This is an experiment I conducted a few weeks ago. It examines the effect that the starer original gravity has on yeast growth. Only yeast growth and viability based on methylene blue staining were examined.
For this experiment a 20 Plato wort was prepared from DME and inoculated with a culture of WLP 036 (Duesseldorf Altbier) at a rate of about 0.04 B/g. After inoculation and thorough mixing the wort was divided into four 500 ml flask. Each flask received about 100 g of the 20 Plato wort. To 3 of the 4 flasks additional water was added. The amount of water added was 100 g, 200 g and 300 g, respectively.
The following chart plots the specific growth in Billion per gram of extract (B/g) for the 4 different experiments. While the 20 P/100 ml experiment was able to develop the best vortex due to the lower wort level, it’s growth was significantly less than that of the other experiments with lower gravity and higher wort levels. Yeast growth saturates at 3 B/g but from this experiment it is not apparent if this limitation is caused by the culture’s access to air or another limiting nutrient. That other limiting nutrient could be nitrogen.
During propagation hardly any vortex was visible in the 5 P/400 ml starter due to a volume that approached the capacity of the flask (500 ml) yet it still showed the same growth as the 10 P/ 200 ml experiment where the vortex was able to draw in more air. A previous experiment (Stir Speed and Yeast Growth) showed yeast growth changes as the stir speed and with it the size of the vortex changes. However, this experiment was done with a different yeast (WY2042) and the yeast used here (WLP036) may hit maximum growth in wort with lower oxygen uptake than WY2042.
When the growth yeast populations were stained with methylene blue to asses their viability slight differences were noticeable.
Yeast grown in the 20 Plato starter showed a viability of about 90% while populations from the other 3 starters had close to 100% viability. That was to be expected given the toxicity of high alcohol environments on yeast cells.
The experiment did not uncover much new information. It showed that high gravity worts but lower starter volumes to not result in the same amount of yeast growth compared to a lower gravity starter with more volume despite their potentially improved access to O2. Furthermore, the resulting higher alcohol concentration in high gravity starters is likely to reduce the viability of the culture.
Nice study, Kai.
I wonder if the higher OG would result in cells with higher alcohol tolerance and thus able to ferment bigger beers…
I think that would be worth a try. I do have a method for testing that, but I’m starting to run out of time until the NHC presentation in Philly. Under alcohol stress, yeast is known to build stronger cell walls if the necessary sterols or oxygen is available. That might be the case in a starter.
Steve over at Woodland Brewing has some interesting data on acohol tolerance and his method might be useful for testing the yeast (http://woodlandbrew.blogspot.com/2013/01/abv-effects-on-yeast.html).
Kai, thanks for the informative post. Would it be beneficial to redo this experiment with only one variable (gravity), rather than two variables (gravity and volume)?
I thought about this, but deemed the results more applicable to brewing practice if I keep the amount of extract the same while varying the gravity. In general brewers want to grow a given amount of yeast and that is the largest factor in determining how much extract is needed.
My takeaway from this is that there doesn’t seem to be much benefit from using starter concentrations below 10P. I’m wondering at what point between 10P and 20P does growth rate and viability start to drop. I brew small batch “starter beers” in lieu of starters most of the time, and I’m wondering how high I can push the gravity before I start to affect yeast health for the next pitch.
Eric, my only concern about that method is the difference in outcomes. With a “starter beer” the beer itself is the outcome simply by its nature. You are encouraging fermentation simply by the unstirred nature. With the stirred starter, you are encourging growth by unlimited access to oxygen and mitigation of dissolved CO2.
It would seem to generate a great deal of yeast, but is it the most healthy yeast possible? Probably not. This is my theory anyway.
Did you done some experiment comparing the growth between a wort prepared with sugar and one with DME ? Some pesoas say that starter made with sugar, the number of viable cells is higher.
Actually I did. And the results were interesting. I can blog about this next. There is some data in the backup slides of my NHC presentation that I posted recently.
Kai: I think you need to repeat the 20 P vs 10 P experiment a few times to actually get some statistics on this point. A one-off experiment is a good start, but to make the point credible, you need more data.
If you innoculated at .04 B/(g extract) and, for the high yield gravities, you yielded 3 B/(g extract), does that mean that your yields were 75 times the innoculation amount?
Yes, that has been my observation. The less yeast you start out with the more it will duplicate while consuming all the available nutrients.