Introduction

Tea, one of the most consumed beverages from past to present, is obtained by infusion of Camellia sinensis (L.) Kuntze plant leaves. It was first discovered in China, and its history dates back to 2000 years ago. Tea is grown in about 50 countries and consumed in more than 160 countries today. Approximately 18–20 million cups of tea are consumed daily [1]. Although black tea is the most preferred, six different tea types can be produced: black tea, green tea, oolong tea, white tea, yellow tea, and brick tea [2].

Epidemiological studies have shown that tea consumption reduces the risk of coronary heart disease and stroke [3]. Besides, tea polyphenols are known to have an anti-carcinogenic effect against the skin, liver, lung, gastrointestinal system, pancreas, and bile cancers [4]. In addition to phenolic compounds, tea contains minerals and trace elements. However, due to the processes carried out during production, tea is exposed to toxic chemical contaminants such as polycyclic aromatic hydrocarbons, pesticides, perchlorate, and heavy metals [5, 6].

Tea is known to be a metal accumulator [5]. The quantities of heavy metals and trace elements in the tea leaves can vary depending on the content and the type of soil in which the tea is grown, the type of tea, the season in which the tea is produced, the maturity of the tea leaves, the duration and kind of the brewing, as well as the teapot type [5,6,7]. Aluminium (Al) is one of the metals contaminating tea [6, 8]. The presence of Al in the soil structure is essential for the development of the tea plant [8]. However, this leads to the natural accumulation of Al in tea plants [8]. It has been previously reported that Al accumulation may lead to Alzheimer’s disease, Parkinson’s disease, dialysis encephalopathy, multiple sclerosis, autism spectrum disorder, and toxicity [9]. Therefore, it is crucial to investigate whether tea consumption causes exposure to toxic levels of Al as tea is a widely consumed beverage in most cultures.

In this manner, some studies have been carried out to determine Al in a variety of tea types and to evaluate its potential adverse effects on health [10,11,12,13]. In addition, Al and some other trace elements were analyzed in tea infusions brewed in stainless steel, ceramic, porcelain, plastic teapots, and some local teapots made of clay, regardless of the infusion time [14, 15]. The effect of infusion time from 15 to 30, 45, and 60 min on Al release was also evaluated in some studies [14, 16].

To our knowledge, no study has been reported to determine Al in black, green, and white tea samples brewed in 5 different teapot materials for three different infusion times. Therefore, the aim of this study was to determine the Al concentrations of black, green, and white tea infusions for the first time in teapots made of aluminium, copper, glass, steel, and porcelain materials for 5, 10, and 15 min infusion times. In addition, the hazard quotient (HQ) of tea consumption was calculated according to the concentrations found in the samples.

Materials and method

Chemicals and materials

All chemicals and solutions used for AI analysis were of high-performance liquid chromatography (HPLC) grade. All solutions were filtered through a 0.45 μm filter before use. Hydrochloric acid (HCl), 5-sulfoquinoline-8-ol (HQS), bis(2-hydroxymethyl)imino tris (hydroxymethyl)methane (Bis–Tris), tetrabutylammonium bromide (TBABr), sodium chloride (NaCl), acetonitrile (CH3CN) and nitric acid (HNO3) were obtained from Sigma-Aldrich (St. Louis, MO, America).

Preparation of tea infusions

Black, green, and white tea samples were purchased from a local supermarket. Tea samples (5 g each) were weighed for each infusion. Five teapots of aluminium, copper, glass, steel, and porcelain were used. Tea infusions were prepared by brewing in water at 100 °C. For this purpose, 100 mL of ultra-pure water was added to five teapots and boiled. Afterward, 5 g of tea samples (black, green, and white) were added and left to brew separately for 5, 10, and 15 min for each tea type. Analyses were performed in three replicates. The study design was summarized in Fig. 1.

