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Old 12-03-2011, 12:18 AM   #1
bessieflames
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Default Add additional oxygen to starter?

Hey all--longtime lurker, first time poster here. I had a question about making a starter on a stir plate. I added pure O2 through a stone and turned the stir plate on. My question is this: is it beneficial to inject pure O2 a second (or third) time? I am assuming that you want oxygen constantly present in the wort so that the yeasties multiply instead of fermenting and that is the purpose of the stir plate. It seems to me that since CO2 is coming out of solution and it is heavier than air, a stir plate won't be able to bring more O2 into solution other than what it starts with. Thanks in advance for the response.

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Old 12-03-2011, 12:21 AM   #2
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i don't think it's necessary. will it hurt? i doubt it. may be more work than it's worth though.

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Old 12-03-2011, 12:21 AM   #3
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You only need oxygen during the early phases of yeast growth and development. Once it's bubbling you don't need it anymore. Oxygen is good, but too much will hurt or kill the yeast. Most stir plates don't require pure oxygen. Save your money.

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Old 12-03-2011, 12:32 AM   #4
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I do a shot of O2 whenever I add wort. Other than that I just let it spin with some foil on top.

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Old 12-03-2011, 04:49 AM   #5
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OK, thanks. It isn't much more effort since I already have the equipment. I had just heard a lot about how a stir plate keeps the wort oxygenated and I didn't see how that was happening. I have been getting good results and high attenuation with the method I am using, so I guess I'll just stick with it. It was more of a question to solidify my understanding than to improve my results. Thanks for all of the responses.

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Old 12-03-2011, 05:33 AM   #6
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Default My experience

I agree... just swish your starter around a bit a couple of times a day, and it will multiply just fine! I did use an airlock on a 5L growler to build my first starter--and never injected oxygen. As long as your fermenter has enough head space, the yeasties will get plenty of O2...

Good luck!

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Old 12-03-2011, 01:06 PM   #7
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Just to add a little bit of background on why the stirring-- in the starter, oxygen diffuses into the wort at the surface of the wort and not much further than that (oxygen is not very soluble in water based liquid). Stirring with a stir plate does 2 things: mix up the wort making the oxygen more evenly distributed and if you stir hard enough to make a vortex, it increases the surface area exposed resulting in more oxygen diffusion

Hope that made sense...

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Old 12-03-2011, 01:37 PM   #8
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I think two points need to be made here before miss information get distributed.

1) While making yeast the cells need O2 to multiply, in normal fermentation (not starters) we add O2 to get the yeast multiplying before they switch from aerobic to anaerobic metabolism making alcohol. Yeast metabolism requires oxygen to survive. When denied O2 it strips oxygen from sugar molecule turning the sugar in to alcohol the anaerobic side of their personality. The easier it is for the yeast the better they grow, and giving them O2 instead of forcing them to find other ways of finding O2 we can just give it to them and make them fat and happy so they will have babies. After all this is the whole point of using foil or foam instead of an air lock.

2) The amount of O2 usually does not require additional O2 being added to the starter but it would not hurt anything. The level of O2 needed to be come toxic is not really possible in a starter (I'm talking about a normal person not someone putting in an O2 turning it on and walking away).

Clem

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Old 12-06-2011, 04:47 PM   #9
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I get that the stirring is supposed to increase the surface area to allow O2 to get into the wort. I also understand why we need O2 in a starter (and at the beginning in a regular fermentation). Maybe I am missing something, but can someone answer the following question:

On my stir plate, after about 6 hours, there is CO2 coming out of the flask. Even with foil on top instead of an airlock, the CO2 has forced the O2 out because the CO2 is heavier. Oxygen can't enter because it can't "sink" past the CO2. Now the interface is between the wort and the CO2. How does the stir plate help now? It seems to me that once fermentation starts, the stirring allows more surface area between the CO2 and the wort and that there is no oxygen in the headspace of the flask anymore.

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Old 12-06-2011, 11:51 PM   #10
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Quote:
Originally Posted by bessieflames View Post
On my stir plate, after about 6 hours, there is CO2 coming out of the flask. Even with foil on top instead of an airlock, the CO2 has forced the O2 out because the CO2 is heavier. Oxygen can't enter because it can't "sink" past the CO2. Now the interface is between the wort and the CO2. How does the stir plate help now? It seems to me that once fermentation starts, the stirring allows more surface area between the CO2 and the wort and that there is no oxygen in the headspace of the flask anymore.
It doesn't really work like that; the whole "magic co2 blanket" mantra that seems to be repeated here is something of a myth.

Take two glasses: fill one with water and one with alcohol. Add some red food coloring to the alcohol and then gently pour the alcohol into the glass of water. The same thing happens with gasses in the air.
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