Flavonols in tea

Fig. 2.66. Chemical structures of flavonols in tea. Reprinted with permission from H. Wang et al. [182].

Fig. 2.67. A typical chromatogram of flavonols in tea leaves. The chromatographic conditions are described in the text. 1 = myricetin 2 = quercetin 3 = kaempferol. Reprinted with permission from. H. Wang et al. [182],...

As relatively few standard compounds are available from commercial or other sources, identification of flavonol glycosides has to be achieved by alternative means, for example UV-, H- and C-NMR spectroscopy. Therefore hydrolysing all glycosides to aglycones followed by HPLC determination offers a practical method for the quantitative determination of flavonoids in tea (Hertog et al, 1993a Wang and Helliwell, 2001). 

FINGER A, ENGELHARDT u H and WRAY V (1991) Flavonol glycosides in tea - kaempferol and quercetin rhamnodiglucosides , J Sci Food Agric, 55, 313-21. 

WANG H and HELLIWELL K (2001) Determination of flavonols in green and black tea leaves and green tea infusions by high-performance liquid chromatography , Foot/Res Intern, 34, 223-7. 

These substances are present in small amounts in tea leaf but there is little quantitative data on their occurrence. Figure 3 shows the structure of several of these that have been identified. They occur both as free flavonols and glycosides.24 They may enter into the oxidative reactions that take place during tea manufacture.50... 

Hollman PC, Van Het Hof KH, Tijburg LB and Katan MB. 2001. Addition of milk does not affect the absorption of flavonols from tea in man. Free Radic Res 34 297-300. 

Synthetic analogues or derivatives of a-tocopherol which have better antioxidant properties can be introduced. Many natural antioxidants such as flavonols, flavones, tea leaf catechins, rosemary antioxidants and spice extracts have been reported to be more active than BHA, BHT or the tocopherols in model systems. The food applications of these compounds need to be explored further. 

Another isocratic elution method was applied for the determination of flavonols in green and black tea leaves and green tea infusions by RP-HPLC. The chemical structures of the flavonols studied are shown in Fig. 2.66. Infusions of teas were prepared by mixing lg of tea leaves with 100 ml of boiling water for 5min, then they have filtered and used for HPLC analysis. The infusion step was repeated three times. Flavonoids were hydrolysed by mixing lg of tea leaves with 40 ml of 60 per cent aqueous ethanol and 5 ml of 6 M HC1. The suspension was heated at 95°C for 2 h, then filtered and the volume was adjusted to 50 ml with 60 per cent aqueous ethanol. Separation was performed in an ODS column (150 X 4.6mm i.d.) operated at 30°C. The isocratic mobile phase consisted of 30 per cent aqueous ACN in 0.025 M KH2P04, and the pH was adjusted to 2.5 with 6 M HC1. The... 

CONTENT OF FLAVONOLS IN DIFFERENT TEA LEAVES (G/KG DRY LEAVES AND IN DIFFERENT INFUSIONS OF GUNPOWDER TEA (MG/L)1... 

CS009 Finger, A., U. H. Engelhadt and V. Wray. Flavonol triglycosides containing galactose in tea. Phytochemistry 1991 30(6) 2057-2060. 

Tea is the only analyzed beverage to contain (-)-epigallocatechingallate (EGCG) in quantifiable amounts. EGCG and (—)-epicatechingallate (ECG) are the most abundant forms, each contributing 27% to the total catechin content (22.2 mg/100 ml) of black tea. Three flavonols (quercetin, kaempferol, and myricetin) are also found in tea. For example, 100 g of decaffeinated tea contains 5.2 mg quercetin, 2.4 mg kaempferol, and 0.1 mg myricetin. 

Identification of black tea as the primary source of flavonols (42.7%), procyanidins (49.5%), and catechins (63.6%) is again consistent with published literature for tea-drinking nations.For example, a study of 1900 Welsh men also observed that tea was the main dietary source of flavonols.Interestingly, 5.4 + 3.0 cups of tea were consumed by these subjects per day between 1979 and 1983 whereas the Scottish participants reported consuming only 2.8 + 2.4 cups of tea per day. This may reflect the current downward trend in tea consumption in the United Kingdom especially by adults under 50 years and also suggests that flavonols are obtained from other food sources in the Scottish diet. Hertog, for... 

A modest but not significant inverse correlation between the intake of flavonols and flavones and subsequent mortality rates was found in a prospective cohort study of US Health Professionals by Rimm et al [206]. The authors do not exclude that flavonoids have a protective effect in men with established coronary heart disease although strong evidence was missing. Also other studies failed to demonstrate a significant statistical association between the intake of polyphenols and CHD. In Great Britain for instance coronary and total mortality even rose with the intake of the major flavonol source, tea [207], The most likely explanation for the latter observation is that in this study tea consumption merely acted as a marker for a lifestyle that favours the development of cardiovascular disease. Indeed, men with the highest intake of tea and flavonols tended to be manual workers, and they smoked more and ate more fat [208],... 

AF Finger, UH Engelhardt, V Wray. Flavonol glycosides in tea—kaempferol and quercetin rhamno-diglucosides. J Sci Food Agric 55 313-321, 1991. 

Volikakis, G.J. and Efstathiou, C.E. (2005) Fast screening of total flavonols in wines, tea-infusions and tomato juice by flow injection/adsorptive stripping voltammetry. Anal Chim Acta, 551 (1-2), 124-131. 

The flavonoids are a group of secondary metabolites widely distributed in the plant kingdom [63]. In 1984, more than 4000 flavonoids were identified in plants [64]. The major dietary sources of flavones are spices and pot herbs, such as parsley, rosemary, and thyme [65], whereas flavonols are predominantly found in onions, kale, broccoli, apples, berries and cherries, and in tea and red wine [66]. The flavanones are mainly restricted to citrus fruits [67], and flavanols are found in considerable amounts in tea, apricots, apples, and cherries [66]. 

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