Beer can only sit on a shelf for so long before it becomes undrinkable, and now German scientists have figured out why.
As time passes, according to new research, the chemical compounds that give beer its smooth bitter edge turn into compounds that are unpleasantly bitter. The discovery paves the way for beers that have a longer shelf life -- a boon to both brewers and consumers.
"Beer is bitter at the start, but the bitter quality changes after some months into a harsh, lingering bitter taste that is very unpleasant for consumers," said Andreas Dunkel, a food chemist at the Technical University of Munich. "We identified several new compounds responsible for that harsh and unpleasant bitter taste that were not known in the literature."
"The study was done in cooperation with several breweries in Germany," he added. "We showed several ways how they might prevent or slow down these processes."
Along with its refreshing nature, its role as a social lubricant and its unique and appealing aromas, beer attracts fans with a bitter flavor profile, which is especially strong in IPAs and other hoppy varieties. When bottled brews sit around for months, however, those complex bitter notes turn into something foul and repulsive.
Scientists have long known that a group of hop-derived compounds called iso-alpha acids are responsible for making beer bitter in the first place and that these compounds break down over time. In recent work, Dunkel and colleagues identified many of those breakdown products for the first time.
For the new study, the German team took the work a step further. They used several high-tech chemistry techniques to analyze in detail each component of a Pilsner-type beer in brown glass bottles. They conducted analyses on multiple samples at various stages of degradation for up to 10 years.
The study turned up two new compounds that, in addition to dozens of others that were previously discovered, contribute to the bitterness of aging in beer, the researchers reported in the Journal of Agricultural and Food Chemistry.
As part of the work, the researchers also documented the breakdown process of bitterness in beer every two weeks for over a year and a half. That allowed them to see exactly which compounds get converted first and how quickly the degradation happens. They even quantified the bitterness of each compound.
The results showed that, by about four to six months in storage, the composition of beer's bitter profile changes dramatically.
"The big take-home message here is that, as beer ages, a whole series of breakdown products grow with time," said Tom Shellhammer, a brewing scientist at Oregon State University in Corvallis. "It's another message to drink your beer fresh."
Knowing how beer changes chemically over time could eventually help inspectors gauge the age of beer on store shelves, Shellhammer said. Commercially, the work also offers a wealth of opportunities, particularly to large- and medium-sized brewers who would love to make beer that lasts longer, thus saving some of the cost of having to throw away old beer.
Keeping beer cool, for one thing, seemed to slow the conversion from good bitter to bad bitter. Slightly changing the pH made a difference, too. Brewers might also be able to avert some of the problems, Dunkel said, by changing the ratio of compounds in their original recipes.
"There are several possibilities and the brewers now are testing them," he said. "I think there will be some new things, not in the next month but in the next years, so that beer can be stored much longer."