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While the complete fermentation may oxidize over 75% of the catechins in tea leaves, which mainly form theaflavins, but the concentration of theaflavins found in black tea was not high. This is probably due to the oxidation of the theaflavins, which leads to the formation of thearubigin (Turkmen et al., 2009). The thearubigins are a group of black tea polyphenols that are still poorly understood.

Brewing of dried black tea leaves produces a dark reddish-brown infusion

 The color as well as the characteristic taste of black tea infusion is mainly given by theaflavins and thearubigins (Muthumani & Kumar, 2007). Theaflavins contribute to the astringent and brisk taste and the golden color, while thearubigins give a rich mouthfeel and a brown-red color 

Dried tea leaves are produced from the fresh leaves of the tea plant, Camellia sinensis (L.) O. Kuntze (C. sinensis). C. sinensis is further classified into two main varieties: assamica, which is larger in terms of the tea plant and tea leaves, and sinensis, which has a smaller leaf size

Figure 2: Structure of flavonoids, including catechins (flavan-3-ols), flavones, and flavonols, found in tea

DOI: 10.1111/1750-3841.16137

Study: weak evidence

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China, India, Kenya, Indonesia, Sri Lanka, and Vietnam are the primary producers, accounting for around 77% of the global tea production and 80% of exports

According to the database of the Food and Agriculture Organization (FAO) of the United Nations [17], China was the leading tea-planting and tea-producing country among India, Sri Lanka, Indonesia, Vietnam, and Kenya in 2018, 2019, and 2020, which accounted for around 70% and 52% of plantation area and production, respectively (Figure 1).

Approximately 75% of tea consumption in the world comprises black tea, which accounts for the highest production and consumption [4]. The ambient temperature range required for a tea plant is 18 °C to 25 °C to grow well. Shoot growth is observed to be slowed down if the temperature is under 13 °C or above 30 °C. Tea plants require at least 1200 mm of rainfall annually. The optimal soil pH range for tea plant growth is 4.5–5.6. The best soil condition for the growth of tea plants is deep, having greater than 2% organic matter, well aerated and drained [5]. Usually, it is made from young shoots with one unopened bud and two or three leaves.

The total annual production and consumption of tea is three billion kilograms in the world

with a distinct fermentation, oxidation, and processing method [11]. As a fermented tea, black tea is typically produced in two ways orthodox and crush, tear, and curl (CTC). The orthodox involves four stages: withering, maceration or rolling, fermentation, and drying, whereas CTC black tea processing involves withering, maceration, cutting, tearing and curling, fermentation, and drying. The CTC process has a horizontal or vertical Rotorvane maceration machine for the maceration of tea leaves instead of rollers [12]. The most important of these stages is fermentation, which affects the smell, color, mouth feel, and, most importantly, the bio functions of black tea

Tea stays flavorful and healthy for over a year in cool conditions and away from air and moisture. However, prolonged storage, particularly in conditions where moisture and light exist, degrades the quality of the tea, which eventually results in “softness”, or lacking briskness and having a “flat” taste [180]. Black tea deteriorates by losing astringency and flavor and sometimes finding undesirable “taints” due to autoxidative reactions and lipid hydrolysis that reduce sugars, TF, photosynthetic pigments, amino acids, and some volatile aliphatic elements and raise volatile phenolic and non-dialyzable pigments. Heat and moisture accelerate these reactions. high moisture content is more detrimental to high temperature.

From days five to eleven of the plucking interval, TFs and TRs, black tea’s most characteristic compounds, vary with leaf maturity. Plucking with 25% coarse and 75% fine leaves balances yield and quality. Coarse leaves and immature shoots are undesirable because standard flush was dependent on standard leaves. Figure 4. Percentage of standard flush for standard leaf plucking and effect of fine and coarse plucking on black tea’s quality.

TFs and TRs, two significant polyphenols used as black tea quality indicators, make up 50–70% of the phenolic components in tea water extract.

Tea is cultivated from the equator to the subtropics, where seasonal changes may be extreme.

