MalFet said:There's a lot of stuff going on.
Some of it is metabolic, as intermediate compounds get broken down by the yeast. Acetaldehyde and diacetyl come to mind immediately, but there are certainly others.
Some of it is chemical, including oxidation.
Some of it is physical, as suspended solids settle out into the trub.
"Cleaning up" is a bit simplified, but it's not all together wrong.
This thread was started in an attempt to continue an off topic conversation from a now closed thread.
Has anyone actually experimented with aging on the yeast cake, vs aging without any yeast present removed by filtration, to see what role yeast plays post fermentation?
As of right now, most are speculating.
MalFet said:It's not really speculation. These things are readily observable.
I can't think of any particular published experiment, but I'm not sure there's need. If you have diacetyl, you need yeast to metabolize it. Same goes for acetaldehyde, some heavy alcohols, pyruvate, acetate, etc. There's a chart here.
If you don't have detectable intermediates, you don't need the yeast any more. If you do, filtration will leave you with a flawed beer. People tend to tell new brewers to leave the beer on yeast for several weeks because it's the safe bet in a "can't hurt, might help" sort of way. To say categorically that beer either needs or doesn't need a long fermentation is naive.
I found and skimmed through the other thread. The person responsible for getting it locked has very little understanding of yeast biology.
The twelve experiment participants were split on whether the process yielded significant flavor differences. Six said there was either no or slight flavor difference, while five reported definite flavor differences. One was disqualified because coriander was added to one sample and not the other. However, in his submission, Hugh Brown of New Westminster, British Columbia, commented that both samples were quite good.
Of those that described a flavor difference, five reported that the racked beer tasted “cleaner” or “smoother.”
Not arguing with you, in fact, it makes perfect sense, but has it ever been directly proven that it's the yeast responsible for this, and not something else? I leave all my beer in the primary for a month before I cold crash and bottle.
I'd really like to do the experiment I outlines in the previous thread, but I don't have force carb or filtration equipment.
MalFet said:The problem is that an experiment like this wouldn't prove much, if anything. Sometimes they yeast has a lot of work to do once the simple sugars have been processed, sometimes it doesn't. I've had beers be ready for bottling after a week, some have taken a month, and a few have taken much longer, all due to yeast intermediate metabolites. In all cases, the simple sugars were all consumed in the first couple of days.
Unfortunately, this experiment is logically equivalent to calculating annual beef consumption by calling a few friends and asking if they're having hamburgers for dinner tonight. Unless you can propose another chemical pathway for things like diacetyl to be broken down, I would say this stuff is relatively proven.
Edit: P.S. I hope I don't sound short or anything here. You've got a fully legitimate question, and I'm glad to see it asked.
I also enjoy homebrewing partially because of the science behind it. I was not satisfied with explanations for several things so I ponied up and bought Briggs' Brewing: Science and Practice. I have only read about 1/2 of it - it's over 800 pages. It is meant as a textbook for professional brewing but has a lot of relevance to homebrewing as well. It is definitely science-based and not experience-based. Unfortunately the science-based aspect means it is just as dry as most science texts. But it does have a few pages specific to the science of secondary fermentation (maturation).
Check it out. It could help in this discussion.
Unless you can propose another chemical pathway for things like diacetyl to be broken down,
In case someone wonders, my opinion (not proved in a brewing set up, just what makes sense to me biologically) was part of the other thread that went too off-topic and appropriately closed by the mod.
I did more research in this subject and this is the best I could find. Brewing is also science, and perhaps that is the main reason I like this hobby so much. For those who don't know, PNAS is a highly renowned journal in scientific research worldwide. It's one of my favorites. This particular article was NIH funded.
I encourage people interested in the matter to read it the article. You may find it overtly technical if you don't have a cell biology or molecular biology background, but you can skip the technical aspects about the results of their experiment and read the general well accepted knowledge about Saccharomyces cerevisiae, the common brewing yeast.
They do suggest that S. cerevisiae can carry on some degree of metabolic activity after starvation (i.e. sugar depletion), albeit minimal. Note that it is a common knowledge and also stated in the article that reentry in the cell cycle and peak of metabolic activity requires re-exposure of the yeast to its energy source, which is sugar.
So, perhaps the yeast play some role in the aging process after all but every reliable scientific resource I found to date confirms it's very minor at best.
They do suggest that S. cerevisiae can carry on some degree of metabolic activity after starvation (i.e. sugar depletion), albeit minimal. Note that it is a common knowledge and also stated in the article that reentry in the cell cycle and peak of metabolic activity requires re-exposure of the yeast to its energy source, which is sugar.
So, perhaps the yeast play some role in the aging process after all but every reliable scientific resource I found to date confirms it's very minor at best.
