Yeast Pitching Dilemma

I'm no expert in repitching, but some of the calculators have models that cover repitching slurry, including tips about how to estimate the thickness of (and thus the cells in a given amount of) a slurry. I'm thinking the Yeast book might too, but I don't know for sure and don't have it with me.

 

“It looked to me that the Wyeast/White Labs pack/vial of 100 billion yeast cells (max is what I thought it would have available at pitch time)…”

As regards Wyeast: “Wyeast’s 125 mL Activator packs are advertised to contain 100 billion cells, but actually average around 120–130 billion cells, according to Les Perkins, microbiologist and quality control manager for Wyeast.”

“ …could have a % less based on age of the pack/vial…”

Yes, yeast cell count will decrease with age. How many yeast cells die over a period of time (e.g., per month) is an area of debate. The Mr. Malty yeast calculator assumes that 21 billion cells die per month (for the first 4 months); this value is grossly conservative.

Cheers!

P.S. The values you quoted of 0.35/0.50 is certainly more appropriate than the value of 0.75.

 

More appropriate based on what?

 

http://www.wyeastlab.com/hb_pitchrates.cfm

I like to use the pictures/chart at the bottom to estimate the amount of cells in washed yeast. Keep yeast in fridge, measure the volume of yeast (ie using diameter of jar and height of yeast). Using these numbers, you get about 2.5 billion cells/mL of concentrated slurry, ie the cells at the bottom. Now you know how many cells you have. (note: the 2.5 billion # is consistent with slurry densities in Yeast.)

You don't know viability or vitality. One of our fellow BA members is studying this right now. You can use one of the proposed models (which may or may not be conservative) lacking other data.

Because of what I would have to guess is some loss in vitality of washed and stored yeast, I would certainly propose making a starter before use, unless it has been a very short time (less than one week) and you have a low OG (you don't need many cells). Be sure to let this starter go a bit longer (say 36 - 48 hours) - since it will have some lag, you want to let it completely finish before crashing again, otherwise you won't get the growth you are expecting.