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01/29/07 03:26 PM  
Oxygen and Brett Sourness
It's been suggested here that aeration of wort (or lack thereof) is a way of controlling the sourness produced by brettanomyces in an all-brett fermentation.

This doesn't really make sense to me. I only know of one way that brett can produce large quantities of acid: burning ethanol to produce acetic acid. In this process one acetic acid molecule (and a water) are created from one ethanol and one O2. If you do the math, then even a very strong oxygenation, to 20 ppm O2, provides only enough oxygen for 37 ppm of acetic acid to be produced. And I think the flavor threshold for acetic acid in beer is 175 ppm. Furthermore, I would suspect this oxygen is "gone" by the time any appreciable ethanol is produced!

Perhaps there is enough oxygen in the headspace of secondary fermenter to make a difference? Suppose a 5 gallon batch is put into a 6 gallon carboy, without purging the oxygen from the headspace. The gallon of headspace contains about 1 gram O2, enough to produce ~2 grams of acetic acid. But this is only enough to achieve 105 ppm in the 5 gallons of beer, again below the flavor threshold.

So, brettanomyces cannot make a beer truly sour on its own (by this pathway), unless it is supplied with much more air than is available from wort oxygenation or a reasonable amount of headspace.

I definitely don't want to discount anyone's real world experiences, though. It seems like there were real observations that led people to think that aeration of the wort can lead to very sour beers in all-brett fermentations. (Although I think Sebastian said he observed the opposite.) In any case, it seems very hard to explain based on the acetic acid pathway described above. Does anyone know of another pathway brett can use to produce significant amounts of acid?


01/29/07 04:06 PM  
Re: Oxygen and Brett Sourness
Sebastian had different results from Al, possibly temperature played a role here. But I tried some of Als early efforts and sourness levels were pretty high, though from what I have seen that seems to be Clausinii specific. He suggested O2 could be tied to sourness so I O2 deprived my barley wine in an effort to keep that down. The result was (before I added finished brett of another strain) a beer that bore a shocking resemblance to an English barley wine. More indepth info would have to come from Al.
Al B
01/29/07 09:57 PM  
Re: Oxygen and Brett Sourness
Baums -

Scientifically, you make a great deal of sense. I would love to see analytically the different acid profiles vs. strain specificity vs. environmental conditions.

Brettanomyces (strain dependant, etc. ) will produce many kinds of acids, not just acetic and lactic. Clausenii produces pineapple esters during fermentation, so it probably produces acids relevant to those esters such as caproic, capric, butyric, or isovaleric (in addition to lactic and acetic). I do not know whether aerobic or anaerobic pathways for those other fatty acids.

Apparently in one reference, it only takes 2 ppm of one of these fatty acids to slam the human palate threshold, while acetic is about 175ppm, and lactic 400ppm. Dynamic little bugs! So I have a hunch that these acids are playing a role with Mr. Clausenii.

I recall Sebastians Brew was fermented at 80 or 90F so I don't know how much O2 was really dissolved or what (maybe He can chime in - don't know what temp. it was pitched).

Its something I may try this Summer myself w/ clausenii (right now I'm experimenting with WY lambicus and a recultured Fantome).

I don't know if that made things more turbid. I keep reading up on these critters and it seems they eat everything and produce all kinds of wacky compounds.....phenolics, tetrahydropyridines -what the hell? Even my microbiological culture referrences say to buffer the growth media with carbonates (since some wild strains may actually kill themselves from the acids they produce!) Hey, lets all have a lambic !!!!

Al lamBicus

01/30/07 10:58 AM  
Re: Oxygen and Brett Sourness

Agreed--there is so much that isn't known.

By no means am I certain that acetic is the only acid that could be produced in enough quantity to sour a beer. But I think it's the only acid that we already know brett can produce in large quantity. (Anyone know of any others?)

As for lactic, I guess some experiments at Leuven found some brett could produce ~500 ppm lactic, but while this is above threshold it is far from truly "sour." I have no idea how the lactic is produced--maybe with the right strain/conditions, levels could get high?

I'm pretty sure some anomala (perhaps even claussenii) strains can produce citric acid as well, so maybe that one can also be added to the list of potential souring pathways. This one is pretty interesting, because I guess there is a lot of citric acid in pineapple, and lambics are sometimes described as citric. Anyone have more info on this?

