Brewing: Fermentation
Fermentation is the process by
which yeast converts the glucose in the wort to ethyl
alcohol and carbon dioxide gas -- giving the beer
both its
alcohol
content and its carbonation. To begin the fermentation
process, the cooled wort is transferred into a fermentation
vessel to which the yeast has already been added. If the beer
being made is an ale, the wort will be maintained at a
constant temperature of 68 F (20 C) for about two weeks. If
the beer is a lager, the temperature will be maintained at 48
F (9 C) for about six weeks. Since fermentation produces a
substantial amount of heat, the tanks must be cooled
constantly to maintain the proper temperature.
These fermentation tanks hold more than 2,400 gallons
(9,085 L), which means that it takes four batches of wort to
fill one tank. Since fermentation takes at least two weeks,
the capacity of the brewery is limited by how many tanks they
have.
When the wort is first added to the yeast, the specific
gravity of the mixture is measured. Later, the specific
gravity may be measured again to determine how much alcohol is
in the beer, and to know when to stop the fermentation.
Click
here for more details on measuring the amount of alcohol
in beer.
The fomenter is sealed off from the air except for a long
narrow vent pipe, which allows carbon dioxide to escape from
the fomenter. Since there is a constant flow of CO2 through the pipe, outside air is prevented
from entering the fomenter, which reduces the threat of
contamination by stray yeasts.
When fermentation is nearly complete, most of the yeast
will settle to the bottom of the fomenter. The bottom of the
fomenter is cone shaped, which makes it easy to capture and
remove the yeast, which is saved and used in the next batch of
beer. The yeast can be reused a number of times before it
needs to be replaced. It is replaced when it has mutated and
produces a different taste -- remember, commercial brewing is
all about consistency.
While fermentation is still happening, and when the
specific gravity has reached a predetermined level, the carbon
dioxide vent tube is capped. Now the vessel is sealed; so as
fermentation continues, pressure builds as CO2 continues to be produced. This is how the
beer gets most of its carbonation, and the rest will be added
manually later in the process. From this point on, the beer
will remain under pressure (except for a short time during
bottling).
When fermentation has finished, the beer is cooled to about
32 F (0 C). This helps the remaining yeast settle to the
bottom of the fomenter, along with other undesirable proteins
that come out of solution at this lower temperature.
Now that most of the solids have settled to the bottom, the
beer is slowly pumped from the fomenter and filtered to
remove any remaining solids. From the filter, the beer goes
into another tank, called a bright beer tank. This is
its last stop before bottling or kegging. Here, the level of
carbon dioxide is adjusted by bubbling a little extra CO2 into the beer through a porous stone.
How Yeast Makes
Alcohol and Carbon Dioxide When the yeast first hits the wort,
concentrations of glucose (C6H12O6) are very high,
so through diffusion, glucose enters the yeast (in fact,
it keeps entering the yeast as long as there is glucose
in the solution). As each glucose molecule enters the
yeast, it is broken down in a 10-step process called
glycolysis. The product of glycolysis is two
three-carbon sugars, called pyruvates, and some ATP
(adenosine triphosphate), which supplies energy to the
yeast and allows it to multiply. The two pyruvates are
then converted by the yeast into carbon dioxide (CO2) and ethanol (CH3CH2OH, which is the
alcohol in beer). The overall reaction is:
C6H12O6 => 2(CH3CH2OH) + 2(CO2)
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