Dear Cecil: The current ad campaign for Samuel Adams beer makes the somewhat dubious claim that the company's beer, stored in brown bottles, is better preserved than beer in — eew! — clear or green bottles. So, time to break out your beakers (and beer bottles) and tell me if there is any validity to this claim, or if it's just the usual marketing babble. David

Cecil replies:

Although we spurn outright trivia, David, we recognize that some of the questions we deal with at the Straight Dope are more consequential than others. The theory of relativity, species collapse, and so on … this is the stuff of party chatter. Every so often, however, we get to settle one of the great questions of our times. Today is such a day.

Despite the occasional introduction of civet feces (no joke) or other eccentric ingredients, beer is an essentially simple product, typically made from water, malted grains, yeast, and hops. These seemingly uncomplicated fixings give rise to more than 600 volatile compounds, with chemical reactions continuing the entire time the beer ages. As with most chemical reactions, heat speeds them up, as can the energy in light. Some of these reactions can yield a mellower flavor. Too much light, however, and your brew may be “lightstruck,” meaning you get skunky beer.

The first reference to lightstruck beer dates from 1875, but the cause was unknown until the late 20th century. The culprit: hops. You may ask: what are hops, anyway? I confess to being a little vague on the subject myself. Hops are the conelike flowers of the climbing plant Humulus lupulus, used to give beer its bitter flavor. When light reacts with certain hop-derived compounds, it creates a variety of unpleasant-smelling and -tasting chemicals, the biggie being 3-methyl-2-butene-1-thiol, or MBT.

There are several ways to prevent beer from becoming lightstruck: brew it without hops, use light-resistant hop extract instead, or add antioxidants. Since all these things affect the taste, though, most brewers prefer to simply keep the beer away from light. Packaging beer in cans is one obvious solution, but beer snobs historically have shunned cans, claiming they impart a metallic taste. Modern high-tech coatings have largely allayed such concerns, and some now claim cans are the ideal way to package beer.

But you asked about glass. Colored glass can filter out both visible and ultraviolet light. Brown glass tends to block more light than green; clear glass, predictably, doesn’t block much at all. Since dark beers absorb more light than light beers, it’s essential to store stouts, bocks, and the like in brown bottles, while lighter beers can be happy in green ones.

Or so goes the theory. To see how things worked out in practice, we turned, as so often, to the lab. My assistants Una and Fierra, both experienced home brewers, cooked up a batch of extra-hoppy German-style beer which they dubbed “Cecil’s Dopetoberfest,” containing a modest 4.6 percent alcohol by volume. They bottled it in brown, green, and clear glass and let it age for six weeks in a cool basement.

Next they grouped the bottles into five sets of three (each comprising one bottle of each color) and left them outdoors in direct sunlight for different lengths of time, keeping control samples safely hidden. The five groups of bottles were exposed to three, eight, 24, 48, and 72 hours of sunlight respectively. Thanks to cold weather, keeping the bottles cool while in the sun wasn’t a problem, although incursions by squirrels and possums required occasional intervention. After their time in the sun, the bottles from each group plus several control bottles were refrigerated to 35 degrees Fahrenheit and sampled in a double-blind taste test.

Results:

After three hours of sun exposure there was no significant difference among the beers, although both testers rated the control beer the least palatable. Which isn’t so odd — some research suggests exceedingly small amounts of MBT can improve beer flavor.

After eight hours of sun the clear-bottled beer had developed a skunky odor and a bitter chemical taste. The other bottles were judged uniformly good.

After 24 hours of sun, the clear-bottled beer produced a strong skunky odor and a taste Fierra noted as “Ewwwww!” The green-bottled beer started to taste metallic.

After 48 hours, the clear-bottled beer became still more disgusting, and upon opening could be smelled from six feet away. The green-bottled beer had acquired a strong metallic taste — Una, summoning her inner barbarian, could choke it down, but not Fierra. The brown-bottled beer remained indistinguishable from the control.

After 72 hours in the sun, even the brown-bottled beer was starting to go.

Conclusions:

(1) In this world of mendacity and fraud, at least one ad claim has a basis in fact — brown bottles do protect beer better than green or clear.

(2) Notwithstanding (1), in the war of beer versus sun, don’t bet against the sun.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.