Clean-up (Can yeast reduce protein for energy?)

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?

tCan said:

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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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.

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

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.

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...

tCan said:

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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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

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

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.

Is it safe to assume that this process of conditioning more or less begins as the FG stabilizes?

Edit - Post 4 seems to address what I'm asking

Yooper said:

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

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That is a great article. Is there anyway we could force every new brewer to read it?

That article is fantastic, but a lot of it is going to be lost on brewers that don't have a background in biochemistry and organic chemistry.

However, I think those of us that do, can definitely help others out by summarizing some of the more salient points from the article. Here's what I got out of it that I think brewers should be reminded of.

1. There is more to fermentation than watching bubbles come out of the airlock. The reduction of diacetyls does not produce CO2 or ethanol, so there is no visible evidence of this occurring. Since the densities of diacetyl and the reduced product, 2,3-butanediol are nearly identical, so I would expect that any change in gravity would be undetectable by a homebrewer. More evidence towards extended time on the yeast.
2. Interesting to note that a lack of valine can lead to increased levels of diacetyl. Also, the author states that extract worts tend to have lower levels of proteins than all-grain worts. Extract brewers might want to consider yeast nutrient (largely a nitrogen source) to mitigate this.
3. Inadequate oxygenation can be a contributor to the production of diacetyls. Appears that excessive oxygenation is not a concern for a homebrewer.
4. Unless working with slants, 1-10 bacterial cells per 10 million yeast cells is an acceptable level to prevent off-flavors due to bacteria.
5. Sterilization is really only needed for the early stage of yeast propagation or when working with semidormant liquid yeast. So, sanitation is really important when making starters from a vial or smack pack that has been in the refrigerator.
6. This entire article really suggests that to ensure the best chances of making good beer, yeast starters are really beneficial.
7. Key aspect of making good starters is using the appropriate media for propagating the yeast - uses a particular example of poor valine uptake, and thus high diacetyl, in worts fermented with yeast grown on starters made from sucrose. I was involved in a discussion a couple weeks ago about this and I will paste a link to the article in that thread as well.

Any brewer that wants to improve the flavor of their beer and protect against unwanted off-flavors should really take the time to read that article, even if a lot of the biochemistry is over their heads. It isn't hard to extract the recommendations that are implicit in it.

Yes, pretty good article. The only problem with it I caught is that the implication that diacetyl is rarely formed outside the cell (i.e. by non enzymatic oxidation) is counter to the usual thinking which is that diacetyl is formed after the yeast have been removed unless all the acetolactate has been removed i.e. it is, for the most part, formed by non enzymatic oxidation. This is the basis for George DePiro's simple test: warm a sample of beer you are about to package (at which point, presumably, you have removed the yeast). If acetolatate is present the warming will speed it's conversion to diacetyl. Thus if you smell diacetyl after warming, you can pretty much count on it forming after the beer is packaged and should krausen.

This model is also responsible for the efficacy of Maturex (alpha acetolactate decarboxylase) which catalyses the conversion of acetolactate directly to 2,3 butane dione.

If you want the last word on diacetyl get a copy on Inoue's monograph "Diacetyl in Fermented Foods and Beverages". More than the average homebrewer probably needs to know but some pretty interesting information in there (Japanese people hate the taste of diacetyl).