Aging: what exactly happens?

[o4_srt]

If the yeast cake is truly dormant, why would bubbles of co2 rise out of it post fermentation?

One could argue that it's trapped, but I would think that the co2 would dissipate far quicker than the yeast flocculate.

I don't necessarily think yeast go dormant the minute they flocculate, from what I have read in the past, right before they go dormant, there is a period of nutrient uptake to prepare for the dormancy/future fermentation.

Could this nutrient uptake be occurring after flocculation?

 

[cruelkix]

Not all (not most?) commercial breweries are adding enzymes, and in any event there's no shortage of yeast in suspension.

Valid Point.

 

[ayoungrad]

That's how I read it too. Once yeast have settled, they can't reduce VDKs.

Every commercial brewery dumps the cone within a few days of fermentation. I would think the fact that diacetyl is much less common in commercial beer than homebrew would be enough to observe that having a big pile of yeast in the bottom of the fermentor isn't the key to reduced diacetyl.

It seems to me that commercial brewers use alternative methods of maturation utilizing techniques aimed at increasing throughput. To say that they dump the cone rapidly does not infer that the beer does not need to be conditioned. It just means (as I mentioned previously) that commercial brewers do not necessarily rely on yeast to clean things up.

For my homebrewing, I don't like the idea of using chemicals. As an example of this, the last two beers I made were made with distilled water or filtered water. I'm trying to avoid campden and my tap water has chloramine. I could use tap and put in campden but I'm going a different route.

I guess my point is that there is more than one way to skin a cat. You could use chemical means to clean-up beer but yeast cells do metabolize diacetyl and until beer is filtered correctly, there are always active suspended cells. And there is also a small amount of available sugar for those cells despite a steady final gravity.

 

[ayoungrad]

I have to say that I am really enjoying the discourse here.

I have read a tremendous amount about brewing but there is so much more to learn. Other people questioning things I accept really forces me to relearn and reevaluate so many concepts.

Don't get me wrong, I'm all for RDWHAHB but I find these subjects really interesting.

 

[Revvy]

See, the D-rest is a different history. There seems to be no reference of what exactly the yeast does after full attenuation in brewing beer. And I looked hard for it. That's why I based my theory that not much is done by the yeast after fermentation is finished based on general knowledge in yeast metabolism I found published in reliable scientific sources. I'm not saying it's the truth, just what I believe now and I will keep looking. Will hopefully start reading the book soon!

I'm not gonna get into this stupid debate- (it's been done to death no matter what the OP thinks) anymore but there are some things out there about diacytal and long primaries, and I've posted it repeatedly..


"THE ROLE OF DIACETYL IN BEER
By Moritz Kallmeyer"

The Abstract begins...

Diacetyl as a product of fermentation is more characteristic of ales than lagers. Diacetyl is produced early in the fermentation, and then most of it is reabsorbed by the yeast and reduced to flavourless compounds later on. Yeast strains differ markedly in their diacetyl reduction ability. Some ales and a few lagers (such as the famous Pilsner Urquell) contain perceptible amounts of diacetyl, but as a rule modern brewers consider it as a fault. This is because certain bacterial infections and other errors in brewing technique will increase diacetyl levels resulting in unacceptable beer aroma and flavour profile. This parameter thus serves as a quality check. However, it is important to remember that diacetyl flavour is a natural by-product of yeast fermentation, and in some beer styles it is an optional or even required flavour component in low amounts.

From here....


Drayman's Brewery and Distillery

There's two methods of rests listed in the Kallmeyer article...one for ales and warmer beers....interesting.

Maturation of beer flavour requires the presence of yeast as a catalyst. There are many methods of finishing that have the sole objective of prolonging the contact of beer with yeast after primary fermentation is completed. I want to emphasize that a diacetyl rest with most of the yeast lying at the bottom of the tank and not enough in suspension is of no use. Most lager breweries, especially those that use Weinhenstephan 308 or similar “diacetyl producing yeast’s” employ a long diacetyl rest, in order to minimize diacetyl in the finished beer.

