More_Hops_Please
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- Feb 9, 2013
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Okay, so I did some kitchen counter science in the interest of seeing just how accurate BeerSmith 2 is at recommending starter sizes & pitch rates. As I had suspected, the results were interesting.
I was using 2nd generation yeast washed from a Robust Porter brew for these calculations. The yeast was Safale US-05 which had been slurried and decanted twice then left to flocculate and stored at 40° F for 40 days. The slurry was stored in 4 one-quart mason jars.
For the purpose of this starter, I only used one of the jars, which I estimated to contain about 50b viable yeast cells. I calculated that by multiplying the volume of my yeast solids(15mL) by 4.5b cells per mL(according to Jamil). I then calculated a 90% viability(about right for good washed yeast), and added a 20% fudge factor on account that at least some of my solids are non-yeast proteins and/or trub. Here's my math:
That's a VERY rough number, particularly because I don't know the actual ratio of yeast solids to non-yeast solids and I was just going with an educated guess. I do feel confident that if anything, I've OVER calculated my beginning yeast count, as I used very optimistic viability and density factors. Which is to say, I have at most 50b viable yeast cells, probably less, but certainly not more.
Okay, on to the starter. I pitched my flocculated 50b yeast cells to a 1.5qt(1400mL) un-hopped starter with an OG of 1.045 with no stir plate. Per BeerSmith 2, when the starter is finished, this should result in 127.8b viable yeast cells. So how many did I end up with? After all starter yeast had flocculated, I measured my yeast solid volume and calculated cells in the same manner. I had a final volume of 87mL yeast solid. Which calculates to:
Over 280 billion viable yeast cells! BeerSmith 2, you're a damn pessimist! But wait, this number is actually too low for one major reason: yeast solid to non-yeast solid ratio.
You see, I know the volume of solids I started with, and I know the volume of solids I ended with, but more importantly I know that the delta is almost entirely yeast solid. No trub or hop sludge was added to the equation, so any change in my solid count can only be attributed to yeast reproduction. Knowing that, lets take the yeast density out of the equation entirely and just look at how many NEW yeast cells we've produced, since that's the number we can have the most confidence in, and is going to give us a more accurate representation of our final cell count:
Counting new viable cells only(and 90% viability is actually rather low considering the age of these new yeast), we're up near 300 billion cells, which is very nearly what BeerSmith 2 calculates using a stir plate(remember, no stir plate was used here) in a 1.5qt starter.
So you're probably now thinking the same thing I was thinking... that seems way too high, something has to be wrong with these numbers. Well lets consider which numbers could be off? My volume measurements were taken with painstaking care, so I am confident in those. Maybe Jamil's estimate of cells per mL was too high? Could be, and there's no way to check without a hemacytometer, which I have to admit I do not own.
So lets say Jamil's number is too high for this particular strain of yeast... what if it was a very conservative 2.5 billion cells/mL? Well then I'd only have about 190 billion viable cells... which is still over 50 billion more than BeerSmith 2 predicts. The only explanation is that BeerSmith is WAY too conservative in their starter calculations(at least for this particular strain at this particular volume).
You know, before I started this, I was considering buying a stir plate. But now I'm thoroughly convinced that I don't need one.
I was using 2nd generation yeast washed from a Robust Porter brew for these calculations. The yeast was Safale US-05 which had been slurried and decanted twice then left to flocculate and stored at 40° F for 40 days. The slurry was stored in 4 one-quart mason jars.
For the purpose of this starter, I only used one of the jars, which I estimated to contain about 50b viable yeast cells. I calculated that by multiplying the volume of my yeast solids(15mL) by 4.5b cells per mL(according to Jamil). I then calculated a 90% viability(about right for good washed yeast), and added a 20% fudge factor on account that at least some of my solids are non-yeast proteins and/or trub. Here's my math:
15mL * 4.5b c/mL = 67.5b c
67.5b c * .9 viability = 60.75b c
60.75b * .8 estimated yeast density = 48.6b viable cells
67.5b c * .9 viability = 60.75b c
60.75b * .8 estimated yeast density = 48.6b viable cells
That's a VERY rough number, particularly because I don't know the actual ratio of yeast solids to non-yeast solids and I was just going with an educated guess. I do feel confident that if anything, I've OVER calculated my beginning yeast count, as I used very optimistic viability and density factors. Which is to say, I have at most 50b viable yeast cells, probably less, but certainly not more.
Okay, on to the starter. I pitched my flocculated 50b yeast cells to a 1.5qt(1400mL) un-hopped starter with an OG of 1.045 with no stir plate. Per BeerSmith 2, when the starter is finished, this should result in 127.8b viable yeast cells. So how many did I end up with? After all starter yeast had flocculated, I measured my yeast solid volume and calculated cells in the same manner. I had a final volume of 87mL yeast solid. Which calculates to:
87mL * 4.5b c/mL = 391.5b c
391.5b c * .9 viability = 352b c
352b c * .8 estimated yeast density = 281.9b cells!
391.5b c * .9 viability = 352b c
352b c * .8 estimated yeast density = 281.9b cells!
Over 280 billion viable yeast cells! BeerSmith 2, you're a damn pessimist! But wait, this number is actually too low for one major reason: yeast solid to non-yeast solid ratio.
You see, I know the volume of solids I started with, and I know the volume of solids I ended with, but more importantly I know that the delta is almost entirely yeast solid. No trub or hop sludge was added to the equation, so any change in my solid count can only be attributed to yeast reproduction. Knowing that, lets take the yeast density out of the equation entirely and just look at how many NEW yeast cells we've produced, since that's the number we can have the most confidence in, and is going to give us a more accurate representation of our final cell count:
87mL(final solids) - 15mL(starting solids) = 72mL delta
72mL * 4.5b c/mL = 324b c
324b c * .9 viability = 291.6 b new cells!
72mL * 4.5b c/mL = 324b c
324b c * .9 viability = 291.6 b new cells!
Counting new viable cells only(and 90% viability is actually rather low considering the age of these new yeast), we're up near 300 billion cells, which is very nearly what BeerSmith 2 calculates using a stir plate(remember, no stir plate was used here) in a 1.5qt starter.
So you're probably now thinking the same thing I was thinking... that seems way too high, something has to be wrong with these numbers. Well lets consider which numbers could be off? My volume measurements were taken with painstaking care, so I am confident in those. Maybe Jamil's estimate of cells per mL was too high? Could be, and there's no way to check without a hemacytometer, which I have to admit I do not own.
So lets say Jamil's number is too high for this particular strain of yeast... what if it was a very conservative 2.5 billion cells/mL? Well then I'd only have about 190 billion viable cells... which is still over 50 billion more than BeerSmith 2 predicts. The only explanation is that BeerSmith is WAY too conservative in their starter calculations(at least for this particular strain at this particular volume).
You know, before I started this, I was considering buying a stir plate. But now I'm thoroughly convinced that I don't need one.