Withering, as the initial step in manufacturing black tea, is crucial to produce high-quality black tea. Tea processing begins with withering, where freshly harvested tea leaves are spread out in a proper withering system to lose moisture before processing [49]. For the subsequent steps, tea leaves go through a series of physical and chemical changes [47,98]. In tea processing, leaf moisture is crucial; tea leaf withering partially dries the surface and core moisture of tea leaves [99]. Black tea leaves are usually withered at room temperature to reduce moisture. Tea shoots undergo withering from being harvested until they are macerated or rolled [47]. Turgid tea shoots become flaccid as moisture levels decrease on a wet basis from ~70–80% to 60–70% during the withering process [100]. As green leaves wither, their moisture content drops to 60–70%, as noticed by [101,102]. Tea leaves are more flexible after withering and can be rolled and twisted without breaking.

Post-withering tea leaves are macerated or rolled to rupture plant cell structures, allowing catechins, polyphenol oxidase, and peroxidase to interact during fermentation [97]. Rolling extracts and twists leaf juice [54]. The main goal of rolling is size reduction and cell destruction to expose new surfaces to air during fermentation. It also presses out juice and coats leaf particles with a thin juice coating to accelerate chemical reactions [119]. This process is crucial for black tea production since most TFs are generated during rolling [112]. During rolling, enzymes are released from the leaf when it is broken and exposed to oxidation.  In Sri Lankan black tea production, orthodox rolling is used to gently roll tea leaves. The rolling method produces black orthodox tea. For tea leaf pulverization in Central Africa, a Lawrie tea processor is used, which can also be used to treat them harsher [120], whereas Indian and Kenyan tea leaves are crushed and macerated using crush tear and curl machines [120,121],

Fermentation When catechins are exposed to oxygen, these enzymes develop oxidized polyphenolic substances like TFs and TRs [40,137,138]. The fermentation environment’s temperatures, relative humidity, time, pH, and oxygen affect these compounds greatly. When black tea ferments properly, it turns green to coppery brown. It also smells fruity, brisk, rich in flavor, and tastes strong. Fermentation needs a set temperature, humidity, and time. During fermentation, good aeration increases TFs and TRs, whereas less aeration results in a reduction of those compounds. In the same way, high temperatures also reduce TF formation

In general, fermentation is carried out at around 20–30 °C for 30–120 min, although 25 °C for 60 min is considered the optimum. Enzymatic reactions require enough oxygen. Maintaining high relative humidity (95–98% is vital during fermentation

Drying stops fermentation by inhibiting enzyme activity, producing dried black tea [41]. Tea particle drying stops oxidation and enzymatic activity, decreases moisture to 3–4% (wb), facilitates handling and transportation, and enhances shelf life. Black tea dried at 110 °C temperature and 1.5 rpm is of excellent quality. The most effective combination was found to be 100 °C for 25 min. The two most common dryers used in tea factories are fluidized bed dryers (FBDs) and endless chain pressure (ECP) dryers.  Tea leaves can be dried at low temperatures using radio frequency, microwave, freeze, and vacuum

DOI: 10.3390/foods12244467

Study: weak evidence

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Table 1. Composition of tea and their impact on taste and color of brew: Quinic acid

Table 2. Volatile compounds responsible for aroma and flavor of tea.

. This paper provides a critical review on phytochemical constituents, anticancer property, and clinical applications of black tea and its bioactive compound

although more clinical experiments are needed at molecular levels to understand its anti-cancer property

Limited number of human intervention studies done on regular intake of black tea demonstrate its chemopreventive potential, while a substantial number of clinical investigations with green tea are documented.

DOI: 10.1080/10408398.2014.994700

Study: weak evidence

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Epidemiological data linking black tea consumption to a reduced CHD risk appeared robust. The meta-analysis by Peters et al. (2001) reported that the incidence rate of MI was estimated to decrease by 11% with an increase in black tea consumption of three cups per day (one cup ¼237 ml) with a fixed-effects relative risk (RR) estimate of 0.89 (95% con-fidence interval: 0.79, 1.01). Two case control studies provided additional evidence of the RR estimate for 3 cups/day. 

