Why does beer go 'skunky'? - Ask a Scientist

In American society, attending college and drinking beer have become coterminous. This is true even at academic bastions such as Hopkins, where one fraternity estimates that it purchases approximately 130,000 fluid ounces of alcohol each year.

While most people who drink at parties do so in order to temporarily escape from Hopkins academics, those same partiers should be aware of the science involved in the bottling and storage of the brew they are imbibing.

While we always hear of parties 'running out of alcohol,' inevitably people will leave unopened beers lying around at room temperature or higher.

Also, it is not uncommon to see people drink unfinished beers that have been left out for extended periods of time. Party hosts may also run out of room in their home refrigerators to store beer.

What people seldom realize is that there is important health and safety science, not to mention flavor science, involved with leaving beer lying around.

Alcohol, like most other food and drinks, can go bad. In beer terminology this is called skunked beer, the colloquial term for a drink in which both the taste and aroma have become less than fresh and unpleasantly sour.

There is a precise science to skunky beer, as researched by scientists Malcolm Forbes of UNC-Chapel Hill and Denis De Keukeleire of Ghent University in Belgium, who published their findings on the subject in Chemistry - A European Journal.

Their attempt to solve the mystery of why beer goes skunky started with the early stages of the brewing process, where a solution of carbohydrates, called wort, is produced by enzymatic degradation of starch.

Wort, combined with hops (which act as flavoring agents) creates a liquid called hopped wort. This liquid contains alpha acids. Most beers have six different alpha acids.

These alpha acids cause beer to taste bitter, and serve an imperative element in the formation of a steady foam head on beer.

Alpha acids are very sensitive to light. When beer is exposed to visible or ultraviolet light the alpha acids undergo photolysis, causing free radicals to form.

These free radicals give beer so-called 'light struck' flavor by creating the same thiols present in a skunk's spray-hence the moniker 'skunky' beer.

This is one of the primary reasons why most beers are bottled in dark colored bottles, with dark brown being one of the most common colors.

According to an interview with Forbes on ScienceBlog.com, at least one major brewery adds a chemically modified hop to their beer which prevents free radicals from forming as quickly.

Bottlers also use certain tricks use to avoid having to take this step. Corona, which is brewed into clear bottles, is often packaged in closed boxes, and Corona's marketing team encourages consumers to add a slice of lime to their beer, most likely to combat stale odors.

Most beer is kept refrigerated in liquor stores at slightly above freezing temperature - around 38 degrees Fahrenheit. The majority of home refrigerators keep beer near this same temperature.

Beer's optimal serving temperature, according to Tod Mott from New Hampshire's famous Portsmouth Brewery, is approximately 45-65 degrees Fahrenheit, with lagers (crisper beers which ferment at lower temperatures) on the cooler end, and ales (more robust and fruity and fermented at a higher temperature) on the warmer end.

Thus, if beer is left out at room temperature, it is being put in an environment that is not conducive to optimal freshness.

So how badly can beer spoil?

Chemistry graduate student Stephen Diegelmann, an amateur brewer who has won three Hopkins University Graduate Representative Organization awards for his brews, noted that storage conditions and certain chemical processes are key in preserving a beer's ideal taste and temperature.

"It is unlikely that beer would actually spoil to a point to be toxic," Diegelmann said. "That being said, it is possible for beer to taste horrible and possibly even make someone throw up. Unfermented beer is the idea environment for any microorganism to grow."

Diegelmann noted that during the fermentation process, after the yeast die, the beer is ready for storage, and is often stored for months of years.

"Long term storage requires keeping the beer in the dark and close temperature control, with constant temperature being more important than specific temperature," Diegelmann said.

According to Ernesto Igot of Clipper City Brewing in Baltimore, even if beer has gone skunky, it won't necessarily make you sick.

"Beer will turn sour due to the presence of bacteria but the specific microorganisms are non-pathogenic. Beer could also taste papery due to oxidation," Igot said.

Since beer and wine are products of fermentation, they will hit a peak and then head downhill. Distilled spirits, or hard liquor, is quite dead and so will store forever if kept tightly sealed.

However there is still the problem of liquid evaporation with spirits, as Whiskey tends to evaporate rapidly if kept in a barrel (approximately 15% evaporation in the first year, and 5% thereafter).

While the News-Letter science section does not endorse underage drinking, we certainly endorse drinking fresh tasting and safe beer.

So if you decide to imbibe some frothy goodness, remember to be responsible, and drink beer that has not gone 'skunky.'