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CO2 Questions

Hi all.

In response to the numerous questions regarding exploding bottles, the
answer is "yes, the bottles will explode if the CO2 has nowhere to go".
The yeast will *not* shut down when the pressure gets too high (no-one
here said that, but it is a common misconception).  This is not a big
deal for using it as a CO2 generator, as we are allowing the CO2 to enter
the tank.  However (and this is a big however), your yeast generator
*can* get plugged up.  If you use a glass bottle, it can explode and
do serious damage to anything in the room.  It is uncommon, but it does
happen (yes, I know -- you'd think that the output hose would come off
before the bottle exploded, but this doesn't always happen).

Someone else asked about using yeast to make root beer.  I've done this
(ginger beer is even better).  The bottles *will* explode, though.  Plus,
the root/ginger beer will be alcoholic.  A rootbeer FAQ is here:
OK, this is off topic, but I posted this to the Homebrew Digest a few
years ago (and it's short).

>> what stops the yeasti-beasties from eating all that
>> unfermented sugar, and blowing bottles all over my
>> kitchen.
>NOTHING!!!  This is very important.  I have exploded more than one
>batch of rootbeer in my time.  I have, however devised a fairly safe
>way to make soda pop (using yeast to carbonate it).  First, use only
>plastic bottles (and only new ones to boot).  Plastic bottles will
>make a mess when they explode, but won't usually kill people.
>Secondly use a yeast that is fairly temperature dependent (an ale
>yeast is good -- bread yeast, lager yeast, champagne or wine yeast
>are all out).  After you bottle, squeeze the bottles periodically
>until they are hard.  Put them in the freezer.  When they get cold
>enough (almost frozen), take them out and decant the liquid off of the yeast.
>rebottle and store the bottles in the fridge.  I have found that this
>method gets rid of most of the yeasty taste and will give you much more
>control over the carbonation level.  It's well worth the extra work.

Finally, someone asked about precise recipes for yeast based CO2 production.
This is very difficult, because the amount of CO2 is dependent upon
the yeast strain, the temperature, the nutrient level in the mixture
and the phase of the moon (well, not really, but...)

Most yeasts work best if the density of the sugar/water mix is about
1.030 (you can use a marine hydrometer to measure this, or buy one at
a homebrew store -- the latter is cheaper).  1 pound of sugar in 1 US
gallon of water will give you a density of about 1.045 (this is a little
over 100 grams per liter -- you can do the math better than me :-)).
At anywhere between 1.020 and 1.060, you can expect the yeast to work
at its highest rate and ferment all the sugar available.

If the density gets higher, the yeast will endure some osmotic shock and
will work more slowly.  Reproduction will be hampered and may produce
mutants.  It is likely that the yeast will not fully ferment all the
sugar available.  Note that most people recommend recipes that result
in densities of 1100-1300.  I believe this is why many people find
DIY yeast generators unpredictible (sometimes going for 3 weeks, sometimes
going for 1 week, etc).

There are many more issues involved here.  David Lorenzen and I were
doing some tests (well, David was doing them and I was kibitzing :-))
to determine how to optimize water/sugar ratios and volume of generators.
The results were pretty inconclusive.  If there is any interest, perhaps
David could post his results.