Oxygenating starters?

[mongoose33]

Here's something that is baffling me.

Yeast need oxygen.

One thing advocated for LODO brewing is boiling one's strike water and cooling it down to strike temp, so as to drive off oxygen that could oxidize the mash.

When making a starter typically people boil DME and water to sterilize the starter wort prior to adding the yeast--but that drives off oxygen!

**************

So here's what I do not get: wouldn't we want to oxygenate the starter after it has cooled to pitching temperature so as to add back the oxygen that boiling drove off?

There is a belief in some circles--I've read it many times--that one wants to have a loose foil cap (or foam stopper) on the flask in which the starter is working, so as to presumably allow oxygen to enter. Not sure how this would happen once the yeast start producing CO2, but even so, there's just not a lot of gas exchange there.

Am I missing something here? Tonite for the second time I gave my starter a shot of O2 just prior to adding my yeast. Why wouldn't this be a pretty normal practice? Or have I just missed it? Or missed something else?

 

[Singletrack]

One thing advocated for LODO brewing is boiling one's strike water and cooling it down to strike temp, so as to drive off oxygen that could oxidize the mash.

Yes, since yeast need oxygen, it is good to oxygenate -- or at least aerate -- the starter wort. Stir plates are good for that.

Not sure what LODO strike water has to do with it, but if you want to confirm that boiled wort is low on oxygen, well, confirmed.


I shake the snot out of my starter after it cools, then put it on a stir plate with the loose foil. Oxygenating may be a little better, but I don't do it.

 

[mongoose33]

Yes, since yeast need oxygen, it is good to oxygenate -- or at least aerate -- the starter wort. Stir plates are good for that.

Not sure what LODO strike water has to do with it, but if you want to confirm that boiled wort is low on oxygen, well, confirmed.


I shake the snot out of my starter after it cools, then put it on a stir plate with the loose foil. Oxygenating may be a little better, but I don't do it.

I mentioned the LODO strike water as a way to note that when boiling the starter wort, we're driving off the oxygen. That's the only reason.

We want the starter wort to have oxygen in it, but we've just taken pains to drive the oxygen from the wort, albeit unintentionally.

I've long been a skeptic that there's a great deal of oxygen sneaking past the foil cap to help the yeast and once they start working and producing CO2, it seems unlikely any gets in at all.

 

[Jtk78]

I shake the snot out of my starter after it cools, then put it on a stir plate with the loose foil. Oxygenating may be a little better, but I don't do it.

Exactly as I do as well.

Curious to other reactions in regards to oxygen introduction with the loose foil.

 

[Singletrack]

Oh, the foil. Yeah, hard to believe there is any significant benefit to using foil over an airlock. Once the foil goes on, I think the air in the initial head space of the starter contains about all the oxygen you're going to introduce. That's why I shake before stirring.


Also, my loose foil is pretty tight. Fruit flies.

 

[kh54s10]

For starters you want to increase cell counts. Boiling the DME is to sanitize, not to drive off oxygen. Shaking or a stirplate add oxygen to the starter. LODO mashing is a different story and as far as I know not related at all. You still want to pitch the optimum cell count of yeast.

LODO is something that I have only seen recently and don't know anything about. It is something that I may look into one day, but I still have a lot to work on with traditional brewing. Latest is water adjustment to styles.

 

[mongoose33]

For starters you want to increase cell counts.

Yes, that's why we need oxygen, so the yeast can build cell walls.

Boiling the DME is to sanitize, not to drive off oxygen.

Yes--but the problem I'm seeing is that it also does the latter, which is not a good thing.

Shaking or a stirplate add oxygen to the starter.

It's never been clear to me how using a stir plate can add much other than what might have been in the headspace, and it's not adding all of that.

LODO mashing is a different story and as far as I know not related at all. You still want to pitch the optimum cell count of yeast.

LODO is something that I have only seen recently and don't know anything about. It is something that I may look into one day, but I still have a lot to work on with traditional brewing. Latest is water adjustment to styles.

I wasn't trying to equate LODO to a starter, except to note that in LODO, strike water may be boiled to drive off oxygen; the issue is that when boiling a starter, we do the same thing.

 

[kh54s10]

It's never been clear to me how using a stir plate can add much other than what might have been in the headspace, and it's not adding all of that.

Once you put a starter on a stirplate or start shaking it you are adding oxygen back into the mixture. The same as shaking a fermenter to oxygenate before pitching the yeast. I hope you don't have the starter sealed off. It should have a cover that is not air tight so that oxygen can get in. If you seal it you are doing it wrong.

