Oxygen Requirements as related to the Volume of Wort

Wyeast's website states the following:


I find this language extremely unhelpful and imprecise. Describing oxygen levels in ppm is not really accurate, because the ppm varies as the volume of liquid increases. But the number of yeast cells is initially fixed.

To clarify this point, suppose that I pitched one Activator pack (100 billion cells) into a very small starter of 500 ml. Then I oxygenated that starter to 10ppm. The total amount of oxygen in solution would be 1/2 that of a 1000 ml starter oxygenated to 10ppm. Obviously the 1000 ml starter and the 500 ml starter, each oxygenated to the same ppm, would be providing very different levels of oxygenation to the yeast.

Does anyone know how to interpret Wyeast's statement, then? Are they assuming that brewers will be oxygenating 5 gallons of wort, or a sufficiently large volume of wort to 10pmm? It seems like a better way of stating oxygen requirments would be to state the oxygen requirements for a given number of yeast in a given volume of wort. For example, 10ppm for 1 million yeast in 1 ml of wort.

I believe they are referring to oxygenating the full wort volume prior to pitching.

Hmm, yeah, that would make sense. So is there any way to determine the oxygen requirements of yeast in a starter? My problem is that I want to fully oxygenate the yeast in the starter, so that I don't have to oxygenate the main batch of wort. But I don't want to over-oxygenate the starter, because I don't want to promote aerobic yeast growth. That is, I want to be able to pitch exactly the right amount of cells, and I will provide that amount by supplying the appropriate amount of Activator packs (for which yeast cell counts are known). I just want to use the starter to provide the yeast with their initial oxygen requirements for cell membrane synthesis.

the problem is you are looking at it wrong.
the oxygen is for cell development and cell division.
the yeast in the smack pack is not the proper amount to ferment a whole 5 gal batch
but it will multiply to the proper amount when combined with FAN, Food and the Oxygen= to 5gal@10ppm.

what do you mean by "because I don't want to promote aerobic yeast growth"
you know there are alot of beers fermented open top. and the most efficent way yeast repoduce is an aerobic process, thay can do it anaerobic but only if thay can steal an oxygen off an unsatirated lipid and its slow and inefficent

stoutaholic said:

My problem is that I want to fully oxygenate the yeast in the starter, so that I don't have to oxygenate the main batch of wort.

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the problem is you cant get the yeast all teh oxygen thay will need in the starter. becase eatch time the yeast is going to split it need oxygen
and after the oxygen is gone it stops spliting
thats is how its self regulates, otherwize it would just keep spliting out of control till all the FAN was gone..

So the only way you can do this is to make a huge starter that has all the yeast need for a vigoris fermentation with no further cell division.
but that is really impractical becase you will increase you batch cost with all the DME need to make the 2+gal starter

why do you not want to have to oxygenate?
its easy all you have to do is let the wort fall outof the hose from the neck of the carboy or lip of the bucket that splashing and bubbling alone is all you need to oxi prechilled wort

The fermentation of beer is an anaerobic process. Fermentation is the process of cell growth and division via anerobic metabolism. The rate limiting factor of yeast growth is not oxygen, per se, but typically sterol reserves (and if not sterols, then fermentable sugars or other nutrients). If yeast is utilizing aerobic metabolism, it is producing CO2 and water, not ethanol. Therefore, the wort sugars are being utilized for anabolism (the production of cell mass) rather than the production of BEER. Moreover, oxygen is implicated in the production of a number of undesirable compounds such as diacetyl, acetyaldehyde, and fusel alcohols, as well as staling reactions. Check out the chapter on Oxygen in "Essays in Brewing Science." Page 133 discusses the argument for oxygenating starters, not wort.

Anyway, assuming that I only want to oxygenate the starter, does anyone know how I would determine the oxygen concentration necessary in a starter for a given strain of yeast? Can anyone think of an experiment that we might perform?

if there is oxygen then it does not need the sterol reserves! oxygen and Free available nitrogen is the limiting factor of cell division in wort average gravity fermentation

Isn't ppm the descriptor in and of itself. Or, doesn't ppm define the ratio you are looking for regardless of volume in that 10ppm defines the saturation of oxygen per liter?

