W&Z's "Yeast" book gives some good data on propagation rates based on innoculation rate at 1.030. Then the authors say "If you are using a stir plate .. the yield will be higher." Thanks guys. How much higher? Empirical answers I can find online vary from 1.53x (mrmalty.com) through 10-15x (http://www.pivarstvo.info/forum/files/yeast_propagation_and_maintenance_128.pdf
). Looking at figure 1 in the latter shows a 5x increase in cell count for stirring over occasional shaking.
I started with some yeast from the end of October, so about 50% viability, or 50 billion cells. I pitched this into 2L of 1.030 wort, for a pitching rate of 25 million cells/ml. W&Z's figure 5.5 gives a doubling factor of 1.8, meaning that I have added 1.8x50 = 90 billion cells, for a total of 140 billion. However, that's for an un-aerated starter. Since I used a stir plate, instead I have somewhere between 214 billion and 2.5 trillion cells, with MB Raines' 5x putting it at 700 billion. So that's pretty unhelpful!
Now, I'm happy to RDWHAH and pitch what I've got, but the engineer in me wants to know what's actually happening. Does anyone have any guidance? Or suggestions to how I could measure this myself without the expense of a cytometer? Perhaps a graduated cylinder, cold-crashing, and some heuristic for the density of compact yeast cake?