HVG Germany: Hopfen

Light struck flavour

As far as light struck flavour is concerned, the chemist means 3-methyl-2-buten-1-thiol (MBT).

In English-speaking countries this thiol, which has a very active odour, is also known as "skunky thiol", and is already perceptible in concentrations of a few nanograms per litre.


The iso-alpha-acids are sensitive to light and disintegrate at wave lengths between 350 nm and 500 nm. This phenomenon, which has been recorded in literature since 1875, occurs if beer is exposed to normal daylight. In the presence of sulphurous amino acids such as cystein the dissolved iso-alpha-acids react to the known thiol. Presumably riboflavin also plays a role in the photolysis of the iso-alpha-acids.

The formation of MBT can be checked by two methods:

1. by protecting the beer in dark glass bottles
2. by using reduced iso-alpha-acids

In the paper by Prof. D. De Keukeleire, M. D. E. Forbes et al. "Mechanism for Formation of the Lightstruck Flavor in Beer" the chemical reactions were investigated during the formation of MBT. With regard to the reduced iso-alpha-acids the scientists came to the following result:

"Apart from storing beer in light-proof containers, such as dark glass or cans, or immediate consumption, the photosensitivity can be circumvented by reduction of the iso-a-acids so that the deleterious photochemical process is prohibited.

This has been conclusively shown in the present work for the dihydroiso-a-acids in which the photoreactive a-hydroxyketone group is reduced to a photoinactive 1,2-diol entity, resulting in complete light resistance.

Conversely, the tetrahydroiso-a-acids are as photoreactive as the iso-a-acids;
however, 3-methylbut-2-ene-1-thiol (";skunky" thiol) cannot be formed from these compounds subsequent to photocleavage. As a consequence, the lightstruck flavor derived from tetrahydroiso-a-acids must be distinctly different from the "natural" lightstruck flavor, perhaps having less obnoxious organoleptic features."

As investigations by the German Research Institute for Food Chemistry show, the intensities of methional and phenylacetaldehyd are also increased through the exposure - compared with the unexposed beer. 

To sum up it can be said:
Although it is possible to counteract the volatile mercaptan odour by reduced iso-alpha-acids, it is essential that beer is not regarded as altogether light-protected: On the one hand e.g. tetra-hydro-iso-alpha-acids can allow hitherto not analysed light struck compounds to arise, secondly light causes a faster, stronger formation of aging components, which considerably changes the original taste. The photo-oxidation of the lipids shown by Prof. Karl Wackerbauer in transparent bottles and the research carried out by Dr. Carsten Zufall on beer aging "light stable beers", which were presented during the EBC Congress 2005 in Prague, should be studied by interested brewers.