Concentrations of selected metals (Cu, Mn, Zn, Cd) in tea leaves were investigated. Samples included black, green, and other (red, white, yellow, and oolong) teas. They were purchased on a local market but they covered different countries of origin. Beverages like yerba mate, rooibos, and fruit teas were also included in the discussion. Metal determinations were performed using atomic absorption spectrometry. In black teas, Mn/Cd ratio was found to be significantly higher (48,091 35,436) vs. green (21,319 16,396) or other teas (15,692 8393), while Cd concentration was lower (31.4 18.3 μg/kg) vs. other teas 67.0 (67.0 24.4). Moreover, Zn/Cu and Cu/Cd ratios were, respectively, lower (1.1 0.2 vs. 2.2 0.5) and higher (1086 978 vs. 261 128) when comparing black teas with other teas. Intake of each metal from drinking tea was estimated based on the extraction levels reported by other authors. Contributions to recommended daily intake for Cu, Mn, and Zn were estimated based on the recommendations of international authorities. Except for manganese, tea is not a major dietary source of the studied elements. From the total number of 27 samples, three have shown exceeded cadmium level, according to local regulations.
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Among other components, tea leaves consist of tanning agents, alkaloids, amino acids, pigments, and trace amounts of mineral compounds. Trace elements present in tea leaves play an important role in human metabolism [2]. They can be released from leaves to the infusion. Thus, tea leaves may become the source of metals in human diet. Some of the metallic elements, such as copper, manganese, and zinc are essential for basic processes in the human body while others (like cadmium) are toxic. The main aim of our study was to determine the content of copper, manganese, zinc, and cadmium in tea leaves and to find out if there are any significant differences among the teas of different types and countries of their origin.
Tea samples (n = 27) were purchased on a local market in southern Poland. All samples were based on pure tea leaves, without additives or flavors. The set of samples included teas in the form of loose leaves as well as ground ones in tea bags. Samples were divided according to their type and country of origin. Among the samples, there were 12 samples of black tea, eight samples of green tea, two samples of red tea, two samples of white tea, two samples of oolong tea, and one sample of yellow tea. All details are given in Table 1.
Manganese concentration in the samples varied from 457 4 to 2210 35 mg/kg (mean SD 962 388 mg/kg). Similar results were published by Street et al. [1], where manganese concentration in 30 samples of different types of teas varied from 511 to 2220 mg/kg. The authors did not notice a major difference between manganese concentration in black and green teas (nor they did for other elements: iron, zinc, and copper).
Comparing black and green teas, Mn/Cd ratio was found to be significantly different between these two groups. When comparing black teas to the others, four parameters showed significant differences: Cd concentrations, Mn/Cd, Zn/Cu, and Cu/Cd ratios. Further studies, including more tea samples, are needed to establish if there is such a general trend for these groups of teas.
In black teas, Mn/Cd ratio was found to be significantly higher vs. green or other teas, while Cd concentration was lower vs. other teas. Moreover, Zn/Cu and Cu/Cd ratios were, respectively, lower and higher when comparing black teas with other teas. This differentiation can be caused by the fermentation process during black tea production. Our results partly agree with the reports of other researchers; however, some differences can be noticed. In particular, zinc content in black tea as well as cadmium content in black and green teas was found to be much lower than reported by other authors. Very high content of manganese in two samples of black teas from Kenya was observed. Tea is a major dietary source of manganese while the intake of other elements is negligible. In three samples, content of cadmium was found to be higher than allowed by regulations of the Health Minister of Poland.
CTC (Crush, Tear, Curl) tea manufactured in Sri Lanka was used in this study. Tea brew was prepared according to the traditional method by adding boiling water to tea leaves. The samples were collected at different time intervals. Total phenolic and flavonoid contents were determined using Folin ciocalteu and aluminium chloride methods respectively. Gallic acid, caffeine, epicatechin, epigallocatechin gallate were quantified by HPLC/UV method. Antioxidant activity was evaluated by DPPH radical scavenging and Ferric Reducing Antioxidant Power (FRAP) assays.
Tea brew was prepared according to the conventional method. Deonized water (500 ml) was boiled in a glass beaker placed on a hot plate. At the onset of boiling, heating was terminated and the tea leaves (5.0 g) were added to boiled water. The beaker was then covered with a watch glass. Magnetic stirrer was used at a constant speed to maintain a homogenous sample. A volume of 1.0 ml was withdrawn at different time intervals (0, 1, 2, 4, 6, 8, 10, 12, 14, 20 min) and centrifuged. The supernatant was assayed for their phenolic and flavonoid content by spectrophotometry. Gallic acid, caffeine, epicatechin and epigallocatechin gallate were quantified by Reversed Phase High Pressure Liquid Chromatography (RP-HPLC). Antioxidant activity was assayed by DPPH radical scavenging and Ferric reducing Antioxidant Power (FRAP) methods.
