Sichuan Inland Organic Agriculture Corporation

Antioxidant properties of tropical and temperate herbal teas

E.W.C. Chana, Y.Y. Lim, a, , K.L. Chonga, J.B.L. Tana and S.K. Wonga

 

Received 23 October 2008;  revised 11 September 2009;  accepted 20 October 2009.  Available online 29 January 2010.

 

Abstract

Antioxidant properties (AOP) of thirteen tropical and five temperate herbal teas were screened. Comparisons were made with green, oolong and black teas of Camellia sinensis. The AOP studied were total phenolic content, radical-scavenging activity, ferric-reducing power and ferrous ion-chelating (FIC) ability. Tropical herbal teas were more diverse in types and more variable in AOP values than temperate herbal teas. Herbal teas generally had lower antioxidant values than teas of C. sinensis. Exceptions were lemon myrtle, guava and oregano teas with AOP comparable to black teas. FIC ability of mint and peppermint teas was significantly stronger than all C. sinensis teas.

 

Keywords: Tropical teas; Temperate teas; Herbal teas; Camellia sinensis; Antioxidant activity; Food analysis; Food composition

 

Article Outline

1. Introduction

2. Materials and methods

2.1. Herbal and Camellia teas

2.2. Extraction of teas

2.3. Extraction efficiency

2.4. Antioxidant properties of teas

3. Results and discussion

Acknowledgements

References

 

1. Introduction

For centuries, eastern countries have been using herbal remedies to treat infections, ailments and diseases. Herbal remedies are often consumed in the form of tea, i.e. an infusion of dried plant parts steeped in boiling water. Herbal teas have been gaining popularity in western countries in recent years (Manteiga et al., 1997). Hundreds of different herbal teas are sold in health food stores. Available as pure or blended samples, herbal teas are popular because of their fragrance, antioxidant properties and therapeutic applications ([Naithani et al., 2006] and [Aoshima et al., 2007]). Tea from Camellia sinensis is the most widely consumed beverage in the world, second only to water (Muktar and Ahmad, 2000). It is an important dietary source of natural phenolic antioxidants ([Lachman et al., 2003] and [Dimitrios, 2006]).

 

Extensive research has been carried out on the antioxidant properties (AOP) of green and black C. sinensis teas ([Manzocco et al., 1998] and [Dufresne and Farnworth, 2001]). AOP of herbal teas of temperate plants, mainly of Lamiaceae, have been well studied ([Triantaphyllou et al., 2001], [Atoui et al., 2005] and [Capecka et al., 2005]). AOP of tropical herbal teas have been studied less, with some analyses carried out on Aspalathus linearis ([von Gadow et al., 1997] and [Erickson, 2003]); Cymbopogon citratus ([Tsai et al., 2007] and [Aoshima et al., 2007]); Hibiscus sabdariffa (Aoshima et al., 2007); Phyllanthus amarus (Lim and Murtijaya, 2007); Psidium guajava and Toona sinensis (Chen et al., 2007); and Thunbergia laurifolia and Orthosiphon aristatus (Chan and Lim, 2006).

 

In this study, AOP of thirteen tropical and five temperate herbal teas were screened, with comparisons with green, oolong and black teas of C. sinensis carried out as positive controls. The AOP studied were total phenolic content, ascorbic acid equivalent antioxidant capacity (AEAC), ferric-reducing power (FRP) and ferrous ion-chelating (FIC) ability. This study, to the best of our knowledge, represents the most comprehensive comparison between AOP of tropical and temperate herbal teas in the literature.

 

2. Materials and methods

2.1. Herbal and Camellia teas

Herbal teas, together with green, oolong and black teas of C. sinensis screened for AOP are listed in Table 1. Tropical herbal teas of misai kucing (O. aristatus), lemon grass (C. citratus), guava (P. guajava), bitter gourd (Momordica charantia), lemon myrtle (Backhousia citriodora), mas cotek (Ficus deltoidea), pegaga (Centella asiatica) and rooibos (A. linearis); temperate herbal teas of rosemary (Rosmarinus officinalis), peppermint (Mentha piperita), mint (Mentha spicata), chamomile (Matricaria recutita) and oregano (Origanum vulgare); and green, oolong and black teas of C. sinensis were purchased from the supermarket. Teas of getto (Alpinia zerumbet) and rang jeud (T. laurifolia) were obtained from Okinawa (Japan) and Bangkok (Thailand), respectively. Teas of legundi (Vitex negundo) and asam gelugor (Garcinia atroviridis) were obtained from Forest Research Institute Malaysia (FRIM). Dried flowers of jin yin hua (Lonicera japonica) were purchased from the Chinese medicine shop. In this study, tropical herbal teas included several plant species from the subtropics such as B. citriodora, A. linearis, A. zerumbet and L. japonica. Teas of C. sinensis, a tropical and sub-tropical species, are not considered as herbal teas.

 

Table 1.

Types of tropical and temperate herbal teas, and green, oolong, and black teas of Camellia sinensis screened for antioxidant properties.