Fig. 1
figure 1

Study design

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High-performance liquid chromatography analysis of Al

A prominence series of HPLC instrument (Shimadzu, Kyoto, Japan) consisting of an LC-20AT pump, a DGU-20A5 degasser, an SIL-20A autosampler, a CTO-10ASVP column oven, and an RF-20A fluorescence detector (FD) was employed to carry out the analyses. GL Sciences Inert Sustain C18 (250 × 4.6 mm, 5 µm) analytical column was used. Analysis of Al in tea samples was achieved by an HPLC-FD method previously described by Shibukawa et al. [17]. The mobile phase was prepared with 10 mM Bis–Tris, 0.3 mM HQS, 7.0 mM TBABr, 70 mM NaCl, and 25% acetonitrile (v/v). A sample preparation solution was prepared using twice the amounts of the reactants. The extracts obtained by mixing the tea infusions and sample preparation solutions (1:1 v/v) were placed in vials after filtering through a 0.45 μm membrane filter. HPLC-FD conditions were set as follows: excitation wavelength 366 nm, emission wavelength 510 nm, column temperature 25 °C, flow rate 1 mL/min, and injection volume 20 μL. Under optimized conditions, the method was linear in the range of 10–1000 μg/L with a calibration equation of y = 2923.1x + 226212 (r = 0.9983) [18].

Calculating the estimated daily intake and hazard quotient

Estimated daily intake (EDI) of metals was calculated according to Eq. 1 using the amount of metal in the food, the amount consumed, and the individual’s body weight. For this calculation, body weight and daily tea consumption were accepted as 70 kg and 10 g, respectively [19].

$${\text{EDI}} = \frac{{{\text{C}}_{{\text{metal}}} \times {\text{W}}_{{\text{food}}} }}{{{\text{BW}}}}({\text{mg}}/{\text{kg}}\,{\text{BW}}/{\text{day}})$$
(1)

Cmetal = Concentration of heavy metal (mg/kg), Wfood = Daily average consumption (g), BW = Body weight (kg).

The hazard quotient (HQ), which was calculated according to Eq. (2), is the metal ratio in the food consumed to the reference oral dose (RfD) of the metal with EDI. The United Nations Food and Agriculture Cover (FAO)—World Health Organization (WHO) on Joint Food Additives Expert Committee (JECFA) has accepted a tolerable daily Al intake of 1 mg/kg per body weight [20, 21]. If the HQ value is less than 1 (HQ < 1), the consumption of relevant food is considered safe [22].

$${\text{HQ}} = \frac{{{\text{EDI}}({\text{mg}}/{\text{kg}}/{\text{day}})}}{{{\text{RfD}}({\text{mg}}/{\text{kg}}/{\text{day}})}}$$
(2)

Statistical analysis

IBM SPSS software (version 16; SPSS, Inc., Chicago, IL, USA) was used for statistical calculations. The suitability of variables to normal distribution was evaluated by visual examination (histogram, probability plots) and analytical methods (Kolmogorov–Smirnov/Sahpiro-Wilk tests). Descriptive statistics were made by displaying the mean ± standard deviation. The relationship between tea types, teapot type, and infusion time factors was determined using the 3-way ANOVA test. Bonferroni correction was applied for multiple comparisons (p = 0.05).

Results

Al concentrations of tea beverage

Al concentrations examined in black, green, and white tea samples according to the infusion times of 5, 10, and 15 min in teapots made of aluminium, copper, glass, steel, and porcelain material are shown in Table 1. The results showed that Al concentrations vary from 38.46 ± 5.08 to 845.75 ± 11.14 μg/L. Tea type significantly influenced Al concentrations (p < 0.001) (Table 1). Mean Al concentrations for black, green, and white tea were 50.36 ± 22.78 µg/L, 715.53 ± 97.32 µg/L, and 271.13 ± 48.76 µg/L, respectively. Accordingly, green tea had the highest Al concentration while black tea had the lowest concentration (Table 1). Teapot type also significantly affected Al concentrations (p < 0.001) (Table 1). Average Al concentrations of teas brewed in aluminium, copper, glass, steel, and porcelain teapots were 287.44 ± 255.98 µg/L, 311.07 ± 254.4 µg/L, 355.04 ± 280.05 µg/L, 387.15 ± 316.67 µg/L, 387.67 ± 317.53 µg/L, respectively (Table 1). In addition to tea type and teapot, infusion times had a significant impact on Al concentrations (p < 0.001) (Table 1). Average Al concentrations were 339.62 ± 285.12 µg/L, 346.04 ± 291.4 µg/L, and 351.36 ± 284.38 µg/L for 5 min, 10 min, 15 min infusion times, respectively. The highest Al concentration was determined for the 15 min infusion time while the lowest was determined for 5 min (Table 1).