Free radical scavenging ability of tea samples collected at different time intervals and authentic samples of tea constituents (gallic acid, caffeine, epicatechin and epigallocatechin gallate) was assayed by DPPH radical scavenging method with slight modifications [28]. Test samples (50 μl) were diluted up to 1000 μl with deionized water. DPPH reagent prepared in absolute ethanol (100 μM, 950 μl) was added to the test sample (50 μl) and the mixture was allowed to stand for 30 min in the dark. The scavenging activity was quantified by measuring the absorbance at 517 nm. Deionized water was used as the blank. The control was prepared by mixing deionized water (50 μl) with DPPH (950 μl). Results were expressed as percentage scavenging of DPPH radical calculated using the following equation:
The ferric ion reducing power of the samples collected at different time intervals was determined according to Sharma and Kumar (2011) with slight modifications [29]. Samples (50 μl) were diluted up to 1000 μl with deionized water. The test sample (100 μl) was mixed with phosphate buffer (0.2 M, pH 6.6, 250 μl) and potassium ferricyanide (1 %, 250 μl). The mixture was incubated at 50 C for 20 min. Trichloroacetic acid (10 %, 250 μl) was added and the samples were centrifuged at 6500 rpm for 10 min. The supernatant was mixed with deionized water and ferric chloride (0.1 %) at a ratio of 1:1:2 respectively. The samples were vortexed and absorbance was measured at 700 nm. The reagent blank was prepared by replacing tea sample with deionized water. L-ascorbic acid was used as the standard antioxidant. The antioxidant capacity was expressed as Ascorbic acid equivalent reducing power (mg/g of tea leaves).
Ideally, tea should be free from contaminants such as heavy metals, which are toxic and harmful to the human body because of their non-biodegradable nature, long biological half-lives and persistent accumulation in different body parts [7]. Tea is consumed in all of Bangladesh throughout the year, and Bangladesh is one of the leading tea producing and exporting countries in the world [8]. In 2006, Bangladesh exported approximately 5 million kg of tea leaves, and this figure continues to increase even while the total local tea consumption in the country is reported to be 39 million kg [8].
In addition, little is known about the relative recovery of heavy metals from tea leaves, and there are no standard official methods in Bangladesh for the digestion of tea to determine heavy metals. Moreover, to our knowledge, there is limited data on the amount of heavy metals in fresh tea leaves, processed tea or soils from tea plantations in Bangladesh. Therefore, the aims of this study were (1) to determine the concentrations of common heavy metals such as Cd, Cr, Pb, As and Se in tea leaves and soils from tea plantations; (2) to report the degree of contamination and daily intake of toxic heavy metals via tea (3); to measure the interaction of heavy metal concentrations in fresh tea leaves, processed tea and soils from tea plantations by analyzing the transfer factor (TF); and (4) to evaluate six digestion methods using different acid combinations and recommend the most appropriate digestion method for determining the levels of five heavy metals in tea samples.
Cr was detected in rather high amounts in black tea (3.581 3.941 µg/g), but it was not detected in fresh tea leaves or tea plantation soils (Fig. 3). Its level is higher than the recommended limit for Cr by the WHO of 0.05 µg/mL [36]. Moreover, Cr concentrations in black tea from India, China, Sri Lanka and Turkey were reported at 0.371, 0.155, 0.050 and 3.000 µg/g [39, 44], respectively. It is plausible that Cr contamination occurred during the fermentation process, which is one of the important processing steps of black tea in Bangladesh. In particular, it may occur during the CTC rolling steps involved in the production of black tea. However, this finding is lower than the previously reported Cr concentration (32.87 µg/g) in some tea samples from Bangladesh [34] which may be contributed to the different types of tea samples used as well as variance in the type of soil in the tea garden.
Similar to the findings for fresh tea leaves, Cr was not detected in the tea garden soil samples (Fig. 4). However, Cr has been reported in agricultural soils in the United States (48.5 µg/g) [45], India (1.23 µg/g) [46] and Kunshan, China (87.73 µg/g) [47]. Low concentrations of Cd (mean 0.222 0.103 µg/g) were observed in all investigated soils from the tea plantations samples (Fig. 3). These levels were lower than that previously reported in U.S. agricultural soils (13.5 µg/g) [45], but higher than in Indian agricultural soils (0.05 µg/g) [46] and soil from Kunshan in China (0.20 µg/g) [47]. 2ff7e9595c
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