Aging: what exactly happens?
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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.
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.
o4_srt
Well-Known Member
MalFet said:
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.
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 either that beer needs or that it 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.
o4_srt
Well-Known Member
MalFet said:
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.
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.
o4_srt
Well-Known Member
MalFet 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?
Indyking
Well-Known Member
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 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.
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 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.
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.
Check it out. It could help in this discussion.
Indyking
Well-Known Member
Thanks! Definitively checking it out!
remilard
Well-Known Member
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.
kcold0403
Well-Known Member
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.
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.
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.
Here is a quote from Briggs re: diacetyl:
"Yeast cells will not assimilate exogenous acetohydroxy acids but will readily take up and reduce diacetyl and 2,3-pentanedione to acetoin and 2,3-pentanediol, which have no adverse flavours... This forms the basis of effective diacetyl removal from green beer..."
It goes on to say the the yeast need to be in a metabolically healthy state as you have mentioned. However, the book earlier mentions that the yeast can be in such a state with residual sugars in the fermented beer as well as by adding new sugar.
"Yeast cells will not assimilate exogenous acetohydroxy acids but will readily take up and reduce diacetyl and 2,3-pentanedione to acetoin and 2,3-pentanediol, which have no adverse flavours... This forms the basis of effective diacetyl removal from green beer..."
It goes on to say the the yeast need to be in a metabolically healthy state as you have mentioned. However, the book earlier mentions that the yeast can be in such a state with residual sugars in the fermented beer as well as by adding new sugar.
That is awesome. I wonder how long there will be free access?
kcold0403
Well-Known Member
Like most Journals, the newer publications aren't free. The ones that are a little older (maybe a year or so older) are free. I think the subscription is just a hard copy to get sent to your house and some additional perks but being right out of college, anything free is a good thing right now. Maybe when I get some extra money (that I don't actually spend on brewing), I'll subscribe just to give my thanks to the hours it kept me busy at work. lol
I think I found the reference from Briggs on beer maturation through google. It has a little more info than Briggs but essentially says similar things. Here is (hopefully) the link:
http://books.google.com/books?id=Uv...nd maturation manual of good practice&f=false
http://books.google.com/books?id=Uv...nd maturation manual of good practice&f=false
HairyDogBrewing
Well-Known Member
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 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.
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.
If you need that much info, look at the link and try to find the sources for that book. You will probably get the x and y of it there. But I really think that is reinventing the wheel. I do know and understand the term "standing on the shoulders of giants" but I think there are some things we can take at face value.
Indyking
Well-Known Member
I'm afraid there is not and there will never be a specific article in such a highly regarded journal that study Saccharomyces cerevisiae metabolism in the context of homebrewing. Well, there better not be anyway, because funding for research has higher priorities.
This was just the closest high quality article I could find to prove common knowledge about general Saccharomyces cerevisiae metabolism. It applies to most situations. That's how evidence for research hypotheses are built in many cases, based on common facts well accepted in the scientific community.
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.
These two sentences are contradictory.
Please read Briggs or the online reference I posted.
FWIW... you can almost always trace back "common facts well accepted in the scientific community" to the original article that proved them to be true. Granted, its a painful process. But if you want to get to the nuts and bolts of the truth, that is the way to truly do it.
I'm satisfied with what I have to read for now.
I'm satisfied with what I have to read for now.
mhenry41h
Well-Known Member
o4_srt 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!
I think all anyone is asking for is an example of this "evidence based in high quality research on Saccharomyces cerevisiae metabolism [that] suggest it would be a very minor role". So far, you have produced a single article, grossly unrelated.
There's nothing wrong with tearing down sacred cows. In fact, it's pretty much the best thing there is. But, right now you are trying to bust up one dogma (that you must keep your beer on yeast for 87 weeks or it will poison and kill you) with another, equally unfounded dogma (that yeast plays a very minor role).
This is an interesting question. You may certainly be right. Our objections are not that you are trying to attack our culture. Our objections are that you are making claims in the name of science...without science.
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
Indyking
Well-Known Member
Dont 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!
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
That is the issue I guess. You will have to wait and see if your unfiltered beer has off-flavors. If it doesn't, you would have to repeat the experiment until you get a batch that does.
If they both taste the same, maybe try repeating the experiment with a lager without a diacetyl rest? Maybe pitching at higher than expected temps? Or with a yeast strain that is know to produce diacetyls.
Of course this would only answer the question for diacetyl and nothing else...
remilard
Well-Known Member
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.
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!
Point taken. I guess what most people are saying is that the idea that yeast play a roll in maturation is not a perception. It is a well accepted fact. The references I gave support this.
I like your article. I used to have to find all sorts of original articles like that one and it was amazing to me how far back the journals go. And, at least at the time, that they actually still had the original copies available.
According to Briggs, the yeast "readily take up and reduce" as I mentioned above. So it is an intracellular conversion. It requires yeast cells.
Well, perhaps I am using terminology wrong, but I have always understood "metabolism" to be a blanket term referring to intracellular activity related to energy production, including various redox reactions. Wikipedia, at least, seems to agree.
Wouldn't saying "It's not metabolism, is reduction" be like saying "I'm not making beer, I'm making a Belgian Wit"?
In any case, this is semantics. The broader question was whether diacetyl, etc., can be broken down in meaningful quantities the absence of yeast, and you don't seem to be disagreeing with me on that.
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).
True, true, though I hear WL and Wyeast are planning to expand their repertoire of commercially available enzymes.
(I'm very jealous of your book, btw. I get a stipend for relevant book purchases from my job, and I've often wondered if I can sneak that through.)
