Yeast starter/stir plate theory confusion

Ok, stupid question because I am sure the answer will come in the first post....

I have been reading about yeast starters and the use of stir plates. The stir plate is used to keep the wort aerated to enhance the growth and multiplication of the yeast cells. Heres my problem:

1)CO2 is a byproduct of yeast metabolism, in both aerobic and anaerobic metabolism
2)CO2 is heavier than air
3) Stirring the yeast in a narrow necked Erlenmeyer flask would just be stirring it under a blanket of CO2... Where is the aeration?

 

CO2 is heavier than air, but that doesn't mean much if it's not sitting still. Obviously since there's plenty of CO2 in the air we breathe, it CAN mix with air, it just doesn't happen instantaneously. This is why open fermentation can be okay, but becomes a problem once primary fermentation subsides - after CO2 stops being generated faster than it diffuses from the 'blanket' into the air, it's no longer a blanket any longer. My guess would be that the agitation due to the movement of the liquid surface would be enough to speed this process.

 

I think that the stir plates are usually used in conjunction with breathable stoppers. The vortexing pulls air into the flask and into solution, actively displacing the CO2.

 

Right, I get what you both are saying, and I understand the whole, "well it works doesn't it?" concept. I am just having trouble visualizing the fact that a CO2 blanket, even over a vortexing mass of liquid (which in itself will be vortexing due to the friction between liquid and gas) will up itself out of a narrow necked Erlenmeyer flask. I could see it in a wide mouthed jar, but the narrow aperture of the flask would act to retain the gas inside the flask, minimizing mixture with outside air.

 

In a starter, I don't think the yeast will go through more oxygen than will have been dissolved in the water beforehand. I could be completely wrong.

 

I don't exactly have any real scientific evidence to back me up, but clearly your intuitive 'feel' for the predicted behavior of the gases is different from mine - can't really say who's wrong or right without knowing more I guess. My thinking is that CO2 is not THAT different in density from air, so I doubt that it will take very much of a disturbance to make it mix rapidly enough that a moderate amount of air remains in the flask while stirring.

Perhaps a logical test would be to take a thin wooden strip (like a coffee stirrer), light it on fire, and plunge it down into the headspace in the flask while the starter is being stirred and is actively fermenting away, and see if it goes out instantly like it's in pure CO2, or if it remains burning (even for just a few seconds)

It would be interesting to find out for sure, because lately I have been more lazy and haven't been using my O2 stone to oxygenate my starters, just relying on the stir plate to do the job. However, I suppose that during the period before fermentation picks up, it'd have enough time to get air in there anyway...

 

Wouldn't regular osmosis take care of that? I mean, like, the concentration of O2 in the flask would be going down, but it'd still be a mix...

 

The stir plate is not used to keep the starter wort aerated. It is used to keep the yeast in suspension, and therefore in better contact with the fermentables, resulting in a faster and more complete fermentation.
You do not want your stir plate to spin so rapidly that it aerates the wort, any more than you would want your beer to be aerated once fermentation has begun. Just keep the yeasties in suspension so they can do their job quickly and completely.

 

^ Bingo
|

 

Ok, that makes MUCH more sense than what I was reading....... I kept seeing "keep it aerated because yeast only multiply under aerobic conditions, so use a stir plate" It just didn't make sense...

Grant, there would be some diffusion of gasses (not osmosis, that requires a semi-permeable membrane, but I got what you were saying) but that depends on the thickness of the blanket, mixing ratio/ concentration gradient etc. etc.

So if the stir plate is only to keep yeast in suspension, there is no reason not to use an airlock over a breathable cork/loose aluminum foil, as you will only be relying on the O2 already dissolved in the wort.

Hummm, still seems like I am missing some blatant concept or something....
I should stop thinking and start drinking

 

I think of gas diffusion similar to adding dye to a jar of water. The brownian motion of the solution causes mixing to occur and is fairly independent of gravity if the molecular mass of the two gasses is similar.

 

NOW you're getting it!!!!

 

A stir plate is also constantly driving off CO2, which inhibits yeast growth.

 

While the primary purpose of a stir plate is to keep yeast in suspension, its also greatly facilitates gas exchange. CO2 is knocked out of suspension and O2 is allowed to diffuse in because the starter liquid is constantly being put in contact with the air above. Oxygen is often a limiting factor in yeast production, so you definitely do want to use a piece of foil or a breathable stopper instead of an airlock (which would prevent O2 from entering), unless there is considerable headspace in your starter vessel. If you do this, stir plates can make a profound difference in the yeast production of your starter. See Figure 1 in this excellent reference for quantification:
http://maltosefalcons.com/tech/MB_Raines_Guide_to_Yeast_Culturing.php

Other useful refs:
http://www.wyeastlab.com/hb_makingastarter.cfm
http://www.mrmalty.com/starter_faq.htm
http://www.brewboard.com/index.php?showtopic=33475

 

As others have posted, the purpose of a stirplate is mainly to keep the yeast in suspension where the food is. The whole process becomes anerobic just like in the fermenter, but a foam stopper can allow for possible air exchange which won't be a bad thing, and is a standard lab process for this exact type of culturing.

