On the bottom, the x-axis is the wort concentration in degrees Plato.
The three plots represent inoculation rates of 39, 77, and 116 million cells per ml.
Several weeks ago I started an experiment to compare inoculation rates and wort concentrations to the number of cells produced in a starter.
Well, the results are in, and they are different than I expected. Cell growth followed fairly close to the "old school" rule of 10 billion new cells per litter per degree Plato. This is also roughly equivalent to to 1 billion new cells per gram of DME. Although, there was some correlation between inoculation rate and cell growth as well. The number of cells produced was roughly twice that on the predictions that the popular calculators estimate.
The biggest surprise was that yeast calc underestimated the cells grown by a factor of two.
This test was conducted with three different strains, in this post I'll cover the results of the US-05 strain. The graph above show a linear relationship between the amount of sugar in the wort and the number of cells produced. This holds for the three cases. The slope is approximately 13 million cells produced per ml or wort per degree plato. Another way to think of this is grams of DME to number of cells produced. Crunching the numbers indicate that 1 billion cells can be grown for every gram of DME. For a standard wort 1:10 by weight of DME that translates to 10ml per billion cells.
So it looks like the starter calculation can be much more simple that the calculators seem to indicate.
For US-05 use 1g of DME and 10ml of water for every billion cells you want to produce.
In addition to this experiment I have been keeping track of the number of cells produced by other starters, and for the most part they follow this simple rule. One thing to consider is that cell death can also play into the number of viable cells in the starter. If the yeast is allowed to sit in alcohol for a period of time after growth they will begin to die. There is a post on that coming up!