**3. Results**

#### *3.1. Ferric-Reducing Antioxidant Power (FRAP) Values of the Tested Teas*

FRAP value was used as an important indicator for the antioxidant capacity with regard to reducing ferric ions to ferrous ions, and FRAP results of the 30 teas are displayed in Table 2. The total FRAP values ranged from 611.18 ± 5.09 to 5375.18 ± 228.43 μmol Fe (II)/g DW with a 9-fold di fference. Dianqing Tea, Xihu Longjing Tea, Dongting Biluochun Tea, Yongxi Huoqing Tea, and Duyun Maojian Tea exerted the top five reducing capacities, namely 5375.18 ± 228.43, 3926.32 ± 56.00, 3845.21 ± 44.17, 3752.52 ± 96.75, and 3664.97 ± 53.33, μmol Fe(II)/g DW, respectively. Tibetan Tea exhibited the lowest reducing ability of 611.18 ± 5.09 μmol Fe(II)/g DW. In addition, according to the statistical description and non-parametric tests (Table 3), the FRAP values for the three fractions met the following order: water-soluble > bound-insoluble > fat-soluble.


**Table 2.** Ferric-reducing antioxidant power (FRAP) values of the 30 common Chinese teas.

Abbreviations: FRAP, ferric-reducing antioxidant power; DW, dry weight; SD, standard deviation. Different superscript lowercase letters (a,b) indicated statistical significance (*p* < 0.05).


**Table 3.** Comparison among FRAP, TEAC, and TPC of different tea fractions.

Note: 1, fat-soluble fraction; 2, water-soluble fraction; 3, bound-insoluble fraction; a, non-parametric test between 1 and 2; b, non-parametric test between 2 and 3; c, non-parametric test between 1 and 3; FRAP, ferric-reducing antioxidant power; K-S, Kolmogorov-Smirnov Test; M, median; MAX, the maximum value; MIN, the minimum value; Moses, Moses Test; M-W, Mann-Whitney test; QL, the lower quartile; QU, the upper quartile; TEAC, Trolox equivalent antioxidant capacity; TPC, total phenolic content; W-W, Wald-Wolfowitz Test.

#### *3.2. Trolox Equivalent Antioxidant Capacity (TEAC) Values of the Tested Teas*

TEAC value was an important index of free radical-scavenging capacity, and the results of the 30 Chinese teas are presented in Table 4. The total TEAC values varied from 326.32 ± 0.48 to 3004.40 ± 112.89 μmol Trolox/g DW with a 9-fold difference. Dianqing Tea, Junshan Yinzhen Tea, Mengding Huangya Tea, Weishan Maojian Tea, and Xihu Longjing Tea showed the top five free radical-scavenging capacities, namely, 3004.40 ± 112.89, 2418.71 ± 26.70, 2303.72 ± 53.67, 2250.40 ± 37.95 and 2125.92 ± 44.43 μmol Trolox/g DW, respectively. Tibetan Tea had the lowest free radical-scavenging capacity of 326.32 ± 0.48 μmol Trolox/g DW. Moreover, according to the statistical description and non-parametric tests (Table 3), TEAC values for the three fractions met the following order: water-soluble > bound-insoluble > fat-soluble bound.


**Table 4.** Trolox equivalent antioxidant capacity (TEAC) values of the 30 common Chinese teas.

Abbreviations: DW, dry weight; SD, standard deviation; TEAC, Trolox equivalent antioxidant capacity. Different superscript lowercase letters (a,b) indicated statistical significance (*p* < 0.05).