Table 1 Al concentrations of tea samples according to their type, infusion time and teapot they brewed in
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Regarding the main factor interactions, relationships between tea * teapot type, tea * infusion time, and teapot * infusion time were found to be significant (p < 0.001). Additionally, the interaction between tea * teapot type * infusion times significantly affected Al concentrations (p < 0.001). When the teapot type * infusion time interaction was evaluated, the Al content of black tea brewed for 5 min in the steel teapot was significantly lower than other teapot types (p < 0.001). Similarly, the lowest Al concentration was found in black tea brewed for 10 min in the steel teapot; however, this was not statistically significant compared to copper (p = 0.426), glass (p = 0.085), and porcelain (p = 0.063) teapot. Al concentrations of black tea brewed for 15 min in the porcelain teapot were significantly higher than in other teapots (p < 0.001). For the green tea, the teapot type and infusion time interaction indicated that the highest Al concentration was observed in the porcelain teapot brewed for 5 min. Furthermore, the Al concentrations of tea samples were the highest in steel teapot brewed for 10 min (p < 0.001). The lowest Al concentration was detected in the copper teapot for 5 min infusion period of green tea. While the white tea sample had the highest Al concentration in the glass teapot for 15 min infusion time, it had the lowest Al concentration for 5 min infusion time in the copper teapot.

Estimated daily intake and hazard quotient

Based on the JECFA’s recommendation, EDIs were below 1 mg/kg for black tea, all teapot types, and all infusion times. For white tea, EDI was above 1 mg/kg only for 15 min infusion time and glass teapot. All teapot types and infusion times were above the limit value for green tea. For green tea, when Al concentrations in tea infusions were evaluated for an individual of 70 kg and 10 g of tea consumption per day, the hazard ratios were above 1 for all teapot types and infusion times. For white tea, EDI was above 1 only for 15 min infusion time and glass teapot. Hazard ratios of black tea for all teapot types and infusion times were below 1 (Table 2).

Table 2 The estimated daily intake (EDI) and hazard quotient (HQ) of aluminium in teas
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Discussion

Teapot material, tea type, and infusion time preferences differ between societies. Although the effect of infusion times and teapot materials on tea infusion and taste is traditionally known, the effect of these parameters on Al release has not been enlightened to date. In this study, it was observed that the type of tea, the teapot material, and the infusion time influenced Al concentrations in tea infusions.

The soils used for growing tea are prepared by acidifying with Al and ammonium sulfate and can be used for 50–80 years [12]. Tea leaves have been shown to accumulate Al in many studies [6, 10, 11, 15, 22, 23]. Al accumulation in the tea plant can be affected by many factors. It has been reported that Al content of black tea is higher than that of green tea [12, 24]. On the other hand, in accordance with our results, there are studies indicating that green tea contains more Al than black tea [25, 26]. Fernández-Cáceres et al. reported that there is no significant difference between the metal content of green tea and black tea [1]. Another study showed that white tea has lower Al concentrations than black and green tea [27]. In this study, the highest concentration of Al was determined in green tea and the lowest in black tea. Since the Al concentrations in tea species are affected by many factors, such as the heavy metal content of the soil and the phenolic composition of the tea, evaluating the Al content of tea samples according to a standardized classification is still unachievable. For this reason, analyzing local tea products in terms of heavy metal and trace element contamination is crucial to determining risk factors related to these compounds.

