Thursday, December 12, 2013

Calculating Slurry Volume for Starters

Earlier tonight -- two days prior to my upcoming Namur brew day -- I made a starter using a portion of the cake harvested from Batch 02 on November 23, 2013.  As I don't have the proper equipment for properly measuring cell counts, I did a workaround that should hopefully be sufficient for now.  To do this, I utilized YeastCalc.

First, I put in my harvest date and OG into YeastCalc, which told me that I needed 175 billion cells.  I then changed my initial starter volume (no aeration) to 1.0 L.  After fiddling with the numbers in the "Liquid Yeast Properties" box, which is where I initially found my viability percentage, I discovered that having an initial cell count of 160 billion would give me 131.19 billion viable cells, which would then yield 175 billion viable cells with the 1.0 L starter.

Once I knew that I would need 160 billion cells from my population, I used the measurements provided by Wyeast for repitching to determine my cells per mL of slurry.  This indicated that there are 1.2 billion cells per mL when the slurry is 40-60% solids.  I estimated my slurry (below) to be 75% solids, so I figured that I had roughly1.8 billion cells/mL.  Doing the math to get to 160 billion cells, I determined that I needed 88.89 mL of slurry from the jar, and that's what I pitched into the starter.

Slurry from Namur (Batch 02) in a 500mL mason jar.

I realize that this method is pretty crude and relies on a lot of estimates, so I have some bigger plans for the future.  My initial thought, and we'll see how this works out, is that I will take two test vials of White Labs yeast.  Given that these purportedly contain roughly 100 billion cells, I will empty these into a coffee filter to drain out the liquid.  I'll then take measurements of the weights of the resulting yeast, and determine the weight of 100 bill cells without any trub, averaging the weights from each vial.  From there, I can then drain out my yeast cakes in the same manner and determine pitching rates based on weight.

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