Oxidation processes polyphenol quinones

FIa.VOnoIOxida.tlon, The fermentation process is initiated by the oxidation of catechins (1) to reactive catechin quinones (13), a process catalyzed by the enzyme polyphenol oxidase (PPO) (56). Whereas the gaHocatechins, epigaHocatechin, and epigaHocatechin gaHate, are preferred, polyphenol oxidase can use any catechin (Table 2) as a substrate. This reaction is energy-dependent and is the basis of the series of reactions between flavanoids that form the complex polyphenoHc constituents found in black and oolong teas. 

Enzymes known as polyphenol oxidases cause enzymatic browning. Other names of the enzyme include phenolases and tyrosinases. The enzymes catalyze the conversion of monophenols and diphenols to quinones. The quinones can undergo a series of non-enzymatic reactions to produce brown, gray and black colored pigments, collectively known as melanins (11). Maillard reactions, caramelizations and ascorbic acid oxidations can produce similar types of colored compounds (12). For some food processing... 

Figure 3. Interrelationship of Oxidative and Reductive Processes Linked to the Production of o-Quinones. POD = peroxidase, PPO = polyphenol oxidase, GSG glutathione, GSSG -oxidized glutathione.

The o-quinones formed from phenolics further enhance the intensity of browning by oxidation of other substrates, complexing with amino acids and protein, and polymerization. Non-enzymatic discoloration is believed to involve metal-polyphenol complexing as reported in the processed potato (Bate-Smith et al., 1958), cauliflower (Donath, 1962), and asparagus (DeEds and Couch, 1948), conversion of leucoanthocyanidins to pink anthocyanidins in the processed broad bean (Dikinson et al., 1957), green bean puree (Roseman et al., 1957), and canned Bartlett pear (Luh et al., 1960), and protein-polyphenol complexing in chilled or stored beer (Schuster and Raab, 1961). 

It is generally considered that polyphenol oxidase, which has at least three isoforms, is the key enzyme in the fermentation processes that produce black teas, but there is evidence also for an important contribution from peroxidases with the essential hydrogen peroxide being generated by polyphenol oxidase (Subramanian et al. 1999). The primary substrates for polyphenol oxidase are the flavan-3-ols which are converted to quinones. These quinones react further, and may be reduced back to phenols by oxidizing other phenols, such as gallic acid, flavonol glycosides and theaflavins, that are not direct substrates for polyphenol oxidase (Opie etal. 1993,1995). 

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