Tag Archives: mixed fermentation

Experiment Time: Does Lactic Acid Skew Refractometer Readings?

In a Facebook group I follow, an interesting problem came up. Somebody had brewed a mixed-fermentation Berliner Weisse with brewer’s yeast, lactobacillus brevis and brettanonymces claussenii. They didn’t own a saccharometer, but instead determined their OG with a refractometer. They now wanted to know whether fermentation was finished, and used the refractometer as well to measure the beer that had been in the fermenter for several weeks.

Before I continue, a quick excursion into how refractometers work. Every translucent material bends light to a certain extent, the light gets refracted. To describe the extent with which the light is bent, the so-called refractive index is used. The refractive index n is defined as n = c / v, where c is the speed of light in a vacuum, and v is the speed of light in the particular medium. Water for example has a refractive index of 1.333. When we dissolve sugar in water, the refractive index of the solution is increased. The refractive index of a 10% glucose solution in water is 1.347, for example. This change in the refractive index can be used to indirectly measure the OG, by looking at the difference between the expected refraction of water vs the measured one, i.e. by how much more light gets refracted.

In fermented beer, this gets trickier, because due to fermentation, the resulting liquid contains ethanol. Ethanol has a refractive index of 1.361, which skews the overall measurement. Ethanol also skews the measurement when determining the FG of a beer, as its specific gravity is just 0.79. In typical fermented beers, the amount of ethanol is large enough that we need to correct our measurements to be able to estimate the actual FG. This is all fine.

The mixed fermentation opens up another problem, though. It has a refractive index of 1.427 which significantly higher than that of either water or ethanol, but in typical fermented sour beers, its content by weight is fairly low compared to e.g. ethanol. According to this presentation, typical Berliner Weisse contains 2 to 4 g/L of lactic acid, i.e. 0.2 to 0.4% by weight.

When I read about the issue of seemingly underattenuated Berliner Weisse, one of the things that came to my mind was exactly whether the lactic acid from the mixed fermentation skewed the measurement enough to cause such a large disparity that a beer that is expected to be overattenuated to come up with an apparent attenuation of just 68%.

So I asked myself the question: if I added the typical concentration of lactic acid in a Berliner Weisse (i.e. 2 to 4 g/L) to distilled water, by how much would my refractometer be skewed?

I have a refractometer at home, 2 litres of distilled water, a big bottle of 80% lactic acid, and pipets with which I can measure out millilitres of lactic acid. But how many grams is a millilitre of lactic acid? A millilitre of pure lactic acid weighs 1.357 grams, therefore a millilitre of an 80% solution would weigh would weight about 1.285 grams. Since I want to test a whole range of lactic acid content, just measuring out by ml is good enough for me.

I poured 1 litre of distilled water into a clean vessel, and calibrated my refractometer so that it shows exactly 0°Brix. I then added 1 ml of lactic acid (= 1.285 g), stirred it well, and measured again with the refractometer. I measured X°Brix. I then repeated this to up to 5 ml of lactic acid (= 6.425g), and got the following measurements:

  • 1 ml (~ 1.3g): 0°Brix
  • 2 ml (~ 2.6g): 0.2°Brix
  • 3 ml (~ 3.8g): 0.4°Brix
  • 4 ml (~ 5.1g): 0.4°Brix
  • 5 ml (~ 6.4g): 0.6°Brix

This was actually less skew than what I had expected. For a quick counter-check, I added 80% lactic acid on the refractometer, and the measurement was off the scale.

Luckily, I have a Berliner Weisse maturing at home. I brewed it a few months ago, mixed fermentation with S-04, Lacto brevis and Brett bruxellensis. I simply lautered and sparged 30 litres of wort from a 50% Pilsner/50% pale wheat malt mash, which turned out at 11°P OG (unboiled, of course). On my refractometer, I measured 5.2°Brix, while with my saccharometer, I measured 2.0°P. I used calculators to get the expected attenuation based on the OG (in Plato) and FG (in Plato) resp. refractometer reading (in Brix). For both values, I got almost the same level of attenuation (81.82 vs 82.11 apparent attenuation) as well as almost the same ABV (4.7% vs. 4.72%). This is fairly consistent with what I’ve measured earlier, namely that the amount of lactic acid in Berliner Weisse has very little impact to skew a refractometer measurement.

Nevertheless, I think it goes without saying though that a brewer should never ever rely on a refractometer alone. While I use one during my home-brewing, I only ever employ it to measure sugar content ad hoc during the brew day: it’s useful to observe saccharification of your mash, the strength of your first runnings, as well as the sugar content in your final runnings, or to get a good idea about the pre-boil gravity and post-boil gravity of your wort. It’s a tool that has its place, but for observing the progress of fermenting beer, I think it’s a much better idea to just use saccharometers. Even quite precise ones, with a scale down to 1/10 of a degree Plato, and thermometers for further correction of the measurement, are not exactly expensive.

