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Yeast and DIY CO2

In the last digest, Dwight asked:
> I don't
> know if a consensus was reached, but does anyone have any scientific
> evidence of the longest and most effective yeast (teaspoons) - sugar (cups)
> - - water (vol.) combination?  Good anecdotal information is also welcome.
> ~D.Chang;

Sorry for the length of this post, but there is alot of material
to cover.

I don't have any references to back me up here, but I've got some
experience with yeast as a homebrewer of 10 years.  Basically, yeast
have three phases in their lifecycle.

1) Respiration phase: The yeast consume oxygen and sugar in order to
reproduce.  The by-product of this is CO2 (but no alcohol).

2) Fermentation phase: The yeast consume sugar anarobically.  The
by-product of this is CO2 and alcohol.

3) Floculation: The yeast give up the ghost and clump together, either
sinking to the bottom or rising to the surface (depending on the

OK.  This is an oversimplification, but that's basically how yeast
work.  All of these phases are happening at all times, but in a closed
system (like in our CO2 generators, or when you are making beer),
most of the yeast will be coordinated.

Phase 1 can continue as long as there is oxygen and sugar available
(although, see below).  If oxygen runs out, phase 2 begins.  If
sugar runs out, phase 3 begins.  Yeast will also replicate to a
maximum density in the water.  Since the CO2 production is a function
of how many yeasty-beasties are eating sugar, the volume of CO2
is a function of the volume of water used and *not* by the amount
of sugar in solution (assuming there is enough O2 and sugar and
various nutrients to keep the yeast population optimal).

Once the O2 runs out, phase 2 begins.  This produces alcohol.  Yeast
will also reproduce in this stage, but at reduced rates (it takes
more sugar to produce enough energy to reproduce anarobically).  There
are also many other by-products of fermentation (most of which
are toxic to yeast).  Once the sugar runs out, the yeast will move
on to phase 3.  Also, if the concentration of fermentation by-products
reaches a certain level, the yeast will also move to phase 3 (the
amount of tolerance is dependent upon the strain of the yeast).  Finally,
if there are not enough other nutrients in the water, the yeast
will floculate.

Finally, stage 3.  The yeast leave suspension and either sink to the
bottom or float to the surface (actually, all strains will do both,
but the tendancy to do one or the other is dependent upon the strain).
Much of the yeast is still viable as long as conditions are restored
to favourable levels.

So how does this affect us?  Well, the first thing to notice is that
the level of CO2 output is dependent upon the concentration of yeast
in the water and the volume of the water.  If we could keep the yeast
population at a constant level, we would have a constant (and 
predictable) level of CO2 produced.  Note that this is (mostly)
independent upon the amount of sugar in the water.

However, as alcohol levels rise and sugar levels fall, the yeast has
a tendency to floculate.  This tendency is dependent upon the yeast
strain.  We need to pick a yeast that tends not to floculate.  Bread
yeast is famous for that.  Most beer and wine yeast are selected for
their ability to floculate and clear the beverage.

We need to choose a level of sugar to add.  Since CO2 production is
not related to sugar levels, it is tempting to add a whole whack of
sugar (as much as we can dissolve) to the water.  Unfortunately,
as the level of dissolved sugar increases, osmotic pressure builds
on the cell walls of the yeast.  For the most part, the yeast can
adapt to this environment, but it takes time.  The higher the
density of the liquid, the longer it takes for the yeast to adapt.
Also as the level increases, the amount of yeast in suspension will
decrease.  So again, we need to select for strains of yeast that
like high density solutions.

Now we must consider oxygen and alcohol.  Ideally, we would keep O2
levels high throughout the process.  This would reduce the level of
alcohol produced and keep the yeast population at optimal levels.
Oxygen is also needed to produce fatty acids needed by the yeast. If
oxygen is not introduced into the system at some point, the yeast
becomes unhealthy and will not reproduce at optimal levels.  Many
homebrewers use aquarium bubblers to introduce oxygen when they are
trying to produce yeast cultures for adding to their beer.
Unfortunately, I can't think of a way to do this, since any air we
add will dilute the CO2.  Given this, we need a yeast that is capable
of tolerating high levels of alcohol.  In my experience, bread yeast
is acceptible for this.

Finally, we must consider other nutrients (mostly nitrogen based
ones).  Since we are adding pure sucrose to the water, the yeast
are using their stored up levels of other nutrients in order to
survive.  This will work acceptably unless you try to continually
reuse the yeast that you have.  In this case, you will need to add
some kind of yeast nutrients.

This message is already quite long, so I think I'll stop here.  This
should give you some ideas on how to improve the predictability of
your yeast systems.  Tommorrow, I will write up my suggestions for
developing a better system.

                  mike.charlton at sympatico_ca