Anthocyanins oxidative degradation

All natural anthocyanins suffer from inherent instability, so they may be degraded to form colorless or brown-colored, often insoluble products. It is evident, that depending on the reaction type, different degradation products are formed however, only little analytical data is available. For example, after reaction of anthocyanins with hydrogen peroxide, substances of the benzofu-ran type were detected (139). Studies with grape must like model solutions indicated that anthocyanins are degraded by coupled oxidation and that they form adducts with caffeoyltartaric acid... 

Polyphenoloxidase (PPO, EC 1.14.18.1) is one of the most studied oxidative enzymes because it is involved in the biosynthesis of melanins in animals and in the browning of plants. The enzyme seems to be almost universally distributed in animals, plants, fungi, and bacteria (Sanchez-Ferrer and others 1995) and catalyzes two different reactions in which molecular oxygen is involved the o-hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of 0-diphenols to o-quinones (diphenolase activity). Several studies have reported that this enzyme is involved in the degradation of natural phenols with complex structures, such as anthocyanins in strawberries and flavanols present in tea leaves. Several polyphenols... 

As plant tissues senesce and die, three processes may ensue almost simultaneously. First, enzymes within the dead but sterile and physically intact cells cause proteolysis and other autolytic degradations. The released amino acids, sugars, tannins, phenols, and quinones may be oxidized by chemical or enzymatic catalysis to produce humus-like pigments, or proto-humus as discussed by Stevenson in Chapter 2. This was well illustrated by Cohen (Given and Dickinson, 1975) who observed cellular material of partially polymerized eaco-anthocyanins in residues of Rhizophora mangle deposited in a mangrove swamp in Florida. The autolytic reactions may be prominent in situations where microbial decomposition is slow due to acidity, anaerobiosis, or lack of basic nutrients. 

Tannins may also be broken down by oxidation (Section 6.3.6), although less readily than anthocyanins (Laborde, 1987). The formation of quinones following tannin degradation has been demonstrated and is sometimes accompanied by the opening of the heterocycle. These structural modifications cause the color to evolve towards yellowish-brown hues and precipitation may occur. These reactions are characteristic of wines with very high tannin contents and low anthocyanin concentrations. 

Cheynier, V, Souquet, J.M., Kontek, A. Moutounet, M. (1994). Anthocyanin degradation in oxidizing grape musts. Journal of the Science of Food and Agriculture, 66,283-288. 

Sulfur dioxide (bisulfites) also reacts with thiamine yielding inactive compounds (see Section 5.6.6), forms adducts with riboflavin, nicotinamide, vitamin K, inhibits ascorbic acid oxidation (see Section 5.14.6.1.6) and reacts with ascorbic acid degradation products, reduces o-quinones produced in enzymatic browning reactions back to 1,2-diphenols (see Section 9.12.4), causes decolourisation of fruit anthocyanins (see Section 9.4.1.5.7) and reacts with synthetic azo dyes to form coloured or colourless products (see Section 11.4.1.3.2). Sulfur dioxide also reacts with pyrimidine bases in vitro, specifically with cytosine and 5-methylcytosine. Important reactions of sulfur dioxide are shown in Figure 11.4. 

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