Every once in a while I see minor pieces of information here that I think it may be wrong, but I don't bother to argue because I'm either not sure or it's just not worth it. But the yeast-cleaning-up-after-fermentation-is-complete thing has gone too far unquestioned I think.
I have strong reasons to believe brewing yeast cells stop any significant metabolism after their main source of energy, sugar, is depleted. I have showed evidence, some less but other purely scientific to prove my point. I'm not a microbiologist but I do have a cell biology background and my PhD was done in a microbiology lab sharing knowledge with a majority of microbiologists, though all of them were bacteriologists.
The idea supported by many here is to not use a secondary fermentation and ferment your beer beyond that period where final SG has been reached, so the yeast can clean-up after themselves and result in a better final product. That just doesn't happen IMHO. I never actually secondary any Ale as a matter of fact and I also keep my primary way beyond after fermentation is complete because I simply believe that stretching that time allow solids, including inactive yeast, to sediment better maximizing the appearance and possibly flavor of the beer. It has nothing to do with yeasts cleaning-up anything. They can't do it without energy and after fermentation is complete, their source of energy is gone and they become dormant.
In the case of D-rest, like I said before, John Palmer himself in the latest edition of his book How to Brew states that it is important to do it in the right time before lagering, which is before fermentation is totally complete, so the yeast will have substrate available to clean up the diacetyl. A lot of people will tell that they actually do a successful D-rest after fermentation is finished. I believe that is wrong too.
Fermentation in lagers is very slow. It may have long passed the peak of fermentation where airlock activity is typically noticed, but there is still some residual fermentable sugars available for the D-rest, only it's very scarce, hence the need to increase the temperature to activate the almost dormant yeast.
Look. I'm sorry for the bold letters, OK, sometimes I just can't help it. You can believe whatever you want too. I'm not asking you to believe in what I'm saying about this, but perhaps you all will think about it whenever that perception comes across again.
Finally, I brewed an English Brown Ale about 3 weeks ago, which I have bottled half of the 5 gal batch right after fermentation was completed and will bottle the other half after resting the remaining in the primary vessel for a month. After all have been carbed, I will taste them in a blind fashion to see if I can tell them apart. Because I don't likely have the most accurate palate in the world, barely that, I'm looking for BJCP-certified folk here who is willing to degust them as well in the same fashion. I highly suspect there will be a noticeable difference between them, not because of the famous, or infamous, yeast clean-up paradigm, but for the reasons aforementioned in this post.
Cheers!
