Streszczenie

Wprowadzenie: Herbata zielona, czarna i pu-erh mają, w porównaniu do innych surowców roślinnych, silne wła­ściwości przeciwwolnorodnikowe. W pracy zbadano15 różnych herbat należących do trzech typów, zielonych, czarnych i pu-erh (po pięć rodzajów z każdego typu) w kierunku ich właściwości przeciwwolnorodnikowych. Aktywność przeciwwolnorodnikową wyciągów i surowców zmierzono za pomocą rodników DPPH i ABTS^•+.
Materiały/Metody: Aktywność przeciwwolnorodnikową herbat zmierzono za pomocą DPPH (rodnika 1,1-difenylo­-2-pikrylohydrazylowego) i ABTS^•+ (soli diamonowej kwasu 2,2′-azyno-bis(3-ethylobenzotiazo­lino-6-sulfonowego)). Zdefiniowano jednostki TAU₅₁₅ i TAU₇₃₄ (jednostki aktywności przeciw­wolnorodnikowej) odpowiednio dla testów z DPPH i ABTS^•+ i określono liczbę jednostek na mg wyciągów (TAU_515/mg i TAU_734/mg) i g surowców (TAU_515/g i TAU_734/g).
Wyniki/Dyskusja: Gdy badano wyciągi, największą liczbę jednostek aktywności przeciwwolnorodnikowej TAU_515/mg i TAU_734/mg stwierdzono w mg wyciągu octanowego otrzymanego z liści herbaty zielonej: odpo­wiednio 57,7±0,8; 106±2,0 jednostek. Gdy liczbę jednostek aktywności przeciwwolnorodnikowej (TAU_515/g) policzono na g surowca, największy potencjał wolnorodnikowy (7601±92) zaobserwowano dla liści herbaty zielonej, naj­większą wartość TAU_734/g (14303±354) obliczono również dla liści herbaty zielonej. Najniższą aktywność przeciwwolnorodnikową TAU_515/g (684±30) i TAU_734/g (1870±180) obliczono dla liści herbaty pu-erh.
Zaobserwowano wysoką, dodatnią korelację pomiędzy aktywnością przeciwwolnorodnikową su­rowców i zawartością związków fenolowych w tych surowcach.

Słowa kluczowe:liście herbaty • związki fenolowe • właściwości przeciwwolnorodnikowe

Summary

Introduction: Green, black and pu-erh teas are known to have strong antiradical properties in comparison to other plant raw materials.
In this study fifteen different teas belonging to three types, green, black, and pu-erh tea (five of each kind), were investigated for their antiradical properties. Antiradical activity of extracts and raw materials was measured using DPPH and ABTS^•+ radicals.
Material/Methods: The antiradical potential of teas was measured using DPPH (1,1-diphenyl-2-picrylhydrazyl) and ABTS^•+ (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt). The TAU₅₁₅ and TAU₇₃₄ (antiradical activity units) were defined with the tests of DPPH and ABTS^•+, respec­tively, and the number of units was calculated per mg of extracts (TAU_515/mg and TAU_734/mg) and g of raw materials (TAU_515/g and TAU_734/g).
Results/Discussion: When the extracts were investigated, the highest number of antiradical units (TAU_515/mg; TAU_734/mg) was found per mg of ethyl acetate extract obtained from green tea leaves assayed with DPPH and ABTS^•+: 57.7±0.8; 106±2.0, units respectively. When the number of antiradical units TAU_515/g was calculated per g of raw material, the highest antiradical potential (7601±92) was observed for green tea leaves. The greatest TAU_734/g value (14 303±354) was also calculated for green tea leaves. The lowest value of antiradical activity units TAU_515/g (684±30) and TAU_734/g (1870±180) was calculated for pu-erh tea leaves.
A high positive correlation was found between the antiradical activity of the raw materials and the content of phenolic compounds in these raw materials.

Key words:tea leaves • phenolic compounds • antiradical properties

Abbreviations:

TAU_515/mg and TAU_734/mg – the number of total antiradical units calculated per mg of extract using DPPH and ABTS^•+ respectively; TAU_515/g and TAU_734/g – the number of total antiradical units calculated per g of raw material using DPPH and ABTS^•+ respectively.