Do some reading from the reliable links such as thos from M.B. Raines at the Maltose Falcons, and MrMalty.com, and you'll then know way too much about yeast

 

I don't understand how much O2 can enter the flask if it is covered. When you first pitch, there shouldn't be much C02 production, but does the vortex actually pull in outside air? After CO2 starts, I don't see how any outside air can enter with positive pressure on the flask.

 

Right -- that is why you need to cover it loosely with foil. Unless you seal the mouth up with an airlock or something, the CO2 will continuously escape the flask and the pressure inside is the same as atmospheric. O2 can easily diffuse back in. The stirring action of the starter probably does help to mix in the air above, as well.

 

I don't see how air will diffuse back in once carbonation has begun because the flask is now under constant positive pressure. Isn't that the principle behind open fermentors? And do you want air in there once carbonation starts?

 

<--- insane in the membrane. Of course osmosis requires a membrane, can't believe I got that mixed up.

 

I guess what I am saying is that there is no fermentation lock on the mouth of the flask, so gases are free to move in our out. Therefore, there is no pressure inside the flask. There may be a partial pressure gradient for the different gas fractions, but remember that the gas inside the flask is likely in motion with the stirred starter, so it is getting physically mixed.

In an open fermenter, the krausen largely protects the fermenting beer underneath. The common belief that an impermeable CO2 'blanket' is built up in fermenters is largely exaggerated. Also, those fermenters are not agitated, so it is easier for things to 'settle' in the vessel, inhibiting gas exchange.

When building large stir-plate starters, it is advisable to decant the liquid because, as you mention, it could be a source of oxygen contamination. But we are talking 2L starters or greater before the impact is felt (according to Jamil Zainasheff, anyway, who seems very knowledgeable about the subject).

Anyways, the refs I cited in a previous post in this thread are quite authoritative and probably have much better explanations than I have attempted here.

 

Hi.

This is a very interesting thread.

I bought a Heidolph 2. hand professional stirplate. I don't get nearly as fast a rotation of the magnet, as I've seen the homemade stirrers are getting.
I have tried different sizes of stirring magnets.

Does the rotation speed have any influence regarding this threads subject?
Is the simple stirring of the bar enough, or will it have to be powerful enough to make a whirl that goes to the bottom of the (erlenmeyer)flask?

Regards.
Lars
www.larchris.dk

 

That is only true if you are not propagating the yeast. Yeast need the oxygen for reproduction. A starter does not have to taste like a beer, but the yeast have to be healthy when reproduction is over. What i am saying here is this; for the most part fermentation in a starter is not necessary or even wanted until just before a pitch. This is because the point of a starter is propagation or to start yeast on the metabolic pathway. For the nay sayers, even in propagation some alcohol will be produced, but only 2 to 3 percent. This however is enough to retard or stop bacteria growth. That said the only purpose of plain fermentation of a starter, is because you don't want the accelerate growth or you do not have the equipment.

As for the air circulation in side of a flask. If you produce a vortex on the liquid you will affect the surrounding air and create a circulation system. This is why ocean currents play such a huge role in weather, that and they usually carry heat. Anyway that is off topic. you can perform experiments on this if you buy a cheap microscope. you can perform a cell count with and without the stirplate, Oxygen can be measured this way because yeast will stop reproduction as soon as oxygen or sugar is exhausted. In regular fermentation oxygen is the limiting factor in reproduction because the wort will only hold so much . S.

 

Yes, I understand the yeast need O2 to reproduce, what i am failing to visuilize is the exchange between room O2 and your starter.

Right, the friction between the surface of the liquid and the overlying gas blanket will cause movement in the gas layer, but there is a MUCH larger surface area when you are talking about the ocean vs. a flask that narrows as it gets to the top. Becasue the mouth is much narrower than the liquid/gas interface + the production of CO2 causing a partial internal positive pressure + the increased density of CO2 vs. room air = my bewilderment at how a stir plate aerates the starter.

Mabey a visuilization will help explain my puzzlement. take a half full flask and set it on a stir plate. Notice the amount of surface rotation and the size of the vortex. Now without altering your stir speed, fill the flask up a little further than its narrowest point. after it equilibrates, notice how the surface is moving much slower that it was at half volume? The gas exchange is going to occur at this level with little movement + some sort of stopper/Foam stopper further impeading movement of gasses. Couple this with a slight outflow of CO2 from the flask and this is where my confusion starts.

Interesting discussion, thanks guys!

