from 2-6 days for ales, or
4-10 days for lagers, depending on conditions.
A head of foamy
krausen will form on top of the beer. The foam consists of yeast and
wort proteins and is a light creamy color, with islands of
green-brown gunk that collect and tend to adhere to the sides of the fermentor. The gunk is composed of extraneous
wort protein, hop resins, and dead yeast.
As the attenuative phase winds down, a
majority of the yeast start settling out and the krausen
starts to subside.
Many canned kits
will advise bottling the beer after one week or after the krausen has
subsided. This is not a good idea because the beer has not yet gone through
the Conditioning phase. At this time the
beer would taste a bit rough around the edges (e.g. yeasty flavors,
buttery tones, green apple flavors) but that will disappear after a
few days of conditioning.