While associations cannot prove cause and effect, particularly where other dietary components are likely to be at work, the case for black tea was strengthened by the existence of experimental data suggesting a plausible mechanism.

Databases were searched for relevant epidemiological and clinical studies published between 1990 and 2004. Clear evidence was found for coronary heart disease (CHD), where an intake of > or = 3 cups per day related to risk reduction. The mechanism could involve the antioxidant action of tea polyphenols.

It has been suggested that plant antioxidants, such as those found in tea, red wine and cocoa, can help prevent and control cancer development. This has arisen from experimental (mainly animal) work on green tea catechins, although polyphenols in black tea are increasingly being studied in both animals and humans. 

Flavonoids may also exert other effects unrelated to their antioxidant capacity, for example, anti-inflammatory effects (Aneja et al., 2004) and inhibition of tumorigenesi

Thus, it is not possible to draw conclusions until further human studies are published. Prospective studies relating to colorectal cancers were more numerous and indicated either no relationship or a protective effect of tea at mean intakes of 1.5 cups per day or more.

Only two studies implied harm and these were Far Eastern studies where black tea consumption was associated with an increase in colorectal cancer (Kato et al., 1990; Inoue et al., 1998). However, Arab and Il’yasova, 2003, suggested that the tea consumers in these studies adopted other Western habits that may have increased their risk of colorectal cancer, for example, high-saturated fat, low-fibre diets.

observed in a cohort study that the total tea intake was significantly associated with a lower prevalence of cognitive impairment in Chinese adults although the effects were most evident for black and oolong teas.

There are innumerous compounds present in tea that have been associated with the beneficial effects on brain function and health such as flavan-3-ols, caffeine and L-theanine. However, it is important to consider that in in vivo conditions these compounds must be first absorbed and/or metabolized, cross the BBB and then overcome the P-glycoprotein (P-gp) efflux in order to exert the potential beneficial effect

It has been hypothesized that manganese in black tea could impact positively on heart disease risk, via the role of manganese superoxide dismutase in supporting cardiac muscle function and attenuating lipid peroxidation. One study examined the effect of tea drinking on markers of Mn status, finding no significant association

Fluoride – both systemic and topical – attenuates the risk of demineralization. A systematic review has suggested that fluoride from tea may benefit dental health (NHS CRD, 2000).

The available evidence suggested that black tea consumption had a moderately positive effect on bone mineral density (BMD), particularly in older women. There was a significant increase in BMD with higher levels of tea consumption (four or more cups per day) (Chen et al., 2003). Black tea was also identified as an independent protective factor for the risk of hip fractures in men in the Mediterranean Osteoporosis Study (Johnell et al., 1995; Kanis et al., 1999; Hegarty et al., 2000) reported that this effect was independent of the addition of milk to tea. 

However, on closer examination, these studies failed to control for confounders associated with tea consumption and CHD risk, for example, lower socio-economic status, long-term smoking and higher dietary fat intakes.

The epidemiological evidence does not appear consistent enough to enable firm conclusions about associations (although it is evident that black tea is not harmful).

DOI: 10.1038/sj.ejcn.1602489

Study: weak evidence

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Epidemiologic studies suggest correlation between tea consumption and reduced risk of type 2 diabetes, although the mechanisms for these observations are uncertain. Odegaard et al. (2008) investigated the prospective association between intakes of coffee, black tea and green tea with the risk of type 2 diabetes in Singaporean Chinese men and women. These authors concluded that consumption of coffee and potentially black tea, but not green tea, was associated with lower risk of type 2 diabete. larger, population-based, randomized, controlled trials are required to further clarify the relationship between tea consumption and its beneficial effects on type 2 diabetes. 

More recently, Halliwell (2012) has commented on the fact that until this moment the contribution of antioxidants is uncertain due to the lack of evidence of the beneficial effects in in vivo studies. In addition, there is a need to take into consideration the interindividual variability for metabolism and absorption of potential bioactive compounds.