Introduction

The leaves of tea (Camellia sinensis (L.) Kuntze) are known to have strong antiradical and antioxidant proper­ties [26]. Brewed tea has been consumed for many centu­ries and health benefits have been attributed to tea since the very beginning of tea history. Nowadays tea of dif­ferent kinds (especially green and black) is the beverage most widely consumed, next to water, in the whole world [18]. Some research made in vitro and using animals pro­vides evidence that the polyphenolic compounds present in teas act beneficially in several chronic diseases [4,7,11]. For example, epigallocatechin gallate (EGCG) is attribu­ted to be responsible for the ability of green tea to promo­te health [19]. Both green tea and black tea are considered to have anti-carcinogenic properties, which has been pro­ved in many animal models [15,25].

Black tea is the most consumed tea in the world. More than 70% of all tea produced is black tea [6]. Many stu­dies have demonstrated the health benefits of black tea consumption [14].

It was demonstrated that risk of myocardial infarction was lower by 11% when consumption of the beverage was three cups a day (about 700 ml) [12]. Other authors reported a considerable decrease of coronary heart disease (CHD) when more than a cup of tea is drunk daily [17]. Some au­thors did not observe any beneficial effect [9], while others observed a positive correlation between tea consumption and a lower risk of CHD mortality [10] as well as hyper­tension, plaques of the carotid artery, and increased homo­cysteine concentration in plasma [14].

Brewed tea is rich in various types of polyphenolic compo­unds [6]. Ruxton [15] investigated the content of phenols in brewed black and green tea. Black tea in decreasing or­der is rich in epigallocatechin (10.43 mg/100 ml), epica­techin (2.33), quercetin (2.07), theaflavins (1.58), catechin (1.52), kaempferol (1.34), gallocatechin (1.26) and myri­cetin (0.45). Green tea is rich in epigallocatechin (17.08 mg/100 ml), epicatechin (8.47), catechin (2.73), querce­tin (2.69), kaempferol (1.42), myricetin (1.10) and theafla­vins (0.05) [14].

Tea polyphenols possess strong antiradical and antioxidant properties, and health benefits of consumption of tea are attributed partially to the antiradical and antioxidant pro­perties of tea [6]. Tea extracts have been shown to scaven­ge singlet oxygen [8], hydroxyl radical [23], peroxyl radi­cal [16], superoxide radical [24] and many other dangerous compounds which lead, among other things, to the destruc­tion of lipids, proteins and nucleic acids [21].

The antioxidant activity of green tea extracts is always hi­gher than that of black tea [1]. Especially effective as an antioxidant is epigallocatechin gallate (EGCG), which was detected in high concentrations in green tea extracts [5]. Green and black tea, in vitro, can inhibit lipoprotein oxi­dation initiated in the presence of Cu²⁺.

The antioxidant and antiradical activity of teas has been me­asured by several methods. The methods can be divided into so-called hydrogen atom transfer (HAT) and electron trans­fer methods (ET). The methods called FRAP (ferric redu­cing ability of plasma, also ferric ion reducing antioxidant power) belong to ET methods and use Trolox (water soluble vitamin E analogue) as a standard. The methods using DPPA and ABTS^•+ radicals belong to the HAT methods [3]. Another problem is the way of demonstration of antiradical features of substances. Some authors use standard substances known for their antiradical activity such as vitamin C or Trolox [14].

However, there is a lack of papers demonstrating antira­dical activity of raw materials, extracts or substances as a general pool of hypothetical, defined units.

In this work the antiradical activity of different types of teas was measured and presented as the number of antira­dical units TAU₅₁₅ and TAU₇₃₄ for the tests with DPPH and ABTS^•+ radicals per mg of extracts and g of raw materials.

Material and Methods

Raw material

Green teas

The leaf of green tea from China bought from PPH Biofluid sp.j. was marked with G₁, the green classic English tea leaf from India was marked with G₂, natural green tea Dilmah Sri Lanka (Sri Lanka) with G₃, the leaf of green tea Sir Roger (China) with G₄, and the green tea Teekanne (China) with G₅.

