wang and helliwell (2001) determination of flavonols in green and black tea leaves

Determination of ¯avonols in green and black tea leaves and green tea infusions by high-performance liquid chromatography Huafu Wang *, Keith Helliwell R&D Department, William Ransom and Son plc, Hitchin, Herts SG5 1LY, UK Received 26 May 2000; accepted 9 August 2000 Abstract Tea ¯avonols are potent antioxidants and make up 2±3% of the water-soluble solids from tea leaves. In this paper, the conditions necessary for hydrolysing and analysing ¯avonols in tea leaves and tea infusions are optimised and an isocratic elution system for the determination of the hydrolysed ¯avonols by high-performance liquid chromotography is presented. Aqueous ethanol was selected as the best solution for hydrolysing ¯avonoids in tea leaves. The contents of ¯avonols on a dry weight base in green tea leaves ranged from 0.83±1.59, 1.79±4.05, and 1.56±3.31 g/kg, and in black tea leaves from 0.24±0.52, 1.04±3.03, and 1.72±2.31 g/kg for myricetin, quercetin, and kaempferol, respectively. It was observed that the particle size of ground tea leaves signi®cantly in¯uenced the yield of ¯avonols. The contents of ¯avonols in dierent green tea infusions are given. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Tea; Flavonols; Myricetin; Quercetin; Kaempferol; HPLC 1. Introduction Flavonoids are natural products widely distributed in the plant kingdom. They are subdivided into six classes: ¯avones, ¯avanones, iso¯avones, ¯avonols, ¯avanols and anthocyanins varying in their structural character- istics around the heterocyclic oxygen ring (Rice-Evans & Miller, 1997). Many ¯avonoids are predominantly present as glycosides rather than non-glycosylated forms (aglycones). For example, ¯avonols occur gen- erally as O-glycosides with a sugar residue mostly at the C-3 position. The type and number of sugar residues, together with chain branching, can result in a very large number of individual glycosides. The main ¯avonoids found in tea are ¯avanols and ¯a- vonols. They are of interest because they have a wide range of pharmaceutical properties including antioxidative, anticarcinogenic and antiarteriosclerotic (Dreosti, Wargo- vich, & Yang, 1997; Jankun, Selman, Swiercz, & Skrzypc- zak-Jankun, 1997; Wiseman, Balentine, & Frei, 1997; Yamamoto, Juneja, Chu, & Kim, 1997; Yang, 1997). Tea catechins are structurally, primarily, ¯avanols, and these form about 20% of the dry weight of green tea, while tea ¯avonols make up 2±3% of the water-soluble solids of tea. Therefore, the catechins have attracted more attention from researchers. However, ¯avonols are also regarded to be potent antioxidants due to the combination of a keto group conjugated to a double bond in the C ring, together with adjacent hydroxyl groups in the B ring (Fig. 1) (Bors, Heller, Michel, & Stettmaier, 1996). Flavonols are structurally more stable than catechins, and it has been shown that tea is a major dietary source of these compounds (Hertog, Hollman, & van de Putte, 1993). Fieschi, Codignola, and Luppi Mosca (1989) mea- sured the total ¯avonoid glycoside and aglycone contents of dierent types of black tea infusions using paper chromatography followed by spectrophotometric measurements. Price, Rhodes, and Barnes (1998) inves- tigated ¯avonol glycoside in tea infusions and tea products using high-perfomance liquid chromotogarphy (HPLC) gradient elution system. However, this approach was not suitable for routine analysis as most reference compounds are not commercially available. Hertog et al. (1993) analysed the content of ¯avonoids in tea infusions, wines, and fruit juices by hydrolysing all glycosides to aglycones followed by HPLC determination. This oered a practical method for the quanti- tative determination of ¯avonoids in foods. In this paper, the conditions for hydrolysing and analysing ¯avonols in tea leaves and tea infusions are 0963-9969/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0963-9969(00)00156-3 Food Research International 34 (2001) 223±227 www.elsevier.com/locate/foodres * Corresponding author. Tel.: +44-1462-437615; fax: +44-1462- 420528. E-mail address: [email protected] (H. Wang).

wang and helliwell (2001) determination of flavonols in green and black tea leaves

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