I would suppose that provided you are using a DO meter that 10ppm in 500mL would be the same saturation as 10ppm in 5 gallons. regardless of the cell count saturation disparity.

And, isn't it this scenario where the whole EVOO thang was found to be most useful?

Clayton, if you don't have access to "Essays in Brewing Science" or Bamforth's "Beer - Tap into the Art and Science of Brewing" or Ian Hornsey's "Brewing", then you could also check out chapter 18 of "Malting and Brewing Science", which is online as a google preview. Page 635 is particulary good on this topic: Malting and Brewing Science, Chapter 18, Yeast Growth.

Again, respiration (aerobic metabolism) does not produce beer. It produces CO2 and water. And in most fermentations it is unlikely to continue for very long, if at all, because: (1) the oxygen requirements for the sythesis of sterols and fatty acids use up a vast amount of oxygen (see page 635 of Malting and Brewing Science) and (2) the Crabtree Effect inhibits the aerobic metabolic pathway until glucose levels have been reduced below 0.4%.

Anyway, the point of this post was not to try to explain yeast cell metabolism, but to try to figure out how to sufficiently oxygenate a starter. There must be some particular concentration of cells to milliters for which Wyeast's 10ppm applies. For instance, maybe 2000 ml per 100 billion cells is a concentration at which 10ppm would be sufficient for most yeast strains. Or maybe the only way to sufficiently aerate a starter is to continuously aerate it. If we knew how long it took for cell membrane synthesis to occur, we could continuously aerate the starter for that amount of time, then cease aeration, allow the yeast to consume the residual oxygen, and then pitch.

GilaMinumBeer said:

Isn't ppm the descriptor in and of itself. Or, doesn't ppm define the ratio you are looking for regardless of volume in that 10ppm defines the saturation of oxygen per liter?

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ppm is "parts per million", so, yes, it is independent of volume. But my point is that 10ppm of oxygen in 500 ml is a hell of a lot less oxygen than 10ppm in 5 gallons. 10ppm in 500 ml is really not that much total oxygen. It's the same as talking about a 10 Plato wort -- 100 billion yeast cells in 500 ml of a 10 Plato wort is going to use up all the fermentables in that wort in no time. Similarly, 100 billion cells in 500 ml of 10 ppm wort is going to use up the oxygen in no time.

stoutaholic said:

But my point is that 10ppm of oxygen in 500 ml is a hell of a lot less oxygen than 10ppm in 5 gallons.

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True but the saturation is constant. Yes?

The problem here is not the saturation of oxygen but the cell count disparity. So, by this my thinking is that you first need to determine the cell count per volume equivalences and adjust the saturation from that.

My point is that the ppm gives you the ion concentration of O2 per liter. From there isn't it just a matter of determining the cell count per liter at pitch time to develope the base ratio you seek?

stoutaholic said:

Or maybe the only way to sufficiently aerate a starter is to continuously aerate it.

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Isn't that pretty much what a stirplate with a good vortex does?

GilaMinumBeer said:

True but the saturation is constant. Yes?

The problem here is not the saturation of oxygen but the cell count disparity. So, by this my thinking is that you first need to determine the cell count per volume equivalences and adjust the saturation from that.

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Yeah, if Wyeast had specified "100 billion cells in 5 gallons of wort at 10ppm" then I could calculate the appropriate concentration of cells for 500 ml. But unfortunately they were not that precise. Also, I don't think that the formula would be linear, and would not scale down to the volumes of a starter. For instance, it is probably not possible to supply enough oxygen in 500 ml of wort, in one shot, to sufficiently oxygenate 100 billion cells. With 100 billion cells in 500 ml, the formula would probably suggest something like 250ppm (I haven't done the math yet).

At 10 ppm wort is effectively saturated with oxygen (IIRC, ca. 12 ppm is actually fully saturated, depending on temp gravity etc). So, if you are going to oxygenate your wort for your starter, you want to blow pure O2 into it for long enough to get it saturated. i.e. this is the most O2 you can get into the wort by this method, and will give you the maximal amount of growth for the amount of cells you pitch in (whatever that happens to be).