 

[mongoose33]

Once you put a starter on a stirplate or start shaking it you are adding oxygen back into the mixture. The same as shaking a fermenter to oxygenate before pitching the yeast. I hope you don't have the starter sealed off. It should have a cover that is not air tight so that oxygen can get in. If you seal it you are doing it wrong.

I don't seal it tight, but I'll be darned if I can see how you can get much if any gas exchange there. In essence, if you're shaking, you're hoping that whatever O2 is in the headspace of the flask is enough for the yeast. Is it? How do we know that?

It is pretty much established that oxygenating the wort in a fermenter is better done via an oxygen tank and a wand. So why is it ok to do it via shaking for a starter and not an O2 wand, but it's recommended to use the wand and pure O2 if it's the fermenter?

See my confusion here?

And as far as O2 exchange through the foil or whatever, how is it doing that? Yeast offgas CO2 which, through positive pressure, should prevent O2 from entering the flask.

I'm still back to the first question: why don't we see people suggesting we oxygenate starters using pure O2?

 

[Singletrack]

It is pretty much established that oxygenating the wort in a fermenter is better done via an oxygen tank and a wand. So why is it ok to do it via shaking for a starter and not an O2 wand, but it's recommended to use the wand and pure O2 if it's the fermenter?/QUOTE]

I use pure O2 for the fermenter wort because it is typically much higher gravity than my starter, so I think there is more benefit.

As for the starter, we're really not looking for air exchange after the yeast start spewing copious amounts of CO2. That air exchange just is not going to happen. We want to dissolve oxygen in the wort before the yeast produce a lot of CO2. Oxygenation is great for that. Stir plates do okay in my experience.

 

[triethylborane]

I don't seal it tight, but I'll be darned if I can see how you can get much if any gas exchange there. In essence, if you're shaking, you're hoping that whatever O2 is in the headspace of the flask is enough for the yeast. Is it? How do we know that?

CO2, O2 and the other molecules that are in the atmosphere are small, an understatement. They can find their way through the nooks and crannies of foil placed over the opening and around the neck of an erlenmeyer flask. Even the most stoutly and perfectly emplaced foil has little nooks and crannies that these small molecules would find to be vast expanses of space to effortlessly move. Bacteria and other infection baddies are orders of magnitude larger and even though they may fit, we can look at Pasteur's experiments to find our concerns can be assuaged.

It is pretty much established that oxygenating the wort in a fermenter is better done via an oxygen tank and a wand. So why is it ok to do it via shaking for a starter and not an O2 wand, but it's recommended to use the wand and pure O2 if it's the fermenter?

See my confusion here?

Shaking the starter would need to be constant. Stir plates do a better job and the undergrads and master students can be freed up from such plebeian duties as shaking.

And as far as O2 exchange through the foil or whatever, how is it doing that? Yeast offgas CO2 which, through positive pressure, should prevent O2 from entering the flask.

I'm still back to the first question: why don't we see people suggesting we oxygenate starters using pure O2?

Unless you have somehow made a hermetic seal with the foil (not happening) or you are making starters at the summit of K2 where O2 is in short supply (watch out for the descent, its a killer) or some other completely unrealistic scenario, a stir plate will provide conditions where plenty of O2 reaches the starter. The movement of the liquid thanks to the motion of the stir bar, the fact the molecules are bouncing around, endlessly pursuing equilibrium exponential distribution.

Here is a paper that is not related to brewing, but can help with the questions:



THE LEGEND OF CARBON DIOXIDE HEAVINESS

GIOVANNI BADINO

Dip. Fisica Generale, Universita` di Torino, Via Giuria, 1, Torino, Italy, [email protected]
Abstract: The false legend of carbon dioxide traps resulting from the weight of carbon dioxide gas is disproved. In spite of water-vapor lightness in comparison with air, no water-vapor trap exists on cave ceilings. In fact, underground atmospheres with specific compositions are not related to gravity, but to the absence of any air movement around the gas sources. The process of double diffusion of oxygen and carbon dioxide during organic compound decomposition in still air is shown to be significant. This phenomenon can form atmospheres that are deadly due to oxygen deficiencies and poisonous because of excess carbon dioxide. Carbon dioxide storage behaves like a liquid and can flow or can be poured, as cold air can, but these are typical transient processes with no relation to a cave’s foul air formation.

https://www.researchgate.net/profil...xide_heaviness/links/00b49523a9aa33101e000000

 

[Totally]

I only use a loose foil on my starters. I do not use oxygen bottles yet.

I feel I got a boost in fermentation start time once I started shaking (or just pouring from high) my wort in the fermenter.

I also believe I got another boost in start time once I started shaking my starters into a bubbling froth before pouring them in.

My last batch was bubbling like mad without about 2-3 hours.

