The role of riboflavin in beer flavour instability: EPR studies and the application of flavin binding proteins

Abstract

The effect of riboflavin and riboflavin binding proteins on the light-induced formation of reactive oxygen species and sunstruck off-flavour was studied in model beer solutions. Under model beer conditions (pH 4.0, 1 ppm riboflavin, 5% ethanol and traces of O2) hydroxyl and hydroxyethyl radicals were formed upon illumination. Radical formation was measured with the spin traps N-t-butyl-α-phenylnitrone (PBN) and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). DMPO appeared to be a better spin trap than PBN for studying the effect of light exposure, since PBN is photochemically active by itself. Addition of isohumulones to the model beer reduced the amount of riboflavin-induced radicals. Two different riboflavin binding proteins were tested both for their ability to scavenge riboflavin and how in turn this influenced free radical formation. The apoform of egg white riboflavin binding protein (RfBP) was more efficient in reducing radical formation than an apo-flavodoxin protein isolated from Azotobacter vinelandii. Organoleptic assessment clearly indicated that the addition of apo-RfBP to model beer solutions, containing stiochiometric amounts of riboflavin as well as isohumulones and cysteine, reduced sunstruck off-flavour formation. The dual role of riboflavin and ethanol as radical propagators in oxidiatve flavour change is discussed.