Kvasný průmysl (Feb 2008)
Colour changes during beer aging.
Abstract
Spectrum changes of yellow, orange and yellow-red oxidation products of epicatechin with their typical absorption maximum around 430 nm were studied. Oxygen is necessary for the non-enzymatic formation of these substances and their formation increases with growing pH value and Fe2+ and Cu2+ concentration. Based on the a.m. reasons, colour products are formed more easily in tap water than in deionized one. The colour substances formed can partially discolour with falling pH value and increasing concentration of reducing substances. Colour substances once formed cannot be fully reversibly reduced, which corresponds to the model of a partially reversible oxidation-reduction reaction and gradual colour increasing of model solutions as well as beer in both aerobic and anaerobic conditions. Oxidized polyphenols as well as reductones can provide further oxidizing agents during the reduction process, such as semiquinone and reductone radicals and further colour substances are formed as a result of their mutual reactions. Mutual reactions between oxidized polyphenols and reductones can be speeded up by the effect of visible radiation and then observed by differential spectrophotometry.Spectrum changes of yellow, orange and yellow-red oxidation products of epicatechin with their typical absorption maximum around 430 nm were studied. Oxygen is necessary for the non-enzymatic formation of these substances and their formation increases with growing pH value and Fe2+ and Cu2+ concentration. Based on the a.m. reasons, colour products are formed more easily in tap water than in deionized one. The colour substances formed can partially discolour with falling pH value and increasing concentration of reducing substances. Colour substances once formed cannot be fully reversibly reduced, which corresponds to the model of a partially reversible oxidation-reduction reaction and gradual colour increasing of model solutions as well as beer in both aerobic and anaerobic conditions. Oxidized polyphenols as well as reductones can provide further oxidizing agents during the reduction process, such as semiquinone and reductone radicals and further colour substances are formed as a result of their mutual reactions. Mutual reactions between oxidized polyphenols and reductones can be speeded up by the effect of visible radiation and then observed by differential spectrophotometry.
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