Totally non-scientific, but works for me and I will continue to do it for now.

I would advocate a little blast of O2 into a starter for sure.

 

[triethylborane]

Oh, the foil. Yeah, hard to believe there is any significant benefit to using foil over an airlock. Once the foil goes on, I think the air in the initial head space of the starter contains about all the oxygen you're going to introduce. That's why I shake before stirring.


Also, my loose foil is pretty tight. Fruit flies.

What happens when you crack open a cold one (with the boys), don't drink it, but place a piece of foil over the opening just as you would with an erlenmeyer flask containing a starter, and place it on a table, allowing it to remain in place for 24 hours at room temps and prevailing atmospheric pressures?

Or simply when you forget to properly torque down the cap on a bottle of carbonated beverage?

 

[kh54s10]

Oxygen from a tank, stirplate or intermittent shaking will all get oxygen into the starter wort. The difference is the amount. Oxygen from the tank will get you the most, stirplate next and intermittent shaking the least. Yeast calculators will give you different values for intermittent or stirplate. I believe they don't have tank values because so few people use that method.

 

[Singletrack]

What happens when you crack open a cold one (with the boys), don't drink it, but place a piece of foil over the opening just as you would with an erlenmeyer flask containing a starter, and place it on a table, allowing it to remain in place for 24 hours at room temps and prevailing atmospheric pressures?

Crack open a beer without drinking it? Now you've stretched the imagination too far!

As a thought experiment, if I concentrate hard, I suppose the beer would go flat. But that's because pressurized CO2 is escaping. That's a far cry from a starter exchanging air through my foil cap prior to fermentation. Once the starter is fermenting, yep, CO2 escapes out those small crevices in the foil cap. But that doesn't help aerate my wort.

Those small air molecules have no trouble getting around the foil -- if there is something driving them into my starter. But my starter has no such driving force, and without it, I am skeptical that there is any significant air exchange.

A better experiment would be to fill a flask with cool smoke and cover the top with foil. Observe how long it takes for the smoke to clear. I'm guessing it takes longer than it takes my starter to start pushing out CO2. That means we don't get a lot of new air that would help with aeration during the lag period.

 

[triethylborane]

Crack open a beer without drinking it? Now you've stretched the imagination too far!

As a thought experiment, if I concentrate hard, I suppose the beer would go flat. But that's because pressurized CO2 is escaping. That's a far cry from a starter exchanging air through my foil cap prior to fermentation. Once the starter is fermenting, yep, CO2 escapes out those small crevices in the foil cap. But that doesn't help aerate my wort.

It seems like you are thinking about CO2 as a liquid, rather than a gas. The CO2 gas molecules are not densely concentrated on the order of a liquid, whereby O2 molecules cannot pass the CO2 molecules . . . at atmospheric pressures encountered with starters this does not occur. Ive explained it in a previous post with @mongoose33, but there is O2 diffusing in when you have a stir plate (even without, for obvious reasons, i.e. no vacuum).

Those small air molecules have no trouble getting around the foil -- if there is something driving them into my starter. But my starter has no such driving force, and without it, I am skeptical that there is any significant air exchange.

Gases don't need anything to drive them, they have energy. PV=nRT, R is the amount of work per degree C per mole. Plenty of O2 molecules bounce into the crevices of the foil and into the starter, just as plenty of CO2 molecules pass by on the way out. These molecules are so small the crevices would be expansive.

A better experiment would be to fill a flask with cool smoke and cover the top with foil. Observe how long it takes for the smoke to clear. I'm guessing it takes longer than it takes my starter to start pushing out CO2. That means we don't get a lot of new air that would help with aeration during the lag period.

You could do this with dry ice in a bit of water. Again, molecular physics didn't break down with home brewers yeast starters, O2/atmosphere will diffuse in. CO2 out, no vacuum, atmosphere in.

 

[Singletrack]

You've seen fermentation. Lots of CO2 pushing out through very tight crevices in the foil provides very little opportunity for air to get in. Gases tend to move from zones of higher pressure to zones of lower pressure, so from inside the flask to outside the flask.

Yes, there will be a bit of air exchange before fermentation takes off. I'm considering that exchange insignificant until proven otherwise.

 

[mongoose33]

You've seen fermentation. Lots of CO2 pushing out through very tight crevices in the foil provides very little opportunity for air to get in. Gases tend to move from zones of higher pressure to zones of lower pressure, so from inside the flask to outside the flask.

Yes, there will be a bit of air exchange before fermentation takes off. I'm considering that exchange insignificant until proven otherwise.

This has been part of what I'm not reconciling--you said it better than I did.

