How to make a yeast starter

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Re: How to make a yeast starter

Postby majorvices » Thu Jul 30, 2009 4:34 am

fossilsrocks wrote:A couple of thoughts: The flask is a sealed system. No mater how much of a swirl you have in the flask, there is no real mechanism there to draw anything into the flask. In fact, the yeast produce CO2 which will actually cause a positive pressure inside. This has been argued in detail before...


I'm still not entirely sure I believe that, especially before fermentation begins - I think you are picking up constant aeration from the vortex before you see any fermentation. Someone with a DO meter needs to prove that one way or another once and for all. I do agree with aerating before hand - however you should be careful aerating a starter with pure O2 as I seem to have overdone it in the past and prohibited fermentation. o2 can be toxic to yeast and in such a small volume I think you can reach toxic levels. That is from my personal experience, I do not use pure O2 in my starters any longer.

As far as using an airlock on a starter - it probably doesn't really hurt anything (which is why I said "in theory" above) - however it is not necessary - I don't use an airlock even on my fermenters until krausen has subsided (in fact there is evidence that even a small amount of back pressure can increase ester production, whether an air lock can create enough back pressure is another story).

BTW: Dr. M. B. Raines tends to agree that a continual stirring on an un-airlocked starter contributes to constant aeration: http://www.maltosefalcons.com/tech/MB_R ... turing.php
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Re: How to make a yeast starter

Postby fossilsrocks » Thu Jul 30, 2009 8:26 am

I agree that before fermentation starts you are keeping the wort aerated with the swirling action. BTW, I don't use O2; I aerate with a pump & stainless "Stone."

I don't think there is anything wrong with using an air lock; I just think foil is easier. the pressure exerted by using an airlock is minuscule, and I don't believe it has any effect on fermentation.

Dr. M. B. Raines is wrong on the continuous aeration point! :shock: :P :lol:

Seriously, think about this: Once fermentation begins, the vessel will quickly be filled with CO2. How can continual aeration be going on under a CO2 blanket?
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Re: How to make a yeast starter

Postby majorvices » Thu Jul 30, 2009 8:58 am

You may be right ... or not. I think that the co2 is constantly mixed with air if the vessel is not sealed. I duno - it's something that we can go back and forth on, and have before. So let's not. :wink:
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Re: How to make a yeast starter

Postby fossilsrocks » Thu Jul 30, 2009 11:54 am

majorvices wrote:You may be right ... or not. I think that the co2 is constantly mixed with air if the vessel is not sealed. I duno - it's something that we can go back and forth on, and have before. So let's not. :wink:

Fair enough! :wink:
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Re: How to make a yeast starter

Postby TG » Fri Aug 06, 2010 3:04 pm

fossilsrocks wrote:I agree that before fermentation starts you are keeping the wort aerated with the swirling action. BTW, I don't use O2; I aerate with a pump & stainless "Stone."

I don't think there is anything wrong with using an air lock; I just think foil is easier. the pressure exerted by using an airlock is minuscule, and I don't believe it has any effect on fermentation.

Dr. M. B. Raines is wrong on the continuous aeration point! :shock: :P :lol:

Seriously, think about this: Once fermentation begins, the vessel will quickly be filled with CO2. How can continual aeration be going on under a CO2 blanket?

Gases mix to fill their available space. That is part of the definition of a gas. And this is the basis for the Law of Partial Pressure.

Air is a mixture of nitrogen, oxygen and CO2 too. The partial pressure of oxygen is 20% of 1 bar (~15 psi), or 3 psi. So there is 3 psi of oxygen trying to get into a flask that is not airtight. An airlock pervents gas diffusion, but not a loose a lid.

Sean Terrill has shown that a starter made with an airlock grows less yeast than the same size starter made with a loose aluminum foil lid. http://seanterrill.com/2010/01/14/aerat ... -starters/

If CO2 were really "heavier than air" my basement would be full of it and I'd suffocate every time I went down there.

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Re: How to make a yeast starter

Postby pclemon » Sat Aug 07, 2010 4:07 am

TG wrote:If CO2 were really "heavier than air" my basement would be full of it and I'd suffocate every time I went down there.

Brew on



I agree that, in general, CO2 will mix with the ambient air pretty quickly but if you fermented in say a chest freezer with the lid shut and went down there after a few days of fermentation and stuck your yead in the freezer for a few minutes, yes, you would suffocate. If you have doubt of this find the episode of mythbusters where they float a tin-foil boat on a tub of gas of sulfur hexafluoride.

Using an airlock on a flask for a starter is only letting gasses flow one way. Keeping the wort moving in the flask (stir-plate) will also keep the air moving above it and will aid in the mixing of the CO2 that's off-gassed with the ambient air. Because it's being mixed it will also allow some O2 back into the flask.
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Re: How to make a yeast starter

Postby fossilsrocks » Sat Aug 07, 2010 6:19 am

TG wrote:
fossilsrocks wrote:I agree that before fermentation starts you are keeping the wort aerated with the swirling action. BTW, I don't use O2; I aerate with a pump & stainless "Stone."

I don't think there is anything wrong with using an air lock; I just think foil is easier. the pressure exerted by using an airlock is minuscule, and I don't believe it has any effect on fermentation.

Dr. M. B. Raines is wrong on the continuous aeration point! :shock: :P :lol:

Seriously, think about this: Once fermentation begins, the vessel will quickly be filled with CO2. How can continual aeration be going on under a CO2 blanket?

Gases mix to fill their available space. That is part of the definition of a gas. And this is the basis for the Law of Partial Pressure.

Air is a mixture of nitrogen, oxygen and CO2 too. The partial pressure of oxygen is 20% of 1 bar (~15 psi), or 3 psi. So there is 3 psi of oxygen trying to get into a flask that is not airtight. An airlock pervents gas diffusion, but not a loose a lid.

Sean Terrill has shown that a starter made with an airlock grows less yeast than the same size starter made with a loose aluminum foil lid. http://seanterrill.com/2010/01/14/aerat ... -starters/

If CO2 were really "heavier than air" my basement would be full of it and I'd suffocate every time I went down there.

Brew on

The foil lid does slow down the oxygen trying to diffuse into the flask. While the starter is fermenting, CO2 is being actively produced. That creates some outflow which further reduces any diffusion that may be occurring. Diffusion isn't an instantaneous process.

Was Sean able to definitively show what caused the increased volume of yeast? Many would say that the reduced pressure of the foil cap is the reason.
Scott

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Re: How to make a yeast starter

Postby fossilsrocks » Sat Aug 07, 2010 6:20 am

You get this week's award for resurecting old threads :D
Scott

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Re: How to make a yeast starter

Postby TG » Sat Aug 07, 2010 10:21 am

Doh! Didn't see the "09".
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