Yeast Attentuation and Flocculation

Attenuation And Flocculation

Brewing terms attenuation and flocculation are defined and
explained in this section.  The material
is derived from articles Chris White wrote for Brew Your Own magazine many
years ago.

ATTENUATION:

Yeast is responsible for turning sweet wort into what we
call "beer".  Yeast consume the
sugar in wort, and turn that sugar into CO2, alcohol, and flavor
compounds.  When yeast finish the
fermentation process, they shut down, clump together, and fall to the bottom of
the fermentor, or
"flocculate".  When
yeast flocculate, it is easy to see that fermentation is done.  But how can the brewer be sure?  What if the flocculation is  minimal, and yeast and CO2 stay in
solution.  How does the brewer really
know when fermentation is done?  The
answer:  by testing the degree of
attenuation.  Apparent attenuation
percentage is the percentage of sugars that yeast consume.   Attenuation varies between different
strains.  The fermentation conditions and
gravity of a particular beer will cause the attenuation to vary, hence each
strain of brewers yeast has a characteristic attenuation range.  The range for brewers yeast is typically
between 65-85%.

How does a brewer calculate attenuation?

First, the specific gravity must be checked with a
hydrometer before the yeast is pitched.
Specific gravity is a measurement of density.  The specific gravity of water is 1.000, and
wort has a higher density relative to water because of the sugars present in
wort.  As these sugars are consumed by
yeast during fermentation, the density and therefore specific gravity
measurement lowers.  The yeast also
produce alcohol, which is lighter than water, so to obtain the
"actual" attenuation, alcohol must be removed by heat and replaced by
water.  Only large breweries go to such
lengths to report the "actual" attenuation, while the attenuation
most homebrewers measure is "apparent" attenuation.  The specific gravity measured before
pitching, after correcting for temperature, is called the "OG", or
Original Gravity. The OG needs to be logged into a brewers notebook.  Then during fermentation, the specific
gravity can be re-checked.  The specific
gravity will fall towards 1.000 during fermentation, and a brewer can learn
much about the fermentation by checking the specific gravity of the beer
daily.  Once the gravity remains the same
for 3 days in a row, the yeast is most likely done with fermentation.  The specific gravity at the end of
fermentation is called "FG", or Final Gravity. To calculate attenuation
percentage, the following equation can be used:

[(OG-FG)/(OG-1)] x 100

The only way to know if a yeast has completed fermentation
is to check the expected attenuation.
Many homebrewers make the mistake of worrying about a beer before they
even check the attenuation.  A simple
check of the specific gravity at the end of fermentation will help in this
regard.  It is not completely accurate
without computing the attenuation.  For
example, if a high gravity beer is made, the FG will be higher then normal, but
the expected attenuation for that yeast strain may have been obtained.  To obtain expected attenuation numbers,
consult a list of yeast strain attenuation figures.

Most manufactures of brewers yeast list the attenuation
ranges of their yeast strains. This can be very useful to a brewer in matching
a yeast strain to a beer style. An example would be a brewer wishing to make an
American-style Pale Ale.  A yeast strain
should be selected that will produce a dry finish, and allow for hop flavors to
come through. A good choice would be a neutral yeast with an attenuation of
70-80%. If a brewer wants to make an English style mild ale, a yeast strain
that does

not attenuate as much would be desired.  An attenuation range of 65-70% would be more
appropriate.  Would a yeast strain that
attenuates to 80% taste bad in an English style mild ale? No, but the beer
would not taste true to style.

FLOCCULATION:

Brewers have created their own unique vocabulary.  Words such as pitching, attenuation, and
flocculation take on special meaning to brewers. Pitching is adding yeast to
wort to start fermentation.  Attenuation
is the percentage of sugars yeast consume during fermentation.  The magical art of yeast coming together,
dropping to the bottom of a fermentor, is called flocculation.

Flocculation is a desirable and important characteristic
that is unique to brewers yeast.  When
brewers yeast nears the end of fermentation, single cells aggregate into clumps
of thousands of cells, and drop to the bottom of the fermentor, leaving clear
beer behind. If yeast flocculate too early, the beer will be underattenuated
and sweet. If yeast do not flocculate, the beer will be cloudy and have a
yeasty taste.

Most strains of yeast, which brewers call "wild"
yeast, do not flocculate well, and remain in suspension for extended periods of
time.  The ability to flocculate is a
product of natural selection.  Brewers
have continually collected yeast either from the bottom or top of a fermentor
and in doing so, selected for increasingly flocculent stains.  The chemistry of flocculation is complex, and
will be the subject of a future article.

Yeast flocculation can be classified as high, medium, or
low.  Ale yeast strains are found in each
category, while lager yeast are predominantly medium flocculators.  An English/London Ale strain would be a high
flocculator, while an California/American Ale strain a medium flocculator. A
Hefeweizen strain is an example of a low flocculator.  It is difficult to tell which category of
flocculator is used to produce individual commercial beers, because most
commercial beers are filtered before being bottled or kegged.

Article used courtesy of https://www.whitelabs.com for the purposes of this
assignment. Original article located here: https://www.whitelabs.com/resources/attenuation-and-flocculation