I finally ran a brewing experiment again and am dusting off this blog to post the results.
The experiment was a repeat of a standard experiment that evaluates yeast growth in still, intermittently shaken and stirred starters. I like how easy it is to set up and plan to repeat it again.
919g of 12 Plato wort were aerated to 100% air saturation on a stir plate. Then 7.8 g WLP 830 yeast sediment from a primary fermentation was added and allowed to un-flocculate completely. The cell density was determined as 22.3 Million/ml with a hemacytometer. If you do the math you find that the slurry contained a total of about 20 Billion cells which means its density was 2.63 Billion/g. Previous slurry density assessments for slurries from yeast propagation ranged from 4-5 Billion/g for this yeast. I assume that trub played a big role in the reduced cell count in this slurry.
Once the yeast was evenly distributed in the wort, the wort was evenly divided into 3 500 ml Erlenmeyer flasks. The flasks were labeled with their empty weight and covered tightly with aluminum foil. I didn’t tighten the cover intentionally. I noticed that this happens when I grab the flasks at their neck to shake them. I made sure that all of them had tightened aluminum foil caps, even though only one would be shaken intermittently.
All starters were allowed to complete fermentation. One sat still, one was shaken intermittently (1-2 times in the morning and 3-5 times in the evening) the last one was placed on a stir plate and stirred continuously. A vortex formed on the stir plate, but was eventually covered with foam when CO2 started escaping. The ambient temperature was around 18 C.
Once fermentation was complete the yeast was allowed to settle into a dense cake until the beer on top cleared. The beer was poured off and the flask’s weight with yeast and stir bar (in one of the flasks) was determined. From that, the empty weight and the stir bar weight the yeast weight was determined:
|starter type||total extract in starter (g)||final yeast weight (g)||estimated sugar utilization (Billion cells per g)|
I only recorded the yeast weight and did not count the yeast with the microscope. Simply, because I did not want to spend too much time on this. To estimate the number of cells grown I assumed that the sediment had a density of 4.7 Billion/g, an average of the numbers I had assessed in earlier experiments.
Discussion and Conclusion:
The wort volumes for the three starters were not exactly the same, hence the difference in the extract weight.
I was surprised to see that the sill and shaken starters showed no difference in the amount of yeast that was gown from the available extract. This is different from the data reported by others (e.g. the great Maltose Falcon article on yeast propagation: Yeast Propagation and Maintenance: Principles and Practices) where a shaken starter grew significantly more than a still starter and a stirred starter outperformed a shaken starter by about 4x. The caption to the chart mentions that the data point for the still starter was taken from a different experiment, though.
Data I recorded on past yeast propagation steps (usually stirred) and fast ferment tests (usually shaken) show more difference in extract utilization for yeast growth, but there are also a number of other parameters that have been different for those experiments. Most notably the starting gravity and the initial cell density.
I’ll have to repeat this experiment in the future to see what covering loosely with aluminum foil does and what an airlock would do to the extract utilization that can be achieved. And maybe I also find the time to count the final number of cells.