|MadSci Network: Chemistry|
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
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