On the other hand, I am not so sure about the larger fatty acids you mention. While they have very low flavor thresholds, I doubt they add much sourness at those levels. I don't have a reference for that--just basing it on the flavor descriptions for those acids, and the fact that the amounts are so small compared to acetic and lactic in lambic, etc. I tend to think acetic, lactic, and citric are by far the most likely candidates for "brett sourness."

Al, when you made your sour claussenii beer, is it possible that the large amount of acetic acid in a well-aerated starter made it into the beer? Just a shot in the dark...


Al B
01/30/07 11:28 AM  
Re: Oxygen and Brett Sourness
I can tell you that aerating continously a clausenii culture as a starter drops the pH in 24 hrs to a pH of 3.

Its possible that the starter contributed to the overall sourness in my clausenii brews. However, I also supplied Steve with the slurry (not the same starter, but preped the same) and he did not aerate the wort.

I don't see citric in any of my refs. Perhaps I was off-target if I thought that an acid at 2ppm acts upon the human palate as another acid at 175ppm, why would alot of acid would be required? Without actual pH readings (and tastings for perceived acidity?) in a designed experiment, I can only guess. But since I made two more wild brews without aerating the wort, the acidity is less in comparison in roughly the same time span.

How does everyone aerate their worts for pitching Bretts in a primary? and what temp?

1- splashing the wort in a jug

2- Air pump and injecting air into wort

3- Conc. O2 injection

4 - none of the above

Mike T
01/30/07 11:49 AM  
Re: Oxygen and Brett Sourness
The version I just did of Sebastianís Brett C beer (OG 1.048, 85% pils, 15% wheat malt) got a decanted 1.5 L starter and a solid 20 seconds of shaking (much less aeration than I would normally give a beer). I pitched at 80 and fermented around 82 ambient (a bit cooler than Sebastian), but it still fermented out (76% AA) in about 3 days. The sample I took a few nights ago was shockingly clean, just a hint of tartness and barnyard, but certainly not sour.

I'm hopefully brewing tonight or tomorrow night and pitching onto the yeast cake from this one. This time with a grist that contains 5% sauer-malt to up the acidity (the lactic acid will survive the boil, right?). Iím also going to aerate more (2 minutes shaking) and ferment cooler (68 ambient), looking for a funkier brew.

01/30/07 11:51 AM  
Re: Oxygen and Brett Sourness
Thanks for the details. For what it's worth I have played around with Wyeast's B. Lambicus strain a little, and generally found no sourness, except a little hint in a continuously stirred starter.

More interestingly: I just looked up Patent 3733253 on the web--it is mentioned in one of the ATCC dekkera descriptions. In that patent (example 6 or 7) someone claimed to produce a very large amount of citric acid (10000 ppm) using claussenii in what amounts to a 12P glucose solution and some nutrients!

Mystery solved? I might not go that far, but that's a pretty good lead I think.

Also I think Frank Boon once said claussenii is the most important brett in his beer.


01/30/07 08:31 PM  
Re: Oxygen and Brett Sourness
Baums, that's some nice analysis, but I think you're forgetting about the oxygen that gets into the beer through the aging vessel during a long secondary.

Check out section 8 on Raj Apte's article on Brewing Flemish Red Ale (google it, sorry can't post the link)


01/31/07 10:28 AM  
Re: Oxygen and Brett Sourness

I was really just trying to say that brett's ethanol-to-acetic-acid pathway doesn't explain how initial aeration of the wort could lead to sour beer in all-brett ferments. And that even a pretty good amount of headspace air is not enough for much acetic acid production either.

I completely agree that a long secondary in a porous vessel could let in enough oxygen to create significant acetic acid. I'm actually not sure Raj's numbers and the ethanol-to-acetic-acid pathway explain it though.

His numbers are in O2 cc/L.year. A cc of oxygen weighs 1.4 mg, so this is the same as 1.4 O2 mg/L.year or 1.4 O2 ppm/year. Since brett can produce about 2 ppm acetic from 1 ppm O2, multiply Raj's numbers by 2.8 to get "ppm acetic acid per year." In anything other than the HPDE bucket, it would take at least 3 years to get above 175 ppm, which is only the taste threshold.

Not sure there's much value in that calculation, though, since I would imagine barrels (and whatever is used to seal them) vary a lot. Some more acetic, some not, and probably the only way to tell is to brew in them, which is a lot more fun than doing these calculations anyway.


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