Previous studies reported the significant effects of infusion time on Al content [5, 10, 27,28,29]. Ghoochani et al., performed the heavy metal analysis in 5, 15, and 60 min tea infusions. Al concentrations were determined between 0 and 1516.27 mg/kg in dry weight. The maximum Al concentrations for 5, 15, and 60 min were found as 633.44, 1020.00, and 983.00 mg/kg, respectively. No correlation had been found between Al concentration and infusion time [5]. Mehra et al., and Moghaddam et al., reported that the Al concentration increased with the infusion time [10, 28]. The highest and lowest Al concentrations were found at 2 min and 10 min infusion time, respectively. The average Al concentration was 821 ± 307 mg/kg [28]. Moghaddam et al. reported significantly higher Al concentrations for 2 min infusion time than for 5 min [10]. Similarly, in this study, there was an increase in the mean Al concentration from 5 min (339.64 ± 291.72 µg/L) to 10 min (346.01 ± 298.20 µg/L) and 15 min (345.66 ± 286.93 µg/L) infusion time. Additionally, in our study, it was determined that Al concentrations increased with increasing infusion time for black tea in aluminium teapots and green tea in glass and aluminium teapots. On the other hand, a different study reported that Al transition was higher in the first infusion (15 min) of tea samples, and was lower in the subsequent infusions (30, 45, 60 min) [14]. Based on these findings, it was recommended to consume tea brewed for 10–15 min instead of 5 min. Al concentrations were found to be high for 5 min infusion time in copper and glass teapots for black tea. For green tea, there was a tendency to increase Al concentration in aluminium, copper, and steel teapots with increasing infusion time. In white tea, there was an increase in Al concentration at 10 min, while there was a decrease at 15 min.

Previous studies investigated the relationship between teapot material and Al concentration without considering infusion time. In the tea samples brewed in six different teapot materials, the highest Al concentration was found in the plastic teapot, and it was the lowest in a teapot called “zisha”, which was produced from a local material [7]. In another study, the metal release of commonly used traditional teapots has been studied. It has been determined that there was a health risk related to zinc and nickel alloys. In addition, Bolle et al., reported increased metal transfer with increasing infusion time [16]. The metal transition from stainless steel and brass teapots was also investigated [16]. The high concentrations of Al and Mn were found in tea for different teapots. The authors stated that high metal concentration was caused by the nature of the tea [15]. In this study, Al was detected at the highest levels in black and green tea when brewed in porcelain teapots. However, these concentrations were at a level that does not pose a risk. The average highest concentration in white tea was detected in the steel teapot. For white tea, while the 15 min infusion time in the glass teapot posed a risk, the other teapots and infusion times did not. The difference in metal alloying of teapot materials should be considered in determining Al concentrations. However, other parameters, such as the soil structure, region where the tea material was grown, and infusion time, could also affect the Al content.

Varying Al content of different tea samples can also be associated with phenolic compounds. Black, green, and white tea is produced by applying different processes to tea leaves. White tea is produced at the end of the withering and drying processes. Variations in production processes may affect phenolic compounds in tea leaves. Epigallocatechin gallate is the most abundant phenolic compound found in white and green tea, while the aflavin is the most abundant in black tea due to the polyphenolic enzymatic oxidation effect of the fermentation process [30]. The polyphenols in tea can reduce toxicity by forming complexes with heavy metals released during brewing [31]. In this manner, future studies should be designed to determine the phenolic content of tea species and to examine their effect on metal transition.

Hazard quotients of tea samples are desired to be lower than 1 under ideal conditions, which indicates the safety of tea consumption. For black tea, hazard quotients were less than 1 for all infusion times and teapots. For white tea, except for 15 min of infusion in a glass teapot, the risk rates were less than 1, and therefore its consumption was safe. However, the consumption of green tea was not safe for all teapot types and infusion times. For a 70 kg individual consuming the tea infusion obtained by the 10 g of the tea, determined average Al concentrations in black, green, and white tea samples correspond to 0.26%, 1.97%, and 0.79% of limits set by JECFA, respectively. Although these rates seem to meet a small amount of tolerable intake, exposure to AI in the long term may play a role in the pathogenesis of some diseases. Apart from tea, foods, cosmetics, medicines, water, and breathing air can cause Al exposure. Today, sources of AI exposure increase day by day with industrialization. Since tea is a beverage widely consumed by all segments of society, necessary measures should be taken to prevent toxicity and minimize exposure.

Conclusion

Tea is among the most consumed beverages today and has the potential for health-promoting effects, but it may also be a source of some contaminating agents. In this study, Al concentrations of white, green, and black tea samples brewed in aluminium, copper, glass, steel, and porcelain teapots for three infusion times were determined. The concentration of Al in green tea was the highest. Results showed that green tea could cause a significant amount of Al exposure. Necessary precautions should be taken to prevent and minimize Al exposure from tea. Determining the type of tea, preferring a tea containing less Al, determining the type of teapot that will cause less Al contamination, and infusion time should be considered in reducing the Al exposure.