That starter I did Friday night, which I oxygenated w/ an O2 wand, added a pinch of yeast nutrient, pitched without crashing or decanting yesterday afternoon, is going to town like nobody's business. I've never seen so much activity.

Here's a youtube video showing that action (very short video, circa 4 seconds):

[ame]https://www.youtube.com/watch?v=ox5gL73DKIw&spfreload=10[/ame]

 

[triethylborane]

Lots of CO2 pushing out through very tight crevices in the foil provides very little opportunity for air to get in.

Negative. The pressure exerted by CO2 on a foil topped erlenmeyer flask as result of yeast metabolism is not significantly higher (or lower) than the ambient atmospheric pressure of the gasses exerting pressure on the flask. In our scenario, CO2 is not blocking O2 or other atmospheric gases from entering (these are gases not liquids). CO2 at the density we are working with doesn't produce a shield of gas that prevents O2 from entering.

The crevices in the foil are vast expanses of space for molecules that are only a few hundred picometers in length. Even if a crevice in the foil were 1 micrometer, this is six orders of magnitude larger than the molecules. It would be comparable to a human walking through a tunnel that is about the diameter of the moon (apologies if this is off, but it is within an order of magnitude, Im doing this off the top of my head on my mobile).

If you plug the top of the flask with an air lock, that is a different scenario (and a stir plate would be a total waste of time). And if you are able to get a hermetic seal with just a square of foil, thats really something vastly unusual. Im trying to keep this as generalized as possible, though the pressure concepts are honestly an advanced HS chem class and the spatial/magnitude concepts are a combination of analytical/physical chem and molecular physics.

 

[mongoose33]

Negative. The pressure exerted by CO2 on a foil topped erlenmeyer flask as result of yeast metabolism is not significantly higher (or lower) than the ambient atmospheric pressure of the gasses exerting pressure on the flask. In our scenario, CO2 is not blocking O2 or other atmospheric gases from entering (these are gases not liquids). CO2 at the density we are working with doesn't produce a shield of gas that prevents O2 from entering.

The crevices in the foil are vast expanses of space for molecules that are only a few hundred picometers in length. Even if a crevice in the foil were 1 micrometer, this is six orders of magnitude larger than the molecules. It would be comparable to a human walking through a tunnel that is about the diameter of the moon (apologies if this is off, but it is within an order of magnitude, Im doing this off the top of my head on my mobile).

If you plug the top of the flask with an air lock, that is a different scenario (and a stir plate would be a total waste of time). And if you are able to get a hermetic seal with just a square of foil, thats really something vastly unusual. Im trying to keep this as generalized as possible, though the pressure concepts are honestly an advanced HS chem class and the spatial/magnitude concepts are a combination of analytical/physical chem and molecular physics.

I don't claim to know everything, or even half the things, but I have difficulty with this, for this reason:

[ame]https://www.youtube.com/watch?v=ox5gL73DKIw&spfreload=10[/ame]

That's the airlock on my fermenter which is going right now. Pitched yesterday afternoon. I had airlock activity within 4 hours (it was a helluva starter!), and it is producing just a ton of CO2

I know that initially there will be some diffusion within the flask, but it's difficult for me to believe there's much. Why? Because what's normally in the flask is....air! So whatever diffuses in will also diffuse out.

Then, as we start to see an increase in CO2 generation by the yeast, there will be a positive pressure inside the flask. It's not like a wind blowing but what you're arguing (I think--if I'm misconstruing it correct it, please) is that those incoming atoms are swimming upstream.

I could even believe a few could do that, or a few million, but against the tide? It would be like a hydrogen-filled ping pong ball which normally might rise, being pelted by lots of normal ping pong balls raining down on it. It might eventually rise but it's going to have to work at it.

One last element of this suggests to me that O2 aeration is going to be superior to whatever else we might do with shaking, stir-bar, etc., and that is that the supposed optimal amount of O2 in the wort is at least 10ppm--and Chris White in his book says even with a sintered stone running continuously off an aquarium pump, you can only reach 8 ppm. And most people aren't doing that.

I'm not trying to be intentionally difficult here, it just seems like it would be far superior to O2 aerate a starter. I'm trying to understand this better, not just the what but the why.

 

[Singletrack]

Negative.

Holy go jumpin off the bridge. :smack:

And get off my lawn.

 

[Minky]

So here's what I do not get: wouldn't we want to oxygenate the starter after it has cooled to pitching temperature so as to add back the oxygen that boiling drove off?

If I understand correctly, in theory, the starter is the only place that oxygen is needed. The oxygen is necessary to build strong cell walls in the reproductive phase of the yeast.

Therefore, if the ideal number of cells for a given batch of beer can be grown in the starter, there would be no need for oxygen in the wort, as fermentation is anaerobic.

