Pitching Rate Experiment

I finally got around to conduct a new set of experiments. These experiments focus on fermentation. Rather than brewing full batches of beer and change parameters I’m using wort left over from full size batches to run a few small scale fermentation experiments.

The first set of experiments focused on pitching rate. Since pitching rate is a parameter that has been evaluated by many brewers I didn’t expect too many surprises. I was mostly interested in testing my approach, in particular using the weight change of the fermenting beer as an indicator of the progression of fermentation. This weight drop is the result of escaping CO2.

300 ml of 12 Plato beer contain about 36g of extract. With an apparent attenuation limit of 80% about 66% or 23.8g of this extract is fermentable. About 1/2 of its weight is converted to CO2 of which the majority escapes. This means during fermentation the beer looses about 11g or 4% of its weight. This weight loss can easily be followed using a scale with an accuracy of 0.1 g. The scale I use is a cheap kitchen type scale with a capacity of 2 kg and a precision of 0.1g. It has shown excellent repeatability of weight measurements.

Setup

For this experiment WLP 830 yeast sediment from a fast ferment test was added in different amounts to about 355g of 13.1 Plato Pilsner wort. The wort was aerated to about 7.2 ppm O2 before it was divided into the individual 500 ml flasks for the experiment. The yeast was brought into suspension and pitching rates were determined though cell counts.

The beer was not agitated during fermentation and the flasks were closed with an airlock. The mean ambient fermentation temperature was 18 C. Weight measurements were taken twice daily. After 7 1/2 days, once the weight stabilized and approached the expected weight loss, the extract was measured using a hydrometer and the beer was filled into individual 12 oz bottles. Sugar syrup and some yeast was added to carbonate the beers.

To asses the level of yeast growth that happened during fermentation all beer was decanted from the flask and the flask with sediment was weighed. The empty weight of all flasks used was known and hence the weight of sediment could be determined. To estimate cell count a sediment density of 4 Billion cells per gram was assumed. This is a number that I found true for yeast sediments from fast ferment tests or yeast propagation.

Results


The following chart shows the beer’s weight drop (in %) during fermentation for all 5 pitching rates:

The following table shows metrics that were measured

test name A B D C E
stating extract 13.1 Plato
attenuation limit 80.0%
yeast strain WLP830 from slant
yeast source Bo Pils FFT
yeast age 2 days
fermentation type still, airlock, in 500ml flask
fermentation temp 18 C
starting O2 7.2 mg/l
pitching rate 32 23 15 9 2 M/ml
extract drop per day 1) 3.5 3.7 3.7 3.0 2.8 Plato
final extract 2.7 2.7 2.7 2.8 2.9 Plato
attenuation delta 2)
0.6% 0.6% 0.6% 1.4% 2.1%
yeast growth 36.0 30.7 37.5 33.9 29.3 B
growth per extract 0.77 0.66 0.81 0.72 0.63 B/g

1) the extract drop per day was for the most active part of the fermentation

2) difference between limit of attenuation and actual attenuation

Discussion

The progression of the extract or weight drop during fermentation was as expected: The higher the pitching rate the faster the fermentation start and the greater the extract drop during the most active part of fermentation. This has been observed before and is related to the greater initial yeast population.

It is also well known that beers with lower pitching rates have a more difficult time or take longer to attenuate fully. This was observed as a higher final extract for the beers with the lowest pitching rates compared to the beers with higher initial pitching rates. The differences were not very dramatic, though.

The amount of yeast growth showed a correlation to pitching rate. Higher pitching rates lead to more yeast growth (the initial cell population was considered in this analysis) but the differences in yeast growth were not large. The highest pitching rate yielded about 25% more yeast growth. This might be due to the fact that yeast growth was limited by sterol reserves. A large initial population has more combined sterol reserves than a small initial population which means that it can sustain more cell divisions.

I have not yet tasted the finished beer and also plan to measure the pH of each of these beers at that point.

4 thoughts on “Pitching Rate Experiment

  1. Kai,

    Nice experiment! I think you got what one would expect with different pitching rates; higher pitch rates would lead to full attenuation faster.

    I’ve done a similar experiment measuring gravity of three very different pitching rates:

    http://sciencebrewer.com/2012/01/31/pitching-rate-experiment-part-deux/

    and results:

    http://sciencebrewer.com/2012/03/02/pitching-rate-experiment-part-deux-results/

    I’m curious whether you see a lower final gravity for your lowest pitch rate compared to you highest pitch rate. In my experiment (repeated this twice!) both the underpitch and overpitch attenuated better than my control pitch rate.

    J

    • Jason,

      The final gravity was highest for the lowest pitching rate and highest for the higher pitching rates. But it could be argued that the difference is within the noise of the measurements.

      I read through your blog posts and I’m not sure if I can agree with Garret Oliver’s explanation. It is my understanding that a given yeast population always has 50% cells without scars 25% with one scar 12.5 % with two scars and so forth. I think I found that in a German brewing book a while back and it made sense to me. I also think that I got more yeast growth the higher the pitching rate was.

      The differences in final gravity, which you observed, were much more pronounced than mine. To get to the bottom of this, I think the experiment would have to be repeated.

      Cheers,
      Kai

      • Well, I’m not one to disagree with Garrett Oliver, and in non-quantitative manner, his explanantion makes sense to me.

        I think you have to think more of it as an equilibrium of new daughters to old cells with different pitching rates. Maybe your quoted figure of 50% cells without scars, 25% with one, etc, etc, may be pushed to one end of the spectrum or the other depending in pitching rate.

        If you do happen to repeat the experiment on a larger scale, let me know so we can compare notes.

        Cheers,

        J

        • I checked with James from Basic Brewing Radio who conducted a pitching rate experiment with his listeners. There was one case where lower pitching rates yielded higher attenuation:

          (yeast was Safale US05, a dry yeast)
          0.5g/gal = 1.005
          2.0g/gal = 1.007
          8.0g/gal = 1.009

          Kai

Leave a Reply

Your email address will not be published. Required fields are marked *


− 2 = four