Method 1
If a very cold primary fermentation was used it involves allowing the beer temperature to rise from the controlled primary fermentation temperature of about 10°C to 15-18°C when the primary fermentation is coming to an end. Normally, the time is determined by the attenuation of the beer. If, for example the wort starting gravity was 1050 and the expected terminal gravity is 1010, then the diacetyl rest would be commenced when the beer has attenuated to about SG 1023 when two-thirds of the total fermentable material in the wort has been consumed. The diacetyl rest normally lasts for 48-72 hours, until primary fermentation is over and secondary fermentation is under way. At this time the temperature is lowered when the more traditional method is followed, probably 1°C per day until the lagering temperature of 0-1°C is reached.

Method 2
If a warmer primary fermentation temperature was used for ale or lager the diacetyl rest involves either lowering the beer temperature 2 or 3°C at the end of primary fermentation or keeping it constant for up to 6 days. In lager yeast strains with low diacetyl production it is common practise nowadays to employ a short diacetyl rest followed by centrifuging to remove excess yeast and then crash cooling to 0°C. When brewing ales, that should have very low diacetyl levels especially German Ales like Alt and Kölsch, the implications are to not use highly flocculent yeast and to allow an extended primary fermentation, albeit at cooler temperatures until sufficiently low diacetyl levels are reached. Yeast that settles in the cone is still removed on a daily basis.

And someone referenced THIS article last week...

Beer Flavors #1: Diacetyl
Modern brewing practice dictates that beer be aged on live yeast until the vast majority of AAL is converted into diacetyl. Brewer’s yeast, while unable to metabolize AAL, will readily absorb and break down diacetyl into relatively flavorless compounds. By giving the beer enough contact time with the active yeast, the brewer can eliminate the diacetyl. It generally takes only about two weeks of aging an ale to assure that it will have no buttery flavors

Secondly people demanding scientific papers backing this stuff up have to realize something important about this....This long primary discussion has only in the last year or so, since Basic Brewing/BYO decided to tackle it, been beyond here. WE experimented/fought/argued/debated for about 4 years or more in relative obscurity until our arguments got brought out to the larger brewing community, and with such a lot of dicussions that folks started to listen an wonder.

Because prior to that the rest brewing community just assumed autolysis was inevitable, repeated the rote chestnuts of Papazian, and then Palmer, and kept repeating it...But noone gave it any thought, no one bothered to write about it or experiment, except maybe us.

AND THEN PALMER admitted that he did the same thing. He just regurgitated the same old belief, not really researching it or anything, just repeating what he heard.

My recommendation was based on the premise that (20 years ago) larger (higher gravity) beers took longer to ferment completely, and that getting the beer off the yeast reduced the risk of yeast autolysis (ie., meaty or rubbery off-flavors) and it allowed more time for flocculation and clarification, reducing the amount of yeast and trub carryover to the bottle. Twenty years ago, a homebrewed beer typically had better flavor, or perhaps less risk of off-flavors, if it was racked off the trub and clarified before bottling. Today that is not the case.

Just because the papers aren't written YET doesn't mean there's no "scientific validity" to this....the papers are being written now.

And a lot of it, just happenning simple by people trying things out and deciding for themselves if it's valid or not.

When people aren't open minded about something, or don't think deeper and just look the old arguments without thinking...then noone bother's researching or caring.

Thing is, once BYO/Basic Brewing picked up on what we were doing, folks outside of here started to look deeper....Palmer is a good example....And once he did it, the culture shifted, and people ARE researching it. Hopefully more thoroughly than the basic brewing guys.

But I predict you'll be seeing plenty of "scientific papers" coming out on this topic.

