The White Labs website says 2.5 Billion cells per ml. http://www.whitelabs.com/yeast/innovation
That figure is more than likely a conservative statistical mean that accounts for the difference in yeast cell size and varying amounts of non-yeast organic matter (the last thing that White Labs needs is some hot shot home brewer with a microscope and a hemocytometer posting that they are inflating their cell count numbers).
With that said, the difference between 2.5 and 3 billion cells per milliliter is in the noise due the nature of exponential growth. As I have mentioned many times, yeast biomass grows at a rate of 2n
, where n is the elapsed time since the culture left the lag phase divided by the length of the replication period. The reason why the biomass grows at a rate of 2n
is because yeast cells reproduce by budding, a process where a smaller daughter cell forms on the cell wall of its mother cell and then splits off.
If we look at the problem from a growth point of view, we can see how insignificant a difference of 0.5 billion cells per milliliter is in the grand scheme of things.
number_of_replication_periods_necessary_to_reach_saturation = log( saturation_cell_count / pitched_cell_count) / log(2)
The maximum cell density (saturation) for 1mL of wort is approximately 200 million cells; therefore, the maximum cell density for 19 liters (5 gallons) of wort is 19,000 * 200,000,000 = 3.8 trillion cells.
3.8 trillion cells = 3,800 billion cells
35 milliliters of slurry that contains 2.5 billion cells per milliliter has a total cell count of 35 * 2.5 = 87.5 billion cells
number_of_replication_periods_necessary_to_reach_saturation = log( 3,800 / 85.5) / log(2) = 5.44
35 milliliters of slurry that contains 3.0 billion cells per milliliter has a total cell count of 35 * 3 = 105 billion cells
number_of_replication_periods_necessary_to_reach_saturation = log( 3,800 / 105) / log(2) = 5.178
Increasing the pitched cell count to 200 billion cells
number_of_replication_periods_necessary_to_reach_saturation = log( 3,800 / 200) / log(2) = 4.25
Therein lies the reason why yeast cultures are like nuclear weapons in that one does not need to worry about being all that close to a calculated required cell count. It's also why the results from yeast calculators should be taken with a grain of salt.
Like all living organisms, no two yeast strains behave exactly the same under the same conditions. A strain will often behave differently in different brew houses. The best way to discern the correct pitching rate for a given strain in one's brew house is to pitch, observe, perform a sensory evaluation, take good notes, and repeat the process with a different pitching rate until the desired result is produced.
On another note, I am curious to know if White Labs separates 100% of the non-yeast solid organic matter from the medium. They definitely use an autoclaved growth medium. From what I can ascertain, the medium used to be malt extract based. The 20bbl brewing kettle that they recently installed is capable of 15 PSI above normal atmospheric pressure operation, and it appears to be plumbed such that it can pump autoclaved wort directly into the propagation facility.