Help with calculations - Home Brew Forums

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03-27-2011, 10:50 PM   #1
jwic

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I'm getting a little frustrated trying to find a simple explanation of the numbers involved in yeast cell counts. I'm not entirely inept but I can't seem to figure it out from How to Brew nor can I find a thread on HBT to help me out.

Can someone please explain it simply?

I don't have How to Brew right in front of me so I found on the WyeastLab web-site a chart which says for Ales, starting gravity <1.060, pitching AND fermentation temperatures >65dF, I need 6 million cells per milliliter.

The 100B WyeastLab smack-pack comes in around 5,283,441 for a 5 gallon batch and so I used a starter to notch it a bit more toward an exact 6M/mL. I used a starter of 1/2 cup Extra Light DME and 2 cups water.

Can someone show me the actual math (or point me to the thread) how many millions cells per milliliter I actually pitched after they multiplied in the starter?

JWiC

03-28-2011, 02:12 AM   #2
MalFet
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The math you are looking for doesn't exist, unfortunately. Neither Mrmalty nor wyeast's formula is published (as far as I know), though Chris White's book Yeast has some inoculation and growth rate tables that you can use. Is there a particular reason you want access to the numbers rather than to just use the calculators as they stand?

Yeast population dynamics are too complex a system to model completely, so all of the estimates are quite rough. If you really want to know how many cells you pitched, you need a hemocytometer and microscope. I don't do many cell counts anymore, but the numbers I saw were often significantly above or below what either of the calculators predicted.

For what it is worth, Wyeast seems to think that you would be pitching about 9M/mL, assuming your smackpack was relatively fresh.

In any case, Wyeast and White Labs both suggests smaller pitching rates than I think most people around here would be happy with. The "standard" pitching rate numbers come from George Fix, and look something like:

.75M cells / mL of wort / degree Plato for an ale, or
1.5M cells / mL of wort / degree Plato for a lager

So, a 1.040 ale would by this rubric want something on the order of 7.5M/mL, or 150B cells. A 1.060 ale would need 11.25M/mL, or 225B cells.

03-28-2011, 05:21 PM   #3
jwic

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Hmm...interesting. When I got home last night I looked at How to Brew and one chart - if I'm reading it correctly - gives some figures regarding the number of yeast cells in a starter. Don't have the book in front of me, but it looked like if I pitched a 100B pack into 1L starter at 1.040, I should get 168B yeast cells. If I remember correctly, the rest of it went something like this: 2L = 211B; 3L = 251B; 4L = 280B.

I found on WyeastLabs.com that it would be about 6M/mL...where did you get the 9M/mL. Or maybe I misread...

At any rate, thanks for clarifying that there's no hard and fast calculation. Also, let me ask it another way: how do I know if I'm underpitching, overpitching, or pitching approx. the right amount?

03-28-2011, 06:22 PM   #4
MalFet
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Quote:
 Originally Posted by jwic Hmm...interesting. When I got home last night I looked at How to Brew and one chart - if I'm reading it correctly - gives some figures regarding the number of yeast cells in a starter. Don't have the book in front of me, but it looked like if I pitched a 100B pack into 1L starter at 1.040, I should get 168B yeast cells. If I remember correctly, the rest of it went something like this: 2L = 211B; 3L = 251B; 4L = 280B. I found on WyeastLabs.com that it would be about 6M/mL...where did you get the 9M/mL. Or maybe I misread... At any rate, thanks for clarifying that there's no hard and fast calculation. Also, let me ask it another way: how do I know if I'm underpitching, overpitching, or pitching approx. the right amount?
The 6M/mL and the 9M/mL are two completely different things. The 6M/mL is Wyeast's recommended pitching rate, as stated here. As I mentioned before, just about everyone would say those numbers are lower than what you should actually use for guidelines. The 9M/mL I mentioned, on the other hand, is an estimated actual cell count pulled from their calculator here.

I don't have How to Brew on hand, but I vaguely remember a table along the lines you mention. In any case, there are several sources providing estimations, but none of them will get you a very precise count. As far as knowing if you are pitching the right amount, any of these options will probably be "good enough". The wyeast calculator lets you manipulate the numbers more directly, but the mrmalty calculator is probably easier to use. Either one will work fine.

03-30-2011, 07:25 PM   #5
jwic

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Dec 2010
Chicago, IL
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Just one more question (using the Wyeast calculator).

I used a .125 gallon starter wort and pitched one pack. It says that my final pitch rate is 6.9.

If I understand correctly, if I want to pitch the 9 you recommend, I'd have to use a starter of .25 gallons. Correct?

03-30-2011, 07:35 PM   #6
gio
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Starter yeast counts are random guesses. It's not worth doing the math as it's going to be wildly inaccurate.