You know there's no point in quoting a paper form 10, or 20, or 30 years ago, we're not brewing with the same yeast or the same quality of yeast we were back then. Even if the strains are the same, they're produced better, and they're in our hands fresher, than anything available to homebrewers than ever before...heck there's even more yeast strains available than before....So maybe Fixe's Book, or Papzizain's or Palmer's or even a brewing science article from the 70's, that we want to bandy about as "proof" isn't exactly going to be the best source- maybe we're writing the new articles now, or just making the discoveries by our own experiences.

This thread is no different than the 10,000s of other threads on here about this....plenty of "scientific" stuff has been brought it in other threads, for or against. This is just another beating of the dead horse....it's no different than the tread that this spun off from.

I'm not gonna engage or debate in here, I just wanted to bring up a couple of points, since someone was mentioning diacytle and there is stuff about it and prolonged yeast contact out there. Some of it's older, like the kalzemyer article, but there is at least one more recent source out there.

And to give some food for thought for those demanding "scientific proof" about this...it's hard to have science done about it if no one took it serious enough to look into it, or just repeated the old info. There had to be a shift in the brewing culture, which there has been largely because of us on here, before it was in a position where people will start to do the research further.

*unsuscribe*

 

[cruelkix]

It seems to me that commercial brewers use alternative methods of maturation utilizing techniques aimed at increasing throughput. To say that they dump the cone rapidly does not infer that the beer does not need to be conditioned. It just means (as I mentioned previously) that commercial brewers do not necessarily rely on yeast to clean things up.

I think to Malfet's point, do you really think most commercial brewers are using enzymes or other means to mature their beer? I would agree with him in that most do NOT do that. Maybe the really big guys do. On the flip side, most commercial brewer's DO dump their yeast early.

I think this is a valid argument in that it shows that the yeat cake is not necessary for the maturation pahse as much as the yeast in suspension is?

 

[Indyking]

That's not how I interpret Briggs or the reference I posted. They both seem to indicate that the dealing with diacetyl can be accomplished at the end of primary or during maturation. There are always yeast cells present and there is always a small amount of sugar present (even after "completion" of primary fermentation).

And I agree with that! Fermentation in lagers is so much slower, full attenuation may take several weeks. There is always some residual fermentable sugar, but since its so scarce after the bulk of fermentation has passed, temperature of the wort must be elevated to re-activate the yeast. The situation in Ales is different. I believe after a while, due to the higher fermenting temps, very little if any residual fermentable sugar at all is left, then the yeast goes into dormancy, and very little yeast-specific activity occurs during the aging process.

 

[cruelkix]

I'm not gonna get into this stupid debate- (it's been done to death no matter what the OP thinks) anymore but there are some things out there about diacytal and long primaries, and I've posted it repeatedly..

Chill buddy. It's not the *if* this thread is asking about, its the why. I don't think anyone is arguing the benifits as much as trying to figure out the root cause.

Thank you for the information that you have posted previously. It will definitely help the discussion.

 

[ayoungrad]

And I agree with that! Fermentation in lagers is so much slower, full attenuation may take several weeks. There is always some residual fermentable sugar, but since its so scarce after the bulk of fermentation has passed, temperature of the wort must be elevated to re-activate the yeast. The situation in Ales is different. I believe after a while, due to the higher fermenting temps, very little if any residual fermentable sugar at all is left, then the yeast goes into dormancy, and very little yeast-specific activity occurs during the aging process.

I understand you believe this. But just present some sort of support for it. Anything that directly addresses your opinion.

I have presented a source and my source's source to support my stance that yeast does actively affect the aging process and beer flavor.

I think Revvy has made several valid points. Not that it is a scientific landmark, but look at how many posts he has. Think about how much more experience he has than all of us put together. He hasn't written a peer-reviewed paper but he has established well founded techniques for homebrewing based on personal experience and first-hand knowledge.

 

[o4_srt]

Chill buddy. It's not the *if* this thread is asking about, its the why. I don't think anyone is arguing the benifits as much as trying to figure out the root cause.

Thank you for the information that you have posted previously. It will definitely help the discussion.

exactly. I don't see any harm in having a civil discussion intended on furthering our knowledge.

