Determination of trace elements and heavy metals content of green and black tea varieties consumed in Iran

Overview

Jannat and colleagues measured zinc, cadmium, lead, and copper in 60 commercial green and black tea samples from the Tehran retail market by differential pulse polarography. The source is routeable for true-tea dry-leaf occurrence evidence because it reports per-brand means, per-class means, and overall ranges for each of the four elements in mg/100 g of dry tea leaves. Cadmium and lead were detected in many samples at concentrations well above the standard limits the authors used as their reference benchmark, even though the paper’s narrative concludes that mean concentrations fell within an acceptable range; this internal disconnect is preserved verbatim in this page rather than reconciled.

Key numbers

All metal concentrations below are in mg per 100 g of dry tea leaves (equivalent to 10 mg/kg per 1 mg/100 g) unless otherwise stated.

Sample set and preparation

The study tested 60 commercial samples drawn from 25 brands and seven flavor categories purchased in Tehran. The split was 33 black-tea samples and 27 green-tea samples. Each sample of approximately 1 g of dry tea leaves was digested in 20 mL concentrated nitric acid until digestion was complete, followed by addition of 2.0 mL of 70% perchloric acid with continued gentle heating for one hour, then diluted to a 100 mL volumetric flask with deionized water before measurement.

Means and standard deviations by tea class

Table 3 of the paper reports the black-tea zinc mean as 0.520 ± 1.632 mg/100 g, which differs in the third decimal from the 0.505 ± 1.632 figure repeated in the abstract and the results-and-discussion narrative; this page preserves both values rather than picking one.

Ranges across all 60 samples

Standard-limit comparison reported in Table 5

The authors compared their per-class means against an unattributed standard-limit column expressed in the same mg/100 g unit: Zn 5, cadmium (Cd) 0.01, lead (Pb) 0.1, Cu 5 mg/100 g. By that reference, mean cadmium in black and green tea was 41.7× and 28.4× the standard, mean lead in black and green tea was 3.25× and 3.95× the standard, and mean zinc and copper fell at or below the standard. The Materials and Methods do not attribute the cited standard limits to a specific agency, regulation, or year, and the source’s overall conclusion that values are in an acceptable range refers to the authors’ own t-test against an unstated WHO acceptable-daily-intake reference rather than against the Table 5 standard limits.

Class-level comparisons

The authors report a statistically significant difference between black and green tea for zinc and cadmium content (P < 0.05) and no significant difference for lead and copper (P > 0.05). Analysis of variance found no significant difference in heavy-metal concentration attributable to flavor (ordinary, lemon, mint, cinnamon, jasmin, honey, or tab). Green-tea samples carried higher mean zinc, lead, and copper and lower mean cadmium than black-tea samples.

Comparison values reported from prior Iranian and regional studies

The discussion contextualizes the new values against earlier work and reports the prior means as: Falahi and Hedaiati 2013 (Iranian black tea) Zn 2.88, Cd 0.0134, Pb 0.021, Cu 1.59 mg/100 g; Nkansah et al. 2016 (Ghanaian commercial green and black teas) Zn 0.02, Cd 0.036, Pb 0.016 mg/100 g; Raj et al. 2011 (Indian black tea) Pb 0.231, Cd 0.089, Zn 2.539, Cu 1.434 mg/100 g and (Indian green tea) Cd 0.159, Zn 2.639, Cu 1.128 mg/100 g with Pb below 0.5 mg/kg LOQ; Hosseni et al. 2013 (Iranian vs imported black tea) Pb 0.04975 vs 0.02933 mg/100 g and Cd 0.0045 vs 0.000914 mg/100 g; Maleki et al. 2010 (Iranian black tea brands) Pb 0.66 ± 0.14 to 15.48 ± 0.58 µg/g and Cd 0.09 ± 0.013 to 1.92 ± 0.38 µg/g. The discussion notes that several of these prior studies report lower lead and cadmium values than the present polarographic survey.

Methods (brief)

Approximately 1 g of dry tea leaves was digested in 20 mL concentrated nitric acid on a thermostat-controlled hot plate until digestion was complete (about one hour), followed by 2.0 mL of 70% perchloric acid with continued gentle heating for one hour. Small additions of deionized water prevented dryness during evaporation. After cooling, the digest was filtered into a 100 mL volumetric flask. Zinc, cadmium, lead, and copper were quantified by differential pulse polarography using a Hanging Drop Mercury Electrode (HDME) with a 10 mL sodium acetate/tartaric acid buffer at pH 4.6-4.8, an initial electrolysis at -800 mV for 90 s, a 10 s rest, and stirring at 2000 rpm. The standard-addition technique with three additions of 100 µL of quadruple standard solution was used for quantification, and calibration curves for all four elements were reported (Zn y = 0.0082x + 0.0294 R² = 0.9998; Cd y = 0.0171x + 0.0008 R² = 1; Pb y = 9.6103x + 5.7374 R² ≈ 1; Cu y = 0.0244x + 0.0476 R² ≈ 1). Reagents (acetic acid glacial; copper(II), lead(II), zinc(II), and cadmium(II) nitrates; sodium acetate; tartaric acid) were sourced from Merck (Germany) and 65% nitric acid from AppliChem (Germany).

Implications

Certification: This is Iranian-retail-market true-tea evidence, not a US- or EU-market benchmark pool input without jurisdictional stratification. The basis is dry tea leaves, not brewed infusion, so transfer-into-cup factors must be applied before treating these values as exposure inputs for tea drinkers. The reported per-class means for cadmium and lead sit far above the standard-limit column the authors themselves print in Table 5, which is a useful reminder that paper-level conclusions about “acceptable range” should be checked against the paper’s own tables.

App: Route to tea, true tea, and the metals pages for lead, cadmium, copper, and zinc. Do not route as tea-infusion exposure evidence because the basis is dry leaves, not brewed liquor. Do not route as herbal-tea, kombucha, or supplement-tea evidence; the sample set is Camellia sinensis only.

Courses: Useful for teaching dry-leaf versus brewed-infusion basis, polarographic determination of base metals as an alternative to AAS and ICP-MS in resource-constrained labs, and the discipline of cross-checking a paper’s narrative conclusion against the standard-limit table the same paper prints.

Related evidence

• Tea
• Camellia sinensis
• Tea
• True Tea (Camellia Sinensis)
• Lead
• Cadmium
• Copper
• Zinc

Update history

The five most recent substantive edits to this page, classified major (evidence or structure moved), correction (a published value or statement was wrong and has been fixed), or minor (narrative rewritten without changing the underlying evidence). Each description is derived from what the edit did to this page; the linked commit is the authoritative record, routine regeneration passes are excluded, and the full version history lives in git.