Does oxygen really enter your starter?

[Jaybrinks]

If this has been covered....forgive me....just hit me upside the head with the links.

I have heard the debate of whether to airlock your starter or just loosely cover it. The Theory being that the loose cover, be it a piece of foil, loose lid, or foam stopper, would allow oxygen to enter the liquid....which of course is beneficial.....and the air-lock of course would allow no oxygen in. I question this theory. Here is why.

Co2 is heavier than oxygen, we all know that. During fermentation the yeast activity is creating a positive pressure inside the vessel......so we have a constant flow of Co2 out of the vessel. Knowing this.... how is leaving the cover loose going to allow oxygen in?

 

[passedpawn]

OK, here is me talking about things I barely remember from college chemistry:

Yes, they mix according to the partial pressures of the gasses. CO2 is clearly heavier (atomic mass) than O2, so the concentration will be greater in the flask / jar, but O2 will expand into the jar and dissolve into the starter.

Hopefully someone with a fresh degree will step up and validate the nonsense I just typed.

 

[KevinM]

Are you looking at a larger closed environment and no turblence that could alter the airflow as well?
I thought that had something to do with the activity, say if you had a constant stir bar going on for example, or a breeze from an air duct that would pass over any opening and therefore creating a subtle mix.

Personally, I keep mine airlocked for the most part because I don't want stuff flying in on its own. Already have a fruitfly problem.

 

[theredben]

The loose cover will let oxygen in if the starter is agitated. That is the purpose of stir plates. If you leave it stable, then yes, the heavier co2 will displace most of the air in the vessel.

Moral of the story here is that if you are not going to agitate the starter in some way, might as well just throw an airlock on it.

 

[passedpawn]

Personally, I keep mine airlocked for the most part because I don't want stuff flying in on its own. Already have a fruitfly problem.

Good call. Those fruit flies are one of the biggest dangers to a loose cover over a starter. You should look into those permeable foam stoppers... probably a great solution for you.

 

[JKoravos]

It's called diffusion. If you want a mental workout, look up fick's law of diffusion.

The concentration of oxygen in air is 21%. As long as the concentration of oxygen in the flask is less than 21% and there is no physical barrier (i.e. loose foil) oxygen will diffuse into the flask to try to even out the concentration differential. As long as the yeast is using the oxygen in the headspace, more will continue to diffuse in from the atmosphere.

 

[remilard]

Look at it this way. Co2 is heavier than oxygen, and you are constantly exhaling gas which has much more co2 and much less oxygen than air. If you stay still, how can you continue to breath?

 

[emjay]

Foam stoppers provides the best of all worlds.

 

[samc]

+1 foam stopper, yeast nutrient & stir plate all combined to make some super starters for me. I mostly use dry yeast but when I make liquid starters I have been getting great results compared to the old days of air locks. Foil - no way will I do the foil method on starters, nor loose fitting lids on fermenting beer. All those ridiculous statements always mentioned about things (bacteria) not flying into your beer seem to skip the bug issues. Anyone who has been hit by the fruit flies knows this!

 

[MBasile]

I usually just use an aquarium pump and HEPA filter for the duration of the starter. I wrap foil around the vessel (usually a 64 oz growler) neck and the air tube. Tie twine around the neck and tube to keep critters out.

+1 foam stopper, yeast nutrient & stir plate all combined to make some super starters for me. I mostly use dry yeast but when I make liquid starters I have been getting great results compared to the old days of air locks. Foil - no way will I do the foil method on starters, nor loose fitting lids on fermenting beer. All those ridiculous statements always mentioned about things (bacteria) not flying into your beer seem to skip the bug issues. Anyone who has been hit by the fruit flies knows this!

Yup, damn flies! My dog knows how to open a sliding screen door, but not close it. Hence the twine to tie it off.

 

[Jaybrinks]

It's called diffusion. If you want a mental workout, look up fick's law of diffusion.

The concentration of oxygen in air is 21%. As long as the concentration of oxygen in the flask is less than 21% and there is no physical barrier (i.e. loose foil) oxygen will diffuse into the flask to try to even out the concentration differential. As long as the yeast is using the oxygen in the headspace, more will continue to diffuse in from the atmosphere.

I think this is a excellent explanation...very interesting.....but I question how any gas could enter a positively pressurized container. Are you saying that diffusion is not affected by the outward pressure and movement of the co2?

With a stir plate, the yeast are kept in suspension equaling more activity, which in turn creates more co2. So what I envision is that there is positive pressure in the flask while the yeast are active and creating co2.....constant co2 pressure pushing out of the vessel.....creating a barrier of co2 "wind" if you will.

I just wonder if the benefit of a stir plate is more for simply keeping yeast in suspension, as apposed to introducing oxygen.

I realize I probably should have posted this in the Brew Science section....it is kind of Nerdy.

 

[norsk]

I use a three piece sans the removeable tube and no fluid. So I guess it's a two piece...

