Fifty-one kinds of commercial herbal and tea infusions were obtained from markets in Guangzhou, which represent main categories of the infusions made in China (Table 1).

The total phenolic contents of 51 infusions were estimated using the Folin–Ciocalteu method, which relies on the transfer of electrons from phenolic compounds to the Folin–Ciocalteu reagent in alkaline medium, and is a simple and rapid method [10–13]. As shown in Table 2, the total phenolic contents varied from 0.032 ± 0.001 to 1.395 ± 0.068 g gallic acid equivalent (g GAE)/L with the difference of 44-fold, and the mean value was 0.480 g GAE/L for 51 infusions. Ping An Tang li yan cha had the highest total phenolic content (1.395 ±0.068 g GAE/L), but Nian ci an run (chun cui hong se ting zhuang) showed the lowest total phenolic content (0.032 ± 0.001 g GAE/L) among the tested infusions.

For the herbal infusions, the total phenolic contents varied from 0.032 ±0.001 to 1.395 ±0.068 g GAE/L with the difference of 44-fold, and the mean value was 0.410 g GAE/L for the 28 herbal infusions (Table 2). Ping An Tang li yan cha (1.395 ±0.068 g GAE/L) had the highest total phenolic content, followed by Ping An Tang shi gan cha (1.192 ±0.011 g GAE/L), Qing Xin Tang jiang huo wang (1.028 ±0.055 g GAE/L), Qing Xin Tang er shi si wei (1.007 ±0.039 g GAE/L), Qing Xin Tang zhi ke hua tan tang (0.909 ±0.037 g GAE/L), Qing Xin Tang hou zheng tang (0.875 ±0.019 g GAE/L) and Qing Xin Tang gan mao cha (0.844 ± 0.013 g GAE/L). Nian ci an run (chun cui hong se ting zhuang) had the lowest total phenolic content (0.032 ± 0.001 g GAE/L) among the tested herbal infusions.

For the tea infusions, the total phenolic contents varied from 0.253 ± 0.005 to 0.867 ± 0.015 g GAE/L with the difference of 3-fold, and the mean value was 0.565 g GAE/L for the 23 tea infusions (Table 2). Kang Shi Fu muo li mi cha (0.867 ±0.015 g GAE/L) had the highest total phenolic content, followed by Kang Shi Fu muo li qing cha (0.808 ± 0.012 g GAE/L), Tong Yi cha li wang (0.724 ±0.009 g GAE/L), Kang Shi Fu lü cha (0.705 ±0.008 g GAE/L), Ya Tian bing lü cha (0.705 ±0.028 g GAE/L) and Qi Lin hua jian qing yuan (0.696 ±0.025 g GAE/L). Que Chao yuan ye bing hong cha had the lowest total phenolic content (0.253 ± 0.005 g GAE/L) among the tested tea infusions.

When the total phenolic contents of these infusions were compared with those of Serbian white wines and Korean wines reported in the literature [8,14], there was no statistical difference (p > 0.05), which indicated that these infusions could contribute the same health benefit as those wines in terms of polyphenols. Taking the negative health effect of alcohol in those wines into account, these infusions would have better health benefits for people [15].

The ferric reducing antioxidant power (FRAP) assay was used to evaluate antioxidant capacities of the 51 infusions. The FRAP assay is based on the capacity of antioxidants to reduce ferric(III) ions to ferrous(II) ions [16,17], which is a simple and widely used method for the evaluation of antioxidant capacity [18–20]. The FRAP values of 51 infusions are shown in Table 2. In general, these infusions had very high antioxidant capacities. As indicated in Table 2, the FRAP values varied from 0.392 ±0.014 to 30.581 ±1.285 mol Fe(II)/L with the difference of 78-fold, and the mean value was 9.189 mol Fe(II)/L for the 51 infusions. Ping An Tang li yan cha had the highest FRAP value (30.581 ±1.285 mol Fe(II)/L), and Nian ci an run (chun cui hong se ting zhuang) showed the lowest FRAP value (0.392 ±0.014 mol Fe(II)/L) among the tested infusions.

