Yeast cleaning up after themselves?

[vicratlhead51]

I've been working on introducing a friend to brewing and he's had a few questions I don't always have the best answer for. I've been addicted to reading this forum since even before I brewed my first batch. I've seen posts that say leaving the beer on the yeast cake after fermentation is complete can help clean up some off flavors in the beer. I kind of just took this statement at face value and have always let the beer go a week or more past reaching FG without really questioning why. Just knowing it can help the beer was enough for me but my buddy was asking me why and all I could say was "It helps clean up some off-flavors." but I couldn't really explain what was happening. Can anyone give me a little more info. on what's going on during this post fermentation phase?

 

[beninan]

I'm not sure of the scientific reasons why, but your yeasts just gone done having a week-long drunken orgy. there's a lot of cleaning to do

 

[brewhooligan]

http://www.howtobrew.com/section1/chapter8-3.html

This section of Palmer's online book explains how the yeast continues to condition the beer even after the bulk of the sugar has been fermented out. His online book explains this idea within the context of a secondary fermentation. But even Palmer has gone on record, supporting longer primary fermentation and foregoing secondary. There's a thread on this messageboard somewhere...

 

[ajdelange]

The two most significant things are both the result of yeast's tendency to maintain the beer in a reduced (chemical term) state. First, this reduced state results in reduction of the carbonyl groups in vicinal diketones (diacetyl and 2,3, pentane dione) to the analogous diols (alcohols) which have much higher flavor thresholds. The buttery flavors that are associated with the diketones are thus dispatched. Second, the carbonyl group in acetaldehyde is reduced to a hydroxyl group i.e. acetaldehyde is converted to ethanol. Another effect is the production of CO2 which "scrubs" nasty volatiles ("jungbuket") from the beer.

 

[vicratlhead51]

http://www.howtobrew.com/section1/chapter8-3.html

This section of Palmer's online book explains how the yeast continues to condition the beer even after the bulk of the sugar has been fermented out. His online book explains this idea within the context of a secondary fermentation. But even Palmer has gone on record, supporting longer primary fermentation and foregoing secondary. There's a thread on this messageboard somewhere...

Perfect, just what I was looking for. I had tried the search option but the search came back with tons of results not really addressing the question. I'm sure if I had gone through and sifted through all of them I would have eventually hit on something but I figured somebody would answer it faster :fro: