New light on bad beer

Sunlight on isohumulones gives beer a skunky off-flavour.

Belgian scientists have been finding out how to stop one of their country’s most famous products - beer - from going bad.

When beer is exposed to light significant taste and flavour changes are observed. An off-flavour called lightstruck flavour (LSF) develops. LSF is attributed to 3-methylbut-2-ene-1-thiol (’skunky’ thiol), and it has been shown that just 1 ngL ^-1 of this thiol can make beers unpalatable. During the boiling of hops, a key ingredient in beer, isohumulones are formed; these are pivotal to the development of LSF but are also essential in the stabilising of beer foam. When irradiated with UV light these isohumulones decompose readily and a key intermediate for the development of LSF forms.

Denis De Keukeleire and co-workers from Ghent University are currently investigating the reaction products formed by the photooxidation of isohumulones with a view to preventing the bad flavour developing in beer. Reaction products were analysed using gas chromatography and mass spectroscopy. Loading the reaction mixtures into transparent syringes and exposing them to visible light at the same time as they were injected into the ionisation source of a quadrupole time-of-flight mass spectrometer gave the team a real-time analysis technique. Photooxidation of isohumulones by excited beer flavins lead to the lightstruck flavour in beer. The work also comprises the use of so-called reduced isohumulones, which are frequently used to brew light-stable beers, by showing that these compounds formed the same reaction products as isohumulones on short exposure times.

Current methods for preventing lightstruck flavour are based on using opaque packaging, however customers prefer transparent packaging. De Keukeleire hopes that the understanding that this work provides could lead to new ways of preventing the deterioration of beers when exposed to light.

Helen Fletcher