 

Hi.

This was just my point. When the stirrer rotates very fast, the whirl in the middle can get quite deep. My guess was that the surface of the liquid gets bigger- and broken up to better let in the O2 and let out the CO2. I made this test-setup to day to illustrate my point:



My Heidolph stirrer isn't fast enough to do this in a 2 liter erlenmeyer. This is made with two neodyme magnets on a computer fan, with a 2 cm. stirbar.

As you can see the air is being heavily thrown in every time the whirl hits the bottom of the flask.

But again, is this needed, or is it enough to just create a stir, that keeps the starter (and yeast) in a constant movement?

Regards
Lars
www.larchris.dk

 

I am almost certain that the vortex does draw air into the flask, we have in floor heat so with no forced air there is little to no dust, yet my foam stopper is always dusty at the end of a starter, but the counter all around my stir-plate is not. I am thinking if I were to suspend some fine powder in the air above my stir-plate I would see it drawn towords the flask mouth. How else does a dry foam stopper collect 100 times more dust than the areas all around it?

 

good point. I will have to try that with some powder and see.

 

Liquids and gases alway try to equalize. There is likely a slightly higher concentration of CO2 in the flask, but there is also plenty of O2. The earlier example of die in water is a good visualization. One would think that without agitation a drop of die in water would stay separate. But try it and you will find that in a few minutes it is mixed.

 

I don't know that it's necessary for the vortex to pull in any new air from the outside. The flask starts out half filled with starter fluid/yeast and half with air. Now this air is 20% oxygen so thats um 200,000 ppm. The starter needs to have 8-12 ppm of oxygen.

So your starter starts to ferment and release Co2 but all the oxygen, nitrogen, and Co2 are mixed up by the vortex so more of that 200,000 ppm stays in the flask compared to a non-mixed starter. As the starter is mixed the oxygen can continue to diffuse into the starter.

 

Static charge.

I would say the two experiments that need to be done to figure out what's happening are the match stick, and doing cell counts on one stir plate with a permeable stopper and one with an airlock. We can speculate all day but science is the way to actually learn something.

 

I always thought the yeast reproduce as long as 02 is available and once the 02 drops it goes into fermentation. So if the starter is in constant motion then 02 will be reintroduced as it being used by the yeast and the yeast keep growing . Once there is enough yeast to strip the 02 they start to ferment. Then the stir plate just keeps them in suspension .... I may be talking out of my ass ....

 

The main point of contention here is whether there is new O2 being introduced into the flask or not. The yeast will be producing CO2 even during aerobic metabolism which will fill the flask and may or may not prevent new O2 from reaching the solution. I was thinking of buying a hemocytometer but I don't have a stir plate so I can't really do these experiments myself.

 

This debate may be elucidated by first clarifying some issues about yeast metabolism.

When the starter is first innoculated, you will also typically aerate it. So the yeast begin with a source of oxygen and a wort rich in saccharides. The first task of the yeast will be to synthesize sterols for the cell membrane via oxidation. This will use up a certain percentage of the oxygen in solution. Different yeast strains have extremely different oxygen requirements for the synthesis of sterols, with some strains requiring only 4ppm and others requiring greater than 19ppm. Therefore, a great deal of the initial oxygen will be utilized just for the synthesis of the cell membrane. Internal glycogen reserves that the cell has already built up, not the saccharides in solution, are used to to fuel this process of cell membrane synthesis.

Once the cell membrane has been synthesized, the yeast will focus on growth and division. It is NOT the case that cell division only occurs via respiration in an aerobic environment, as some posters have stated. In fact, because the solution at this point will contain greater than 0.4% glucose, aerobic metabolism will initially be inhibited. Only after the glucose content of the solution has been reduced below 0.4% will the catabolic repression of the Crabtree Effect be removed. At that point, if there is still oxygen left in solution, then the yeast WILL employ aerobic respiration and will reproduce quite efficiently until the oxygen is used up. However, once this source of oxygen is used up, they do not cease division. They will just transition to fermentation. Fermentation is the process of cell growth and division via anerobic metabolism.

So, IF a stir plate DOES continuously add oxygen to the solution, this WILL keep the yeast in a state of aerobic metabolism (after the initial glucose is consumed and the sterols are synthesized) and will therefore increase the efficiency of yeast metabolism by 14 times that of anerobic metabolism. However, the total amount of yeast growth will be limited, of course, by the total fermentable sugars in the wort. The stirring activity will also increase the efficiency of growth by keeping the yeast in suspension.
-------------
Edit: I was wrong on one point here. The Crabtree Effect applies not just to glucose levels, but to other hexose sugars that can be converted to glucose intracellularly, i.e. glucose, fructose, and maltose, but not sucrose. Since at least one of these sugars exists during the full course of the fermentation, and the fermentation is aneraobic after the first hour or two, aerobic metabolism (respiration) never occurs in a brewery fermentation. Also, in a typical fermentation, sterols levels are typically the limitation of growth, not sugars, though it is possible, given a large enough pitching rate and sufficient oxygenation, for some nutrient besides sterols to become limiting. But typically nitrogen will become limiting before carbohydrates.

