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DIY CO2 -- yeast

Greetings all --

	I have been following the DIY CO2 thread with some interest, and
decided it was time to stop lurking.  As a user of DIY CO2 and an avid
homebrewer, I have gained some insight into the behavior of yeast that
may be useful.   The most important thing to understand about yeast is
its life cycle.  Treating yeast (Saccharomyces cerevisiae for beer/wine
yeast) as a “chemical” and not as a microorganism leads to most problems
encountered with fermentation.  The life cycle of Saccharomyces
progresses in three stages.

1)  Respiration -- the first stage in the life cycle is aerobic.  When
yeast is added to an unfermented nutrient broth (called wort in brewing,
or must in winemaking), it utilizes free oxygen in the solution.  No
alcohol is produced in this stage, and CO2 production is low.  During
respiration, yeast stores energy in various chemical forms to be used
later during reproduction and fermentation.  Aerobic respiration will
generally continue until most of the dissolved oxygen is exhausted.

2)  Fermentation -- this is the stage during which most CO2 is
produced.  When no oxygen is available, yeast will switch to an
alternate metabolic pathway utilizing sugars for energy and producing,
primarily, CO2 and ethanol.  Yeast divides rapidly in this phase,
reaching its carrying capacity (about 50 million cells/ml) in the wort
or must, and remains suspended in solution in order to expose maximum
surface area to nutrients.  Assuming no oxygen is added back to the
fermenting wort, yeast will continue fermentation until one of two
things happens;  either alcohol concentration will exceed tolerance, or
the yeast run out of food.

3)  Sedimentation -- once conditions are no longer amicable to
fermentation, yeast will stop dividing and start storing energy in the
form of glycogen, a polysaccharide.  Yeast will flocculate and fall out
of solution, creating a cake of sludge on the bottom of your
fermentation vessel.  At this point, the yeast are no longer
metabolically active, and await better conditions in a state of

With the life cycle of yeast in mind, I have a few suggestions about how
to achieve better DIY CO2 results.

I have noticed that most CO2 setups use only table sugar as fuel for the
yeast.  Yeast require sucrose as their primary source of energy, but
also require amino acids, lipids, and trace elements for proper
metabolic action.  In beer brewing, this is normally not a problem, as
the malted barley used contains sugar as well as anything else the yeast
needs.  However, in winemaking, which is pretty similar to adding sugar
to water, it is necessary to add extra nutrients to the must in order to
get proper fermentation.  If proteins run out before the sugar does,
what you have is a “stuck” fermentation, in which the yeast have plenty
of fuel, but cannot process it.  There are a number of ways in which you
can provide nutrients to your yeast.  I use the malt extract that is
used for beer making.  This is available in any homebrewing store in
either liquid or powder form.  A few tablespoons added per liter of
sugar water is usually sufficient.  Homebrewing shops also sell
something called “yeast nutrient” which works nicely.  I am not sure
about the flour that others have tried, as the sugars and proteins are
locked up in complex forms (starch, polypeptides) that the yeast cannot
utilize without enzymatic degradation. 

If you want to have a quick starting, long lasting fermentation, it is
necessary to provide your yeast with plenty of oxygen when they are
first added to your nutrient broth.  Shake it, stir it, or put an
airstone in it, but saturate it with O2 before you put your yeast in.

Yeast are microorganisms, and microorganisms compete.  So, when setting
up your fermenter, keep it clean.  Rinse it out with hot water.  Bleach
treat it.  Whatever, just provide your yeast with a sanitary environment
in which to get started.  Once the yeast is established, it will
outcompete most other bugs, but initially, bugs like Lactobacillus and
Acetobacter (both found in the air and all over your body) can inhibit
yeast reproduction.  If your nutrient broth smells sour or vinegary, you
have problems with one of these bugs.

Treating your broth with bicarb is fine, but go easy.  Yeast functions
best at a pH of 5-5.5.   

Use beer or wine yeast.  Bread yeast is OK, but poops out before beer or
wine.  The primary difference is in alcohol tolerance.  Beer yeast will
go up to about 5-7%/vol EtOH, and wine yeast 14-18%, depending on
strain.  Bread yeast is below beer yeast in alcohol tolerance, from my
experience.  If you want to superload sugar in your initial mix,
Champagne yeast will work the longest, as it has the highest alcohol
tolerance.  Due to alcohol tolerance in your strain of yeast, it is
possible to put too much sugar in your initial mix.  The extra won’t
hurt the yeast, but it won’t be utilized after a certain alcohol
concentration and will be wasted.  If you have access to a hydrometer,
which measures specific gravity, 1.060 is a good starting point for most
beer yeasts.  This can be attained with roughly one cup of table sugar
per liter of water.  Oh, and if you are using beer yeast, use ale
yeast.  Lager yeast (S. uvarum) ferments best at between 35 and 50
degrees and may be inhibited at room temp.

If one wishes to get yeast fermenting and producing CO2 quickly (e.g. if
your tank CO2 generater just stopped producing and your pH is going up),
starter cultures are in order.  However, talking about those could be
the subject of a post in itself.  If there is interest, please let me

You can keep your yeast going from one addition pretty much indefinitely
by using dormant yeast to inoculate a fresh nutrient broth.  Just a
small amount of the dormant, flocculated yeast is necessary to get a new
fermentation going.  However, if you are doing this, you will be most
successful if you get the yeast within one week of the onset of
sedimentation.  Considerable breakdown of the yeast can occur after
that.  Use of a clear container for your fermentation vessel can be
helpful in determining how much time your yeast has left.  Once you can
see through the solution, sedimentation is in full swing, and your CO2
production is decreasing.

Finally, I have an observation that I cannot explain, but that seems to
work.  Make sure your nutrient/yeast broth comes to within one inch of
the top of your fermentation vessel.  For some reason, lots of dead
airspace within the vessel seems to cause problems.  I don’t know
whether this is due to partial pressures of CO2 affecting pH or what.

Those are the major suggestions I have for now.  Hope this generates
some interest.
							Dave Gauthier
							gauthie9 at pilot_msu.edu