You need to get a microscope and a hemacytometer and do cell counts. A cheap microscope isn't too expensive and it's not that difficult to do.
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03-30-2011, 07:54 PM   #7
jwic

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So, then, what do people do then: NOT whip up a starter? just eyeball a starter? use the calculators on Wyeast or MrMalty as "best guess"? Or do people not care?

I guess, maybe I should ask it this way: how do people ensure they're not at least over-pitching or under-pitching?

03-30-2011, 09:12 PM   #8
gio
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Quote:
 Originally Posted by jwic So, then, what do people do then: NOT whip up a starter? just eyeball a starter? use the calculators on Wyeast or MrMalty as "best guess"? Or do people not care? I guess, maybe I should ask it this way: how do people ensure they're not at least over-pitching or under-pitching?
Most people underpitch. It's hard to overpitch although many beers will taste better slightly underpitched. People use MrMalty and related calculators as a best guess but in reality you can't tell to any degree of accuracy.
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03-30-2011, 11:48 PM   #9
MalFet
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Quote:
 Originally Posted by jwic Just one more question (using the Wyeast calculator). I used a .125 gallon starter wort and pitched one pack. It says that my final pitch rate is 6.9. If I understand correctly, if I want to pitch the 9 you recommend, I'd have to use a starter of .25 gallons. Correct?
Yep, those are the numbers I see too. Ideally, you should probably degrade the pack value a bit unless you are certain that it is extremely fresh. MrMalty has a function built in to do this, but I prefer to do it manually. A three month old pack kept well is probably in better shape than a one month old pack kept poorly. You can simulate your best guess of this kind of thing by entering .9, .8, .7, etc. packs into the #of packages field. The mrmalty.com calculator can give you a ballpark sense of how packs degrade over time.

Quote:
 Originally Posted by gio Starter yeast counts are random guesses. It's not worth doing the math as it's going to be wildly inaccurate. You need to get a microscope and a hemacytometer and do cell counts. A cheap microscope isn't too expensive and it's not that difficult to do.
I think you might be overstating the case here a bit. It is certainly the case that the formulas are only estimates, but they're a long way from being random guesses. By my experiments, the growth ratios were actually relatively consistent in controlled circumstances. A good chunk of the error comes from difficulty estimating the number of healthy cells in the initial pitch.

For any given set of parameters, the numbers that the calculators offer are median values on a probabilistic bell curve. The curve is definitely wider than anyone would like, but it's not as though final cell numbers are true fictions. They're more like the fuel efficiency estimates listed for cars: you can't be sure that on any given day that you'll get exactly 32mpg, but a car listed at 32mpg will generally get better mileage than one listed at 27.

I used to count all of my yeast pitches, but I've long since given that up. The numbers the calculators offer are indeed close enough for government work, at least according to my tests. Have you gotten wildly different cell counts on your hemocytometer?

03-31-2011, 01:48 AM   #10
gio
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Quote:
 Originally Posted by MalFet Yep, those are the numbers I see too. Ideally, you should probably degrade the pack value a bit unless you are certain that it is extremely fresh. MrMalty has a function built in to do this, but I prefer to do it manually. A three month old pack kept well is probably in better shape than a one month old pack kept poorly. You can simulate your best guess of this kind of thing by entering .9, .8, .7, etc. packs into the #of packages field. The mrmalty.com calculator can give you a ballpark sense of how packs degrade over time. I think you might be overstating the case here a bit. It is certainly the case that the formulas are only estimates, but they're a long way from being random guesses. By my experiments, the growth ratios were actually relatively consistent in controlled circumstances. A good chunk of the error comes from difficulty estimating the number of healthy cells in the initial pitch. For any given set of parameters, the numbers that the calculators offer are median values on a probabilistic bell curve. The curve is definitely wider than anyone would like, but it's not as though final cell numbers are true fictions. They're more like the fuel efficiency estimates listed for cars: you can't be sure that on any given day that you'll get exactly 32mpg, but a car listed at 32mpg will generally get better mileage than one listed at 27. I used to count all of my yeast pitches, but I've long since given that up. The numbers the calculators offer are indeed close enough for government work, at least according to my tests. Have you gotten wildly different cell counts on your hemocytometer?
I completely agree. The numbers are good enough estimates that you'll get a decent beer. You'll only really care about exact pitching rates if you are tweaking a particular recipe and comparing the resulting beers side by side say if you are trying to get more ester production in a beligan, for example. My point is you shouldn't worry about the exact numbers you'll get from a calculator as they aren't that exact anyways. I think MrMalty is a bit misleading in this sense and sometimes gives confusing and perhaps contradicting results. It doesn't matter anyways since there are so many more variables involved when making a yeast starter and the exact pitching rate doesn't really matter all that much anyways.
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