#### *3.3. Total Phenolic Content (TPC) of the Tested Teas*

TPC was adopted to measure the total contents of phenolic compounds in the 30 Chinese teas, and the results are shown in Table 5. Briefly, the range of total TPC values was 37.25 ± 0.16 to 254.29 ± 15.51 mg GAE/g DW with a 7-fold difference. Dianqing Tea, Xihu Longjing Tea, Junshan Yinzhen Tea, Dongting Biluochun Tea, and Yuan'an Luyuan Tea possessed the top five total phenolic contents, namely 254.29 ± 15.51, 215.39 ± 11.87, 214.72 ± 3.22, 211.20 ± 2.52, and 210.05 ± 7.84 mg GAE/g DW, respectively. Tibetan Tea was observed with the lowest TPC of 37.25 ± 0.16 mg GAE/g DW. In addition, based on the statistical description and non-parametric tests (Table 3), the TPC values for the three fractions met the following order: water-soluble > bound-insoluble > fat-soluble.

#### *3.4. Correlations among Ferric-Reducing Antioxidant Power (FRAP), Trolox Equivalent Antioxidant Capacity (TEAC), and Total Phenolic Content (TPC) Values*

The correlations among FRAP, TEAC, and TPC values (based on the total values of three fractions) were determined by the simple linear regression model, and the results are presented in Figure 1. Both FRAP and TEAC values were significantly and positively correlated with TPC (*R*<sup>2</sup> = 0.883, *p* < 0.001 and *R*<sup>2</sup> = 0.941, *p* < 0.001, respectively). These results suggested that the phenolic compounds could be the main components contributing to the antioxidant activities of tea. In addition, FRAP values were positively and remarkably correlated with TEAC values (*R*<sup>2</sup> = 0.928, *p* < 0.001). Therefore, the antioxidants in tea could possess multiple functions regarding reducing oxidants (like Fe(III)) and scavenging free radicals (like ABTS•<sup>+</sup>).


**Table 5.** Total phenolic content (TPC) values of the 30 common Chinese teas.

Abbreviations: DW, dry weight; GAE, gallic acid equivalent; SD, standard deviation; TPC, total phenolic content. Different superscript lowercase letters (a,b) indicated statistical significance (*p* < 0.05).

**Figure 1.** Correlations among FRAP and TPC values (**A**), TEAC and TPC values (**B**), FRAP and TEAC values (**C**). Abbreviations: FRAP, ferric-reducing antioxidant power; TEAC, Trolox equivalent antioxidant capacity; TPC, total phenolic content.

#### *3.5. Systematic Cluster of the Tested Teas*

Based on the FRAP, TEAC, and TPC values, a systematic cluster analysis for the 30 teas was conducted with cluster numbers from 2 to 6, and the results are summarized in Figure 2. After that, the outcomes of cluster number = 4 were further analyzed using Online Analytical Processing (OLAP) accompanied with variance analysis (ANOVA), and the results are presented in Table 6. In detail, cluster 1 contained 12 teas, which were 4 black teas, 4 dark teas, 3 white teas, and 1 green tea, with the lowest values for FRAP, TEAC, and TPC (1050.03 ± 317.40 μmol Fe(II)/g DW, 600.57 ± 194.85 μmol Trolox/g DW, and 84.66 ± 27.90 mg GAE/g DW, respectively). In addition, cluster 2 comprised all the 4 oolong teas, 1 dark tea, and 1 yellow tea, with relatively low values for FRAP, TEAC, and TPC (2207.42 ± 342.61 μmol Fe(II)/g DW, 1416.88 ± 146.80 μmol Trolox/g DW and 167.83 ± 17.30 mg GAE/g DW, respectively). Moreover, cluster 3 consisted of 7 green teas and 4 yellow teas, with apparently high values for FRAP, TEAC, and TPC (3528.45 ± 265.76 μmol Fe(II)/g DW, 2089.27 ± 180.60 μmol Trolox/g DW, and 203.61 ± 11.11 mg GAE/g DW, respectively). Furthermore, cluster 4 included only 1 tea (green tea), with FRAP, TEAC, and TPC values the highest (5375.18 ± 228.43 μmol Fe(II)/g DW, 3004.40 ± 112.89 μmol Trolox/g DW, and 254.29 ± 15.51 mg GAE/g DW, respectively). Based on the result of ANOVA, all of the differences among the 4 clusters regarding FRAP, TEAC, and TPC values were significant (all *p* < 0.001).

