Diacetyl anecdotes and solutions please
I recently came across an article by the late George Fix, which refers to diacetyl appearing in apparently normal beer:
"a widely observed but little discussed phenomenon occurs when diacetyl appears spontaneously in a beer that seemed to have normal flavors. Strong evidence indicates that this can occur when marginally dysfunctional yeast have been used in the main fermentation -- they tend not to metabolize all the acetolactic acid in the wort. The acetolactic acid spills over into the finished beer and later is oxidized to diacetyl. Mechanical abuse of packaged beer can promote this; headspace air is the oxidizing agent. Elevated temperatures augment the effect. I have seen cases in which wort constituents (melanoidins and tannins), oxidized on the hot side in wort production, were passed on to the final beer, only to play the role of oxidizer there."
-which could describe what happened in some of our beer of late. As our pale ale is bottle conditioned, we have reabsorbed the diacetyl flavours. But is it possible for a bottle conditioned beer to present with oxidised characteristics, as the yeast within the bottled beer functions as an anti-oxidant? And if so, what are these characteristics? I would imagine that they would differ from oxidized characteristics in a filtered, artificially carbonated product like most macro lagers, where there is little or no yeast present.
Would like to hear about your experiences with diacetyl appearing in normal tasting beer - and how you have remedied this.
for those interested, the article is found here:
What you can experience is yeast autolysis( self-digestion).
It will give the beer a sharp bitter taste and/or a round yeasty taste, simular to deacethyl.
Conditioning at a lower temperature to aid yeast settlement, and/or use a smaller amount of yeast to the bottle is one of many solutions.
There is also a limit to how much the yeast-cells can work as a anti-oxidant in the confining space of a bottle with limited food-sources.
No - it's definitely not autolysis - our diacetyl occurs during the bottle conditioning, which means that autolysis couldn't occur while there is a food for the yeast.
I am just wondering whether it is normal for diacetyl flavours to arise during bottle conditioning (we use wyeast 1056), or is it possible that we are hot side aerating which is contributing to this? We are very careful not to aerate hot side, but we have identified one possible point in the brewhouse where this may be occuring, albeit in what I consider to a fairly minor way.
Is 1056 a strain that is a big time diacetyl producer, or do I have to look further at my brewing process?
Age your beer and keep bacteria out.
Diacetyl has 2 sources: Diacetyl precursor from yeast and bacteria.
The Diacetyl precursor becomes Diacetyl later. That is why it is possible to not detect it and then get an explosion of it later. How long are you aging your beer?
The second source is beer spoilage bacteria like the notorious Pediococcus, the bacteria that travels in tetrads.
Let me know if you have any questions.
Forget to mention....
The Acetolactic acid mentioned in that article is the Diacetyl precursor I mentioned before. Yeast consume it in the aging process.
In re-thinking your process, what with the bottle conditioning and all I have a question: do you have good temperature control for the warm storage of the bottles while conditioning and are you pitcing fresh healthy yeast back into the beer for bottle conditioning?
Here's what I am alomost sure the issue is: your yeast are tired and rapidly go dormant in the bottle. They probably carbonate the beer fine, then get shocked when the food source drops and don't re-metabolize the Diacetyl precursor. As the beer ages, Diacetyl forms.
Your tired yeast can be compared to a triathlete who mishandles his body and can't finish the last bit of the race at the home stretch of a mile or so. What he did is impressive, but he can't quite do the last bit, which in your case is consume the Diacetyl precursor.
Let me know how you are bottle conditioning and maybe I can help.
One last possibility: you have a yeast mutation for respiratory deficiency, which again renders yeast unable to digest the last bit of food source in the beer, namely Diacetyl precursor. This is caused by poor aeration of the wort at fermenter full, going too many generations on your yeast handling, and/or waiting excessively long to crop your yeast after fermentation, which can be especially damaging if you have a tall fermenter.
I could be wrong, but...
One last comment here: I think Dr. Fix, RIP, was incorrect about the Acetolactic acid in the malt. Diacetyl precursor is affected by Valine levels in the malt because the Valine/Isoleucine pathways, but as far as I know the precursor is formed by the yeast in fermentation.
Originally Posted by simon
Thanks for your thoughts, Steve
- the diacetyl does get completely reabsorbed by the yeast in the bottle eventually. We don't have great temp control over the conditioning process, which is something that we are looking at. And it seems to occur with which ever generation of yeast we use.