How to Brew my Award-Winning Berliner Weisse

I recently brewed a Berliner Weisse, submitted it to SLOSH SOUR 2019, a homebrewing competition dedicated to Berliner Weisse, and I won! So here I’m documenting how I brewed and what’s the rationale behind it.

Since the homebrewing competition was about brewing straight Berliner Weisse, with no twists and no innovations, I decided to simply follow how Berliner Weisse used to be brewed according to historic brewing literature.


Naturally, I wanted to use large amounts of pale wheat malt. Historically, the wheat malt used in Berliner Weisse was high in protein and quite undermodified. Malt like that is practically impossible to get, so instead, I chose to use a large amount of chit malt. To bulk the grist up a bit more, I also added some floor-malted Bohemian Pilsner malt. The final grist looked like this:

  • 2 kg pale wheat malt (Weyermann)
  • 1 kg chit malt (BEST)
  • 0.5 kg floor-malted Bohemian Pilsner malt (Weyermann)


Berliner Weisse is classically mash-hopped, so I simply added 6 g of Tettnanger hops (3.4% alpha acid) to the mash.


I mashed in all the malt with 10 liters of hot water. To stay authentic, I used untreated Berlin tap water. I slightly underestimated the required temperature, so the resulting mash temperature was 46°C. I then heated it up to 53°C and did a protein rest of 40 minutes.

I then heated up the mash up to 62°C, and did a brief saccharification rest of 15 minutes, then further heated it up to 72°C to rest and convert for 45 minutes. After these 45 minutes, the mash was completely converted.

Now this is obviously a weird mash schedule, but in fact I based it on the mash schedule documented by the former Groterjan brewmaster A. Dörfel. At Groterjan, an initial protein rest was done at about 53-54°C, then the mash was slowly heated up to 75°C. I did a brief 62°C rest because I assumed that on my homebrew kit, heating up would be much quicker than on Groterjan’s big kit. I also kept the temperature a bit lower to prevent any issues from potentially overshooting the target temperature. I also left out one step that Groterjan did: they boiled parts of the mash as a final step and slowly mixed it back, trying to keep the temperature at a maximum of 76°C. This just did not make sense for me to do on my homebrew kit.

After conversion was finished, I moved the mash to my lauter tun, briefly did a lauter rest, and then started lautering and sparging. I collected about 20 liters of wort, which I then brought up to 80°C to pasteurize it for 10 minutes. I then chilled the wort down to 20°C and pitched all my microorganisms.

Yeast and Bacteria

I pitched half a sachet (5g) of S-04 dry yeast, a pack of White Labs WLP672 Lactobacillus brevis and a pack of White Labs WLP650 Brettanomyces bruxellensis at the same time, and let it ferment together.

If you want to recreate this and are worried about infections: my protocol is to simply have a separate fermenter for anything involving Brett and Lacto fermentation, that hasn’t caused me any issues in the past.

Primary fermentation quickly started and was finished after about 3 days. The souring process took a bit longer, but after about 3 weeks, the young beer had attained a good acidity. Around the same time, a secondary fermentation, presumably from the Brettanomyces kicked off, which was done after about a week. I then let the beer sit in the fermenter for another few weeks, and then bottled it. As priming sugar I used common sugar, and went for a carbonation level of 6 g/L. Then I put all the bottles in a quiet corner and let it mature.

The OG of the beer was 8.8°P (1.035), slightly higher than the 8°P I had actually planned. At bottling time, the FG was measured to be 0.8°P (1.003). The calculated amount of alcohol is therefore 4.2% ABV. A bit higher than what I wanted to go for, but still low enough to make the beer low and refreshing.

At the time of the competition, the beer was about 2.5 months old, so still fairly young. According to one taster who described himself as hypersensitive to sulphur, it still had some hints of sulphur left. The overall flavour and aroma was quite pleasant though: tart, citrusy, but not overly sour, so that the beer was still balanced and refreshing.

I still have a few bottles left, so my plan is to keep some of them and age them for longer. This should definitely help the Brettanomyces develop more complex and interesting flavours. I also kept the yeast from the bottom of the fermenter, and I plan to repitch it in future batches. I’m actually thinking about brewing another Berliner Weisse soon, just to have something to age and maybe hand in for next year’s SLOSH SOUR competition.

Thanks to everyone involved, in particular THE MASH PIT who organized the homebrewing competition, and Berliner Weisse Kultur e.V. who made it possible for me to present my home-brewed beer to a wider audience at Berliner Weisse Gipfel.