And thanks for the links!

and to everyone else involved, thanks for keeping this civil, let's keep it this way. There is much for everyone to learn about in the brewing process.

 

[Revvy]

Chill buddy. It's not the *if* this thread is asking about, its the why. I don't think anyone is arguing the benifits as much as trying to figure out the root cause.

So you're saying there's NEVER been a thread that discussed or looked at the why's before THIS thread, even in the Science section? Or you just haven't bothered to look. Just about every thread looks and tries to get a handle on the why's of it....

You can't have the what without the why...I just can't see how this thread any different than the CIVIL discussions all over here already about this?

Like THIS ONE for instance.
https://www.homebrewtalk.com/f163/secondary-not-john-palmer-jamil-zainasheff-weigh-176837/

I'm just asking....There's a million civil and uncivil discussions about this, including the one that this thread spun off of, so how is this one any different?

 

[ayoungrad]

exactly. I don't see any harm in having a civil discussion intended on furthering our knowledge.

And thanks for the links!

and to everyone else involved, thanks for keeping this civil, let's keep it this way. There is much for everyone to learn about in the brewing process.

I hope no one has taken my comments as uncivil.

Like I mentioned above, I think this is great stuff. As Revvy has stated, we are unlikely to uncover anything new. But it's all in good fun. And it will hopefully help all of us out.

 

[o4_srt]

So you're saying there's NEVER been a thread that discussed or looked at the why's before THIS thread, even in the Science section? Or you just haven't bothered to look. Just about every thread looks and tries to get a handle on the why's of it....

You can't have the what without the why...I just can't see how this thread any different than the CIVIL discussions all over here already about this?

Like THIS ONE for instance.
https://www.homebrewtalk.com/f163/secondary-not-john-palmer-jamil-zainasheff-weigh-176837/

I'm just asking....

revvy, we are not talking about whether to age or not, we are talking about the processes that occur during aging. Not an argument, not a debate, just passing information and ideas back and forth.

So far, the focus has been on diacetyl, but much more happens to beer when aging than just diacetyl reduction. Things like IPA's getting less hoppy with time, flavors blending together, alcohol heat dying down, etc. Is it the yeast that is directly responsible for this? or something else?


I thought this forum was about sharing information and ideas with other homebrewers, however, the tone that you have taken in this thread has been rather unwelcome.

Think of how many discussions there were about the shape of the earth before it was discovered that it was round, not flat? I'm sure a majority of those discussions went absolutely nowhere, until someone said "you know what? I'm going to figure this out once and for all"

Stifling discussion certainly isn't the attitude I expected to see.

 

[cruelkix]

So you're saying there's NEVER been a thread that discussed or looked at the why's before THIS thread, even in the Science section? Or you just haven't bothered to look. Just about every thread looks and tries to get a handle on the why's of it....

You can't have the what without the why...I just can't see how this thread any different than the CIVIL discussions all over here already about this?

Like THIS ONE for instance.
https://www.homebrewtalk.com/f163/secondary-not-john-palmer-jamil-zainasheff-weigh-176837/

I'm just asking....There's a million civil and uncivil discussions about this, including the one that this thread spun off of, so how is this one any different?

I'm not saying that at all. I'm sure there have been plenty of discussions about it. I started reading this one and found it interesting. I can understand that you don't like to see the same thread pop up a ton of times, but it happens all the time. I can't tell you how many "Beer Line Length" threads I've read. I just ignore them now unless I can help. So if you don't want to discuss. Just ignore it. I doubt you will be able to stop duplicate threads from appearing. I thank you for providing the information you did.

 

[Indyking]

Chill buddy. It's not the *if* this thread is asking about, its the why. I don't think anyone is arguing the benifits as much as trying to figure out the root cause.

Thank you for the information that you have posted previously. It will definitely help the discussion.

Yep, excellent point, thanks!!!

