tCan

Recipes 

So I've been advised that it's a good idea to let the fermenter sit for 1-2 weeks after fermentation appears to have stopped because the yeast go through a 'clean up' process, consuming funky protein flavors.

Does yeast have the ability to do this and is there a good article on it that anyone knows of?

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On this day, I wish a cold brew and warm company upon thee.

Yooper

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No, they don't consume everything.

What they DO consume is some of the by-products of fermentation, like diacteyl. Sorting of cleaning up their own waste products, if you will.

But this is not a long proces, certainly not 1-2 weeks, more like a day or two. They consume all of the easily fermentable sugars, then the tougher ones, then finally their own by products when no fermentable sugars are available.

I will see if I can find a real scientific description of this, as my "dumbed down" version is all that I am capable of.

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Broken Leg Brewery
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tCan

Recipes 

Good luck! I was only able to find brewing guides and such in my search.

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On this day, I wish a cold brew and warm company upon thee.

josephus23

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I think the key to getting the yeast to clean up diacetyl is to ramp up your fermentation temp near the end (when the Gravity is a few points from terminal. You can determine how long you need by testing for diacetyl. There is plenty on the web about how to do this. Two weeks would be really long for something like a hoppy beer for freshness sake.

tCan

Recipes 

I'm actually working with an apfelwein right now so it doesn't matter too much about freshness. I am curious what you guys are refering to when you say diacetyl. Acetyl is a functional group and not a chemical of it's own, so I'm assuming it's short-hand for something...

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On this day, I wish a cold brew and warm company upon thee.

Yooper

Recipes 

Shorthand? No.

Diacetyl is produced during fermentation as a by-product of valine synthesis, when yeast produces α-acetolactate, which escapes the cell and is spontaneously decarboxylated into diacetyl. The yeast then absorbs the diacetyl, and reduces the ketone groups to form acetoin and 2,3-butanediol, relatively flavorless compounds. (from wikipedia for the definition).

Diacetyl has come into the news recently as being toxic to workers, particularly those in the "movie theater popcorn" work, where diacetyl can cause lung damage.

It's a natural by product of fermentation by the yeast in beer, though.

Here's some info on it (from George Fix): http://www.brewingtechniques.com/library/backissues/issue1.2/fix.html

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Broken Leg Brewery
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tCan

Recipes 

Thanks boss that's exactly what I was looking for.

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On this day, I wish a cold brew and warm company upon thee.

ajdelange

Recipes 

Diacetyl is di-acetyl CH3COCOCH3 (2,3 butane dione) one of 2 vicinal diketones of interest to brewers (the other is 2,3 pentane dione). It is formed by extracellular oxidation of alpha acetolactic acid which is excreted by the cell as a byproduct of valine synthesis and is found in most fermented foods. Living yeast are capable of enzymatically reducing it to acetoin (CH3COHCOCH3) and that to 2,3 butane diol (CH3COHCOHCH3) both of which have much higher flavor thresholds than diacetyl. The key to getting rid of it is exposing the conditioning beer to active yeast. This is done traditionally by slowly lowering the temperature of the finishing beer to near freezing (lagering), by adding a portion of actively fermenting beer (krausening), more recently by raising the temperature of the near finished beer (diacetyl rest) and most recently by adding acetolactate decarboxylase (Maturex) to the finishing beer. Reduction of acetaldehyde to ethanol is also part of the cleanup process and also requires active yeast. There are additional things that go on during conditioning/lagering such as the scrubbing of sulfurous volatiles (jungbuket ~ beerstench) and the coagulation and settling of proteins (cold lagering) and the precipitation of calcium oxalate.