I picked up a dissolved O2 meter. Kind of crazy, but my sagas with ethyl acetate continue. I got a starter going tonight for some fermentation experiments. In the mean time, I did some messing around.
I filled a gallon jug with water from the cold tap, which was about 50-degrees. That gave me 4.2 ppm of O2. I put an air stone and aquarium pump in there for 5 minute and that bumped it up to 6.3 ppm. I left it on for 5 more, and it bumped it up to 6.9 ppm. I then put in a 0.5 micron stone with O2 for 60 seconds, and it peaked the meter at 20+ ppm. This setup has the potential to deliver 40 ppm of O2 (Palmer, How to Brew). After an hour (open top, no stopper/airlock), it was still peaked. After 24 hours, it still was peaked. After four days, the jug read 15.8 ppm.
A couple interesting observations here. First, the lower levels of O2 gained with air pump, although I should have let it run 20-30 minutes or so. Still, this is only a gallon of water and it got a head start with the tap water. Second, even with an open top, an oxygenated solution seems to be holding a lot of O2.
So, here are some interesting tidbits to consider:
According to Fix (Principles of Brewing Science) O2 uptake is very rapid by the yeast, usually within a few hours. Palmer states less than an hour. I assume this depends on how active the yeast are when pitching, but if pitching from a fresh starter, it seems reasonable that they’re going to be taking up a lot of this O2 pretty quickly.
Too much O2 will result to overly vigorous fermentation and excessive yeast growth, potentially resulting in off flavors, particularly esters (Lewis/Young, Brewing), (Fix, Analysis of Brewing Techniques).
Generally an Ale requires 8-12 ppm, and lagers 10-15 (Raines-Casselman, Maltose Falcons) Furthermore, greater the viability, less the O2 requirements. The yeast may require as much as half that at 95% viability for 12 Plato beer (Fix, Principles of Brewing Science). Again, it would seem reasonable that a pitch of yeast fresh from a starter should be pretty viable.
So… Some things to think about here. How much O2 is getting into the wort with pure O2, and how cold pitching temps may affect that? How long does that O2 stay in the beer? Does it really diffuse to harmless levels? What happens to yeast with all that available O2? What happens when it’s a big pitch of viable yeast? And if using a stir plate, which should be a well aerated starter resulting in perhaps ample yeast glycogen reserves, what happens then?
I’m planning to do a couple gallon test batches with extract. Both will pitch about about 1.5 million cells/ml/P of yeast, one will have 10 ppm at 50-degrees, the other will be 20+ ppm at 50-degrees.
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I think that regardless of the other variables, you should be shooting for about 10 ppm DO. I would expect that the oxygen is mostly used right away, so pitching temp wouldn't matter so much, assuming you pitch at a temp that your yeast is happy with. I also think that 10 ppm DO assumes a fully viable and vital yeast pitch with the appropriate cell count for your volume and gravity.
If you're using air and not O2, it's a lot harder to oversaturate your wort with oxygen. Air is only about 21% oxygen, so it makes sense you'd be able to get a lot more DO with pure O2.
Your experiments should be interesting. Especially since it is a little harder to accurately measure yeast vitality and viability unless you have an elaborate microbiology lab.
From all that I've read, I think 10 +/- 2 ppm work just fine for all styles, perhaps a little more for high gravity ales. Should be an interesting road ahead.
A look at over oxygenating wort w/ pure O2.
Thanks for your input. If you're up to it, try getting as much DO as possible in a 5 gallon bucket, and check it periodically. See how much you can get in there and how long it takes to get down to 8-12 ppm.