Wednesday, March 6, 2013

Starter Calculators Revisited

YC-I : Yeast Calc (same as Mr. Malty) with "Intermittent Shaking"
YC-N : Yeast Calc (same as Mr. Malty) with "None" selected for Aeration
YC-SP : Yeast Calc (same as Mr. Malty) with "Stir Plate" selected for Aeration
S-04 : Fermentilist Safale yeast shaken only at the start of fermentation.
US-05 : Fermentilist yeast shaken only at the start of fermentation.
Balling : Karl Ballings observation applied to starters.
Wyeast : Calculator from W Yeast site

This data is a compilation cell counts on more than 50 starters.  The lower inoculation rate starters have a higher spread of values.  This deviation is  likely error due to the smaller magnitude of cells counted.  All starters were shaken vigorously at the onset of fermentation but otherwise left still.

When it comes to still starters, the easiest and most accurate way I have found to calculate cell growth is with simple multiplication, but if you have to use an online calculator then using the "intermittent shaking" selection seems to be the most accurate.

To calculate the number of cells to expect in a still starter the following eqation can be used:

Cells Grown = 10 * Volume of starter (in litters) * Gravity of starter (in degrees Plato)

Although only Yeast Calc is shown here the other two popular calculators have a similar growth curve.  Mr. Malty is almost identical in results to Yeast Calc because the latter is a derivation of the former.  The Wyeast calculator has a similar curve to the other two popular calculators.

Low Inoculation Rates

One thing that is most apparent when comparing measurements to the calculators is the performance at low inoculation rates.  The calculators all show considerable roll off in growth at inoculation rates less than 20 million per ml.  This could be due to time.  The lower the inoculation rate, the slower the fermentation will progress.  If the starter was used 48 hours after inoculation the cell count may very well be at the low levels shown by these calculators. 

Moderate Inoculation Rates

The center portion of the calculators is very flat.  For inoculation rates of 40 to 140 most of the curves are fairly flat.  This fits well with what would be expected of normal fermentation.  The primary limiting factor of cell division is available sugar making the cell growth directly proportional to sugar.  Mr. Malty and Yeast Calc have setting for various types of aeration.  Given more oxygen,  yeast will preform aerobic respiration. (1)  The larger amount of energy produced from aerobic reparation allows more bio mass to be generated.  In the presence of high sugar concentration the production of biomass is the preferred. (2) 

Cell growth is proportional to both sugar and available oxygen.

High Inoculation Rates

At high inoculation rates the calculators show diminishing return.  This indicates that the yeast are reaching a maximum cell density.  At cell density of 200 million per milliliter cell productions slows significantly.  At a cell density of 300 million per milliliter cell growth nearly stops.



  1. Steve, you should plot the inoculation rate based on yeast per extract (or sugar) as opposed to plotting it over yeast per volume. I think that way you can better incorporate the yeast growth in different gravity wort. All the yeast calculators you cited assume ~10 Plato wort.

    1. That would be a better metric. Instead of normalizing on the x-axis it is normalized on the y-axis. The data was all normalized based on the gravity recommended by the calculator. These were from 1.035-1.040.

  2. What was the range sizes of your 50 starters?

    1. These ranged from 30 million per ml to 120 million per ml which would be equivalent to one tube or pack of yeast in 0.8 to 3 liters. This is a compilation of several sets of experiments, but most were about 10ml in volume.

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