Tonight I was finally able to do cell counts on an experiment that I wanted to do for a while now: “How does the access to air affect yeast growth in starters”. The experiment compared stirred starters capped with airlock, aluminum foil, no cover and injected air.
The setup was very simple. 8.9 Plato wort was prepared using DME and water (no hops) and inoculated with a 2 day old culture of WY2042 (Danish Lager). The innoculation rate was 10.8 B/l or 0.12 B/g (billion cells per gram of extract). The inoculated wort was split between four 500 ml Erlenmeyer flasks. Each flask received about 275 g wort.
All 4 flasks were placed onto identical stir plates set to identical speed in a temperature controlled incubator. The temperature during the experiment was 22 C.
The air injection was done with sterile air and the end of the glass tube was positioned about 5 mm over the wort surface. This was done to avoid foaming and allowed air to be absorbed through the vortex.
All starters were allowed to grow for 2 days. After that time no more CO2 escape was noticeable in the airlock covered starter.
Results and Discussion
The following chart shows the amount of growth as billion new cells per initial gram of extract that grew in each of the starters. The error bars are based on a counting error of 10% that was applied to both the initial and the final cell count. Given the small number of initial cells the error of the final cell count is dominant in this case.
It is apparent that increased access to air results in more yeast growth. In a previous experiment (not published) that compared an airlock against a foil coveted stirred starter the air lock covered starter showed a growth of 1.0 B/g for the air lock covered starter and 1.8 B/g for the foil covered starter. That result was more dramatic than the difference shown here.
The stream of air injected into one of the starters was so intense that it actually caused significant evaporation. It lead to a weight loss of 20% compared to the 2.2-3.2% for the other starters. Final cell counts considered the actual final volume.
The increased yeast growth could be caused by 2 different mechanisms.
- the more air that is available the more aerobic metabolism the yeast is able to perform. That means more energy for growth since aerobic metabolism is able to generate more ATP per mole of glucose than anaerobic metabolism. But in worts with high levels of sugars, as it is the case here, aerobic metabolism is limited by the Crabtree Effect.
- yeast growth in wort is limited by available oxygen for sterol production. That means that access to more O2 allows more cells to be grown.
This experiment is not able to shed light onto this and more experimentation is needed. Since yeast are able to absorb sterols from the growth medium the addition of olive oil to stirred and airlock covered starters could show if sterol synthesis is a limiter to growth in starters.
The better access a starter has to fresh air the more yeast can be grown. More experimentation is needed to better understand the limiters to yeast growth in starters.
While a completely uncovered flask is not practical for yeast propagation in a brewery it was included to show how how much can can be gained by not restricting the gas exchange. For practical yeast propagation a more sanitary alternative would be necessary.