Black teas

The leaf of classic English tea, Tetley from India was mar­ked with B₁, Premium tea Ceylon, Dilmah (Sri Lanka) was marked with B₂, Yellow Label tea, Lipton, a mixture of about 20 kinds of black teas, with B₃, Yunnan Black Tea, Oskar International Trading sp. z o.o., from China, with B₄, and Assam, Teekanne from India with B₅.

Pu-erh teas

Pu-erh Bio-Active sp. z o.o. (China) – P₁, pu-erh PPH Biofluid sp.j. (China) – P₂, pu-erh Vitax Preminum Foods SA (China) – P₃, pu-erh Bastek Coffee & Tea (China) – P₄, pu-erh Herbapol Lublin SA (China) – P₅.

Preparation of extracts

50 g of raw material was mixed with 900 ml of metha­nol and water (1:3). The raw material was heated at 70°C for 30 min, then was extracted at room temperature for 24 hours. The extract was filtered (filter discs, grade 388, Filtrak). Methanol was evaporated under reduced pressu­re and the remaining water was extracted three times with ethyl acetate (3×200 ml). The ethyl acetate extract and re­maining water was evaporated to dryness under reduced pressure to obtain the dry residues E (ethyl acetate) and A remaining water.

The obtained extracts were additionally marked with the letter of the raw material. For example, ethyl acetate extract obtained from natural green tea Dilmah Sri Lanka (Sri Lanka) G₃ was marked as G₃E.

Colorimetric measurement of phenolic compounds

The amount of phenolic compound was measured with the method of Singleton and Rossi [20].

Reagents

Reagent 1

5.77 g of sodium tungstate (Na₂WO₄ × 2 H₂O) was dis­solved in 75 ml of water. Then 8 ml of 85% solution of H₃PO₄ was added. Then the reagent was heated at 100°C for 3 hours. After cooling the volume was adjusted to 100 ml with the water.

Reagent 2

An 18% (w/o) aqueous solution of Na₂CO₃ was prepared.

Measurement of phenolic compounds

0.5 ml of reagent 1 was added to 0.5 ml of the sample (extract in 50% methanol solution at the concentration 0.17 mg/ml). Then 8.5 ml of 2 was added. The absorbance at 750 nm was measured 2 min after 2 reagent addition. The absorbance was measured in a 1-cm glass cuvette against a blind sample (0.5 ml of 50% methanol was added instead of a sample with the extract). The measurement was repe­ated three times and the standard deviation was calculated.

Measurement of antiradical activity of extracts with the DPPH radical

The antiradical or antioxidant activity could be demonstra­ted in many different ways. Among others it is compared to the known antiradical standard substances such as vita­min C or Trolox. However, no authors try to demonstrate antiradical activity as a general pool of units in a determi­ned mass of raw material and/or extracts.

For that reason we define the antiradical units in a similar way as it is done for the demonstration of enzyme activi­ty [22]. We calculated these hypothetical units per mg of extracts and g of raw materials. We calculated the number of antiradical units per g of raw material as the sum of units calculated for extracts obtained from that raw material. The antiradical activity of extracts presented as the number of units per mg is compared to that of Trolox.

The obtained tea extracts contained compounds soluble in methanol as well as in water. Therefore two different tests were chosen to study the antiradical activity of these extracts: the method using the DPPH radical, which is suitable for fractions soluble in methanol, and the method with the ABTS•+ radical, more suitable for fractions soluble in water.

The method with DPPH was described by Brand-Williams et al. [2]. Antiradical properties were presented as the num­ber of antiradical units TAU₅₁₅ per mg of extract (TAU_515/mg) and per gram of raw material (TAU_515/g).

One unit is defined as the amount of substance in 1 ml of reaction mixture which causes a decrease in absorban­ce of 1 after 1 minute at 515 nm in a glass cuvette with a 1-cm optical path.

Preparation of reagent

DPPH radical (Sigma-Aldrich) was dissolved in methanol (Merck) at the concentration of 0.094 mmol/l. The reagent was stored at 4°C for 24 h before use. Before the test the absorbance of DPPH solution was adjusted to the value equal to 1 with methanol addition.

