HOMEBREW Digest #5529 Mon 23 March 2009

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  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 Return to table of contents
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 Return to table of contents
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