According to Shen et al. (2009) in terms of bone mineral density (BMD), a positive, a weak inversion or no correlation between tea drinking and osteoporosis has been reported.

DOI: 10.1016/j.foodres.2013.01.038

Study: weak evidence

Systematisches Review mit Meta-Analyse

In addition, subgroup in terms of study population (Asian vs. non-Asian), type of tea (black vs. green) and study design (cohort vs. case–control) were stratified.

In summary, our current meta-analysis suggests that available evidences from this epidemiological study are insufficient to conclude that tea consumption could reduce the risk of prostate cancer. Further epidemiological studies are warranted to examine the relationships of tea and coffee with prostate cancer risk.

DOI: 10.1186/1477-7819-12-38

Study: strong evidence

Systematisches Review mit Meta-Analyse

a prospective cohort design. We observed a modest but statistically significant inverse association between tea consumption and risk of stroke. An increase of three cups/d in tea consumption was associated with a 13% decreased risk of stroke (RR 0.87; 95% CI, 0.81–0.94).

DOI: 10.1631/jzus.B1201001

Study: strong evidence

Fall-Kontroll-Studie: This study comprised 1,706 histologically diagnosed cases of digestive tract cancers (185 esophagus, 893 stomach, 362 colon, 266 rectum) and a total of 21,128 non-cancer outpatients aged 40 years and over. 

The OR of esophageal cancer decreased statistically significantly among occasional black-tea drinkers (OR = 0.44, CI = 0.26-0.74). Daily intake of black tea moderately increased the risk of colon cancer (OR = 1.59, CI = 1.06-2.37).

Regarding confounding factors, a decreased risk was observed for cancer at all sites with regular physical exercise and fruit intake.

. The OR of stomach cancer decreased statistically significantly among those who drank seven or more cups of green tea per day (OR = 0.69, CI = 0.48-1.00). Slightly decreased risk was observed for esophageal and stomach cancer, and OR of rectal cancer decreased only among those who drank three or more cups of coffee daily (OR = 0.46, CI = 0.26-0.81

comparative case-referent

DOI: 10.1023/A:1008890529261

Study: moderate evidence

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One study compared green and black teas (Jochmann et al. 2007), finding that they were equally effective at improving vasodilatation in a small sample of women, while three studies reported no impact of tea consumption on a variety of CHD markers.

These examined a variety of CHD risk factors, including lipid levels, lipid oxidation, inflammatory markers, blood pressure, platelet aggregation, vasodilatation and plasma homocysteine. Three studies reported improvements in blood vessel dilatation, but one also found an unexpected rise in systolic blood pressure.

Tea contains around 17 mg of caffeine per 100 ml (40 mg per 235 ml cup), with a wide range of 1–90 mg per 100 ml, reflecting different brewing times (FSA 2004). Some authors cite high caffeine consumption as a risk factor for a number of conditions (e.g. hypertension, dehydration, anxiety or insomnia). Caffeine intakes of 38–400 mg per day, equating to 1 to 8 cups of tea, appeared to deliver benefits, such as alertness and mood elevation, without adversely affecting sleep quality or hydration.

It has been suggested that phenolic compounds in black tea could inhibit the bioavailability of non-haem iron, and thus adversely affect the iron status of at-risk groups, such as children, vegetarians, pregnant women and the elderly. Two reviews (Temme & Van Hodonck 2002; Nelson & Poulter 2004) examined the published data and concluded that there was insufficient evidence to identify tea consumption as a predictor of iron status. Mennen et al. (2007) assessed whether consumption of green or black tea influenced the baseline iron status of 2500 healthy French adults recruited to take part in the randomised, controlled Supplementation en Vitamines et Mineraux Antioxydants (SU. VI. MAX) trial. It was found that serum ferritin, an objective measure of iron status, was not associated with tea consumption, tea strength or brewing time.