For the herbal infusions, the FRAP values varied from 0.392 ± 0.014 to 30.581 ± 1.285 mol Fe(II)/L with the difference of 78-fold, and the mean value was 6.897 mol Fe(II)/L for 28 herbal infusions (Table 2). Ping An Tang li yan cha (30.581 ± 1.285 mol Fe(II)/L) had the highest FRAP value, followed by Ping An Tang shi gan cha (26.314 ±0.663 mol Fe(II)/L), Qing Xin Tang jiang huo wang (25.454 ±1.175 mol Fe(II)/L), Qing Xin Tang er shi si wei (13.252 ±0.225 mol Fe(II)/L), Qing Xin Tang hou zheng tang (12.490 ± 0.615 mol Fe(II)/L), Qing Xin Tang zhi ke hua tan tang (11.697 ±0.777 mol Fe(II)/L) and Qing Xin Tang gan mao cha (10.382 ±0.845 mol Fe(II)/L). Nian ci an run (chun cui hong se ting zhuang) had the lowest FRAP value (0.392 ±0.014 mol Fe(II)/L) among the tested herbal infusions.

For the tea infusions, the FRAP values varied from 6.454 ±0.280 to 22.724 ±0.758 mol Fe(II)/L with the difference of about 4-fold, and the mean value was 11.979 mol Fe(II)/L for 23 tea infusions (Table 2). Kang Shi Fu muo li mi cha (22.724 ± 0.758 mol Fe(II)/L) had the highest FRAP value, followed by Kang Shi Fu muo li qing cha (20.332 ± 0.543 mol Fe(II)/L), Kang Shi Fu lü cha (18.783 ±0.378 mol Fe(II)/L), Kang Shi Fu wu long ming cha (17.361 ±0.267 mol Fe(II)/L), Kang Shi Fu tie guan ying cha (16.222 ± 0.433 mol Fe(II)/L) and Kang Shi Fu bing lü cha (15.136 ±0.336 mol Fe(II)/L). Tong Yi bing hong cha had the lowest FRAP value (6.454 ±0.280 mol Fe(II)/L) among the tested tea infusions.

The correlation between antioxidant capacities and the total phenolic content of the 51 infusions is shown in Figure 1. The result showed a positive linear correlation between the antioxidant capacities and total phenolic content (R² = 0.7929), which indicated that phenolic compounds could be one of the main components responsible for antioxidant activities of these beverages.

The antioxidant capacities of samples may be influenced by lots of factors, such as test system, and cannot be fully described by one single method. Most natural antioxidants are multifunctional. A reliable antioxidant evaluation protocol requires different antioxidant activity assessments to be performed to take into account various mechanisms of antioxidant action [21]. Therefore, the Trolox equivalent antioxidant capacity (TEAC) assay was used to evaluate free radical scavenging capacities of 51 infusions. The TEAC assay is based on the ability of antioxidants to scavenge ABTS^•+ radicals. It can measure antioxidant capacities of lipophilic and hydrophilic components in a sample, and is a method usually used for the evaluation of antioxidant capacity [22]. The TEAC values of 51 infusions are given in Table 2. Generally, these infusions had very strong free radical scavenging ability. As seen from Table 2, the TEAC values varied from 0.250 ±0.006 to 19.296 ±0.692 mol Trolox/L with the difference of 77-fold, and the mean value was 5.074 mol Trolox/L for the 51 infusions. Ping An Tang li yan cha had the highest free radical scavenging capacity (19.296 ±0.692 mol Trolox/L), and Nian ci an run (chun cui hong se ting zhuang) showed the lowest free radical scavenging capacity (0.250 ±0.006 mol Trolox/L) among the tested infusions.

