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observation of dissolved oxygen loss in wort

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jjflash:
Just finished brewing today and have an interesting observation to share/discuss.

Brewed an Imperial Stout OG 1.094. I use an in-line oxygen system into a 7.5 gallon glass carboy with a Hach luminescent dissolved oxygen meter in the carboy.  Wort is at 65 degrees.  I use the standard 1ppm/degree Plato oxygen per the "Handbook of Brewing", Priest and Stewart.  I have been running this identical oxygen set up for about 2 years now.  I run the wort into the carboy and adjusted the oxygen flow to hit my desired number, then immediately pull the Hach LDO out of the carboy.  Today for the first time I decided to leave the LDO in the carboy and take a few readings over the next couple hours. Initial reading upon filling the carboy 20+ppm O2. Did not pitch the yeast.  Over the next hour the oxygen saturation is down to 5.2!  No yeast, carboy sitting at 65 degrees in the refrigerator.  I was blown away how quickly the oxygen came out of solution. Pitched the yeast and thirty minutes later oxygen saturation of zero!

Appears to me that my in line oxygen set up is near worthless as most all the oxygen came quickly out of solution in the carboy. I always thought an in line oxygen system was the most efficient method.  I now suspect any other delivery system would suffer this same problem. Perhaps a better way is to rack to the carboy and run a very slow, continuous oxygen flow via airstone into the carboy for the first 24 hours?

majorvices:
I'm just impressed you have a DO meter!

narvin:
Sounds expensive  ;)

I had expected that O2 would come out of solution quickly, especially above its saturation point, but it is surprising that it got down to 5ppm so quickly.  Still, you're fermenting beer, not growing yeast... I'd stick to continuous aeration in the starter.  Unless you're fermenting an ultra-high gravity beer, too much oxygen in the wort could have a deleterious effect.

tschmidlin:
I would not oxygenate after pitching the yeast.  How confident are you that the DO meter is accurate for long periods of time?

If the temp is 65 the max O2 is higher than 5.2 for water, but I'm not sure how the presence of sugar affects that.  You may be better off corking the carboy when you are done filling it - any O2 that comes out of solution will be sitting in the headspace and be reabsorbed given time, assuming the wort is not at saturation.  Once the yeast is pitched and the dissolved O2 is taken up, some of the O2 in the headspace will dissolve into the beer until fermentation is vigorous enough to drive it out of the carboy.

I think you need to do some more testing :)

cornershot:
I have heard before that yeast consume all dissolved oxygen within the first half hour. So wouldn't it simply be most efficient to oxygenate after the yeast has been pitched (and acclimated?)?

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