 

[lordbeermestrength]

I think this is a excellent explanation...very interesting.....but I question how any gas could enter a positively pressurized container. Are you saying that diffusion is not affected by the outward pressure and movement of the co2?

With a stir plate, the yeast are kept in suspension equaling more activity, which in turn creates more co2. So what I envision is that there is positive pressure in the flask while the yeast are active and creating co2.....constant co2 pressure pushing out of the vessel.....creating a barrier of co2 "wind" if you will.

I just wonder if the benefit of a stir plate is more for simply keeping yeast in suspension, as apposed to introducing oxygen.

I realize I probably should have posted this in the Brew Science section....it is kind of Nerdy.

There is very little positive pressure coming from a starter vessel if it is loosely capped. It wouldn't be enough to stop diffusion from happening if there is any sort of turbulence. A stir plate actually does help introduce oxygen, though two events:

1) it creates a turbulence of air above the liquid that pulls oxygen in more readily,
2) it breaks up the surface of the liquid more, thus allowing oxygen permeation

One more benefit is that it degasses CO2 out of the liquid more than if it were sitting still, also making more room for oxygen permeation into the liquid.

I can't remember where it was that I read about it, but I think Hightest has posted some very interesting material in the past.

 

[SpanishCastleAle]

I just wonder if the benefit of a stir plate is more for simply keeping yeast in suspension, as apposed to introducing oxygen.

I believe this, plus degassing the CO2 in the beer itself, to be the case. I think the amount of air/O2 in the flask during the main fermentation is tiny. I did a little experiment at work using an O2 meter, a large beaker (with a MUCH larger opening at the top than a flask), a stirplate, and Argon (similar weight to CO2) and the amount of O2 near the bottom of the beaker during a simulated fermentation was in the 2-3% range.

I don't know how important O2 is after the yeast has fermented the sugars and is going to sleep. The air would diffuse back into the flask but many people stop the stirplate once it's done fermenting. I can see how the stirring action would constantly add O2 to the wort during the very first, initial stages (prior to the air being pushed out) when yeast need the O2 the most and is taking it in quickly.

 

[mabrungard]

The stir plate improves the transfer of oxygen from the atmosphere over the starter wort. But if there is very little oxygen in that atmosphere, then all the stir plate is doing is keeping the yeast in suspension. I am of the opinion (supported above) that there isn't enough exchange of oxygen from the exterior atmosphere into the atmosphere over the wort to provide much oxygen to the yeast. In addition, the blanketing effect of the CO2 further degrades that possibility.

I personnally pump filtered (0.45 micron) air into the starter vessel with an aquarium pump to keep that atmosphere at the typical 19 to 21 % O2 range which will allow the stir plate agitation to do some good. You don't need to have an airstone on the air supply since the stirring does the work for you. I can tell you that having an airstone only creates a foamy mess.

 

[Catt22]

clipped from wiki:

The partial pressure of an ideal gas in a mixture is equal to the pressure it would exert if it occupied the same volume alone at the same temperature. This is because ideal gas molecules are so far apart that they don't interfere with each other at all. Actual real-world gases come very close to this ideal.

A consequence of this is that the total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of the individual gases in the mixture as stated by Dalton's law

There will be a gas exchange if the flask is vented to the atmosphere and the blanket theory is all wet

 

[SpanishCastleAle]

I'm not saying the CO2 blankets the wort because it's heavier than air, I'm saying it does it because there is a positive pressure (of CO2 gas) inside the flask that evacuates the air.

We had some 'inert gas abatement' studies done at work to insure workers were not entering oxygen deficient atmospheres (due to a mishap in a launch pad elevator that had become O2 deficient and then traveled to another level and somebody then entered the O2 deficient elevator). We now have O2 monitors all over our facility and I can see what happens when, for example, an air-driven pump operates in a smallish room. I can assure you that the CO2 pushes the air out during the main fermentation.

 

[passedpawn]

clipped from wiki:

The partial pressure of an ideal gas in a mixture is equal to the pressure it would exert if it occupied the same volume alone at the same temperature. This is because ideal gas molecules are so far apart that they don't interfere with each other at all. Actual real-world gases come very close to this ideal.

A consequence of this is that the total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of the individual gases in the mixture as stated by Dalton's law

There will be a gas exchange if the flask is vented to the atmosphere and the blanket theory is all wet

Thanks, that is what I wanted to say (at the top) but didn't have the words or confidence.

 

[mabrungard]

clipped from wiki:

The partial pressure of an ideal gas in a mixture is equal to the pressure it would exert if it occupied the same volume alone at the same temperature. This is because ideal gas molecules are so far apart that they don't interfere with each other at all. Actual real-world gases come very close to this ideal.

A consequence of this is that the total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of the individual gases in the mixture as stated by Dalton's law

There will be a gas exchange if the flask is vented to the atmosphere and the blanket theory is all wet

Now that's using a good argument under a condition that doesn't apply!