Which tends to suggest that your suggestion about aeration may be correct. Our oxygenation is perhaps less than what it should be.
Interestingly, we don't experience such diacetyl when we brew our hefeweizen, which is brewed with wyeast 3068.
I still wonder whether diacetyl is to be expected in some level with some yeast strains during the conditioning process.
Your wheat beer not giving a lot of Diacetyl could certainly be due to the amount and type of free amino nitrogen/amino acid makeup in the wort, or it could be due to yeast strain.
One other consideration is that the additional body that you most likely have in your Pale Ale may give the perception that the Diacetyl is higher than the Wheat Beer. I have done sensory experiments in my lab that show that the same analytical levels of Diacetyl by GC method are not detectable in thinner, lighter beers and in beers with harsh hop bittereness. You can on the other hand have a beer with high AE and pleasant, smooth bitterness that fails taste test for unacceptable Diacetyl levels! Fascinating stuff!
I take it that you use the same processing of fermentation time and temperature for each beer, with principal differences being in mashing regime.
The fact that you get reduction of the Diacetyl with conditioning means that you really have nothing to worry about. If you want a lower level prior to filling the bottle, a little extra tank aging won't hurt. I wonder how long you leave the beer on the yeast after reaching the terminal AE?
Consider this: Diacetyl is known to form over time. This is why labs incubate beer samples at 60 C for an hour plus before running Diacetyl analysis on GC: that is the only way to get the true value by converting all the precursor ahead of time. It is perfectly normal to say a beer has perfectly acceptable characteristics, only to later pasteurize it, converting all the precursor to Diacetyl, and find that it tastes terrible!
This is a common misconception. It is NOT taken up by the yeast and is NOT converted to alcohol (common misconception #2). Diacetyl is reduced to 2,3 pentandione by enzymatic activity of yeast.
Originally Posted by simon
2,3 pentandione has flavor threshold of around 1.0 mg/l (much weaker than diacetyl)
Last edited by MoreBeer; 07-27-2006 at 07:35 PM.
Diacetyl, 23B, is not reduced to 23P. They are sister compounds, different pathways, and as someone points out 23P has a much higher taste threshold than 23B. Diacetyl is reduced to 2-3-butandiol. 23P is reduced to 2-3-pentandiol. "one" to "ol". Ketone to alcohol. Pardon the abbreviations.
Correct. Yeast produce at the beginning of fermentation, and reduce at the end of fermentation, both 2,3 Butanedione and 2,3 Pentanedione. These are reduced to the corresponding alcohols.
Originally Posted by bigfatjoe
It is always worth noting that some spoilage bacteria produce 2,3 Butanedione almost exclusively.
Normal yeast fermentation will have a ratio of Vicinal Diketones (VDK) of 1 to 2 ppm Butanedione to every ppm of Pentanedione throughout the fermentation. In the event of contamination with Diacetyl forming bacteria like Pediococcus this ratio will increase as high as 20 to 1!
Wrong on my part, I stand corrected.
Originally Posted by bigfatjoe
A Brief Summary
Yeast create 2-acetolactate during fermentation which is dependent upon the amount of valine available. If there’s enough valine, the yeast will produce less 2-acetolactate, because it is produced through the yeast having to synthesize this particular amino acid.
Outside of the yeast cell, 2-acetolactate is changed to diacetyl (non-enzymatically through oxidative decarboxylation). This is why some people use a diacetyl rest at the end of fermentation, to speed up this process; it is not to make the yeast work harder.
The yeast then take up the diacetyl and change it first to acetoin, then to 2,3-butandiol. This occurs faster than the oxidative decarboxylation of 2-acetolactate to diacetyl.
Should your yeast either flocculate out too soon (e.g. Pilsner Urquell yeast) or be separated from the beer too soon (centrifuge, filter, etc.), then the 2-acetolactate will change into diacetyl afterwards (puzzling countless brewers). 2-acetolactate is not detectable in beer in the concentrations normally present at this stage. Only after it has changed into diacetyl, which has a very low flavor threshold, is it noticeable.
Probable causes of this problem:
1. free amino nitrogen is low
2. fermentation temperatures or dropping the temperature too soon
3. yeast health and/or flocculation characteristics
4. nutrients in wort not optimal for the yeast
As mentioned above, the ratio of diacetyl to 2,3-pentanedione should normally be 3 : 2 in beer. If it reaches around 5 : 1 then you have a pediococcus problem.