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BEER drinkers can raise a glass to the chemists who have finally worked out
how their favourite tipple decomposes into smelly chemicals when exposed to
light. The work could lead to better beers that last longer in bottles.

Beer is made by fermenting malted barley and hops, but light quickly breaks
down chemicals from the hops called isohumulones. The highly reactive fragments
then combine with sulphur compounds to make a substance that chemists call
鈥渟kunky thiol鈥� because it is almost identical to the smelly secretion produced
by North American skunks.

Chemists have studied this process for decades, but no one knew precisely how
isohumulones came apart. Now Malcolm Forbes of the University of North Carolina
at Chapel Hill, Denis De Keukeleire of Ghent University in Belgium and their
colleagues have filled in the missing steps.

The researchers exposed a solution of isohumulones to ultraviolet light and
monitored the reaction using a technique called electron paramagnetic resonance
spectroscopy, which is similar to the nuclear magnetic resonance technique used
in medicine to make images of the human body. This gave them a microsecond by
microsecond insight into the precise make-up of the fragments as they were
created.

But the researchers used pure isohumulones, cautions Mick McGarrity, a
chemist with Labatt Brewing Company in London, Ontario. 鈥淭his work dovetails
nicely with research that we鈥檝e done,鈥� he says, 鈥渂ut it鈥檇 be nice if someone did
this in beer.鈥�

Now that the precise chemistry is known, Forbes and De Keukeleire think it
might be possible to add something to beer to prevent the production of skunky
thiol on exposure to light. That should lead to better brews in cheaper
bottles.

Most beers are bottled in protective brown glass, which costs more than clear
glass. Of the beers that do come in clear or green bottles, many are made with
chemically modified hops extracts that don鈥檛 produce skunky thiol, but taste
different from beer made with unmodified hops.