This has been a great discussion. The best here in a long while. There is no reason we should not have a civilized discussion about the technical aspects of brewing just because the experience niche here keeps feeding us the IFs but not the WHYs of the process.

With all due respect to the book authors and the experience guys out there, I have learned from you too, but experience does not mean precise knowledge.

I am a strong believer that aging is good for homebrews, but trying to learn exactly what causes it is far more clever than pushing the idea that yeasts will do the majority of the work despite of the lack of the basic component to fuel their machinery, which is sugar.

Like I said before, I have my theories but not owner of the truth by any means. I would love if somebody proves me wrong with convincing scientific support. Ask my wife about it.

 

[Flywheel]

I have loved reading this thread, even if it has been talked about before. I don't see any issues with a different conversation happening about the same topic (I think this thread has a different spin than the others mentioned).

 

[ayoungrad]

Yep, excellent point, thanks!!!

This has been a great discussion. The best here in a long while. There is no reason we should not have a civilized discussion about the technical aspects of brewing just because the experience niche here keeps feeding us the IFs but not the WHYs of the process.

With all due respect to the book authors and the experience guys out there, I have learned from you too, but experience does not mean precise knowledge.

I am a strong believer that aging is good for homebrews, but trying to learn exactly what causes it is far more clever than pushing the idea that yeasts will do the majority of the work despite of the lack of the basic component to fuel their machinery, which is sugar.

Like I said before, I have my theories but not owner of the truth by any means. I would love if somebody proves me wrong with convincing scientific support. Ask my wife about it.

Back to the debate...

Did you read through the prior google reference? That reference seems to be the basis for Briggs' stance on the maturation process. What about the reference do you not find to be "convincing scientific support"?

What do you need to convince yourself that yeast cells aid in secondary maturation? What is it, specifically, that you are looking for? I feel like I have provided fairly good back-up information.

 

[Revvy]

I'm just getting at the fact that a lot of info that folks are demanding or saying doesn't exist (like I pointed out with the diacytl info) DOES exists and has been posted in other threads about this topic. Just because some of you haven't seen some of the "why's " answered, doesn't mean we already haven't answered them. You'd be surprised what's already been covered in the 5 years of this...especially in the Jamil discussion, you might find exactly what you're looking for or claim doesn't exist. You imply that the why's haven't been addressed, and a lot of them have.

 

[ayoungrad]

To add to this... Give us a reference that specifically disputes the idea that yeasts play an active role in secondary fermentation.

I have never seen one. But I'm happy to read one if you've got it.

 

[cruelkix]

I'm just getting at the fact that a lot of info that folks are demanding or saying doesn't exist (like I pointed out with the diacytl info) DOES exists and has been posted in other threads about this topic. Just because some of you haven't seen some of the "why's " answered, doesn't mean we already haven't answered them. You'd be surprised what's already been covered in the 5 years of this...especially in the Jamil discussion, you might find exactly what you're looking for or claim doesn't exist. You imply that the why's haven't been addressed, and a lot of them have.

Good to know. I have started reading through that thread. Its a long one. Thanks for the info.

 

[Indyking]

Back to the debate...

Did you read through the prior google reference? That reference seems to be the basis for Briggs' stance on the maturation process. What about the reference do you not find to be "convincing scientific support"?

What do you need to convince yourself that yeast cells aid in secondary maturation? What is it, specifically, that you are looking for? I feel like I have provided fairly good back-up information.

My apologies. I have not had time to track down any of the google references. I'm not sure how useful they would be anyway since they seem to apply more to the D-rest, but I will take a look when I can.

I'm not talking about secondary maturation either. I'm talking specifically about that aging process in Ales where there seems to be a improvement in beer quality if the beer is allowed to rest for prolonged periods of time after fermentation is completed (Final SG is reached).

The "convincing scientific support" I talked about here and in the other thread that was closed refers to scientific articles I showed that confirm that S. cerevisiae activity and metabolism decreases dramatically at starvation in situations different from homebrewing. In our case, starvation means depletion of fermentable sugars, which is essentially their source of energy.