DOI: 10.1111/j.1467-3010.2008.00691.x

Study: weak evidence

Cross-over-Design

To elucidate whether these findings are also applicable to humans, flow-mediated dilation (FMD) and nitro-mediated dilation (NMD) were assessed by ultrasound in twenty-one healthy women before and 2 h after consumption of green and black tea (2 h of FMD and NMD), in comparison with water (control).

In human subjects, ingestion of green and black tea led to significant increases in FMD: from 5·4 (sd 2·3) to 10·2 (sd 3) % (baseline-adjusted difference (BAD) for 2 h of FMD, green tea v. water: 5·0 (95 % CI 3·0, 7·0) %; P < 0·001) and from 5 (sd 2·6) to 9·1 (sd 3·6) % (BAD for 2 h of FMD, black tea v. water: 4·4 (95 % CI 2·3, 6·5) %; P < 0·001), respectively. The increase in FMD was not significantly different between the two tea preparations (BAD for 2 h of FMD, green tea v. black tea: 0·66 (95 % CI − 0·76, 2·09) %; P = 0·36). NMD did not vary between any of the groups. In conclusion, green and black tea are equally effective in improving endothelial function.

DOI: 10.1017/S0007114507838992

Study: moderate evidence

In allen Altersgruppen leisten „Getreide und Getreideprodukte“ sowie „Wasser und wasserbasierte Getränke“ mit je 15 % bis 20 % den höchsten Beitrag zur durchschnittlichen Bleiaufnahme. Bei Erwachsenen trägt zusätzlich die Gruppe „Kaffee, Kakao, Tee“ mit etwa 16 % stark zu der durchschnittlichen Exposition bei. Hier ist vor allem der Kaffee als stark beitragendes Lebensmittel zu nennen.

Toxische Wirkungen von Blei betreffen u. a. die Nieren, das Herz-Kreislauf-System sowie das zentrale Nervensystem.

DOI: 10.17590/20251119-141201-0

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 is concluded that most contaminants leaching into tea infusion are not detected or are detected at a level lower than the regulatory limits. However, the traditional practice of over-boiling tea leaves should be discouraged as there may be a chance for more transfer of contaminants from the tea to the brew.

The target was to examine the transfer rate of contaminants – pesticides, environmental pollutants, mycotoxins, microorganisms, toxic heavy metals, radioactive isotopes (radionuclides) and plant growth regulators 

Mercury (Hg), lead (Pb), arsenic (As), cadmium (Cd), considered as contaminants by the European Commission (2006), and other toxic elements (such as aluminium (Al), chromium (Cr), and nickel (Ni) have been found in tea leaves

DOI: 10.1080/19440049.2014.958575

Study: weak evidence

Analytische Laborstudie (Messung von Inhaltsstoffen)

In order to determine the fluoride content of various products of black tea and to make safety evaluations, 37 commodities in the different manufactured forms of tea were assessed by means of ion-selective electrode and spectrophotometric methods. The results showed wide differences in fluoride levels. The fluoride content was found 0.95–1.41 mg/L in black tea sticks, 0.70–2.44 mg/L in black tea granules and 1.15–6.01 mg/L in black tea bags. Of the products tested, the fluoride content was greatest in black tea bags, presumably because black tea bags are made of low cost, and older tea leaves. According to WHO’s recommendation, fluoride exposure threshold for children is 2 mg of daily intake and for adult, 4 mg. Assuming that children’s daily intake of black tea was 800 ml and adult’s 1500 ml, 56% of black tea bags should be considered unsafe for children and 44% should be considered unsafe for adults. At average rates of consumption, 24% of the black tea bags could result in an increasing risk of osteofluorosis and fractures (WHO). In the areas with drinking water fluoridation or other fluoride supplements, excessive fluoride exposure can become even more severe with even lower intake tea made from black tea bags. 

