Low Oxygen Brewing

In the past weeks two of my followers have pointed me to a paper released by the folks from the German Brewing Forum. That paper talks about low oxygen brewing and how very low oxygen levels (so low that even very detail oriented brewers will have to change their brewing technique to achieve them) are responsible for what we have been calling the Elusive German Flavor in beer. That delicate malt flavor with a subtle background of fresh hops.

I myself have not been able to test any of this but am very intrigued to do so after reading through the paper. The main points are that one needs to deaerate the strike and mash sparge water, use sodium metabisulfite as an additional scrubbing agent and be very careful with doughing and any oxygen uptake later during the brewing process. A DO (Dissolved Oxygen) meter helps with keeping track of the O2 levels throughout the process.

If you think this is the HSA (Hot Side Aeration) discussion all over again, you are somewhat right with the caveat that there is new evidence and a new theory. This theory postulates that the current standard home and craft brewing process already allows for enough oxidation that additional splashing of the wort does not do any noticeable change. As a result HSA experiments have shown inconclusive results so far.

I have heard that this started to spark controversy and I can see that. If the authors are correct, and many seem to have been able to repeat the results, brewers will have to take a closer look at their brewing process and possibly equipment if they want to achieve these results. But keep in mind that not all types of beers are expected to benefit from this. There are many excellent beers, especially craft beers around the world, that are brewed in conventional brewhouses without a low oxygen process.  It’s more along the lines of decoction where an more elaborate brewing process is used for some beers to achieve the desired flavor profile.

At this point we have something new to try and to look into and need to see how this develops over time. Especially as these beers are showing up in competitions and we can see less biased results.

An anecdotal observation that supports this is that craft brewed beers in Germany (usually brew pubs) lack that delicate flavor that many commercially brewed beers in Germany have. Those commercially brewed beers are more likely brewed in a low-oxygen brewhouse.

Here is a link to the paper: On Brewing Bavarian Helles: Adapting to Low Oxygen Brewing

Back to brewing?

I have been taking a noticeable leave of absence from brewing over the last few years. Once I was done with presenting at the Homebrew Conference in Philly I felt that I had been spending so much of my spare time on brewing that I needed to take a break. A long break. After all, this is just a hobby and if that’s true I should be able to stop anytime I want. Deadlines are something left to my day job.

At said day job, responsibilities were also picking up and combined with other hobbies that left little time for brewing science and and brewing itself. While I brewed on and off to keep myself from buying every beer I drank, it felt more like a chore than a hobby.

Key to having fun brewing again was the move to 10 gal batches instead of 5. That reduces down the number of brewdays needed to supply myself to just one or two sessions per month with a nice opportunity for doing split batch fermentation experiments. And getting a 10 gal system to work efficiently is a whole new challenge. More on that in future posts

While I don’t want to jump back into the frenzy of brewing message boards, I might find the time to attend to some of the unanswered emails and comments.

At the time of writing this I’m enjoying a home brewed American Stout. I’m starting to like dark beers now and I had, and still have, a whole lot of dark grain samples from my malt pH experiments.


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.