 

 

2.2. Extraction of teas

In tea extraction, 1 g of tea in powder form was extracted with 50 mL boiling water. Infusions were allowed to steep for 1 h with continuous swirling. Extracts were filtered and stored at 4 °C for further analysis. Analyses of aqueous tea extracts were done in triplicate.

 

2.3. Extraction efficiency

Extraction efficiency of boiling water was tested with Boh green tea. After the first extraction, the residues were filtered and extracted successively for the second and third time. Extraction efficiency in percent was based on total phenolic content of first, second and third extractions.

 

2.4. Antioxidant properties of teas

Total phenolic content (TPC) was determined using the Folin-Ciocalteu assay (Kähkönen et al., 1999). Samples (300 μL, in triplicate) were introduced into test tubes followed by 1.5 mL of Folin-Ciocalteu’s reagent (10 times dilution) and 1.2 mL of sodium carbonate (7.5%, w/v). The tubes were allowed to stand for 30 min before absorbance at 765 nm was measured. TPC was expressed as gallic acid equivalent (GAE) in mg per 100 g material. The calibration equation for gallic acid was y = 0.0111x − 0.0148 (R2 = 0.9998), where y is the absorbance and x is the gallic acid concentration in mg/L.

 

Radical-scavenging activity (RSA) was assessed using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay (Miliauskas et al., 2004). Different dilutions of extracts (1 mL) were added to 2 mL of DPPH (5.9 mg per 100 mL methanol). After 30 min, absorbance was measured at 517 nm. RSA, expressed as ascorbic acid equivalent antioxidant capacity (AEAC) in mg ascorbic acid/100 g, was calculated as IC50(ascorbate)/IC50(sample) × 105. IC50 of ascorbic acid was 0.00387 mg/mL.

 

For assessing ferric-reducing power (FRP), the assay described by Chu et al. (2000) was adapted. Different dilutions of extracts (1 mL) were added to 2.5 mL phosphate buffer (0.2 M, pH 6.6) and 2.5 mL of potassium ferricyanide (1%, w/v). The mixture was incubated at 50 °C for 20 min. After trichloroacetic acid solution (2.5 mL, 10%, w/v) was added, the mixture was separated into aliquots of 2.5 mL and diluted with 2.5 mL of water. To each diluted aliquot, 500 μL of ferric chloride solution (0.1%, w/v) was added. After 30 min, absorbance was measured at 700 nm. FRP of extracts was expressed as mg GAE/g. The calibration equation for gallic acid was y = 16.767x (R2 = 0.9974), where y is the absorbance and x is the gallic acid concentration in mg/mL.

 

The ferrous-ion-chelating (FIC) assay was adapted from Singh and Rajini (2004). FIC ability was determined by mixing FeSO4 (0.1 mM, 1 mL) with different dilutions of extracts (1 mL), followed by ferrozine (0.25 mM, 1 mL). Absorbance was measured at 562 nm after 10 min. The ability of extracts to chelate ferrous ions was calculated as chelating effect % = (1 − Asample/Acontrol) × 100. FIC ability was expressed as chelating EC50 (CEC50) in mg/mL, or the effective concentration of extract to chelate ferrous ions by 50%.

 

3. Results and discussion

Based on three successive extractions of Boh green tea (1 g) with boiling water (50 mL) for 1 h, TPC of first, second and third extractions was found to be 75 ± 1.3, 17 ± 0.5 and 8 ± 0.7%, respectively. As the yield of first extraction was high, all analyses of teas were extracted with boiling water. Significantly higher yields of hot water than cold water extraction of green tea (Lin et al., 2008) and stronger RSA of oolong tea extracted with hot water of increasing temperature (Su et al., 2006) have been reported. For green, oolong and black teas, extraction with water at 100 °C for 3 min yielded higher total flavan-3-ol content than extraction with water at 60 and 80 °C (Horžić et al., 2009).

 

Tropical herbal teas screened were diverse in types, parts used and morphology. They comprised leaves, flowers, fruits and stems of 13 species of trees, shrubs, vines and herbs belonging to 13 genera and 11 families (Table 1). AOP were variable with TPC, AEAC and FRP values ranging from 644 to 7560 mg GAE/100 g, 354 to 13,600 mg AA/100 g and 3 to 61 mg GAE/g, respectively (Table 2). Teas of Myrtaceae (lemon myrtle and guava) displayed the best TPC, RSA and FRP, with values ranging from 5930 to 7560 mg GAE/100 g, 7430 to 13,600 mg AA/100 g and 35 to 61 mg GAE/g, respectively. Getto, mas cotek, misai kucing, lemon myrtle and guava teas displayed strong FIC ability with CEC50 values ranging from 0.9 to 1.2 mg/mL. Their strong AOP could be due to the content and composition of major phenolic compounds (Table 3).

 

 

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Table 2.