DOI: 10.1016/j.fct.2006.01.010

Study: weak evidence

Analytische Laborstudie (Messung von Inhaltsstoffen)

Although all THQ values for fluoride remained well below the safety threshold (THQ < 1), the highest values were observed in elderly individuals with low body weight, particularly women consuming green tea, suggesting increased vulnerability in this subgroup. Conclusions: Among the analyzed samples, black teas demonstrated the most distinct chemical profile, characterized by the lowest pH and the highest acidity, buffer capacity, and fluoride and phosphorus content—especially in teas originating from Africa and Central Asia. While fluoride exposure from leaf tea infusions does not appear to pose a direct health risk, older adults, particularly low-weight women, may be more susceptible to potential non-carcinogenic effects and should moderate their intake of high-fluoride teas.

DOI: 10.3390/foods14172944

Study: weak evidence

klinische Interventionsstudie mit einem prospektiven Vorher-Nachher-Design ohne Kontrollgruppe.

The aim of the study was to evaluate the use of wet dressings with black tea for treating facial dermatitis. We performed a prospective, open, uncontrolled before-after study enrolling 22 patients with atopic or contact facial dermatitis who were treated with black tea dressings and an emollient cream over 6 days. 

A dramatic and highly significant reduction of all four disease activity scores occurred within the first 3 days of treatment and the patients continued to improve between days 3 and 6. No side effects were observed.

Conclusion: Black tea dressings represent an effective treatment option for facial dermatitis. Its advantages include lack of side effects, low cost, and easy availability.

DOI: 10.1080/09546634.2019.1573306

Study: moderate evidence

To compare with other studies, this article used the functional unit of “a cup of tea” (assuming a cup of tea requires 2 g of dry tea and 250 ml of boiling water).

Fig. 1. Photographs of all five tea products considered in this study. Note: The first row are the tea brand names, and the tea categories are shown in the brackets according to the classification theory by Zheng et al. (2015). 

Fig. 6. Comparison of carbon footprint results of a cup tea with previous literature black tea of this study: wuyangkungfu

However, from a whole life cycle perspective tea consumption was a major hotspot. The most sensitive contributing factors, including the amount of boiling water and carbon emission factors of electricity, had great impacts on the accuracy of the final results.

The greatest reduction potential is observed during consumption when using alternative cleaner energy sources, such as liquefied petroleum gas, coal gas, and natural gas. However, it is the most difficult to realize as it involves changing consumer behaviors.

DOI: 10.1016/j.jclepro.2019.06.136

The water footprint of coffee (130 l cup−1 , based on use of 7 gram of roasted coffee per cup) is much larger than the water footprint of tea (27 l cup−1 , based on use of 3 gram of black tea per cup)

Green and black tea: green: 7232, blue: 898, grey:726, total: 8856

DOI: 10.5194/hess-15-1577-2011

The seven flowers of autumn that are often spoken of as traditional signs of the fall in Japan (Miscanthus sinensis, Lespedeza bicolor var. japonica, Pueraria lobata, Eupatorium japonicum, Patrinia scabiosifolia, Dianthus superbus var. longicalycinus) used to be familiar plants seen in the rural satoyama of Japan. However, four of them have been listed as endangered or threatened species as the degradation of Japan's semi-natural grasslands has become significant. However, Chagusaba could be considered places where we can observe these important plants closely and in abundance. It has become clear that tea production plays a role in conserving the region's biodiversity. The agricultural processes of tea production protect grasslands, and tea cultivation benefits from those grasslands. In this way, the traditional tea-grass integrated system can be said to balance tea production with biodiversity, and utilize satoyama resources for high-value-added farming. It

he dried grass is laid in the furrows of the tea plantation

Currently, semi-natural grasslands, especially Miscanthus sinensis (Japanese pampas grass), are rapidly disappearing from Japan. In the past, grasslands in rural areas existed across vast areas and provided resources such as livestock feed, fertilizer to maintain soil fertility in fields, and materials for thatched roofs. In 1880, these grasslands covered over 30% of the country and in 1950 (Ogura 2006), they covered over 10% of it, but now they cover only 3%.

DOI: 10.5814/j.issn.1674-764X.2016.03.001