Side by Side Starters 4 of 4

There are three big take aways from this collection of data.

1. For reasonable inoculation rates and gravities cell growth is not a function of inoculation rate or volume.  Cell growth is simply a direct function of sugar, and may be more accurately predicted by observing sugar consummation.
2. Yeast taken from storage at 40°F (5°C) can out preform yeast taken from an active starter likely due to the glycogen content of the yeast.
3. Attenuation is a function of both inoculation rate and gravity.  Higher inoculation rates and lower gravities lead to higher attenuation.

Growth

Cell growth observed over this wide range of inoculation rates and initial gravities showed cell growth proportional to the amount of initial sugar present.  In the case of the active culture of S-04 7.2 billion cells were grown per litter per initial degree Plato.  Considering that the average attenuation was 62% this indicates that 11.6 billion cells were grown per liter degree Plato of consumed sugar.  This tracks very well with the daily observations.  In a similar fashion the inactive starters grew 10.4 billion cells were grown per litter per initial degree Plato with an average attenuation of 68% making 15.3 billion cells grown per litter degree Plato of sugar consumed.  Both of these starters closely followed the equations derived from Ballings observation for starter cell growth.  Based on these observations the equation can be adjusted to account for the higher cell growth that is likely linked to proper aeration at the onset of inoculation.

Cells Grown = 14 * Volume of wort (Litters) * [Initial Gravity of wort (°P) - Final  Gravity of wort (°P)]

Glycogen

The yeast taken from refrigeration outperformed the yeast taken from a starter in terms of total attenuation and in cell growth.  When fermentation is allowed to run to completion yeast cells will build up a glycogen reserve.  It is possible that this extra glycogen allowed the refrigerated yeast to grow more cells given the same amount of sugar.  Further testing would be required to confirm this including measurement of the glycogen levels.

Attenuation

The percentage of sugar consumed seems to be a function of both the initial gravity and the inoculation rate.  A linear fit for these two parameters shows an excellent r squared fit of 0.9139 and 0.9795.  Although this shows wonderful correlation I am hesitant to say there is causation.  There are a other considerable factors, such as temperature, that are not accounted for here.  The equation also breaks down quickly at inoculation rates beyond the data set used.  It does seem reasonable at inoculation rates and gravities typically used for fermentation of beer.

The following equation can be derived from the linear fits:

A = Real Attenuation (as a decimal.  ie 0.71 = 71%)
G = initial gravity (in °P)
I = Inoculation rate (in million per ml)

A = 9.54E-4(G)-2.44E-4(G)(I)+4.23E-3I)+5.19E-1

Shaped markers are measured data points. 
The number of cells at zero initial gravity is the inoculation rate. 
The "Active" chart represents data taken from an active starter.
The "Inactive" chart represents cells taken from refrigeration.



Real Attenuation in %sugar by weight.
Each column is the number of cells used for inoculation. 
Divide by 10ml for the inoculation rate.