I don't think the book addresses that specifically. That's what I’m trying to find out.

 

[MalFet]

To add to this... Give us a reference that specifically disputes the idea that yeasts play an active role in secondary fermentation.

I have never seen one. But I'm happy to read one if you've got it.

+1.

Indyking, you keep making references to your "reliable scientific references", but don't give us any kind of reference to follow. Citations or they don't exist.

Also, you keep saying that you haven't seen any scientific evidence to the contrary, but ayoungrad's references have been pretty solid in my book. If there is something you find problematic in them, bring it up. Everybody here is enjoying the discussion. Right now it looks like you are just ignoring the references.

Please play science with the rest of us.

 

[Indyking]

+1.

Indyking, you keep making references to your "reliable scientific references", but don't give us any kind of reference to follow. Citations or they don't exist.

Also, you keep saying that you haven't seen any scientific evidence to the contrary, but ayoungrad's references have been pretty solid in my book. If there is something you find problematic in them, bring it up. Everybody here is enjoying the discussion. Right now it looks like you are just ignoring the references.

Please play science with the rest of us.

Did you see my direct link to the PNAS article? Can't remember if you post a comment about it or not.

ayoungrad's references has not shown anything that applies to Ale yeast working under starvation and what they exactly do under that situation, unless I completely missed it.

 

[MalFet]

Did you see my direct link to the PNAS article? Can't remember if you post a comment about it or not.

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 article has nothing to do with the topic at hand.

ayoungrad's references has not shown anything that applies to Ale yeast working under starvation and what they exactly do under that situation, unless I completely missed it.

They talk extensively about the processing of diacetyl after terminal gravity has been reached. Furthermore, these things are readily observable. I've got a beer going right now that, for whatever reason, spat out a lot of acetaldehyde. Over the last few days, the amount has gone down significantly (and will soon be gone completely), and yet the gravity has remained constant.

It's not a starvation environment, either. There are relatively low residual sugar levels in the beer, and don't forget that processing things like diacetyl and acetaldehyde produces energy for the yeast as well. Diacetyl and acetaldehyde are food, though less preferred to simple sugars.

 

[o4_srt]

Perhaps a professor at one of the brewing schools would have some insight on this. I might fire off a few emails this weekend to people at uc Davis or one of the other programs and hope I get a response.

I think there is definitely more going on in the aging process than just diacetyl reduction via yeast, the question is why and how.

Hell if I have free time tomorrow between classes, I might wander down to the bio department and start asking questions. Might track down my chem professor from last semester and pose some questions as well.

As an electrical engineering student, microbiology is not my forte, but I love science.

 

[IffyG]

Did you see my direct link to the PNAS article? Can't remember if you post a comment about it or not.

ayoungrad's references has not shown anything that applies to Ale yeast working under starvation and what they exactly do under that situation, unless I completely missed it.

Your citation failed to show that yeast under starvation conditions have no effect on beer. Rather, the PNAS paper showed that passing through the stationary phase accelerates aging in yeast. This is hardly relevant to the question being asked here.

 

[Indyking]

That article has nothing to do with the topic at hand..

I disagree. That article is one of the most elegant articles I have seen that deals with S. cerevisiae metabolism under starvation. Just because it was not done with samples taken from a homebrewing, it does not mean the general knowledge from it does not apply to our situation.

They talk extensively about the processing of diacetyl after terminal gravity has been reached. Furthermore, these things are readily observable. I've got a beer going right now that, for whatever reason, spat out a lot of acetaldehyde. Over the last few days, the amount has gone down significantly (and will soon be gone completely), and yet the gravity has remained constant...

.

Point taken. In fact, like I said before, I agree with that in the context of lagering, but I believe it does not apply to Ale aging.

