post-fermentation processes

[beerisyummy]

Can anyone explain exactly what yeast does after fermentation is done? One hears about letting the yeast "finish up," reabsorb unwanted flavor compounds, yadayada. Besides lager yeast and diacetyl, because the diacetyl rest if I understand correctly is part of the fermentation process, what exactly goes on post-fermentation and for how long? Tia.

 

[bkboiler]

there's a lot of great literature out there that has done a very thorough job of describing this. A lot of it is free (e.g. How to Brew by Palmer).
If you're looking for the simple 1 paragraph answer: after the yeast eat all the sugar, there is no sugar left, they change their metabolic state so that they can eat other compounds. Like when the human body goes under 130 bpm heart rate and switches from carb burning to fat burning. When the yeast eat these other compounds, they generally leave behind smoother more pleasant aromas and flavors. It is highly yeast dependent and beer dependent (how strong was the beer). It can take anywhere from 2-3 days for a low alcohol English mild to "condition" after primary to several months for a much stronger barleywine.
Part of the longer time is that the yeast are more "tuckered out" after primary of a bigger beer...theres also more work to do in secondary (more harsh flavors are present since more primary fermenting was done to produce the higher ABV), and the working conditions are harsher (higher alcohol and lower pH makes the yeast more stressed).
For bigger beers you can look into nutrient additions, moving the beer off the cake after a few weeks, a different yeast for secondary even...
or dry hop it like crazy and nobody will notice all the acetaldehyde, diacetyl, etc...