| Abstract
| - Phenolic compounds present in beer were shown by fluorescence spectroscopy and laser flashphotolysis to deactivate both singlet- and triplet-excited states of riboflavin with bimolecular rateconstants close to the diffusion control ranging from 2.8 × 109 to 1.1 × 1010 M-1 s-1 and from 1.1 ×109 to 2.6 × 109 M-1 s-1, respectively. Enthalpies of activation were low (up to 33.2 kJ mol-1), andentropies of activation were positive, ranging from 17 to 92 J mol-1 K-1, as derived from temperaturedependence, indicating a compensation effect. From a Stern−Volmer analysis of the singlet-excitedriboflavin quenching by phenols it was found that high amounts of phenolic compounds (>0.3 M)would be needed to hinder triplet-excited riboflavin generation. On the other hand, a phenolic contentof 0.36 mM is likely to quench 90% of the triplet-excited state. Phenol photodegradation was foundto be complex, and using ESI-MS analysis it was not possible to identify specific photooxidationproducts of the phenolic compounds; only the photoproducts of riboflavin could be detected andstructurally assigned. The rate of reaction of triplet-excited riboflavin with phenolic compounds inacetonitrile/citrate buffer (pH 4.6, 10 mM) is 550 times faster than the reaction with iso-α-acids fromhops, indicating that triplet-excited quenchers such as phenols may be involved in the early steps inlight-struck flavor formation in beer through radical formation. Terpenes present in herb-flavored beerswere found to be nonreactive toward singlet- and triplet-excited-state riboflavin, and any protectiondepends on other mechanisms. Keywords: Beer; light-struck flavor; riboflavin; phenolics; terpenes
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