The PNAS article confirms that yeast metabolism does not necessarily ceases upon starvation, but they do say it's dramatically reduced. What I wonder, and probably won't find a reference in a brewing context, is how much of this presumably residual metabolism after the SG has been reached influences the changes in flavor in the aging beer compared to other factors unrelated to yeast metabolism.

 

[remilard]

It seems to me that commercial brewers use alternative methods of maturation utilizing techniques aimed at increasing throughput. To say that they dump the cone rapidly does not infer that the beer does not need to be conditioned. It just means (as I mentioned previously) that commercial brewers do not necessarily rely on yeast to clean things up.

I know of many brewpubs that do nothing different than what many homebrewers do (they have temperature control and use gelatin, say). No chemicals, no filtration.

They all dump the yeast in the first week. I can keg almost any ale at 7 days with no chemicals/fining/filtration and they are generally flaw free (my friends like it and I win a bunch of medals).

The bottom line is, leaving the beer in contact with a pile of yeast at the bottom of the container well beyond TG is not required to make clean beer. The BBR/BYO study seemed to echo this for those who look to homebrewer kitchen science as the pinnacle of knowledge rather than industry practice and academic writings.

What nobody seems to get is that conditioning the beer in the presence of yeast does not require an inch of yeast at the bottom of the container. The yeast in suspension is generally sufficient.

 

[Indyking]

Your citation failed to show that yeast under starvation conditions have no effect on beer. Rather, the PNAS paper showed that passing through the stationary phase accelerates aging in yeast. This is hardly relevant to the question being asked here.

So, show me than an article that yeast under starvation conditions have effect on beer!!! Specific to ales, which is what I'm talking about.

 

[pjj2ba]

Wow, this thread took a turn. This is NOT as thread about autolysis and primary or secondary. We are discussing the maturation of a fermented beer and what is going on and what role(s) yeast play in this, with a bit of an emphasis on diacetyl.

I believe everyone is in agreement that diacetyl exists, it is desired in some beers and a flaw in others, and that yeast play a major role in its creation and removal

Maturation of beer flavour requires the presence of yeast as a catalyst. There are many methods of finishing that have the sole objective of prolonging the contact of beer with yeast after primary fermentation is completed. I want to emphasize that a diacetyl rest with most of the yeast lying at the bottom of the tank and not enough in suspension is of no use.

Paraphrasing
Method 1
If a very cold primary fermentation was used it involves allowing the beer temperature to rise from the controlled primary fermentation temperature of about 10°C to 15-18°C when the primary fermentation is coming to an end. ..... then the diacetyl rest would be commenced when the beer has attenuated to about SG 1023 when two-thirds of the total fermentable material in the wort has been consumed. .........

Method 2
.......When brewing ales, that should have very low diacetyl levels especially German Ales like Alt and Kölsch, the implications are to not use highly flocculent yeast and to allow an extended primary fermentation, albeit at cooler temperatures until sufficiently low diacetyl levels are reached. Yeast that settles in the cone is still removed on a daily basis.

I interpret the "contact of beer" to simply mean there is yeast in the beer - somewhere, anywhere, and not removed by some method (filtering or cold crashing). It then goes on to say that "not enough in suspension is of no use" which I interpret to mean that settled yeast is not going to do much to reduce diacetyl.

For both of these methods above it looks to me that, at least commercially, they are relying on yeast in suspension to clean up any diacetyl. I don't see how this would change in a homebrew setting. Yeast in suspension are going to take up and metabolize diacetyl. With respect to diacetyl, yeast in suspension in a 300 bbl fermenter are going to behave the same as those in our 5 gal fermenters. There certainly is different behavior by yeast in the two systems, but I don't think the metabolism of diacetyl is one of them

So if a persons stated reason for leaving the beer on the yeast cake is to reduce diacetyl, this is not technically accurate. The remaining yeast in suspension are the ones doing the job. There are plenty of reasons why one might leave their beer on the cake, but removal of diacetyl is not one of them. It has nothing to do with if one should use a secondary or do primary only.