I would think that the ideal PPM of O2 would be easier to achieve in a starter than in a much greater volume of wort. Pure O2 seems risky due to the fact that yeast can be killed with too much, but maybe a constant bubbling of filtered air through an aquarium pump would do.

For those advocates of the low oxygen approach, I would think that the residual oxygen in the starter could be driven off somehow before pitching.

 

[mongoose33]

If I understand correctly, in theory, the starter is the only place that oxygen is needed. The oxygen is necessary to build strong cell walls in the reproductive phase of the yeast.

Therefore, if the ideal number of cells for a given batch of beer can be grown in the starter, there would be no need for oxygen in the wort, as fermentation is anaerobic.

The yeast produce many more cells after pitching into the wort. First they go through a lag phase where they are bringing in nutrients and oxygen from the wort, then they go into an exponential growth phase where they're producing a lot more cells.

I would think that the ideal PPM of O2 would be easier to achieve in a starter than in a much greater volume of wort. Pure O2 seems risky due to the fact that yeast can be killed with too much, but maybe a constant bubbling of filtered air through an aquarium pump would do.

I'd guess the opposite--the smaller the amount of wort, the faster you should be able to oxygenate it, and controlling that is difficult.

Normally I oxygenate the fermenter full of wort for about 60 seconds, maybe a bit more. I did the starters for about 20 seconds; one side benefit of that is I saturated the gas above the starter wort with oxygen, added the foil cap, and then when it went on the stir plate there was more oxygen in the gas above than would normally be the case.

For those advocates of the low oxygen approach, I would think that the residual oxygen in the starter could be driven off somehow before pitching.

No, you don't want to do that. You *need* oxygen in your wort to allow the yeast to build cell walls. The LODO approach is primarily on the hot side, i.e., crushing, mashing, and so on. Once the wort is boiled oxygen needs to be added back for the yeast to function well.

BUT--once you get into the fermentation, adding more oxygen isn't going to be helpful and may hurt (exception: high gravity wort may benefit from another shot of oxygen at 12-18 hours). And of course, once fermentation is over, oxygen is not good--oxidation is the result, and we want to avoid that as much as possible.

 

[mongoose33]

I went into Chris White's "Yeast" book to look up something in response to a comment here, and I kind of found some of the answer to my questions. Here's what he says in relation to starters (p 134):

"If you have pure oxygen handy, you can add a dose of oxygen to your starter at the beginning. You will get far healthier yeast and far more yeast growth if you provide a small, continuous source of oxygen throughout the process. Oxygen is critical to yeast growth, and not providing any oxygen to the yeast can have a long-term negative impact on yeast health. Yeast use oxygen to synthesize unsaturated fatty acids and sterols, which are critical to creating a healthy cell membrane and good cell growth. With oxygen present, yeast grow rapidly With no oxygen, yeast grow far more slowly and reach a lower total mass of cells."


He goes on to note that the best way to agitate a starter is a stir plate, and he says there's oxygen pickup and gas exchange using the stir plate.

So I suppose there must be gas exchange going on though it still seems like there wouldn't be much. Since I'm oxygenating the starter wort I suspect I'm getting a head start on it.

 

[RM-MN]

if we want oxygen to be added during the stirring of the starter, why do we encourage people to use erlenmeyer flasks instead of a beaker? The restriction in the neck of the erlenmeyer would limit the entry of oxygen as the yeast produce the excess CO2 that must escape through that restricted area. When you further restrict that by putting a cotton swab or worse, a foil cap, how does any oxygen get in?

 

[Singletrack]

He goes on to note that the best way to agitate a starter is a stir plate, and he says there's oxygen pickup and gas exchange using the stir plate.

So I suppose there must be gas exchange going on though it still seems like there wouldn't be much. Since I'm oxygenating the starter wort I suspect I'm getting a head start on it.

Is he referring to gas exchange between head space air and the wort? That makes sense to me.

 

[triethylborane]

if we want oxygen to be added during the stirring of the starter, why do we encourage people to use erlenmeyer flasks instead of a beaker? The restriction in the neck of the erlenmeyer would limit the entry of oxygen as the yeast produce the excess CO2 that must escape through that restricted area. When you further restrict that by putting a cotton swab or worse, a foil cap, how does any oxygen get in?

You could use a beaker, but an erlenmeyer flask is more appropriate because it allows you to "shake" the contents without spilling.

Those small air molecules have no trouble getting around the foil -- if there is something driving them into my starter.

Rather than regurgitate everything over again, this is close. Gas molecules have energy, they don't need anything driving them in (wind, fan, etc). They bounce around at random and getting around the foil is not a problem, nor the CO2 produced by the yeast given our conditions.

