Theaflavins oxidation

Theaflavin content increases initially as the oxidation process proceeds but falls off rapidly on prolonged oxidation. The mechanism for its further reaction in the oxidizing leaf system is not definitely known. 

Extractable caffeine levels increase on storage, presumably because of decomplexation from theaflavins as the latter decrease. Since caffeine binding with thearubigens is weaker than that with theaflavins, more caffeine may become available.94 An increase in free caffeine may be responsible for decreased value because of the bitterness imparted to the beverage. Caffeine in combination with the normal tea complement of oxidized polyphenolic matter does not exhibit bitterness.95... 

Flavonoids exhibit protective action against LDL oxidation. It has been shown [145] that the pretreatment of macrophages and endothelial cells with tea flavonoids such as theaflavin digallate diminished cell-mediated LDL oxidation probably due to the interaction with superoxide and the chelation of iron ions. Quercetin and epicatechin inhibited LDL oxidation catalyzed by mammalian 15-lipoxygenase, and are much more effective antioxidants than ascorbic acid and a-tocopherol [146], Luteolin, rutin, quercetin, and catechin suppressed copper-stimulated LDL oxidation and protected endogenous urate from oxidative degradation [147]. Quercetin was also able to suppress peroxynitrite-induced oxidative modification of LDL [148],... 

RP-HPLC has also been used for the analysis of flavan-3-ols and theaflavins during the study of the oxidation of flavan-3-ols in an immobilized enzyme system. Powdered tea leaves (20Qmg) were extracted with 3 X 5 ml of 70 per cent aqueous methanol at 70°C for lQmin. The combined supernatants were filtered and used for HPLC analysis. Flavan-3-ols were separated in a phenyl hexyl column (250 X 4.6 mm i.d. particle size 5 /im) at 30°C. Solvents A and B were 2 per cent acetic acid in ACN and 2 per cent acetic acid in water, respectively. Gradient elution was 0-lQmin, 95 per cent B 10-4Qmin, to 82 per cent B to 40-5Qmin 82 per cent B. The flow rate was 1 ml/min. Theaflavins were determined in an ODS column (100 X 4.6 mm i.d. particle size 3pm) at 30°C. The flow rate was 1.8 ml/min and solvent B was the isocratic mobile phase. The data demonstrated that flavan-3-ols disappear during the oxidation process while the amount of theaflavins with different chemical structures increases [177],... 

CS023 Tanaka, T., Y. Betsumiya, C. Mine and I. Kouno. Theanaphthoquinone, a novel pigment oxidatively derived from theaflavin during tea-fermentation. Chem Commun 2000 2000(15) 1365-1366. 

Tanaka, T., Inoue, K., Betsumiya, Y., Mine, C., and Kouno, I., Two types of oxidative dimerization of the black tea polyphenol theaflavin, J. Agric. Food Chem., 49, 5785, 2001. 

During fermentation in the preparation of black tea, oxidative polymerization of flavanols occurs with the formation of theaflavin, theaflavingallates, thearubigins, and epitheaflavic acid [44]. 

The inhibitory effects of tea polyphenols on xanthine oxidase (XO) were investigated. Theaflavins and EGCG inhibit XO to produce uric acid and also act as scavengers of superoxide. Theaflavin 3,3 -digaUate (TE-3) acts as a competitive inhibitor and is the most potent inhibitor of XO among these compounds. TE-3 also inhibited the superoxide production in HL-60 cells. Therefore, the antioxidative activity of tea polyphenols is due not only to their ability to scavenge superoxides but also to their ability to block XO and relative oxidative signal transducers. ... 

In addition to theanaphthoquinone, two new oxidation products of theaflavin, bistheaflavin A (7) and dehydrotheaflavin (8), were isolated in similar model fermentation experiments using banana fruit and tea leaf homogenates, respectively. On the other hand, auto-oxidation of theaflavin in a phosphate buffer at pH 7.3 yielded a yellow pigment, bistheaflavin B (9), along with 6 (figure 5.4). The dimer 9 is deduced to be formed by coupling of the reduced form (6a) of theanaphthoquinone with theaflavin-quinone (5b). The results demonstrated that theanaphthoquinone was also formed nonenzymatically from theaflavin. 

In the oxidation of tea catechins, the initial reaction gave simple catechin quinones. However, subsequent cross-coupling reactions of the quinones yielded many products. Some of these products were unstable and decomposed to give further complex products. These cascade-type reactions account for the complexity of the black tea polyphenols (figure 5.9). Although many of the oxidation products described here were obtained by model fermentation experiments, the presence of compounds 21, 24,26,28, and 29, as well as theaflavins (theaflavin and its galloyl esters), theasinensins (14-17), and oolongtheanins (18), in commercial black tea were confirmed. 

Robertson, A. and Bendall, D. S. 1983. Production and HPLC analysis of black tea theaflavins and thearubigins during in vitro oxidation. Phytochemistry 22 883-87. 

Bajaj, K. L., Anan, T., Tsushida, T., and Ikegaya, K. 1987. Effects of (-)-epicatechin on oxidation of theaflavins by polyphenol oxidase from tea leaves. Agric. Biol. Chem. 51 1767-72. 

---