Yeast Growth and the Question of Quality vs. Quantity

In the Fermentation Test for Starter and Air Access Experiment I showed results that suggested that quantity in yeast growth does not necessarily mean best fermentation performance. I repeated these experiments with a different yeast (WLP 036, Duesseldorf Alt) and a pattern seems to be emerging. Like WY2042, WLP036 is a poor flocculator, which makes cell counting easier. Unlike WY2042, it is an ale yeast.

Below is a chart showing the specific yeast growth (Billion cells grown per gram of extract) plotted for 4 different starter configurations:

  • airlock, no access to air, except for what was in the head space
  • aluminum foil, loosely crimped
  • cotton ball to emulate a breathable stopper
  • open: no cover on the flask. This should be seen as best case for passive gas exchange but it is not necessarily practical for yeast propagation due to contamination concerns

 

Yeast growth over different propagation conditions

Qualitatively these results are very similar to the same experiment done with WY2042 as shown here. The open flask leads to an increase of about 40% over the airlock covered flask. But WL036  is able to grow more yeast per gram of extract compared to  WY2042. These are strain to strain variations that have to be examined in a different experiment.

 

Just like in the previous experiment the yeast grown in this experiment was used to ferment a high gravity wort. The original gravity was increased to 30 Plato (~ 1.130 sg) in order to increase the stress on the yeast. Just as before, the wort was prepared from dried malt extract (Briess extract light DME).

Once again the yeast with the least access to air took off the fastest as can be seen in the following plot of weight loss over the first 7 days:

Weight drop during the first 7 days of the fermentation test

But this time fermentation slowed down significantly well before the 80% attenuation limit of the wort. Based on the weight drop fermentation slowed down significantly at about 40% ADF (Apparent Degree of Fermentation). This time all the fermentation experiments were conducted in 500 ml flasks covered with an air lock. This does seem to lead to a better correlation between weight loss and attenuation.

On day 6 a refractometer reading was done on all 4 experiments and the result correlated with the weight loss at that time. In addition to the refractometer reading the health of the yeast population was assessed with methylene blue staining and all 4 yeast populations. The “airlock” yeast population showed about 10% lower viability (more stained cells) compared to the other 3 populations which showed virtually no stained cells:

WLP036_airAccess_day6_airlock

Micrograph of a methylene blue stained yeast sample from the “airlock” fermentation test 6 days after pitching the yeast. About 10% of the cells stained with MB.

WLP036_airAccess_day6_foil

Micrograph of a yeast sample from the “foil” fermentation test stained with methylene blue at day 6 after pitching. The “cotton ball” and “open” experiments also showed virtually no stained cells.

 

During an additional 2 weeks of fermentation, the yeast propagated with air lock cover remained stalled at about 5% weight loss while other yeast populations were able to pull ahead, albeit much slower than they did during the first 4 days.

WLP036_airAccess_21days

Weight loss during 21 days of fermentation

After 21 days the yeast covered with a cotton ball during propagation is showing the most attenuation so far.

The comparatively poor performance of the yeast grown with an open flask, which showed the most growth per initial extract, points to the conclusion that more growth during propagation is not necessarily better. At this point I suspect 2 mechanisms that could be at work here

  1. yeast propagated with an open flask has access to too much air and as a result becomes less efficient at working in a completely anaerobic environment. If this is the case yeast companies should not propagate yeast in aerobic environments, yet they do.
  2. While more oxygen during yeast growth allows for more growth, the amount of available nitrogen is limited. As a result yeast gown at a high growth rate without supplemental nitrogen will be nitrogen deprived which largely affects their protein level. This may lead to less efficient metabolism thus the slower fermentation rate that was observed.

I suspect that it is the reduces nitrogen level that causes the yeast poorly during the initial phase of fermentation. To test that I plan to repeat this experiment with the following 4 yeast propagation conditions:

  • airlock covered
  • airlock covered + DAP (diammonium phosphate) as an added nitrogen source
  • open
  • open + DAP

 

5 thoughts on “Yeast Growth and the Question of Quality vs. Quantity

  1. Cool experiment! I’m betting that your theory regrading oxygen/nitrogen imbalance is the culpret in the case of the open fermentation. What’s your guess as to why the cotton ball seems to achieve the highest attenuation–do you think it simply optimizes gas exchange?

    • Dave, what aspect of open fermentation are you referring to? As for “cotton ball” being the leader in attenuation I would not read too much into this. The experiment would have to be repeated to show that there is a true trend here.

  2. I think the effect you’re seeing here is that the yeast uses the oxygen to extract energy from the sugar much more efficiently. To get the same amount of growth it therefore needs to use up a lot less sugar. So you see that the weight loss is less, while growth is much higher than the other cultures.

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