European researchers have unravelled the missing step in understanding how light causes the flavour of beer to go off. 

It has long been known that when beer is exposed to light an unpleasant 'lightstruck flavour' can form, causing quality problems for brewers. Denis De Keukeleire and colleagues at Ghent University, Belgium, and the Royal Veterinary and Agricultural University, Denmark, have now studied how isohumulone compounds derived from the hops used in the brewing process break down under light.

They found that some of the light energy absorbed by riboflavin, a vitamin in the beer, cleaves sulfur-containing proteins and amino acids. This produces sulfur radicals which recombine with the isohumulone radicals, ultimately leading to the substance which causes the bad flavour - a compound known as 'skunky' thiol, due to its bad smell. Only a few nanograms of this thiol in one litre of beer are enough to give the offensive flavour. 

Using advanced spectroscopic methods, the scientists were able to study the highly reactive intermediates in the reaction and answer the question of how sulfur is incorporated into the skunky thiol - the missing link in how lightstruck flavour forms. 

De Keukeleire hopes that now the full mechanism has been clarified, measures can be taken to prevent lightstruck flavour. 'Some hurdles must be overcome,' he said, 'but our past experiences in applied hop research gives us confidence to be able to realize a most challenging goal, to the pleasure of the beer-loving population.'

Caroline Moore