For the herbal infusions, the TEAC values varied from 0.250 ± 0.006 to 19.296 ± 0.692 mol Trolox/L with the difference of 77-fold, and the mean value was 3.664 mol Trolox/L for the 28 herbal infusions (Table 2). Ping An Tang li yan cha (19.296 ± 0.692 mol Trolox/L) had the highest free radical scavenging capacity, followed by Ping An Tang shi gan cha (16.269 ± 0.230 mol Trolox/L), Qing Xin Tang zhi ke hua tan tang (6.695 ± 0.114 mol Trolox/L), Qing Xin Tang hou zheng tang (6.499 ±0.046 mol Trolox/L), Qing Xin Tang jiang huo wang (6.474 ±0.019 mol Trolox/L), Qing Xin Tang er shi si wei (6.310 ±0.321 mol Trolox/L) and Qing Xin Tang gan mao cha (6.188 ±0.238 mol Trolox/L). Nian ci an run (chun cui hong se ting zhuang) had the lowest free radical scavenging capacity (0.250 ±0.006 mol Trolox/L) among the tested herbal infusions.

For the tea infusions, the TEAC values varied from 3.815 ±0.087 to 14.020 ±0.324 mol Fe(II)/L with the difference of about 4-fold, and the mean value was 6.791 mol Trolox/L for the 23 tea infusions (Table 2). Kang Shi Fu muo li mi cha (14.020 ± 0.324 mol Trolox/L) had the highest free radical scavenging capacity, followed by Kang Shi Fu muo li qing cha (9.828 ±0.261 mol Trolox/L), Kang Shi Fu lü cha (8.977 ±0.363 mol Trolox/L), Tong Yi cha li wang (8.604 ±0.121 mol Trolox/L), Kang Shi Fu tie guan ying cha (8.361 ± 0.110 mol Trolox/L) and Kang Shi Fu wu long ming cha (8.324 ± 0.069 mol Trolox/L). Qi Lin wu hou hong cha (yuan wei hong cha) had the lowest free radical scavenging capacity (3.815 ±0.087 mol Trolox/L) among the tested tea infusions.

The correlation between antioxidant capacities and the total phenolic content of the 51 infusions is shown in Figure 2. The result showed a positive linear correlation between the antioxidant capacities and total phenolic content (R² = 0.8043), which indicated that phenolic compounds could be one of the main components responsible for antioxidant activities of these infusions. In addition, the correlation between total antioxidant capacities obtained from FRAP and TEAC assays are shown in Figure 3. The results show a positive linear correlation (R² = 0.865) between them, which suggested that antioxidant components in these infusions could reduce oxidants (such as ferric ions) and scavenge free radicals. This result is in agreement with those of medicinal plants and wild fruits [23,24]. Maybe, this was because FRAP and TEAC assays are all electron transfer-based methods [25].

Seven herbal infusions and six tea infusions had the strongest antioxidant activities among the 51 infusions based on a combinative consideration of the results obtained by FRAP and TEAC assays as well as the Folin-Ciocalteu method. They are Ping An Tang li yan cha, Ping An Tang shi gan cha, Qing Xin Tang jiang huo wang, Qing Xin Tang er shi si wei, Qing Xin Tang hou zheng tang, Qing Xin Tang zhi ke hua tan tang and Qing Xin Tang gan mao cha as well as Kang Shi Fu muo li mi cha, Kang Shi Fu muo li qing cha, Kang Shi Fu lü cha, Kang Shi Fu wu long ming cha, Kang Shi Fu tie guan ying cha and Tong Yi cha li wang. The main polyphenolic components in these infusions have been identified according to the method reported in the literature [26], and are shown in Table 3. Because of their high antioxidant activities, it could be speculated that these infusions will be beneficial for the diseases caused by oxidative stress.

For total phenolic content, FRAP value and TEAC value, the differences between herbal infusions and tea infusions, between bitter herbal infusions and sweet herbal infusions as well as between green tea infusions and black tea infusions were significant, but the difference between herbal infusions produced by Ping An Tang and those by Qing Xin Tang was not significant (Table 4). In addition, antioxidant capacities of tea infusions, bitter herbal infusions and green tea infusion were higher than those of herbal infusions, sweet herbal infusions and black tea infusion, respectively. Polyphenols are the most important antioxidants in the tea, and catechins are the major phenolic compounds in green tea. Black tea belongs to fermented tea, and its content of catechins was reduced to 20% of that in green tea [27]. Therefore, green tea usually had higher antioxidant capacity than black tea, which resulted in that green tea infusion might have higher antioxidant capacity than black tea infusion. The bitter herbal infusions are usually made from the medicinal plants under the ‘heat-clearing’ category according to the classification of Chinese medicinal plants [12], or those used for prevention and treatment of cold, flu and cough [20], most of which showed the high antioxidant capacities [12,20], while sweet herbal infusions often contain fewer medicinal plants compared with bitter herbal infusions, resulting in lower antioxidant capacities.