If you want to continuously oxygenate your wort, you are going to need more equipment that the typical homebrewer has. You would need a dissolved oxygen probe, an O2 cylinder, a (possibly computer controlled) regulator, a sparger (in this case, sparging means blowing gas in, not water). I can do this in my lab. And yes, the yeast probably grow a bit better. But most of us *at home* just use a stir plate, and continuously aerate that way. A starter grown this way will reach a higher density of cells compared to one just oxygenated at the beginning. (check mrmalty.com).

Now, it sounds to me like you want to pitch enough yeast such that they do not need to undergo aerobic growth before fermentation starts. I think this is possible, but would require a HUGE starter. For some styles, you will also miss out on the flavor profile generated by the growth phase. The aerobic growth phase is a good thing, before anaerobic fermentation starts!

AnOldUR said:

Isn't that pretty much what a stirplate with a good vortex does?

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Yeah, exactly, but then I just have to figure out for HOW LONG and also the size of the starter. I think the volume of the starter would be a significant variable.

ETA - you might want to look at Proper Yeast Pitching Rates for a discussion of pitching. It really does depend on the yeast, on the style of beer and other factors. Wyeast cannot be precise as there are too many variables.

Anyway, the point of this post was not to try to explain yeast cell metabolism, but to try to figure out how to sufficiently oxygenate a starter. There must be some particular concentration of cells to milliters for which Wyeast's 10ppm applies. For instance, maybe 2000 ml per 100 billion cells is a concentration at which 10ppm would be sufficient for most yeast strains. Or maybe the only way to sufficiently aerate a starter is to continuously aerate it. If we knew how long it took for cell membrane synthesis to occur, we could continuously aerate the starter for that amount of time, then cease aeration, allow the yeast to consume the residual oxygen, and then pitch.

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A continuously agitated starter i.e. one on a stirplate with a foil or foam cap is proven to be the best way to grow yeast (the aim, afterall, of a starter) proven by both experiments by Jamil Zainasheff and Chris White (and built into the Mr. Malty pitching rate calculator). One only need play with the OG of the wort vs. vials/smackpacks to see the ideal number of cells necessary to properly inoculate 5 gallons of wort at a said specific gravity.

The industry pitching rate is 100 million cells per ml wort per degree Plato.

Aeration of the wort is still important because while you are pitching the proper amount of cells, those cells have been grown in a 1.040 wort and will need to adapt to the conditions of the target wort which is now extremely deficient in oxygen post-boil; heavy splashing or direct O2 injection is needed to introduce a sufficient level of oxygen so the yeast can again reproduce aerobically and adapt the colony to the new wort conditions, promoting a healthier fermentation and minimizing lag time. It is also well documented that stressed yeast will throw esters in spades.

stoutaholic said:

Yeah, exactly, but then I just have to figure out for HOW LONG and also the size of the starter. I think the volume of the starter would be a significant variable.

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Anything less than a liter is deficient, because the yeast will churn through the available sugar far too quickly; likewise, a gravity <1.040 ism't providing enough food for the yeast.

It is generally understood that 36-40 hours of continuous aeration via a stirplate is ideal for maximizing yeast growth.

yeah 100 billion asumes an oxigenated wort and cell divistion
i dont know how much AB oxigenates there low gravity wort but i did find this snipet

(Homebrewers pitch too little yeast
Brewers usually pitch 10 million yeast cells for every milliliter of wort. To match commercial pitching rates, homebrewers should pitch 200-400 billion yeast cells. A dry yeast packet typically contains 50 billion cells. A liquid vial contains 70 to 140 billion cells. The smack packs contain around 100 billion cells. This means if you do not create a starter, you are under pitching. Smack packs are not starters. They activate the yeast metabolism, but they do not increase cell counts.)
http://www.fermentarium.com/content/view/175/55/

stoutaholic said:

Yeah, exactly, but then I just have to figure out for HOW LONG and also the size of the starter. I think the volume of the starter would be a significant variable.

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its not time! its volume
you need a set volume of yeast for the wort you want to ferment , who cares how long that will chage every time depending on the yeast and the target wort

I didn't sufficiently explain the approach I am taking, because the post was initially on how to calculate the appropriate oxygenation level for a starter.

First, let me point out that I understand all the traditional approaches to oxygenation and pitching, and how to calculate pitching rates. I understand that most people use starters for yeast propagation, not purely as an oxygenation environment for the yeast.

