From 'Brewing Techniques' magazine, Nov/Dec 1993...
If air is introduced during wort production--in other words, on the hot side --these substances will be oxidized and later, in the finished beer, they can turn around and give up their oxygen to alcohols that were created during fermentation. An oxidized alcohol is an aldehyde, and aldehydes are the bad guys that are responsible for all the stale, old-beer flavors we have all encountered in far too many imported brews.
and...
Now for the good news: Brewpubs and home brewers don't have nearly as much to be concerned about as shipping breweries because they keep their beer close to home rather than sending them out into the cruel world where they can suffer all kinds of insults. Storage temperature has a lot to do with how fast beer oxidizes. So does agitation. There really is something to the old saying that beer doesn't travel well. Getting knocked around in the back of a truck does take a toll on packaged beer, and temperature is an even bigger factor. But even at low temperatures, kept absolutely still, beer will oxidize eventually. That is why the best plan is to store it cold and drink it quick.
Full article available online at :
http://www.brewingtechniques.com/library/b...r.html#aeration