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Re: DIY CO2 Generation

James Purchase wrote:

> In reviewing some material on the web site of Wyeast USA
> (http://www.wyeastlab.com/index.htm) I noticed some comments concerning live
> yeast's need for Oxygen and how a lack of Oxygen can cause fermentation to
> stall and sometimes stop altogether.

Several years ago some homebrewers on the list gave us some fairly
complete posts on the how-tos of growing yeast.  They were excellent
posts.  One was by Dave Gauthier, who I think is still on the list.  I
saved his letter for a long time, but now can't seem to find it.

What I recall is that the yeast need oxygen in the early part of the
process to get establshed, then after that they need to be under
anaerobic conditions to do the sugar->CO2+ethanol reaction.  I don't
know what sort of metabolism they do in the presence of oxygen, probably
sugar->CO2 + H20, which would be great if you could get a pure product.

When I make a new yeast batch I aerate the sugar solution thoroughly for
a while before I add the yeast, and continue it at least until the yeast
is dissolved.  That seems to provide enough oxygen to get the yeast
established.  I prepare the solution by dissolving sugar in boiling
water, which drives off oxygen and other dissolved gasses.  Without
aeration it would be pretty-much oxygen free.  I doubt the yeast would
do well.

> Has anyone ever tried putting an airstone inside of their CO2 generator to
> keep the sugar-water-yeast mixture well aereated as the yeast grow?

The catch here is that the gas produced would be just CO2-enriched air,
and that wouldn't dissolve in water much better than regular air.  In
fact, you don't have to dilute CO2 much at all to slow down the rate
that it dissolves in water.  By my calculation, just the water vapor in
DIY CO2 slows its dissolution rate down by about 10% compared to dry
CO2.  Anything else you add to a DIY system that might dilute the CO2
with other gasses -- an air-filled overflow bottle, for instance -- will
also make it more difficult to get the CO2 dissolved.

So pumping air through a yeast batch might get you some pretty good CO2
production, but then you would have a tough time getting much of the CO2
into the water.

Roger Miller