Also, I have a dissolved oxygen meter and an oxygenation system.

Based upon my understanding of how fermentation works, and how the various by-products of fermentation are generated, I have come to believe that the traditional techniques are sub-optimal if your goal is produce the best beer possible.

First of all, they introduce too many variables into the process. If you are propagating your yeast in a starter, you have to rely on estimates from a pitching calculator to figure out how many cells your starter produced. Whenever you rely on estimates, you are losing preciseness and repeatability. I would rather buy another package of yeast (for which the cell count is known) than to rely on estimates about how much yeast a starter with very particular characteristics is going to produce. Yes, it is an added expense, but my perspective on homebrewing is that we have the freedom to ignore the bottom line, unlike macro- or microbrewers, and it is this freedom that allows us to make better beer.

Secondly, if yeast are pitched at the end of their lag phase, after they have synthesized all required sterols and fatty acids, then there should be practically no lag phase -- they should begin fermentation almost immediatley (especially if the environment they are pitched into has the same characteristics as the starter).

Third, by not exposing the wort to oxygen, you are avoiding all kinds of undesirable chemical reactions. The production of diacetyl, acetylaldehyde, and fusel alcohols all influenced by the presence of oxygen.

Therefore, it seems to me that the best method of controlling a fermentation is to vary your temperature and pitching rate according to the rate and amount of yeast growth that you want (i.e. how many fermentation by-products such as esters you want for the style you are brewing), and to avoid oxygenating the wort itself. If you pitch the appropriate amount of pre-oxygenated yeast, you will obtain predictable levels of yeast growth (and therefore predicable levels of by-products), while avoiding many of the undesirable oxidative reactions that would take place by oxygenating the wort.

So, IF you are pursuing the approach of oxygenating the starter and not the wort, the problem is then determining how long to maintain the yeast in the oxygenated starter environment. If you leave them in the starter too long, the yeast will fulfill their initial sterol requirments and then begin multiplying like mad, thereby causing you to overshoot your targeted pitching rate. If you don't keep them there long enough, they will not have fully synthesized the cell membrane and subsequent fermentation will be compromised (i.e. either the yeast will run out of sterol reserves before finishing fermentation, and not fully ferment the beer, or they will finish but still throw off a lot of esters and other by-products during fermentation). So this is where my inital question came from -- is there a way to figure out the base oxygen requirements of a particular strain of yeast, and what conditions are necessary to provide that? Continuous aeration for X period of time in Y amount of wort for Z cells of yeast, or initial oxygenation of X amount of starter wort to Y ppm for Z cells of yeast?

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UPDATE: I figured out the answer to this question. The technique is discussed in Brewing Yeast and Fermentation, Boulton and Quai (2001), pages 433- 435. The trick is to monitor the oxygen uptake RATE of the yeast. Once the uptake rate reaches its maximum, the yeast have maximized their sterol levels and are ready to pitch. So you just have to measure the point at which oxygen uptake rates start decreasing (at that point the maximum uptake rate has already been achieved). Obviously, this would be a major PITA initially, but once you had performed the experiment with a given yeast strain a couple of times, you could make estimates for future batches. I'll publish my results once I perform this experiment.

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UPDATE 2:
The original question is answered by Greg Doss from Wyeast Labs (who was just published in Brew Your Own):

Hi Shaun,

Thank you for the email.

We do not have a chart on specific O2 requirements per strain, but this
may be something we will provide in the future. 10-15 ppm will cover
most strains.

You are correct in needing to breakdown ppm O2 per cell. Multiple small
(<15 ppm) O2 additions will most likely be necessary. It will not be
beneficial to dissolve high levels (>15 ppm) of O2 into you starter.
Other factors including time, stone (porosity, surface area, pore size),
temperature, starter density, and O2 flow rate will affect DO levels
actually achieved. Try measuring DO in a flask of wort (same starter
vol.) at different times of O2 injections.

I hope that this helps.

Cheers,

Greg


Greg Doss
Quality Control Manager
Microbiologist/ Brewer
Wyeast Laboratories, Inc.
888-WYEAST-1 / Fax: 866-WYEAST-1
[email protected]
www.wyeastlab.com