I’m Brewing Again

I updated the look of the Braukaiser blog and never posted again. Until now.

After last year’s NHC and the work I did on yeast growth I just felt that I needed to take a break and take it easy with the brewing hobby for a while. And I did. And I ran out of home brew and had to buy beer at the store again, ouch. But a week ago I finally brewed again to get the pipeline started. Except for the temperature, I did not take a single measurement on that beer. No OG, no pH, no Fast Ferment Test. I used dry yeast and I may not even test the FG. But I brewed again!

So hopefully I may even come up with some interesting posts in the near future.

New Look

I decided to change the theme of the blog once again. I wasn’t to happy with the old theme and it also had some formatting bugs. Furthermore it wasn’t too well supported. So I decided to go with one of the standard WordPress themes. That will make maintenance and modifications much easier. The blog’s layout also looks much cleaner now. Let me know in case I broke something. One issue I know about is the missing side-bar on mobile devices. It’s actually not missing, it’s all the way at the bottom.

Yeast Growth on Malt and Sugar

Yet another experiment I did a while back. I wanted to see how yeast growth is affected by the ratio of malt extract and corn sugar. The motivation was not to find a way to save on the amount of malt extract needed for starters but to get an idea of how much yeast growth depends on the nutrients that are in wort in addition to the wort sugars.


Wort and a glucose (corn sugar) solution both with with an extract content of 10.6 Plato were prepared. Both solutions were mixed to yield growth media containing 100%, 75%, 50% and 0% malt extract. They were inoculated with WY2042 at a rate of about 0.3 B/g (billion cells per gram of extract or sugar).

This experiment was repeated with the addition of 3% DAP (diammonium phosphate as percent of extract weight) as an additional nitrogen source.

All starters were done under these conditions:

  • ambient temperature: ~20 C
  • volume: ~250 ml in a 500 ml flask
  • stirred and covered with foil


The following chart shows the yeast growth as billion new cells per initial extract (malt extract and added glucose)

Yeast growth over malt content in starter wort

Yeast growth over malt content in starter wort

With and without the added nitrogen there is a steady increase in yeast growth as the percentage of malt in the stater wort is increased. Pure sugar solution, even with the added nitrogen, did not yield any significant growth which is likely due to other nutrients (vitamins, trace elements) that are found in wort. The addition of nitrogen boosted the yeast growth, however, yeast growth was still limited by the malt content and an increase of the latter also increased yeast growth in the presence of additional nitrogen.

The added nitrogen (3% of extract+sugar weight) was about the amount needed to grow 1.5 Billion new cells per gram of extract/sugar if these assumptions are made:

  • yeast biomass formula: CH1.61O0.56N0.16 (source)
  • dry cell density: 20 B/g
  • DAP molar weight: 132 g/mol

Another interesting observation was the attenuation levels that each of the no DAP starters achieved:

Starter Apparent Attenuation over Malt Content

Starter Apparent Attenuation over Malt Content

The low attenuation of the all sugar starter is likely due to the virtual absence of growth in that starter. That resulted in a small yeast population which was not able to consume much of the sugar it had available. These starters were fermented for about 2 days. But even the 50% and 75% malt starters had lower attenuation than the 100% malt stater. That’s surprising since the apparent attenuation limit of the 50% malt starter is about 100% and the yeast population should have been large enough to completely consume all available sugars.


All malt wort provides vital nutrients for yeast growth. When these nutrients are diluted with sugar overall yeast growth suffers. The addition of additional nitrogen in the form of DAP (diammonium phosphate) can compensate for the reduced level of nutrients. But even with plenty of added DAP the best yeast growth was seen in all malt worts.

Yeast Growth Over Roasted Malt Content in Starter Wort

This experiment accompanies Yeast Growth in Hopped Wort and it examines the effect that roasted malts have on yeast growth. I did this experiment about a month ago but haven’t found time to write about it until now. The result was that roasted malts do have an impact on yeast growth but it is unclear if that’s due to less fermentable sugars or the inhibitory effects of the melanoidens.


Just like the hopped wort experiment I prepared 500 ml purely DME based wort and 500 ml DME+carafa wort. For the latter 40 g DME and 40 g Carafa II special (ground to a powder) were boiled with water. The resulting wort was filtered and adjusted to 10 Plato extract content.  An extract potenial of 75 % was assumed for the carafa. That means that the resulting wort is equivalent to wort from a grist with ~43% carafa II and ~57 % pale malt.

