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

The effect of riboflavin and riboflavin binding proteins on the light-induc ed formation of reactive oxygen species and sunstruck off-flavour was studi ed in model beer solutions. Under model beer conditions (pH 4.0, 1 ppm ribo flavin, 5% ethanol and traces of O-2) hydroxyl and hydroxyethyl radicals we re formed upon illumination. Radical formation was measured with the spin t raps N-t-butyl-alpha-phenylnitrone (PBN) and 5,5-dimethyl-1-pyrroline-N-oxi de (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. A ddition of isohumulones to the model beer reduced the amount of riboflavin- induced radicals. Two different riboflavin binding proteins were tested bot h 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 move efficient in reducing radical formation than an apo-flavod oxin protein isolated from Azotobacter vinelandii. Organoleptic assessment clearly indicated that the addition of apo-RfBP to model beer solutions, co ntaining stiochiometric amounts of riboflavin as well as isohumulones and c ysteine, reduced sunstruck off-flavour formation, The dual role of riboflav in and ethanol as radical propagators in oxidative flavour change is discus sed.