At least three chemicals

May 31, 2007 19:41 GMT  ·  By

A beer is a dynamic molecular world that does not stop evolving once it is in the bottle.

Over 1,000 chemicals have been found in beer, giving it nearly 125 different flavors, referring to tastes, odors, and mouthfeel. About 40 flavors are common to the two broad types of beer, ales (pale ales, amber ales, and stouts) and lagers (pilsners and bocks).

Primary flavors are present in concentrations at least double the minimum needed to be felt.

Alcohol, carbonation, and bitterness are primary to all beers, but also hop aroma in pale ales. Secondary flavors, like banana or pear, are present in levels near or just above the detection concentration. Tertiary and background flavors are at concentrations below detection.

But off flavors cover a beer's bouquet, like stale flavor linked with aging and the skunky flavor when beer is exposed to light. "In the first step of brewing, called malting, barley kernels are steeped, permitted to germinate, and then heated to temperatures as high as 110?C until dry. The higher the temperature, the more roasted, toffee-like flavors are present in the beer", said Charlie Bamforth of the University of California, Davis.

After malting, the barley is mashed with hot water and barley enzymes turn starch to maltose.

Brewers next add hops to the wort (the liquid part of the mash) which by boiling turn their alpha acids into bitter-tasting iso-alpha acids. "Hops also contain oils, which contribute characteristic aromas to the brew. Depending on the hop variety, this aroma could be citruslike, piney, or herbal," said Karl J. Siebert of Cornell University.

Another dose of hops at the end of the boil increases the aroma.

In the cooled wort yeast is added to start the fermentation stage, through which maltose is turned into alcohol (ethanol), a warming characteristic, and carbon dioxide, which affects the mouthfeel of the beer. "Some of the yeast's metabolic by-products add flavor as well. Esters, for example, lend fruity tones: Iso-amyl acetate smells of banana, and ethyl acetate smells of pear," said Bamforth.

After fermentation, the beer is stored at cold temperatures for a few days for ales, to a few weeks for a lager. After that, the beer is filtered, further carbonated, and packaged. Brewers already have flavor-stabilizing methods to limit the development of off flavors during the brewing process. The skunky flavor of beer can be attributed to 3-methylbut-2-ene-1-thiol, encountered also in the skunk spray, and appears when beer is exposed to light. "Humans are very sensitive to this compound," said Denis De Keukeleire of Ghent University in Belgium. "3-methylbut-2-ene-1-thiol is detectable in quantities as small as 0.004 microgram (?g) per liter. This amount can form in minutes when beer is exposed to bright sunlight," he said.

Riboflavin, a yeast chemical, absorbs light energy with wavelengths of 350 to 500 nanometers (nm). This energy makes the iso-alpha acids release free radicals that combine with sulfur chemicals produced by the yeast, resulting the stinky thiol. When the beet is put in brown bottles, wavelengths under 500 nm are blocked.

If brewers prefer clear or green bottles, which do not stop those wavelengths, they employ hop pellets with chemically modified alpha acids, which cannot form the thiol, but this process is not 100 % effective. Free radicals were still found to form sulfur compounds with rotten eggs flavor.

Beer flavor also gets staler with the age due to trans-2-nonenal. The compound is detectable at levels as low as 0.035 ?g per liter, but it can reach 0.5 ?g per liter after 3 to 5 months of storage at room temperature. "The primary chemical pathway to stale flavor remains elusive. Oxidative reactions can produce this flavor, so brewers assiduously monitor the oxygen level during the brewing process, keeping it at or below 0.1 milligram per liter," said Bamforth.

But in 1999, Belgian scientists found a nonoxidative mechanism behind the development of trans-2-nonenal. Recently, Danish researchers have found a barley enzyme that converts trans-2-nonenal into nonanal. "While too much of this compound would add an unpleasant, astringent character to the beer, its detection concentration is 150 times that of trans-2-nonenal," said Gustav Hambraeus of the Carlsberg Research Center in Copenhagen.

Barley varieties with enhanced enzyme activity could be achieved, either by selecting mutated plants or by genetic engineering. A team led by Bart Vanderhaegen of Katholieke Universiteit Leuven in Heverlee, Belgium, has found that furfuryl ethyl ether also contributes with an unpleasant aging flavor, especially in ales. "The concentration of furfuryl ethyl ether increases gradually during aging. It can reach its flavor threshold, about 6 ?g per liter, in beer stored at room temperature for a month or so and can reach an overpowering 200 ?g per liter after 6 months", said Vanderhaegen.

Its precursors are alcohol and furfuryl alcohol. "Because beer is slightly acidic, its furfuryl alcohol can pick up an extra proton," explained Vanderhaegen. It can interact with ethanol to form an intermediate molecule, which splits off water and another proton to form furfuryl ethyl ether.

Prolonged wort boiling at higher temperatures produced more furfuryl alcohol. "If brewers reduce the heat applied, they can decrease the amount of furfuryl alcohol in the wort and, therefore, the amount of furfuryl ethyl ether that forms in the stored beer." said Vanderhaegen. And "The most important way to slow down aging is to store beer at cold temperature," Vanderhaegen added. Heat increases the rate of the reactions that induce the stale flavors in stored beer. "You can ruin the flavor in a week, by storing beer in a hot garage. Most brews, kept at room temperature, can be expected to keep their good flavor about 3 months," said Siebert.