**Table 6.** Online Analytical Processing (OLAP) Cube based on systematic cluster analysis for 30 Chinese teas (cluster number = 4).


Abbreviations: FRAP, ferric-reducing antioxidant power; TEAC, Trolox equivalent antioxidant capacity; TPC, total phenolic content.

**Figure 2.** Dendrogram using average linkage (between groups) from systematic cluster analysis for 30 Chinese teas. B, black tea; D, dark tea; G, green tea; O, oolong tea; W, white tea; Y, yellow tea.

#### *3.6. Contents of Phytochemical Compounds in Teas*

Main phytochemicals in 30 Chinese teas, including main catechins (Table 7), caffeine, theaflavine, and other polyphenols (Table 8), were determined by HPLC-PDAD. The chromatograms of the mixed standards and the samples of Dianqing Tea and Tibetan Tea under 254 nm are shown in Figure 3. Eight catechins, including catechin, epicatechin, gallocatechin, epigallocatechin, catechin gallate, epicatechin gallate, gallocatechin gallate, epigallocatechin gallate, four other phenolic compounds, including gallic acid, chlorogenic acid, ellagic acid, and kaempferol-3-*O*-glucoside, caffeine, and theaflavine, were identified in the 30 teas, with epigallocatechin gallate, gallic acid, and caffeine detected and quantified in all Chinese tea samples.


DW, dry weight, "-" means not detected.



DW, dry weight, "-" means not detected.


**Figure 3.** High-performance liquid chromatography (HPLC) Chromatograms under 254 nm of the standard compounds (**A**); Dianqing Tea (**B**); Tibetan Tea (**C**). The numbers in brackets referred to the compounds: gallic acid (**1**); gallocatechin (**2**); epigallocatechin (**3**); catechin (**4**); chlorogenic acid (**5**); caffeine (**6**); epigallocatechin gallate (**7**); epicatechin (**8**); gallocatechin gallate (**9**); epicatechin gallate (**10**); catechin gallate (**11**); ellagic acid (**12**); myricetin (**13**); quercitrin (**14**); kaempferol-3-*O*-glucoside (**15**); quercetin (**16**); theaflavine (**17**); kaempferol (**18**).

Catechins are the most abundant bioactive compounds in teas. In this study, it was found that epigallocatechin gallate was rich in the tested teas, with a range of 0.539 ± 0.013 to 59.354 ± 1.131 mg/g DW, but the difference was apparently large (up to a 110-fold difference). Green, yellow, and oolong teas were comprised of abundant epigallocatechin, but dark, black, and white teas were not. Dianqing Tea, Yuan'an Luyuan Tea, Xihu Longjing Tea, Yongxi Huoqing Tea, and Junshan Yinzhen Tea contained the top-five contents of epigallocatechin gallate, showing 59.354 ± 1.131, 57.230 ± 0.253, 51.734 ± 0.240, 50.947 ± 0.396, and 50.777 ± 0.224 mg/g DW, respectively. Dianhong Tea, with 0.539 ± 0.013 mg/g DW of epigallocatechin gallate was the lowest one. Additionally, these tea samples, especially green, yellow, and oolong teas, were also detected with remarkably high contents of epigallocatechin (2.288 ± 0.050 to 139.854 ± 1.075 mg/g DW), epicatechin (0.477 ± 0.030 to 13.723 ± 0.216 mg/g DW), and epigallocatechin gallate (0.455 ± 0.037 to 35.395 ± 0.568 mg/g DW). Tieguanyin Tea (139.854 ± 1.075 mg/g DW), Luohan Chenxiang Tea (125.439 ± 0.678 mg/g DW), Lu'an Guapian Tea (100.684 ± 0.561 mg/g DW), Taiping Houkui Tea (74.212 ± 0226 mg/g DW), and Lushan Yunwu Tea (53.447 ± 0.326 mg/g DW) possessed the top-five contents of epigallocatechin. Tieguanyin Tea (13.723 ± 0.216 mg/g DW), Fuzhuan Brick Tea (10.357 ± 0.268 mg/g DW), Weishan Maojian Tea (10.062 ± 0.040 mg/g DW), Duyun Maojian Tea (8.700 ± 0.429 mg/g DW), and Taiping Houkui Tea (8.580 ± 0.211 mg/g DW) contained the top-five contents of epicatechin. Dianqing Tea (35.395 ± 0.568 mg/g DW), Juanshan Yinzhen Tea (30.491 ± 0.101 mg/g DW), Dongting Biluochun Tea (27.893 ± 0.426 mg/g DW), Weishan Maojian Tea (24.710 ± 0.247 mg/g DW), and Mengding Huangya Tea (23.805 ± 0.075 mg/g DW) were shown with the top-five contents of epicatechin gallate.

