confusion about yeast starter formula

[DillyB]

i have recently been studying yeast starters, and have came across this from Northern Brewer http://www.northernbrewer.com/documentation/YeastPitchingRates.pdf

to calculate yeast starters. If i use their formula i am getting contradicting information.

I opened a sample ale from BeerSmith and using its numbers to enter into this formula as an example.

AVERAGE PITCHING RATES
ALES
OG < 1.055 (<14°P) = ~0.5 MILL CELLS/ML/°P OG>1.055(>14°P)=~1.0MILLCELLS/ML/°P
LAGERS
OG<1.055(<14°P)=~1.0MILLCELLS/ML/°P OG>1.055(>14°P)=~1.5MILLCELLS/ML/°P

NOTE - mill cells/ml/°P = Millions of cells per ml of wort per degree plato. For example, an ale of 12°P would use approximately 6 mill cells/ml/°P

this sample Beersmith recipe has an estimated OG of 1.074 which gives it a degrees plato of 18.

according to their formula 18 x 0.5 = 9 mill cells/ml/°P correct?


YEAST CELLS NEEDED:

Pitch rate chosen: ___ millions of cells/ml/°P x 1,000,000 = ___ cells/ml/°P

Predicted OG: ___ / 4 = ___ degrees Plato (°P)

Wort volume: ___ gal x 3785 ml/gal = ___ ml

Total cells required = ___ pitch rate x ___ °P x ___ ml = ___ cells required

Convert to billions of cells: ___ cells / 1,000,000,000 = ___ billions of cells


This is their blank formula. below is my numbers filled in



YEAST CELLS NEEDED:

Pitch rate chosen: _9__ millions of cells/ml/°P x 1,000,000 =9,000,000 cells/ml/°P

Predicted OG: 1.074___ / 4 = _.2685__ degrees Plato (°P)

Wort volume: _10__ gal x 3785 ml/gal = __37850_ ml

Total cells required = _9,000,000__ pitch rate x _.2685__ °P x _37850__ ml = _91464525000__ cells required

Convert to billions of cells: _91464525000__ cells / 1,000,000,000 = _91__ billions of cells

I am only needing 91 billion yeast cells. Their information above the formulas in the document i provided from northern brewer says the white labs has about 100 billion per tube.Meaning i don't need a starter? The receipe actually calls for 4 pkg of white labs california ale yeast at 100 billion per tube. thats 400 billion cells, not 91 which I'm getting from the formula.

Is the predicted OG the predicted original or predicted final? am i doing this completely wrong? Thanks so much for the help

 

[LLBeanJ]

You'll need WAYYYYY more than 91B cells. I'm not sure where the math went wrong, but something is definitely not right.

According to Yeastcalc, 10 gals of 1.074 OG requires about 510B cells.

And, yes, you go by OG and batch size to determine how many cells are needed.

Edit: In studying the formulas above, I see where you went wrong. When you divide the OG by 4 to get plato, you need to take the points to the right of the decimal and divide them by 4. So, 1.074 = 74 points / 4 = 18.5.

Edit2: Also, your pitch rate of 9 million cells/ml/°P is too high. If following the rule given above for pitch rate based on an ale OG >1.055, you should be going with 1 million cells/ml/°P.

Therefore:
YEAST CELLS NEEDED:

Pitch rate chosen: _1__ millions of cells/ml/°P x 1,000,000 =1,000,000 cells/ml/°P

Predicted OG: 1.074___ / 4 = _18.5__ degrees Plato (°P)

Wort volume: _10__ gal x 3785 ml/gal = __37850_ ml

Total cells required = _1,000,000__ pitch rate x _18.5__ °P x _37850__ ml = ___ cells required

Convert to billions of cells: _700,225,000,000__ cells / 1,000,000,000 = _700__ billions of cells

Additionally, it is more common to use .75 million cells/ml/° for ales (and 1.5 million for lagers), which would be 700B * .75 = ~525B cells, which is pretty close to what yeastcalc comes up with.

 

[DillyB]

thank you so much LLBeanJ , i understand where i went wrong now.

 

[LLBeanJ]

You're welcome.

A little more...

The discrepancy between yeastcalc and the NB formula is almost entirely a result of the OG to Plato conversion. OG points / 4 is a close approximation, but a more accurate conversion would be:

°P = 259-(259/SG) --> 259-(259/1.074) = 17.85°P. (source: http://www.winning-homebrew.com/Plato-to-specific-gravity.html)

Plug 17.85° P into your example and you'll find that the total cells needed is 507B, which is only 3B less than what yeastcalc came up with.

 

[DillyB]

yes, I've actually come across that formula for calculating plato also. i figured the one on the northern brewer document is a "dumbed down" version or just one to get you close. Thanks for all the help

 

[flars]

Have you looked at the calculator on Brewers Friend site.