Since there is a barrier to free transfer of atmospheres between the exterior and interior spaces, there is no opportunity to maintain that free atmospheric ratio of gases in the starter vessel. Recognize that there is a net consumption of oxygen and production of CO2 in the vessel. That excess of CO2 and deficit of O2 would correct itself if the vessel had a large open contact with the free atmosphere, excepting that we long ago figured out 'germ theory' and now know that we need to restrict the contact with the free atmosphere and the hazards it presents to our starter culture. Therefore, we have a highly restricted contact with the free atmosphere.

The 'free-vented' theory is all wet and the blanket theory has been proven by direct measurement. I welcome a scientific experiment that proves that a low oxygen atmosphere does not exist in a loosely covered or foam plugged starter vessel and its wort, but I suggest that the researchers concentrate on turning lead into gold or creating cold fusion. That should be a bigger payoff.

 

[orangehero]

What is then the basis of the yeast propagation calculators showing such improved yeast growth under constant agitation? On Mr. Malty it is indicated to be even higher than continuous aeration.

 

[SpanishCastleAle]

What is then the basis of the yeast propagation calculators showing such improved yeast growth under constant agitation? On Mr. Malty it is indicated to be even higher than continuous aeration.

Less CO2 stress from CO2 being driven off and keeping the yeast in suspension.

 

[kappclark]

So last night I made a 1L starter for my WPL001...100G DME + 1L water ... Did all boiling in a 2L flask, then couldn't get the stupid stirplate to get the stir bar moving right (2L flask too big for the stirplate I guess)

Transferred to sanitized 1L flask (and now stir bar working fine), covered loosly with foil

SOOOO - my question is -- the starter goes up to the neck of the 1L flask .. not much headspace..will lack of headspace in flask affect O2 exchange ?

(checked the flask this AM, and plenty of bubbles at top and magnet still whirring a whirlpool)

I know - RDWHAHB !

 

[JKoravos]

Now that's using a good argument under a condition that doesn't apply!

Since there is a barrier to free transfer of atmospheres between the exterior and interior spaces, there is no opportunity to maintain that free atmospheric ratio of gases in the starter vessel. Recognize that there is a net consumption of oxygen and production of CO2 in the vessel. That excess of CO2 and deficit of O2 would correct itself if the vessel had a large open contact with the free atmosphere, excepting that we long ago figured out 'germ theory' and now know that we need to restrict the contact with the free atmosphere and the hazards it presents to our starter culture. Therefore, we have a highly restricted contact with the free atmosphere.

The 'free-vented' theory is all wet and the blanket theory has been proven by direct measurement. I welcome a scientific experiment that proves that a low oxygen atmosphere does not exist in a loosely covered or foam plugged starter vessel and its wort, but I suggest that the researchers concentrate on turning lead into gold or creating cold fusion. That should be a bigger payoff.

Low, not NO.

After 36 hours on the stirplate my starter wort tastes oxidized as all hell.

Designing an experiment to prove it would be pretty easy, although, kind of a waste of time because the positive effect of using a stirplate is pretty well established already.

 

[SpanishCastleAle]

Low, not NO.

After 36 hours on the stirplate my starter wort tastes oxidized as all hell.

Designing an experiment to prove it would be pretty easy, although, kind of a waste of time because the positive effect of using a stirplate is pretty well established already.

I don't think he's saying a stirplate doesn't have a positive effect. I use a stirplate everytime but I also don't believe much O2 is getting in. My simple experiment mentioned above was enough to make me believe it's mostly just the CO2 being driven off and keeping the yeast in suspension that makes stirplates so beneficial.

One of the stirplate mfrs implies similar:

Q: How big of a vortex will this stir plate create?
A: It depends on the size of the flask, but the pictures you see (including my own!) of huge whirlpools to the bottom of the flask are misleading. All you need is a gentle stirring motion to keep the yeast in suspension and drive-off CO2. A large vortex to the bottom of the flask unnecessarily stresses the yeast, increases the risk that the stir bar will be thrown and doesn’t increase cell count.

You'd think that if O2 getting in was a major factor that a huge vortex would increase cell count because it should draw in more O2.

 

[bumstigedy]

I typically aerate the starter wort with pure oxygen via an air stone for a minute or two before I pitch the yeast. My logic is this will provide plenty of oxygen for the yeast as they get started.

I have read that some yeast strains need a higher concentration of oxygen than what can be obtained from air. So even if the stir plate does oxygenate the wort over to
time it may not provide the ideal amount.

My other question is he amount of time needed to reach the equilibrium oxygen concentration in the starter wort with just a stir plate. Sine you have just boiled you are starting with no oxygen. Will you provide enough by the time the yeast need it?

 

[SpanishCastleAle]

I wouldn't think so bumstigedy. But by the same token, if you supersaturate the wort with pure O2 and then start the stirplate, I would expect the stirplate to actually assist in knocking out the extra O2.