 

[TeamAshBrewing]

I'm no scientist, but here is how I understand it.
When using a stir plate, air (oxygen and whatever other molecules) is being pulled into starter. Yeast produce CO2 which has to escape some how, so it must be flowing thru the same holes as the air coming in.
If you oxygenate with a tank you would be giving the yeast a jump start of oxygen.

 

[dyqik]

You could use a beaker, but an erlenmeyer flask is more appropriate because it allows you to "shake" the contents without spilling.

I do use a 3l beaker. Since I'm using a stir-plate, I don't need to shake the starter anyway. And using a beaker means I can fit a mini-immersion cooler coil into the beaker to cool the starter down after boiling, which means it takes less than half an hour from start to pitching in my starter.

I guess the foil I've been using should be replaced by cheesecloth or similar though - it'll keep fruit flies out, and allow O2/CO2 exchange.

 

[Minky]

Hi mongoose33,
I think you may have misunderstood the intent of my post. My point about oxygen was that it is only needed for reproduction. Since the purpose of a starter is reproduction, and the purpose of oxygen is for healthy cell walls, IN THEORY, the starter would be the place you would absolutely want the oxygen. IN PRACTICE, you would still probably want to oxygenate the wort, but would need much less. The idea is to build up an adequate supply of healthy yeast before pitching so there isn't as much need for reproduction in the wort.

When you state that "the smaller the amount of wort, the faster you should be able to oxygenate it", we were actually saying the same thing. Another reason that it's easier to oxygenate a starter is that it generally has a lower gravity, which helps with oxygen solubility.

There's a good discussion of this stuff at the following link. The thread gets a little derailed in the middle, but Jamil Z chimes in with:

"The way that works for me is to provide continuous aeration for the starter via either filtered air and/or a stir plate. This is pretty much error free, since you can't add too much O2 this way. If you use pure O2, you need to be a bit more careful on your starter, you can hit it a few times with O2, but I wouldn't make it continuous. You're going to have to experiment to find a rate that works best for your yeast and your style of brewing."

http://thebrewingnetwork.com/forum/viewtopic.php?f=2&t=4000

 

[ibrubeer]

A couple of things come to mind for me personally regarding this conversation. I also use a stir plate and a 2L flask for the starter. Before I add the yeast to the starter, I shake the crap out of it in the flask with a stopper on the top. When you unplug the stopper, you can tell that a small vacuum formed because you can hear it and see it on the foam layer being pulled back down. I take this to demonstrate that some of the air dissolved into the liquid. When I unplugged the stopper, the gas pressures equalized again by letting more air in. The same thing happens with a starter on a stir plate I believe, but much more slowly. O2 is being dissolved into solution due to the stirring action (the whirlpool effect). This would pull more air into the headspace, right? Here are a couple of great links to (better) illustrate this, and also provide more information:

http://www.stirstarters.com/purpose.html

https://www.maltosefalcons.com/tech/yeast-propagation-and-maintenance-principles-and-practices

 

[Hwk-I-St8]

Wow, a lot of discussion about a process that simply works.

First, to the person who said oxygenating a starter means little yeast population growth is needed in the actual wort....I think that's way off...especially if your wort is anything but super low gravity.

Second, the starter growth process is pretty well documented and millions of gallons of beer have been brewed following that process. It works, why try to tear it down?

Shrug - I shake the crap outta my starter (which is relatively low gravity compared to the beers I brew), then put it on a stir plate. My beers start quickly, ferment amazingly fast (usually at final gravity in about 3-5 days) and taste great.

If it ain't broke, don't fix it.

 

[triethylborane]

Second, the starter growth process is pretty well documented and millions of gallons of beer have been brewed following that process. It works, why try to tear it down?

I think some have a desire to make home brewing more complex and/or involved.

 

[mongoose33]

Well, I'm going to come back to my original question/premise: we boil the wort for the starter which presumably drives O2 out of solution, we pitch the yeast, and hope it all works.

It obviously works, but might it be done better? It just struck me as odd that we would do something guaranteed to make it harder for the yeast to do their thing--so why wouldn't we oxygenate a starter like we oxygenate a fermenter of wort?

That was my question. When I see things that appear to be inconsistent, I want to resolve them. That's either a character flaw or an admirable trait, you decide.

I'd never shaken a starter and that sounds like a good way to overcome the lack of O2 that should characterize a boiled starter. I have brewed a lot of beer with starters in the last 2 years and while they all worked, I wondered if I could improve the process. Since yeast health and vitality are held up as things that influence beer, I wondered why we wouldn't see something like oxygenating starters as a best practice.

