Sunstruck flavour

Bondeel, G.M.A., De Keukeleire D. and Verzele M. (1987) The photolysis of trarc.v-isohumulone to dehydrohumu-linic acid, a key route to the development of sunstruck flavour in beer. J. Chem. Soc. Perkin Trans, i, 2715-17. 

It is obvious that this absorbance includes other beer-extractable components, which absorb at 275 nm, in addition to the isohumulones. It is well established that iso-octane extracts about twice the weight from beer than the amount of iso-alpha acids present. The error in the absorbance measurement is, however, probably small, since these other components consist mainly of oxidized fatty acids (precursors of the sunstruck flavour. 15 to 20 mg.l " in lager beer) and other, non-UV active substances. 

As stated before, beer has to keep for very long periods in these modern times of global commercialization. The slow generation of off-flavours, in this context, is of great importance. Sunstruck flavour is connected with iso-alpha acids, as previously discussed, but the other well known beer off-flavours mentioned in this Chapter, are not. However, a brief discussion is required, because research exploring possible connections with hop bitter acids has been carried out. 

Sodium borohydride treatment of alpha acids or of iso-alpha acids leads to the so called rho-iso-alpha acids. This reduced form of the iso-alpha acids accounts for stabilization against sunstruck flavour, while still giving acceptable bitterness (see earlier chapters). 

The abovementioned reducing procedures can be combined to yield hexahydro-iso-alpha acids. These compounds would be very bitter and stable against light-induced sunstruck flavour. 

Upon exposure to sunlight, beer develops an offending odour and taste, known as sunstruck flavour. Kuroiwa attributed this off-flavour to the formation of 3-methyl-2-butene-1-thiol via photolysis of the iso-alpha acids in the presence of sulfur-containing amino acids or proteins (12,13). This compound has been synthesized and compared to the sunstruck flavour (6). Also, the thiol has been identified unequivocally by irradiation of beer, concentration of the solution and gas chromatographic determination via flame photometric detection (14). The maximum concentration is 10" mg.F. Even in these minute quantities the effect on beer quality is disastrous. Similar results have been obtained starling from model mixtures that simulate the beer medium. For this purpose ascorbic acid (0.2 g), isohumulone (0.035... 

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