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Contents:
O2 diffusion (mabrooks)
Re: stir plate oxygen limit (Fred L Johnson)
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Date: Mon, 23 Mar 2009 09:31:52 -0400
From: mabrooks at vt.edu
Subject: O2 diffusion
When it comes to "oxygenation" of most liquids, there are numerous way to go
about it. The simplest way to get to O2 saturation level in a room temperature
yeast starter, assuming you have a stir plate and proper size container (large
surface/vol area) and proper size stir bar, is to simply cover the flask
loosely with foil and turn on the stir plate. The O2 transfer from air will
work wonderfully and you can assume saturation levels will be maintained fairly
well. Diffusion and transfer laws will provide all the exchange needed for this
task. I really cant see the need to go through the trouble of putting in an air
diffuser stone and using a stir plate unless for some reason the yeast O2 demand
was
really excessive? or if the container was "sealed" tight, which I don't see the
need for? If the starter is at fermentation temps the issue is even
less of a concern as O2 has a higher sat level at lower temps.
Typical situations which would necessitate the use of a diffuser stone, is if
you have a
very large yeast starter (> 3 gallons) and the surface area (volume exposed to
air) is very small and the yeast O2 demand is very high (very thick yeast
starter). Typically one would
use pure O2 for this type application. Otherwise, the natural transfer of
O2 from air into a dilute wort/yeast starter (or other liquid) being stirred is
very
efficient, especially below 60 degrees F. Its all about the "demand" for O2 in
the starter, and for starters of less then 2 quarts or so, you don't need to
use a diffusion stone, especially if you have a stir plate.
Matt B.
Northern VA.
BTW: I have an extra stir plate I bought and didn't use (still new) if anyone is
interested I would be willing to sell. Stir bars are included.
Date: Sat, 21 Mar 2009 10:09:43 -0500
From: "Devonna Dieterle" <djdieterle at hughes.net>
Subject: Subject: Re: stir plate oxygen limit
Matt stated he didn't want to pump filtered air directly into the starter do
to the infection risk. Fred stated he pumps air into the starter headspace.
With a good quality inline air filter which is rated to filter to .023
micron is infection a risk and which way is more effective, pump into the
liquid or the headspace(if there is a difference in oxygen absorption)
I ask because I pump air directly into the liquid using 2 filters in series
through a stainless bubbler wand on a stir plate, with the starter vessel
sealed from the outside air and generally follow Mr. Maltys starter volume
calculator for quantities of 1.040 canned wort for stepping up.
Jeff Dieterle
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Date: Mon, 23 Mar 2009 22:19:17 -0400
From: Fred L Johnson <FLJohnson52 at nc.rr.com>
Subject: Re: stir plate oxygen limit
Jeff asks if pumping air directly into the wort is more effective or
less effective than pumping into the head space of a stirring
starter. Undoubtedly, pumping air into the wort directly is more
effective at dissolving oxygen in the wort than pumping into the head
space, but it probably isn't much more effective if the air supply is
continuous and the starter is stirred. One only needs to dissolve
oxygen in at the rate it is used by the growing culture. When the
yeast are at their peak rate of daughter cell production, one might
see a benefit of pumping directly into the medium, but I have no data
to support this. Pumping air directly into the wort leads to lots of
foam, so one would have to turn down the air flow rate and/or add an
agent to break the surface tension to minimize foaming--something I
prefer to avoid.
Fred L Johnson
Apex, North Carolina, USA
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End of HOMEBREW Digest #5529, 03/23/09
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