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Old 08-01-2012, 03:41 AM   #1
CrustyBrau
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Default Yeast propagation

Sorry for the blatant double post, but I dropped this under "science" (seems somewhat reasonable, right?) and then realized this forum was more suitable...


I am trying to gain a practical understanding of yeast propagation, as it relates to starters. I keep seeing something that I just can't make any sense of.

White Labs says that a single stage starter will create a 4-5 times increase in cell count, and Wyeast says it's more like 6 times, almost entirely regardless of the volume of the starter (ignoring the lower end, because obviously the yeast cannot continue once they have fully attenuated). This is for starters that are aerated and agitated.

This means that if you put 100 yeast cells into a 1.040 starter that is 100 gallons in volume, you will end up with between 400 and 600 yeast cells, and then they will just stop reproducing. Obviously, at such a ridiculously low cell count to volume ratio, the properties of the wort would be almost perfectly unchanged, and yet the yeast just stop reproducing. So here you have a 1.039999 wort with almost exactly as much oxygen in it as before you started, and for some reason the yeast in it won't do anything anymore. If you decant off the beer and repitch the yeast into a nearly identical but different 100 gallons of wort, they'll then reproduce up to 1600 - 3600 cells, and then again stop.

Does anyone understand what's going on here? Can you explain it it me?

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Old 08-01-2012, 04:07 AM   #2
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Where are you seeing the claim that starter volume doesn't matter? That goes against the yeast growth tables published by White Labs and Wyeast themselves.

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Old 08-01-2012, 01:35 PM   #3
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You should start with a more basic understanding of the math on how yeast propagate. Read Yeast by White and Zainascheff (the relevant part is just a few pages). Or see the data and formulas available on yeastcalc.com. The question you're asking doesn't seem to be very relevant. What exactly are you after?

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Old 08-01-2012, 01:39 PM   #4
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By the way, I think what you are missing is that yeast growth is a function of the ratio of cells pitched to volume of wort.

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Old 08-02-2012, 03:01 AM   #5
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Quote:
Originally Posted by SpeedYellow View Post
What exactly are you after?
In general, understanding. Specifically, I set out to find a formula that would tell me how much wort I would need to grow X cells from a vial, *in one step*. All the calculators insist you need to go in steps. Steps are bad because each time you open the vessel, you risk letting in contaminates (not to mention the extra labor involved), so it would be very advantageous to be able to grow up arbitrarily large starters in one step.

So now I'm just trying to wrap my head around why that doesn't work. Why do I care? That's just the way I work

Since I know that both sugar and oxygen can easily be sufficient to promote cell propagation through a sufficiently large starter and good constant aeration, the only thing that makes any sense is CO2 build up. I had thought that constant aeration would drive off the CO2, but maybe that's not how it works.
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Old 08-02-2012, 03:08 AM   #6
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Quote:
Originally Posted by CrustyBrau View Post
In general, understanding. Specifically, I set out to find a formula that would tell me how much wort I would need to grow X cells from a vial, *in one step*. All the calculators insist you need to go in steps. Steps are bad because each time you open the vessel, you risk letting in contaminates (not to mention the extra labor involved), so it would be very advantageous to be able to grow up arbitrarily large starters in one step.

So now I'm just trying to wrap my head around why that doesn't work. Why do I care? That's just the way I work

Since I know that both sugar and oxygen can easily be sufficient to promote cell propagation through a sufficiently large starter and good constant aeration, the only thing that makes any sense is CO2 build up. I had thought that constant aeration would drive off the CO2, but maybe that's not how it works.
Yeast population dynamics are driven primarily by cell density and the availability of sugar. Other nutritional factors can play a role too (particularly sterols, oxygen, etc), but primarily as limits. CO2 quantities in the levels that brewers deal with will not limit yeast growth.
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Old 08-02-2012, 03:09 AM   #7
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I remember seeing in Yeast was that they mentioned that once you get to a low enough innoculation rate, yeast reproduction looks more like it does in beer production versus what it does in yeast propagation. Don't recall that they said why.

Larger volume of wort, properly aerated and with adequate nutrients, will produce more yeast. But your returns diminish past a certain point.

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Old 08-03-2012, 01:39 PM   #8
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Quote:
Originally Posted by CrustyBrau
In general, understanding. Specifically, I set out to find a formula that would tell me how much wort I would need to grow X cells from a vial, *in one step*. ..
You should definitely read Yeast, as it explains the formulas and even gives sample calculations. It's then easy to verify the online calculators.

You'll see that indeed you can use a huge single stage starter, but it's extremely inefficient. So take your pick: high DME cost with less hassle, or less DME cost with higher hassle of making two starters.
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