Re: Why does beer get oxidated?

Hi Reynoso.
There are many chemicals that give beer their flavor (many are in a class 
of chemicals called esters).  These are generally volatile (low boiling), 
fairly reactive toward oxygen, and decompose easily in the presence of 
sunlight.  Upon opening a container of beer, these substances begin to 
evaporate.  Those that don't evaporate can become oxidized (that is, react 
with oxygen) as oxygen begins to dissolve into the beer, replacing the 
carbon dioxide that has bubbled out (water holds a fair amount of 
dissolved oxygen...it is this dissolved oxygen that allows fish to 
"breathe" underwater).  These new chemicals formed by oxidation of flavor 
chemicals can become unpleasant in taste and smell (in many cases, these 
are acids).  For example, when butter goes rancid, one of the principal 
stinky substances formed is butyric acid. 

Unpleasant flavors and smells can also arise from the material the beer is 
packaged in.  Beer packaged in clear bottles or green bottles allows light 
of a certain wavelength to enter, and can initiate reactions involving the 
flavor molecules.  This is why some beers such as Heinekin (green bottle) 
are often said to smell/taste "skunky".  Beer contained in aluminum cans 
often has a slightly metallic flavor to it, but is shielded from light and 
doesn't have a skunky flavor.

I hope this answered your question.
Please email me if you have further questions.

Best Regards,
Mike