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New oxidation destructive analysis (NODA)
Savel, J., Kosin, P., Broz, A.

Beer pigments compose of caramels, melanoidins and polyphenols pigments, which can be prepared by heating or boiling of mild colored or colorless precursors, such as sugar or (-)-epicatechin. The natural pigments behave as natural pH and redox indicators. During preparation of caramels by sugar heating at 175C in oven, various sugars came into slight yellow (maltose), brown (glucose, mannose, ribose, xylose), or dark brown (arabinose, fructose) products. At heating, reductones were formed prior to caramel pigments. Redox indicators were added to beer and the absorbances (at 666 nm for methylene blue, 520 nm for methyl red and 610 nm for indigocarmine) were recorded before and after illumination with visible light (5 min) under aerobic and anaerobic condition. Natural or synthetic indicators can change their color reversibly or irreversibly. Reducing compounds can initialize oxygen free radical formation as well as their scavenging. 1,2-diaminobenzene addition supported the oxygen consumption in beer.Increasing beer color has long been considered to be reliable marker of beer ageing. The absorbance of beer can also decrease at the short wavelengths especially after illumination or hydrogen peroxide addition, which is usually imputed to colored compounds degradation [3, 6, 12].The consumers use their eyes to assess beer color and recognize the degree of beer ageing. Spectrophotometric methods for beer color measurement recommend various wavelength (e.g. 530 nm was lately replaced by 430 nm). The absorption of beer gradually decreases with increasing wavelength. Nowadays various tristimulus measurements are replacing the absorption value at single wavelength. A new method called differential spectroscopy has been also developed to recognize gentle color changes, which are caused by electron transport during oxidation of compounds that are naturally present in beer and absorb in the visible region [12]. The beer color is related to redox state of beer which changes during beer ageing. The measurement of beer absorption provided the 'gred color index' expressed as the ratio of beer absorbance at 465 and 500 nm.The usage of beer color measurement can be enriched using redox indicators to beer followed by absorbance measurement at suitable wavelength under well defined reaction conditions. There are many methods for the redox potential measurement, which can be divided into chemical, radical and physical methods. The color increase can be artificially generated by addition of oxidative compounds such as potassium peroxodisulfate, potassium dichromate or potassium permanganate, which is also accompanied by formation of haze and stale flavor compounds. Oxidation agents are efficient under both anaerobic and aerobic conditions but the presence of oxygen made the effect more distinct.This procedure was called Oxidative Destruction Analysis (ODA). The strong correlation between increasing beer color and decreasing colloidal instability was proved. The formation of the degradation products of sugars such as furfural and hydroxymethylfurfural was also recognized [13].Sugar pigments and reductones are also formed by thermal sugar degradation during malting or brewing process e.g. on the hot wall of brewing kettle. Aminoacids can catalyze Maillard reaction which provides colored melanoidins. On the other hand the natural reducing compounds such as reductones or polyphenols can inhibit oxidation reactions based on radical or non radical processes. Prooxidant or antioxidant effect of those compounds have been described [1, 2, 10]. Color indicators have been traditionally used for wort and beer redox state determination usually in the presence of air without measurement under anaerobic condition. Some indicators used e.g. L-dipyridyl reclaimed strongly acid condition, which disabled reaction under natural condition, occurring in packaged beer. Classical redox indicator 2,6-dichlorophenolindoindophenol (DCIP) has been used for the estimation of redox potential of wort and beer in the presence of oxygen. The usage of colored indicators in brewing has been reviewed by several authors [8, 9, 11]. ...

Descriptors: reductone, caramel, antioxidant, prooxidant, 1,2-diaminobenzene, indigocarmine

BrewingScience - Monatsschrift fr Brauwissenschaft, 62 (September/October 2009), pp. 155-163