04_srt: I'm a chemist and am really intrigued on the actual chemistry of brewing. Being a nerd, on my free time I read through the Journal of the Institution of Brewing.
http://www.scientificsocieties.org/jib/
This Journal will give you the scientific evidence you are looking for when it comes to the question, "How does this actually happen". I haven't seen an article directly pertaining to your question but I assume with a little work something will show up. I'll try and look through it as well to give you some hard scientific evidence.
That is awesome. I wonder how long there will be free access?
Redox.
That was easy.
I've always wondered about this too.
I know that hundreds of organic compounds are created and consumed during fermentation.
But, except for sugars and ethanol, I haven't seen anywhere that compound A has x ppm to start and y ppm after a certain amount of time.
It's even more complicated for products that are barrel aged.
I read the article. And by read I mean I read it like I read any such article - I read the abstract, the introduction and the discussion. So if I missed something, let me know...
But this article seems to be aimed at explaining why dormant yeast cells have a limited life span compared with those yeast cells that remain active. It is an article about the aging of yeast, not the aging of beer. The points you bring up have little to do with the thrust of the article.
But, in all sincerity, if you find an scientific article about the aging of beer, I would love to read it.
You'll have to direct me to the part of the article that says anything about yeast activity levels in a sugar-poor environment, because I can't find a darn word. The article talks about transgenerational effects of stress, and in particular compares daughters of stressed cells to naturally aged cells. I might be missing something, but I don't see a connection to the role of yeast in processing metabolic intermediates once the majority of fermentation has completed.
That's how evidence for research hypotheses are built in many cases, based on common facts well accepted in the scientific community.
I find it very hard to argue against perceptions that have been perpetuated for too long in any community, online or not; its almost a change of culture effort, and that is not my goal here by any means.
o4_srt said:Not at all! I started this post for educational purposes, and to get away from the petty arguing in the other thread.
If no one asked questions, and performed experiments, mankind would still be in the stone ages.
If I split the same batch in two, fermented them both, then immediately after fermentation removed the yeast from one via filtration, and then aged for the same length of time, then filtering the other and force carbing both, couldn't it be feasible for any taste difference to be attributed to the yeast, and nothing else?
And, if a lack of difference was noted, couldn't one deduce that yeast don't play a part in "cleaning up" post fermentation, and taste differences are due to some other process?
I am not going to fight this any longer. I can assure you, yeasts may play a role during that aging process after starvation (when sugar in the wort is depleted and final SG is reached), but all evidence based in high quality research on Saccharomyces cerevisiae metabolism suggest it would be a very minor role.
I find it very hard to argue against perceptions that have been perpetuated for too long in any community, online or not; its almost a change of culture effort, and that is not my goal here by any means.
mhenry41h said:That would be a nice expirement. I thin most of us would expect the aged beer that wasn't filtered to taste better. I'd love to know how it turned out.
coming soon...to a fridge near you!
If I split the same batch in two, fermented them both, then immediately after fermentation removed the yeast from one via filtration, and then aged for the same length of time, then filtering the other and force carbing both, couldn't it be feasible for any taste difference to be attributed to the yeast, and nothing else?
And, if a lack of difference was noted, couldn't one deduce that yeast don't play a part in "cleaning up" post fermentation, and taste differences are due to some other process?
These two sentences are contradictory.
Please read Briggs or the online reference I posted.
Sure thing, and it would be a great experiment...If you did that, you could reasonably demonstrate that they yeast is active. But, finding no difference wouldn't demonstrate that, in other circumstances, yeast aren't active. I say go for it, if you can
Are you suggesting that diacetyl, acetaldehyde, etc. undergo substantial redox in the absence of yeast? I've never heard such a thing, but I'd be delighted to learn it.
Don’t want to get too philosophical but I said it is difficult to fight against perceptions, which is very different from common facts well accepted in the scientific community.
Like in this case, the perception that yeasts clean-up after themselves after fermentation is completed. The notion is widespread here and the best I can do is to show scientific facts that challenge the perception.
I know very well we can trace the common facts well accepted in the scientific community too its very roots, like you pointed out, I just don't think it's necessary here and it's often a struggle to find them.
Here is a nice example of a common fact well accepted in the scientific community... legendary!
I'm suggesting that the only way to get rid of diacetyl is reduction, not yeast metabolism like you stated.
In beer it happens via enzymatic reduction and the enzymes come from the yeast.
I'm suggesting that the only way to get rid of diacetyl is reduction, not yeast metabolism like you stated.
In beer it happens via enzymatic reduction and the enzymes come from the yeast.
FWIW... I am a broken record, but Briggs implies that many modern commerical breweries utilize enzymes and other atypical or unnnatural means of speeding up maturation. That is to say, means other than natural yeast cell processes. Homebrewers typically do not go that route (not that you couldn't).
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