 

[dyqik]

Well, I'm going to come back to my original question/premise: we boil the wort for the starter which presumably drives O2 out of solution, we pitch the yeast, and hope it all works.

It obviously works, but might it be done better? It just struck me as odd that we would do something guaranteed to make it harder for the yeast to do their thing--so why wouldn't we oxygenate a starter like we oxygenate a fermenter of wort?

That was my question. When I see things that appear to be inconsistent, I want to resolve them. That's either a character flaw or an admirable trait, you decide.

I'd never shaken a starter and that sounds like a good way to overcome the lack of O2 that should characterize a boiled starter. I have brewed a lot of beer with starters in the last 2 years and while they all worked, I wondered if I could improve the process. Since yeast health and vitality are held up as things that influence beer, I wondered why we wouldn't see something like oxygenating starters as a best practice.

Yeast don't just need O2 at the start of fermentation, but throughout the exponential growth phase, which we want to extend as long as possible in a starter, for maximum yeast multiplication. That's why a stir plate or very frequent shaking works so much better than a starter aerated only at the beginning, with much higher yeast growth factors. What doesn't work well at all is just boiling the starter, cooling then pitching yeast with no aeration/oxygenation at all.

The aim with a starter is different to our aims during fermentation of the actual beer, where we want yeast to express the appropriate flavors in the beer, and efficiently turn sugar into ethanol and CO2. There we want the yeast to go into anaerobic respiration after the initial growth phase (which is the process that turns sugars into alcohol), and we want to avoid oxidation, so we don't want to add oxygen after the growth phase (in very high gravity beers, we might extend the growth phase by adding oxygen a second time 12-18 hours after pitching). In a starter, we don't care about oxidation or alcohol production, and we want the maximum possible yeast multiplication. That's best done by keeping the yeast in aerobic respiration throughout the process with continuous addition of oxygen, which also allows for more cell division, and which can be achieved with continuous aeration alone.

You could oxygenate at the start to a higher level than can be achieved with shaking only at the start, but with frequent shaking or a stir plate, you don't need to - oxygen will be drawn in throughout the process, so you don't need a high initial dose of oxygen. But just oxygenating at the start without further additions of oxygen probably isn't sufficient. So the KISS option is to just stir/shake throughout the starter fermentation.

Stirring/shaking throughout the starter fermentation also degases the starter, which may reduce stress on the yeast by preventing CO2 build-up in solution, although the typical starter gravity is probably too low for that to be a major factor.

 

[hocktim]

It seems like you are thinking about CO2 as a liquid, rather than a gas. The CO2 gas molecules are not densely concentrated on the order of a liquid, whereby O2 molecules cannot pass the CO2 molecules . . . at atmospheric pressures encountered with starters this does not occur. Ive explained it in a previous post with @mongoose33, but there is O2 diffusing in when you have a stir plate (even without, for obvious reasons, i.e. no vacuum).







Gases don't need anything to drive them, they have energy. PV=nRT, R is the amount of work per degree C per mole. Plenty of O2 molecules bounce into the crevices of the foil and into the starter, just as plenty of CO2 molecules pass by on the way out. These molecules are so small the crevices would be expansive.



You could do this with dry ice in a bit of water. Again, molecular physics didn't break down with home brewers yeast starters, O2/atmosphere will diffuse in. CO2 out, no vacuum, atmosphere in.

If you think about it in terms of pressure and path of least resistance, when you put and airlock on your primary it is creating continual pressure pushing co2 out which would also be the case in a starter. If a container is continually building positive pressure and releasing it would be very difficult for air to enter that space that is continually in a state of positive pressure. Even more so if the entrance and exit are reduced volumes .

 

[triethylborane]

If you think about it in terms of pressure and path of least resistance, when you put and airlock on your primary it is creating continual pressure pushing co2 out which would also be the case in a starter. If a container is continually building positive pressure and releasing it would be very difficult for air to enter that space that is continually in a state of positive pressure. Even more so if the entrance and exit are reduced volumes .

An air lock fitted to a yeast starter on a stir plate? Nope.

I think about "it" in terms of thermodynamics and statistical mechanics. Ive written posts ad nauseam on the thermodynamics of the gases involved on yeast starters. Regardless of the CO2 that is being generated by yeast metabolism, O2 will enter through the relatively massive crevices in a foil covering, sometimes "bouncing" off a CO2 molecule that is exiting the flask, but be guaranteed . . . O2 is entering the flask.