Gallic acid, 6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (Trolox), Folin–Ciocalteu’s phenol reagent, 2,4,6-Tri(2-pyridyl)-s-triazine (TPTZ) and 2,2’-azinobis(3-ethylbenothiazoline-6-sulfonic acid) diammonium salt (ABTS) were obtained from Sigma–Aldrich (St. Louis, MO). Sodium carbonate, potassium persulphate, Iron (III) chloride 6-hydrate, iron (II) sulfate 7-hydrate, acetic acid and sodium acetate were purchased from Tianjing Chemical Factory (Tianjing, China). Hydrochloric acid, ethanol and methanol were obtained from Kelong Chemical Factory (Chengdu, China). All chemicals used in the experiments were of analytical grade, and deionized water was used.

Twenty-eight kinds of herbal infusions and twenty-three kinds of tea infusions were bought from local markets (Table 1), which are commercial preparations and in the form of tin with aquatic solution. The samples were kept in the refrigerator at 4 °C until analysis. The various infusions were centrifuged at 3,500 rpm for 30 min, and the resulting supernatants were used for the determination of total phenolic contents and antioxidant capacities.

Total phenolic content of the infusion was determined according to the literature [10,28]. Briefly, 0.50 mL of the diluted infusion (a dilution factor of 10-times with water) was added into 2.5 mL of 1:10 diluted Folin–Ciocalteu reagent. After 4 min, 2 mL of saturated sodium carbonate solution (about 75 g/L) was added. The absorbance of the mixture was measured at 760 nm after incubation for 2 h at room temperature. Gallic acid was used as a reference standard and the results were expressed as gram gallic acid equivalent (g GAE)/L of infusion.

The FRAP assay of the infusion was carried out according to the procedure described in the literature [16,17]. Briefly, the FRAP reagent was prepared from sodium acetate buffer (300 mM, pH 3.6), 10 mM TPTZ solution (40 mM HCl as solvent) and 20 mM iron (III) chloride solution in a volume ratio of 10:1:1, respectively. The FRAP reagent was prepared fresh daily and warmed to 37 °C in a water bath before use. One hundred microliters of the diluted infusion was added to 3 mL of the FRAP reagent. After 4 min, the absorbance of the mixture was measured at 593 nm using a Shimadzu UV-2450 ultraviolet-visible spectrophotometer (Japan). The standard curve was constructed using FeSO4 solution, and the results were expressed as mol Fe(II)/L of infusion.

The TEAC assay of the infusion was carried out according to the method established in the literature [22]. Briefly, the ABTS^•+ stock solution was prepared from 7 mM ABTS and 2.45 mM potassium persulfate in a volume ratio of 1:1, and then incubated in the dark for 16 h at room temperature, which should be used within 2 days. The ABTS⁺ working solution was prepared by diluting the stock solution with ethanol to an absorbance of 0.70 ±0.05 at 734 nm. All infusions were aptly diluted to provide 20–80% inhibition of the blank absorbance. One hundred microliters of the diluted infusion was mixed with 3.8 mL ABTS⁺ working solution. After 6 min of incubation at room temperature, the absorbance of the mixture was measured at 734 nm, and the percent of inhibition of absorbance was calculated. Trolox solution was used as a reference standard, and the results were expressed as mol Trolox/L of infusion.

All the experiments were carried out in triplicate, and the results were expressed as mean ± SD (standard deviation). Statistical analysis was performed using SPSS 13.0 and Excel 2003. The p value less than 0.05 was considered to be statistically significant.