## Saturday, January 19, 2013

### Starter Cell Growth

On the Left, the y-axis is the number of new cells in millions per ml.
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.

http://woodlandbrew.blogspot.com/2012/12/how-many-cells-are-produced-by-starter.html

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!

1. So it looks like you're getting the amount of growth Jamil's calculator predicts with a stir plate, without using a stir plate. Did you aerate the starter at all?

1. Yes, directly following the pitch the container is shaken until 75% or more of the volume is foam. This should achieve approximately 8ppm of oxygen. After that no further oxygen is introduced.

2. Thanks for running these experiments Steven. I very much welcome if others are looking into starter yeast growth and don’t blindly trust Jamil’s calculator.

Where these still, shaken or stirred starters? It’s not obvious from your write-up, but the numbers seem to align with what I found for stirred starters (http://braukaiser.com/blog/blog/2012/11/03/estimating-yeast-growth/). But there are some differences. When I plotted your data as new billion cells per gram of extract over initial billion cells per gram of extract I noticed that you are getting much more growth at high initial cell densities than I do. My experiments, done with liquid yeast, showed a drop in yeast growth around 1.5 billion initial cells per gram. In your data even with 4 B/g initial cells you were getting ~ 1 billion new cells per gram of extract. I think this is because you were working with dry yeast and I was working with liquid yeast. They both are in different metabolic states when they are put into fresh wort. The dry yeast doesn’t have to replenish reserves before it can start growing which allows it to grow more when there is less extract per yeast cell.

Kai

3. Thanks Kai!

What you are suggesting sounds reasonable. The results I have here are far from conclusive, but hopefully shows a relation between sugar concentration and cell growth. Further experimentation is necessary.

The yeast was re-hydrated US-05. Twenty five test tubes were inoculated and allowed to culture at ambient air conditions for about two weeks. There were an additional two higher inoculations that showed unusual results that I could not interpret, so rather than try to explain something that I did not understand I did not report that data thinking that there must have been some error. These may follow what you saw.

There are a number of other variables to consider. Two weeks ensured that the cultures with the lowest inoculation rates and highest sugar concentration completed, however the cultures on the other side of the spectrum were sitting at completion, in the alcohol they had produced, for a long period of time.

But that's just the tip of the iceberg. It seems every experiment leads to a whole slew of follow up experiments.

4. Interesting that these were not stirred. I wonder if you saw the high growth b/c dry yeast comes with lots of internal reserves or because the tall beer column caused circulation that was equivalent to stirring. The aspect that dry yeast has more reserves than liquid yeast should only matter foe the higher inoculation rates.

Are you also seeing the same growth rate for non stirred regular sized starters?

Kai

1. I've been keeping a log of my starters as well. Some of them to follow the 10 million per °P per litter. Unfortunately many of my starters are experimental in some way, so I'm not sure that it indicates correlation.