 

Sounds like it should be really easy to tell if O2 is being constantly introduced then. Like you shouldn't even need a hemocytometer, you should be able to see a difference in the amount of yeast that precipitates after fermentation is complete or the starter is refrigerated.

 

OK, I read the whole thread. I think the guy that wants to know about how the 02 gets into solution can be explained.

Look at the solution in the vortex. The center is being pulled DOWN. That also means the outside is going up. Just look at the mark on the side when the stir plate is running and when its off. The outside is driven up while the center is sucked down.

IF the air inside the flask is spinning too which it has to be at SOME speed, then its doing the same thing. Pushing up on the outside and drawing down in the center. With aniairlock, all it does is keep + pressure inside the flask. With no airlock, towards the rim of the small necked vessel pressure is higher than in the center. There ya go, gas exchanged. I can't tell you what speed, but its going to happen with loosely fitting foil.

Did I get it?

My Stir plate arrived TODAY. Its on the counter with some sugar water and a little knotty for a test run.

Also look at MR. Malty, he sez you need almost 2 liters of starter with out a stir plate and only one with.

David

 

Your explanation makes sense, but it is hard to say how quickly the air is spinning, since the only thing causing it to move is friction between it and the surface of the wort. Even if some air is exchanged, that doesn't guarantee that it is enough to displace all the CO2 above the wort so that O2 actually makes it down in there.

 

Each gas in an environment moves down its own concentration gradient independent of each other. This means that the CO2 in the flask will get out to air and O2 will get in, independent of what each other is doing. It's the same thing that's happening in your lungs and they do just fine . The CO2 produced will only "drive off" the oxygen if there is an airlock or still air so it can create a blanket. There is neither in a proper stirred starter with a foil cap or foam stopper.

If you want to see how much the air is moving in there, drop a couple pieces of dry ice in the flask with some water, cover with foil, wait until the airspace is full of vapour, then stir

 

Fill a bottling bucket with water in the tub. Put your arm in and swirl it around as fast as you can till you get a whirlpool going. You can feel the air pushing up and out the side of the bucket. Same thing with a flask. The excahnge is faster because of the narrow entrance. If air is blowing out that means its being sucked in. The co2 production is too slow. It can't out pace the o2 injection from the atmosphere.

The whole point of the stir plate is to keep the yeast in suspension and airated with o2. This helps the yeast grow fast and strong.

One of the biggest steps in brewing is airating the wort before you pitch your yeast. Some guys use o2 stones, some pour bucket to bucket, shake the fermeter. I used a paint stirrer on a drill. This months BYO mag has a whole section dedicated to create a wort airation system.

I usually make a 2 liter starter using my stirplate before every batch. unless I am repitching on a yeast cake. I have made starters with an airlock and with the stirplate. I found the stirplate made a much morw rigorous starter.

Download Jamils brew strong show on yeaster starters. he goes through the whole process and answers the o2 co2 diffusion.

I hope this helps you out. Good luck, and Brew strong!

 

I'm sure that a stirred flask will keep the concentration of the various gasses even throughout the headspace. But there is no way that air will diffuse into the flask through the foam stopper during active fermentation. The constant production of CO2 pushes the diffusion front out much faster than diffusion can happen. This is an intuitive guess. For the initiated, you could solve Fick's equation for diffusion through the stopper and compare the net transport of a gas to the rate of CO2 production. You wouldn't even need to know actual diffusion coefficients as you could assume they are about the same for all gasses involved. I'm fairly sure you would find that my intuition is correct. I'll bet it would be for the foil case as well - not sure how much space you'd need between the neck and foil for diffusion to actually play a role. I keep the foil tight to keep out bugs.

SM

 

You know I had completely forgotten about the crabtree effect until I read your post. So, is the only effect of oxygenating wort at the start of fermentation to give the yeast materials to build cell membranes? If so then this will be less necessary with large starters. That is, none of the added oxygen (at the start of fermentation) is being used for metabolism but only for making more yeast. If you start with a very large starter and there will only be 2 or 3 cell divisions then early aeration will be less needed (assuming that you are using a healthy starter)

Sorry, that was a bit of a Hijack.

 

This was my first batch of oatmeal stout with the stir plate. I used o2 like I always have. I used to make a 3 litre starter in a gallon jug three or four days before brewing. Now I can make a 1.6 litre starter in one or two days.

This is what I got after 24 hours.



The only thing that changed is the stir plate. Yeast was pitched at 68 like always.