Measurement of DPPH radical scavenging

To 1.46 ml of DPPH methanol solution, 0.04 ml of extract solution in methanol was added. Absorbance at 515 nm was measured in glass cuvettes with an optical path of 1 cm at 1 minute after extract addition to the sample. All measu­rements were repeated three times and standard deviation was calculated. The control sample was prepared by the ad­dition of 0.04 ml of methanol to 1.46 ml of DPPH radical.

The number of antiradical units TAU_515/mg was calculated per mg of extract according to the equation:

where TAU_515/mg = number of antiradical units calculated per mg of extract; A₀ = absorbance of sample at the be­ginning of the reaction; A₁ = absorbance of sample after 1 minute of reaction; m = weight of extract [mg] in 1 ml of measured sample (in cuvette).

The number of antiradical units TAU_515/g was calculated per 1 g of raw material according to the equation:

where TAU_515/g is the number of antiradical units calculated per g of raw material; TAU_515A/mg is the number of antiradi­cal units calculated per 1 mg of aqueous extract; ma is the whole mass of aqueous extract [mg]; TAU_515E/mg is the num­ber of antiradical units calculated per mg of ethyl acetate extract; me is the whole mass of ethyl acetate extract [mg]; and 50 is the weight of raw material taken for extraction [g].

The maximal error (ΔTAU_515/mg and ΔTAU_515/g) was calcu­lated according to the total differential method.

Measurement of antiradical activity of extracts with the ABTS^•+ radical

Antiradical activity of extracts was measured with the me­thod of Re et al. [13].

Reagents

ABTS solution at 7 mmol/l in water and aqueous solution of K₂S₂O₈ at 2.45 mmol/l were prepared. The two solutions were mixed in the volume ratio 1:1 and stored for 16 ho­urs in a dark place at room temperature. During that time ABTS^•+ radical was formed. The final solution was diluted with water to the value of absorbance at 734 nm equal to 1.

Measurement of ABTS^•+ scavenging

0.015 ml of aqueous solution of extract was added to 1.5 ml of aqueous solution of ABTS^•+. The absorbance at 734 nm was measured at the beginning and after 1 minute of reaction. The measurement was performed three times and standard deviation was calculated.

The number of antiradical units TAU₇₃₄ per mg of extract (TAU_734/mg) and g of raw material (TAU_734/g) was calculated in the same way as shown above for the test with the DPPH radical. The maximal error (ΔTAU_734/mg and ΔTAU_734/g) was calculated on the basis of the total differential method.

Results and Discussion

The weight of extracts [mg], number of antiradical units per mg of extracts (TAU_515/mg), distribution of the entire number of antiradical units between ethyl acetate and aqu­eous extracts [%], amount of phenolic compounds [%] cal­culated per dry weight of raw material, and number of an­tiradical units per g of raw material (TAU_515/g) are shown in Table 1. The diagram demonstrating the number of an­tiradical units per g of raw material (TAU_515/g) is shown in Figure 1.

Table 1. Weight of extracts, number of antiradical units (TAU_515/mg) per mg of extract, number of antiradical units (TAU5_15/g) per g of raw material, amount of phenolic compounds expressed in percentage per dry weight of raw material, the antiradical unit distribution between ethyl acetate and aqueous extracts. Value of TAU_515/mg for Trolox was 51.5±0.9

Figure 1. The number of antiradical units TAU_515/g calculated per g of raw material. G1-G5 – green tea, B1-B5 – black tea, P1-P5 – pu-erh

The highest number of antiradical activity units per mg of extract (TAU_515/mg) was noted for green tea extract G₄E (57.7±0.8), the lowest for pu-erh tea extract P₄A (3.0±0.1). The number of units (TAU_515/mg) per mg of Trolox was equ­al to 51.5±0.9. The highest number of antiradical units cal­culated per g of raw material (TAU_515/g) was calculated for green tea classic English leaf from India G₂ (7601±92), the lowest for pu-erh Bastek Coffee & Tea (China) tea P₄ (684±30).

The green teas exhibited the strongest antiradical proper­ties, black teas lower and pu-erh the lowest.