 

[ESBrewer]

Yes, I think generally more oxygen (and less restricted flow of gases) will be beneficial in a starter. For practical reasons (ease and sanitation), foil topped and frequently or constantly agitated starters will be good enough. Some oxygen will enter but this is not the optimal amount. You could inject more oxygen with expensive instruments and get more growth per volume but on the other hand you can simply use more cells and volume and you will end up with practically similar results (in terms of cell count and beer quality) . Have a look at this link

http://braukaiser.com/blog/blog/2013/03/19/access-to-air-and-its-effect-on-yeast-growth-in-starters/

Boiling the starter wort is something that we have to do, because of sanitation. Oxygen IS diminished, but all the oxygen will come back when we cool it down and agitate it for a while. In The presence of air. Even better If we can inject pure oxygen to get a little bit higher o2 concentration. Btw, there is also a certain limit with oxygen concentration. Going too high will be poisonous for the yeast. But this level is usually not reached using home brewing equipment.

When using the standard foil and agitation it is a good idea to use a large head space in the container (some common lab knowledge). When using stir plate we are looking for a good stir, not just moving things round a little. In labs erlenmayer shakers are often used for best growth and those will really spread the small amount of media along the flask walls by shaking hard. This increases the surface area of the wort/media and allows for rapid exchange of gases (CO2 and o2) between the large head space (initially filled with air) and the wort. This kind of circular motion also puts a lot of motion in the gas inside the container and maybe even accelerates gas exchange through the loose foil. Unfortunately, shakers tend to be larger and noisier than the stirrers.

 

[Dcpcooks]

Adding o2 to your starter is good practice. Yeast need 02 to synthesize fatty acids for cell wall stability and healthy growth. O2 insures healthy yeast and optimized growth.

To be perfect you’d want to continually add small amounts of o2 during the entire starter until growth has stopped. That not very practical on a homebrew scale. A stir plate is the best way to drive off co2 and keep the yeast in suspension. Just add a bit of 02 at the beginning and then use a stir plate through completion.

The reason you’d use foil or a foam stopper is it will allow co2 to escape but still allow air to flow back into the starter bringing 02 with it. That’s why you never want to use an airlock on your starters. Wild yeast can’t crawl so a loose fitting cover is all you need to prevent something falling in to spoil your starter.

With that said a stir plate will proved enough o2/co2 exchange and it will limit the risk of cross contamination from the air stone. I always oxygenate prior to pitching yeast in a starter or a beer to limit the risk of cross contamination. I do use a 5 liter flask for my starters and I usually only make 3 liter starters. That combo leaves me with plenty of surface area for gas exchange.

 

[hopScottch]

Yeast don't just need O2 at the start of fermentation, but throughout the exponential growth phase, which we want to extend as long as possible in a starter, for maximum yeast multiplication. That's why a stir plate or very frequent shaking works so much better than a starter aerated only at the beginning, with much higher yeast growth factors. What doesn't work well at all is just boiling the starter, cooling then pitching yeast with no aeration/oxygenation at all.

The aim with a starter is different to our aims during fermentation of the actual beer, where we want yeast to express the appropriate flavors in the beer, and efficiently turn sugar into ethanol and CO2. There we want the yeast to go into anaerobic respiration after the initial growth phase (which is the process that turns sugars into alcohol), and we want to avoid oxidation, so we don't want to add oxygen after the growth phase (in very high gravity beers, we might extend the growth phase by adding oxygen a second time 12-18 hours after pitching). In a starter, we don't care about oxidation or alcohol production, and we want the maximum possible yeast multiplication. That's best done by keeping the yeast in aerobic respiration throughout the process with continuous addition of oxygen, which also allows for more cell division, and which can be achieved with continuous aeration alone.

You could oxygenate at the start to a higher level than can be achieved with shaking only at the start, but with frequent shaking or a stir plate, you don't need to - oxygen will be drawn in throughout the process, so you don't need a high initial dose of oxygen. But just oxygenating at the start without further additions of oxygen probably isn't sufficient. So the KISS option is to just stir/shake throughout the starter fermentation.

Stirring/shaking throughout the starter fermentation also degases the starter, which may reduce stress on the yeast by preventing CO2 build-up in solution, although the typical starter gravity is probably too low for that to be a major factor.

Nuf said dyqik. Although I do enjoy reading all the discussions pontificating the science of brewing, isn't this all we need to consider. Or perhaps, as a novice homebrewer, I have yet to attain the level of sophistication where I see the benefit.

Dyqik's post stops short of answering a lingering question that I have yet to research. I would think that yeast from a 48 hr starter are viable (not necessarily exponentially growing). Perhaps they have exhausted the wort and cell division rate has slowed, but are nonetheless primed and "hungry" to metabolize more sugars. As long as the number of yeast is sufficient for the volume and OG of your beer, why do you need to oxygenated your beer if pitching from a starter?