I often drink a bottle 2 or 3 days after bottling, and then often every day or 2 after that, and it's sometimes quite amazing to see those "rough edges" getting smoothed out.
Can someone explain what happens during that first week or two after bottling, which transforms "green" beer into mature beer?
I ferment my beer and usually let it sit on the primary cake for quite some time. Mainly because I am busy with other things. When my beer comes out of the fermenter I have learned to recognize it as being good or great but without carbonation.
I think that what happens in the bottle is that you get a slight amount of oxidation that you can't control/prevent. Not all oxidation is bad, and this might help with that maturation flavor you are thinking of.
Secondly, I think the addition of CO2 starts to bright the beer up a bit too as a the beer's acidity increases a bit too. I think the lack of CO2 and what it does to the beer is the real contributor to the green character.
Albeit these are small changes but they help to round out the flavors and start to create a harmonious experience in the beer from all of its parts. (Sort of like soup the day after you make it)
That said I don't think there is one true answer for this. These are just my thoughts, which is why I chose to also make this answer a wiki.
Acetaldehyde is a compound that causes off flavors and aromas in beer, often described as tasting and smelling like green apples, cut grass or green leaves, pumpkin, or latex paint, and is sometimes described as giving beer a green character. I think this is what you're tasting. This compound is formed by an intermediate step in the conversion of sugar to ethanol by yeast. Under ordinary circumstances, any acetaldehyde formed during fermentation will eventually be taken up and converted by the yeast. The most common cause is removing the beer from the yeast too early, before the yeast has a chance to complete fermentation.
I don't know the exact process (perhaps someone else can offer more insight?). However I do know that the additional fermentation of the yeast under stressful (high pressure) environment does contribute to the taste change.
Well it varies from batch to batch, but something that happens which can "clean up" bottled beer is that a fresh round of fermentation done at room temps can clean up any lingering diactyl in the beer. However, I would strongly recommend that you get out of the practice of samples your bottles so young. If you recognize that the beer is getting better after some time, then just leave the bottles to do their thing for a few weeks.