The correlation coefficient „r” between the number of TAU_515/g units and phenolic concentration expressed as a percentage is 0.89 (Figure 2).

Figure 2. The correlation between number of antiradical units TAU_515/g and amounts of phenolic compounds C [%]

The number of antiradical units calculated per mg of extract (TAU_734/mg), per g of raw material (TAU_734/g), and the distri­bution of units between ethyl acetate and aqueous extracts, for the test with ABTS^•+ radical, are shown in Table 2. A diagram showing the number of antiradical units per g of raw material (TAU_734/g) is presented in Figure 3.

Table 2. The number of antiradical units per mg of extract (TAU734_/mg),nu_{mber o}f antiradical units per g of dry raw material (TAU734_/g), ant_iradical units distribution in percentage between ethyl acetate and aqueous extracts. Value of TAU734_{/mg fo}r _Trolox was 81.2±1.5

Figure 3. The number of antiradical units TAU_734/g per g of raw material. G1-G5 – green tea, B1-B5 – black tea, P1-P5 – pu-erh

The highest number of antiradical units (TAU_734/mg) was cal­culated for extract G₄E (106±2) and the lowest for extract P₃A (11.3±1.3). The value TAU_734/mg for Trolox is 81.2±1.5. The TAU_734/g value was highest for G₄ (14 303±354) raw material, and the lowest number of TAU_734/g units was cal­culated for P₃ (1870±180). The correlation coefficient „r” between the number of TAU_734/g units and the amount of phenolic compounds in the raw material is 0.93 (Figure 4).

Figure 4. The correlation between number of antiradical units TAU_734/g and amounts of phenolic compounds C [%]

The largest amount of phenolic compounds expressed in percentage per mass of raw material was found in G₂ (5.84±0.36) and the lowest in P₃ (1.26±0.12).

The distribution of antiradical units between ethyl acetate and aqueous extract for each of the raw materials is shown in Table 1 for tests with the DPPH radical and Table 2 for tests with the ABTS^+• radical. For green tea leaves, 60-78% of TAU_515/mg and 58-74% of TAU_734/mg units were calculated for ethyl acetate extracts. For black tea, the respective va­lues were 32-53% of TAU_515/mg and 26-37% of TAU_734/mg units. For pu-erh tea, the amount of TAU_515/mg units in ethyl acetate extracts was 30-44%, and the amount of TAU_734/mg units was 20-26%.

Our results show that the strongest antiradical properties are exhibited by green tea leaves and, in decreasing order, black tea and pu-erh tea. Similar results were described in the literature [1]. The ethyl acetate extracts of green tea leaves showed stronger antiradical activity than extracts obtained from leaves of black or pu-erh teas. The antira­dical activity correlated well with the amount of pheno­lic compounds determined with the colorimetric method.

Analysis of the distribution among extracts of antiradical activity units TAU_515/mg and TAU_734/mg led to the following observations. For green tea leaf extracts a greater number of antiradical units was present in ethyl acetate extracts (Tables 1 and 2). When black and pu-erh teas were inve­stigated, a greater number of units was found in the aqu­eous extracts (Tables 1 and 2).

An explanation for this observation is that phenolic com­pounds present in green tea leaves such as epicatechin gal­late and epigallocatechin gallate (strong antioxidants) are more soluble in ethyl acetate than water.

During the fermentation process, green tea phenols are oxidized and turn into more polymerized structures, the­arubigins and theaflavins. These macromolecular substan­ces have a better affinity for water than for ethyl acetate.

We can conclude that if we take the green tea leaves for the preparation of extracts with antioxidant activity, ethyl acetate is a better solvent, while in the case of black tea and for pu-erh the better solvent is water.

One can draw the following conclusions from the investigation:
a) The leaves of green tea appeared to have the strongest antioxidant properties but those of pu-erh tea the lowest.
b) For green tea, a higher total number of antiradical units TAU₅₁₅ and TAU₇₃₄ was distributed to ethyl acetate extracts than to aqueous extract, but for black and pu-erh teas, more units were calculated for aqueous extracts.
c) There is a strong positive correlation between the con­tents of phenolic compounds in raw materials and the­ir antiradical activity.

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The authors have no potential conflicts of interest to declare.

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