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Mean oxalate content of loose-packed black tea infusions after different brewing time (2g/240 ml)/ All analytical data are the mean of triplicate measurements of three independent samples SEM/ A one-factor ANOVA and the post hoc test showed that there were significant differences between the oxalate content of different brewing times (P
Acidity Of Different Samples Of Tea Leaves.pdf
Mean oxalate content of loose-packed black tea infusions after different dilutions (2g/240 ml)/ All analytical data are the mean of triplicate measurements of three independent samples SEM/The results of independent t-test showed that there were significant differences between the mean oxalate content of different dilutions (P
As shown in Fig. 1, the range of oxalate in Iranian consumed loose-packed black tea after different brewing times was from 4.4 to 6.3 mg/240 ml. The brewing of tea in hot water extracts that portion of total oxalate that is soluble, leaving the insoluble oxalate (mainly oxalate bound to calcium) in the tea leaves [25]. The oxalate contents of black tea samples from tea bags for 1 and 5 min infusions were 10.8 and 20.8 mg oxalate/250 ml, respectively, for one brand and 10.8 and 12.0 mg oxalate/250 ml for another brand [26]. By contrast, no significant increase in oxalate content of Chinese green teas was observed by longer brewing time (5 vs. 10 min) [27]. The results of present study revealed incremental increases in soluble oxalate with increased brewing times all the way up to 60 min although after the 15 min time point, there was a marked decrease in the magnitude of this increase per unit of time.
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.
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.
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).
A pattern recognition method by multivariate statistical analysis, such as principal component analysis (PCA) and orthogonal projection on latent structure-discriminant analysis (OPLS-DA), was employed for the entire 1H NMR dataset for visualizing the global differences in tea leaf metabolites according to age of tea plant. An unsupervised PCA model was used to see the initial spectral features of the 1H NMR dataset and the metabolic relationships between tea samples. The PCA model showed the metabolic dependence of tea leaves on growing vintage or year described by the first principle component with 67.3% variations and on the age of tea plants explained by the second principal component with 9.83% (Fig. 2a). These metabolic dependences were more clear in the OPLS-DA model, as shown in Fig. 2b. The tea leaves collected in 2015 and 2016 were further differentiated in the corresponding OPLS-DA models, as shown in Fig. 2c, d, respectively, which demonstrated strong dependences of tea leaf metabolites on the age of tea plants.
Comparing results (Table 2), obtained by the QUENCHER method and the results from extracts, it can be seen that the yellow tea and the green tea had higher antioxidant activity obtained by the direct application of free radicals, while for the black tea, higher antioxidant activity was obtained when the extraction was used prior determination of the antioxidant activity. Evaluating extracts, higher antioxidant activity was obtained by application of the ABTS+ free radicals. Results of the QUENCHER method had a different tendency. Only antioxidant activity of the black tea was higher when the ABTS+ free radicals were used, while the green tea had almost the same values, and the yellow tea leaves had higher antioxidant activity when the DPPH free radicals were used. Pastoriza et al. [26] evaluated antioxidant activity in different foods using the ABTS+ free radicals and obtained higher antioxidant values for the most evaluated foods when the QUENCHER method was used in comparison to the extracts. Some exceptions (like fresh carrots, apples, potato chips, boiled ham and salami-type sausage) were noted, and in those samples, higher antioxidant activity was evaluated in extracts. Delgado-Andrade et al. [27] also determined much higher antioxidant activity in the wheat bread and the wheat bran bread by the QUENCHER method for ABTS+ and DPPH free radicals than in extracts. Also, they determined that values obtained by the ABTS+ were higher than with the DPPH free radicals. In some cases (like lentil, strawberry, apples, almond, pistachio nut, hazelnut or insoluble fractions of lettuce, orange, lemon, cocoa, biscuits, snacks and bread crust), antioxidant activity determined by the DPPH was similar or higher than with the ABTS+, when the QUENCHER method was used [4,5]. The DPPH has higher solubility in less polar solvents; thus, more reactivity towards lipid-rich containing structures can be expected. The lipid globules in samples could not act as a molecular barrier for DPPH that can reach both the lipophilic and hydrophilic antioxidant compounds presented in the core of the solid particles. The diffusivity phenomenon could be one of the primary explanations for the higher antioxidant activity when using the DPPH compared to the ABTS+ [5]. 2ff7e9595c
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