As the summer heat drags on, one Explainer reader wonders why soda or beer from a can tastes so much fizzier than the stuff that comes out of a bottle. Is there really any difference in the carbonation?
Yes, but only if the beverage has been sitting around for a while. Manufacturers dissolve the same volume of carbon dioxide into their plastic- and aluminum-bound products, but polyethylene terephthalate plastic is somewhat more CO2-permeable than aluminum. That means the fizz will leak out of a plastic bottle of Coke at a higher rate than it would from a can. Over 12 weeks of storage in poor conditions—a hot place with direct sunlight—soda in plastic bottles can lose up to 15 percent of its carbon dioxide. (That's with the new, improved PET used today. Until recently, manufacturers didn't use plastic bottles smaller than 1 liter due to concerns over degassing.)
There may be other reasons why the taste of soda would vary with the packaging. Despite enormous advances in can-lining technology, canned soda dissolves bits of metal through microscopic holes in the coating, affecting the flavor. Again, the degree of degradation is a function of storage time and conditions—the reactions happen more quickly if the can is warm. The problem was worse when cans were made of steel, but some people are still capable of detecting the aluminum flavor. (Glass is both impermeable and flavor-neutral, but soda in glass bottles is not as common in the United States as it once was.) *
Temperature has a much greater effect on carbonation than the packaging. Carbon dioxide is more than twice as soluble in ice-cold soda as it is at room temperature. If the soda gets warm while still in the can or bottle, the gas will leave the soda and enter the head space—that's the area between the soda and the cap used to regulate pressure—and release into the air the moment you open the container. The carbon dioxide will re-dissolve if you chill it, but that process can take some time. Different kinds of soda also have varying carbonation levels. Coca-Cola is slightly more carbonated than its diet counterpart. Ginger ale has almost twice the carbonation of most colas.
The carbonation in beer depends on some other factors. Mass-marketed beers, like sodas, are carbonated by forcing CO2 into the liquid under pressure and begin at the same level of fizziness regardless of container. Many bottled microbrews, however, are carbonated the old-fashioned way—with brewer's yeast and a little sugar. The resulting fermentation—yeast plus sugar yields alcohol plus carbon dioxide—carbonates the beer. (Many aficionados are convinced that the "naturally" carbonated beer contains finer bubbles with a softer mouth-feel, but the science is still unsettled.) Aluminum and plastic are rarely used for this process, because the beers often spend a long time "conditioning" in the bottle, raising the possibility of decarbonation and metallic flavors.
You can't really "taste" carbonation. You feel it the same way you feel pain. When soda exits the pressurized environment of the can or bottle and strikes your tongue, carbon dioxide gas rushes out of solution. It then mixes with water and carbonic anhydrase (an enzyme that helps your body move carbon dioxide into and out of cells) to form carbonic acid. When the concentration of carbonic acid reaches a certain level, nerve endings called nocireceptors send pain signals to the brain. This is the reason soda leaves a tingling sensation in your mouth after you swallow it—the carbonation is gone, but the carbonic acid is still around.
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Explainer thanks Charlie Bamforth of University of California-Davis, John E. Baur of Illinois State University, and Tracey A. Halliday of the American Beverage Association.
Correction, July 23, 2009: The original version stated that soda in glass bottles is nearly impossible to find in the United States. While the most familiar brands now come in plastic bottles, many smaller companies still use glass. (Return to the corrected sentence.)