To create worts with increasing amounts of Carafa both worts were mixes as follows:

  • 100 DME wort: no carafa
  • 67% DME wort + 33% Carafa+DME wort: equivalent to about 15% Carafa in the grist
  • 33% DME wort + 67% Carafa+DME wort: equivalent to about 28% Carafa in the grist
  • 100 Carafa+DME wort: equivalent to about 43% Carafa in the grist

The mixed worts were inoculated with about 2 ml WY2042 yeast slurry. The counted cell density was 0.15 B/g extract. All starters were 250 ml wort in 500 ml flasks and left open while on the stir plate.

For this experiment the resulting yeast was not filtered and dried.



The results are shown below.


Yeast growth over roasted malt content

Yeast growth over roasted malt content

There is a clear relationship between roasted malt content and resulting yeast growth: the more roasted malt the less yeast was grown per extract.


At roast malt levels found in most dark beers (10 % or less) the impact on yeast growth is expected to be minimal and such worts should be suitable for yeast propagation. Extremely high levels of roasted malts should be avoided as the impact on yeast growth can be significant.

The experiment did not asses whether the reduced yeast growth was due to reduced fermentability (roasted grains provide less fermentable extract)  or the inhibitory effect that melanoidens are known to have on yeast metabolism.


Yeast Growth in Hopped Wort

In this experiment I looked at the influence that iso-alpha acids have on yeast growth. This was motivated by curiosity and the fact that there are a number of brewers, like me, who are using leftover wort from brewday for future starters.


For this experiment I prepared 500 ml of unhopped wort and 500 ml of heavily hopped wort. Both worts had an initial extract content of about 11 Plato. The hopped wort was boiled for 30 min with the addition of 4 g of German Magnum (13% alpha acid) pellets. The unhopped wort was also boiled for the same amount of time.

According to the Tinseth IBU model the hopped wort should have 290 IBU, but it is likely that the actual IBU number is less due  iso-alpha acid solubility and or isomerization limitations.

By mixing these two worts starters with increasing bitterness were created ranging from

  • 100% unhopped wort
  • 67% unhopped and 33% hopped wort
  • 33% unhopped and 67% hopped wort
  • 100% hopped wort

All starters were inoculated with 4 ml of yeast slurry from the same yeast culture. The initial cell density was estimated as 0.05 B/g, which is fairly low. All starters were left open and stirred at 20 C.

Yeast dry weight was determined by filtering a known amount of well suspended yeast in starter beer through 1 micron nominal filter paper. The filtered beer was clear which means that all yeast was held back by the paper. The paper and yeast was then dried using a microwave until all water was removed from the yeast. The paper and dried yeast was the weighed using a balance scale with a resolution of 0.01 g.



The chart below shows the growth as B/g for the 4 different starters


Yeast growth over starter IBU levels

Yeast growth over starter IBU levels


Because the yeast in the control flocculated and made counting difficult its yeast count was estimated using the dry weight of a filtered amount of suspended starter assuming 20 Billion per gram.  I’m fairly confident that this is correct since the the yeast from the 33% and 65% hopped wort starter both had a dried yeast cell density of ~ 20 B/g which also matches previous dried yeast measurements for WY 2042.

Yeast from the 100% hopped wort starter showed with 14 B/g slightly less cells per dried weight but the overall yield in yeast biomass was still less than the yield in less hoppy starters.

Biomass Yield Over IBU

Biomass Yield Over IBU


When looking at the biomass yield there is a constant decline as the IBU level of the starter wort is increased.


While hopped wort has a negative effect on yeast growth it is only significant at very high hopping levels. Thus the use of Pale Ale, Pilsner and even most IPA worts for brewing yeast propagation should not pose any problems. Very highly hopped worts may show a noticeable decline in yeast growth and are thus less well suited for yeast propagation.

NHC 2013 presentation

My presentation at the NHC today was very well received. Yeast growth in starters does interest a lot of brewers and I had a lot of new data to present. Now that I finished the presentation I can devote more time on experiment documentation here on braukaiser.com.

A copy of the presentation can be found here.

Starter Wort Gravity and Yeast Growth

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.

Experiment Setup

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.


Yeast Growth

The following chart plots the specific growth in Billion per gram of extract (B/g) for the 4 different experiments. WortOGAndYeastGrowthWhile 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.

Culture Viability

When the growth yeast populations were stained with methylene blue to asses their viability slight differences were noticeable.

20 Plato starter

20 Plato starter

10 Plato starter

10 Plato starter

7 Plato starter

7 Plato starter

5 Plato starter

5 Plato starter

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.


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:


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.


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.


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