Total phenolic content (TPC), ascorbic acid equivalent antioxidant capacity (AEAC), ferric-reducing power (FRP), and chelating EC50 (CEC50) of tropical and temperate herbal teas.

Teas are ranked based on TPC and values are means ± SD (n = 3). For each column, values followed by the same letter are not statistically different at P < 0.05 as measured by the Tukey HSD test. ANOVA does not applying between tropical, temperate, and Camellia teas. Green (g), oolong (o), and black (b) teas of Camellia sinensis were used as positive controls.

 

Table 3.

Major phenolic compounds of herbal teas studied.

Abbreviations: GA = gallic acid, RA = rosmarinic acid, EGCG = epigallocatechin gallate, EGC = epigallocatechin, and GC = gallocatechin.

 

 

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Temperate herbal teas screened were less diverse in types, parts used and morphology. They comprised leaves and flowers of five species of herbs belonging to four genera and two families, Lamiaceae being dominant (Table 1). AOP were less variable with TPC, AEAC and FRP values ranging from 1370 to 5860 mg GAE/100 g, 966 to 7240 mg AA/100 g and 8 to 49 mg GAE/g, respectively (Table 2). Temperate herbal teas of oregano, mint and peppermint displayed strong AOP with outstanding CEC50 values of 0.8, 0.3 and 0.4 mg/mL, respectively. With the exception of chamomile tea (Asteraceae), rosmarinic acid is one of the major phenolic compounds in all the other herbal teas (Lamiaceae) (Table 3). The strong AOP of temperate herbal teas might be attributed to the predominance of rosmarinic acid and other phenolic acids.

 

With higher percentage of teas having strong AOP, particularly in FIC ability, temperate herbal teas investigated were superior to tropical herbal teas. Temperate herbal teas with strong primary AOP (RSA and FRP) also had strong secondary AOP (FIC ability). This was not evident in tropical herbal teas as some teas with strong primary AOP displayed poor secondary AOP (e.g. rooibos tea) and vice versa (e.g. getto tea). RSA and FRP are measures of the hydrogen and electron-donating abilities of primary antioxidants, respectively (Lim et al., 2007). FIC ability measures the ability of secondary antioxidants to chelate metal ions. Primary antioxidants prevent oxidative damage by directly scavenging free radicals, while secondary antioxidants act indirectly by preventing the formation of free radicals through Fenton’s reaction.

 

Teas of C. sinensis had TPC, AEAC, FRP and CEC50 values ranging from 6060 to 14,120 mg GAE/100 g, 7510 to 25,000 mg AA/100 g, 36 to 143 mg GAE/g and 1.0 to 1.9 mg/mL, respectively (Table 2). Ranking of TPC, RSA, FRP and FIC ability was green > oolong ≈ black; green ≈ oolong > black; green > oolong > black; and black > green ≈ oolong, respectively. Findings from this study deviated from Yen and Chen, 1995 G.C. Yen and H.Y. Chen, Antioxidant activity of various tea extracts in relation to their antimutagenicity, Journal of Agricultural and Food Chemistry 43 (1995), pp. 27–32. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (463)Yen and Chen (1995) who reported that ranking of RSA was green > oolong > black, and ranking of reducing power was oolong > green > black; and from von Gadow et al. (1997) and Yokozawa et al. (1998), who reported that ranking of RSA was green > black > oolong. Green teas have been reported to have significantly higher TPC, RSA and FRP, but poorer FIC ability, than black teas ([Chan and Lim, 2006] and [Chan et al., 2007]).

 

The outstanding AOP of green, oolong and black teas of C. sinensis could be attributed to their high flavanol content of epigallocatechin gallate (EGCG), epigallocatechin (EGC) and gallocatechin (GC) (Table 3). Recently, Horžić et al. (2009) reported that green, oolong and black teas, extracted with deionised water heated to 100 °C for 3 min, had total identified flavan-3-ol contents of 999, 666 and 672 mg/L, respectively. Temperate herbal teas of linden and chamomile yielded flavan-3-ol contents of only 127 and 22 mg/L, respectively.

 

Compared to teas of C. sinensis, tropical and temperate herbal teas generally had lower antioxidant values. Exceptions were lemon myrtle, guava and oregano teas with AOP comparable to black teas. CEC50 values of mint and peppermint were significantly larger than all C. sinensis teas. FIC ability of oregano, getto, mas cotek, misai kucing, lemon myrtle and guava teas was comparable to black teas. It can be seen that some tropical and temperate herbal teas had AOP that are superior or comparable to green, oolong and black teas.

 

In conclusion, tropical herbal teas were more diverse in types and more variable in AOP than temperate herbal teas. Herbal teas generally had lower antioxidant values than teas of C. sinensis.

Acknowledgements

The authors are thankful to Monash University Sunway Campus (MUSC) Malaysia for financial support of this project, to Forest Research Institute Malaysia (FRIM) for providing tea samples of legundi and asam gelugor, and to Dr. Mami Kainuma for purchasing the getto tea from Okinawa, Japan.

 

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