Here is the information I have from White Labs about primary fermentation with WLP644. As you can see, you probably didn't under pitch. You also can't really use calculators to estimate Brett growth that were designed to be used with Sacc yeast. Check this info, and in the future, I find emailing White Labs to be a very good way to get decent information about their products.
From White Labs
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The cell count on the Brettanomyces we offer is 1.0e8 cells/ml or higher. A home brew vial contains 35ml of slurry so that leaves the vial with a total count of 3.5 billion cells, meaning that for the first time ever, the internet is basically right!
As far as how many cells you need, I would always side with more when it comes to Brett. It is much slower acting than Saccharomyces strains and a higher pitch rate can help yo achieve your final gravity quicker. The Brett is very high attenuating, so although it takes a while to get there, it will usually ferment all the available sugars resulting in a dry finish.
Typically we would call for 1 liter per BBL (31 US gallons) on a primary ferment and this would give you about 852 million cells per liter or 850,000 cells/ml after inoculation. A secondary ferment rate would be almost one fifth of that rate.
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First Follow Up Below
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The 850,000 cells/ml should leave a 20L batch in need of 17 Billion. Yes, you are right in that direct pitching is not an option with only a single vial. This means it would require and ideal pitch of about 4.8 vials direct, or possibly a single starter.
The pitch rate calculators are taking a lot of information for granted. The oxygen content of the wort, the temperature, the viability and vitality of the seed yeast, and many more factors can cause there to be a large difference in the projected growth and the actual counts of the propagated yeast. This is why there is such a discrepancy between the many calculators out there. The only way to be positive of a count is to use a microscope with hemocytometer or even more complex equipment.
The fact is, without direct pitching, or a microscope, you will only be guessing at the rate you are pitching. This does not mean that you will not have good results, it is just more difficult to replicate exactly.
The starter size and number of steps is subjective to the rate of cell growth. As you can see in your calculator, the target pitch rate can vary based upon the desired cell counts. The calculators are also taking the typical cell count of Saccharomyces strains which (for White Labs) has a cell density of about 2.5 billion cells/ml. This is much more than the 1.0e8/ml than the Brett is counted at.
With a normal 1 liter sized starter you should have sufficient growth to inoculate for a 20L batch. The count will be unknown unless you are to use a microscope and hemocytometer. Alternately, if you direct pitch 5 vials, you should have about 900,000 cells/ml after inoculation.
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Then I got this information as another follow up
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In reality there is very little difference between a beer at 1.050 and a beer at 1.055. The pitch rate is not going to make a reasonable impact between these gravities. This is part of the reason the calculators tend to skew out of acceptable ranges, especially at the extremes.
I am not sure why you think the 17 billion is odd. Yes a Saccharomyces would have a drastically different pitch rate, but it is an entirely different organism, so it should be different. Based upon the known successful dosage rates of the Brett at 1 liter per BBL (for 100% Brett ferments), I reverse engineered the cell counts. This is where I reached the 850,000/ml number. Multiplying that out by the 20,000 ml in a 5 gallon batch (approx), I reached 17 billion total cells required to reach a saturation of 850,000/ml.
You are correct in that the "Rule of Thumb" is measured as cells/ml/degree plato. I can call it 71,000 cells/ml/degree plato if that helps. Pitching rates are not a defined set of rules. People may manipulate the rate depending on the desired outcomes. In practical application, a single degree change in gravity does not usually require a change in pitch rate, however drastic changes may require modification in the pitch rate. Often we recommend a larger pitch rate for beers above 1.065.
We have found the pitch rate we offer is sufficient and successful for beers ranging from 11*p to 16*P
In the end the "exactness" of these pitch rates will be impacted by the ability to measure them properly. If you can't measure the difference between 17 billion cells and 18 billion cells, then it will not matter that your brew is 1 degree higher or lower in gravity.
Very few brewers are actually having their pitches adjusted based upon the gravity of their wort. Most simply use a sized pitch for all their applications, aside from maybe a high gravity beer (above 1.065) or a larger pitch for lagers. They will then use aeration as a method for controlling the growth of the yeast.
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