Phenol and terpene quenching of singlet- and triplet-excited states of riboflavin in relation to light-struck flavor formation in beer

Phenolic compounds present in beer were shown by fluorescence spectroscopy and laser flash photolysis to deactivate both singlet- and triplet-excited states of riboflavin with bimolecular rate constants close to the diffusion control ranging from 2.8 x 10(9) to 1.1 x 10(10) M-1 s(-1) and from 1.1 x 10(9) to 2.6 x 10(9) M-1 s(-1), respectively. Enthalpies of activation were low (up to 33.2 kJ mol(-1)), and entropies of activation were positive, ranging from 17 to 92 J mol(-1) K-1, as derived from temperature dependence, indicating a compensation effect. From a Stern-Volmer analysis of the singlet- excited riboflavin 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 content of 0.36 mM is likely to quench 90% of the triplet-excited state. Phenol photodegradation was found to be complex, and using ESI-MS analysis it was not possible to identify specific photooxidation products of the phenolic compounds; only the photoproducts of riboflavin could be detected and structurally assigned. The rate of reaction of triplet-excited riboflavin with phenolic compounds in acetonitrile/citrate buffer (pH 4.6, 10 mM) is 550 times faster than the reaction with iso-R-acids from hops, indicating that triplet-excited quenchers such as phenols may be involved in the early steps in light-struck flavor formation in beer through radical formation. Terpenes present in herb-flavored beers were found to be nonreactive toward singlet- and triplet-excited-state riboflavin, and any protection depends on other mechanisms.