Involvement of flavanoids in beer color instability during storage

besides Maillard reactions, structural rearrangements of flavan-3-ol monomers cause color changes in beer during storage. Acetone/water-soluble fractions (70/30, v/v) of three lager beers of the same batch, differently stabilized before bottling in glass or poly(ethylene terephthalate) (PET) bottles, were monitored by normal-phase HPLC-ESI(-)-MS/MS over a 1-year period of storage at 20 degrees C. In parallel, beer color was monitored by the European Brewery Convention assay. The evolution of color was similar in the silica gel-filtered beer to that in identically bottled and stored poly(vinylpolypyrrolidone)treated samples, despite the high flavanoid dimers content of the former. On the other hand, color evolved more rapidly in the PET bottle, suggesting a key role of oxygen. The kinetics was still increased in model media containing (+)-catechin, while no color was detected when normal-phase HPLC-fractionated dimers or trimers were investigated. (+)-Catechin emerged as the precursor of less polar products, characterized by a yellow-brown color. MS/MS enabled us to identify these products as issued from the oxidation and intramolecular additions of clehydrodicatechin B4. Similar structures were found. in aged beers spiked with (+)-catechin. Beer storage in the absence of oxygen and at low temperature is recommended so as to minimize the synthesis of such pigments.