In addition, for other phytochemical compounds besides catechins in teas, the content of gallic acid was low in all tea samples, ranging from 0.294 ± 0.021 to 3.822 ± 0.111 mg/g DW with a 13-fold di fference. Huoshan Large Yellow Tea, Yichang Congou Black Tea, Fenghuang Shuixian Tea, Fuzhuan Brick Tea, and Keemun Black Tea possessed the top-five contents of gallic acid, which were 3.822 ± 0.111, 3.546 ± 0.050, 3.284 ± 0.141, 3.097 ± 0.122, and 2.706 ± 0.117 mg/g DW, respectively. Tieguanyin Tea was shown to have the lowest content of gallic acid, which was 0.294 ± 0.021 mg/g DW. Similarly, the contents of chlorogenic acid, ellagic acid, and kaempferol-3- *O*-glucoside were also relatively low in the tested teas.

As polyphenols were suggested as the main antioxidants in teas (Figure 1), we next analyzed the relationships of di fferent polyphenols and antioxidant activities of teas. It was found that the content of catechins had moderate positive correlations (Figure 4A,B) with FRAP values ( *R<sup>2</sup>* = 0.476, *p* < 0.001) and TEAC values ( *R<sup>2</sup>* = 0.515, *p* < 0.001), while the content of noncatechin polyphenols had no evident linear correlations (Figure 4C,D) with FRAP values ( *R<sup>2</sup>* = 0.001, *p* = 0.867) and TEAC values (*R<sup>2</sup>* = 0.002, *p* = 0.819). These results indicate that catechins can be one of the main antioxidants in tea, but noncatechin polyphenols should not be the main contributors.

In addition to this, each tea contained relatively high ca ffeine content, which varied from 12.273 ± 0.040 to 41.631 ± 0.312 mg/g DW with a small di fference (only a 3-fold di fference). Yichang Congou Black Tea, Junshan Yinzhen Tea, Yuan'an Luyuan Tea, Dianqing Tea, and Xihu Longjing Tea comprised the top-five content of ca ffeine, namely 41.631 ± 0.312, 41.457 ± 0.322, 40.737 ± 0.116, 39.764 ± 0.382, and 38.508 ± 0.117 mg/g DW, respectively. The 12.273 ± 0.040 mg/g DW of ca ffeine in Qingzhuan Brick Tea was the lowest.

Furthermore, Wuyi Rock Tea (oolong tea) and Fuzhuan Brick Tea (dark tea) as well as all of the 4 black teas (Yichang Congou Black Tea, Keemun Black Tea, Dianhong Congou Black Tea, and Lapsang Souchong Black Tea) were found with a spot of theaflavine, and the contents were 0.545 ± 0.011, 0.480 ± 0.008, 0.559 ± 0.018, 0.542 ± 0.010, 0.526 ± 0.019, and 0.488 ± 0.012 mg/g DW, respectively.

**Figure 4.** Correlations between FRAP and catechins (**A**), TEAC and catechins (**B**), FRAP and noncatechin polyphenols (**C**) and TEAC and noncatechin polyphenols (**D**). Abbreviations: FRAP, ferric-reducing antioxidant power; TEAC, Trolox equivalent antioxidant capacity.
