## 1 Introduction  
Tea beverages have been known for at least 5000 years [1]. Currently, more than two billion cups of tea are consumed daily worldwide [2]. Tea leaves come from shrubs of the _Camellia_ genus representing the Theaceae family. Leaf processing procedures and preparation for consumption largely influence the chemical composition, which is characteristic of each type of tea [3]. Depending on the method of leaf processing, several types of tea can be distinguished, including black tea (deeply fermented) and white tea (not fermented). White tea undergoes only a mild drying process [4], while black tea undergoes a complex fermentation process that oxidises, e.g., phenols [5]. From a phytochemical point of view, tea infusions are rich in bioactive compounds (mainly polyphenols). The health-promoting properties of tea depend on the type and amount of bioactive molecules in the drink, which, in addition to the industrial process of preparation of a commercial product, is also influenced by the infusion preparation method [6]. Its frequent consumption contributes to the removal of free radicals from the body and regulates the activity of oxidases, thus protecting humans against numerous civilisation diseases [7, 8]. Tea infusions prevent the occurrence of cardiovascular diseases or glucose metabolism disorders and, additionally, tea consumption accelerates metabolism,which prevents obesity; hence, tea can be used as a dietary supplement [9]. Furthermore, it has antibacterial, anti-inflammatory, or anti-cancer properties [10]. Flavan-3-ols (cate-chins) are the main substances present in teas that are associated with these confirmed effects [11; 12; 13; 14]. The basic functions of these compounds include, e.g., antioxidant activity: scavenging reactive oxygen species and the inhibition of the formation of free radicals and lipid peroxidation. Their antioxidant activity is associated with the presence of many hydroxyl groups [10; 15]. Tea contains several catechins, with epicatechin (EC), epigallocatechin (EGC), epicatechin-3-gallate (ECG), and epigallocatechin-3-gallate (EGCG) as the main compounds, playing a considerable role in the health-promoting properties of this beverage [16; 17]. The type and concentration of catechins vary depending on the tea species. Green tea is particularly rich in these compounds [11]. The country of origin, i.e., the habitat conditions (climate and soil) of cultivated tea bushes, also modulates the concentration of these compounds in the final infusion. [PERSON] et al. [11] analysed, among others, the chemical composition of green teas from China, Japan, and Kenya and noted that the concentration of total catechins was in the range of 3.3-14.99 mg/g DM in the teas from China and Kenya, respectively [11]. Black teas that were subjected to the fermentation process contained several times lower concentrations of catechins, and the dominant catechin was ECG [11]. In turn, other authors reported that EGCG was the dominant catechin in black teas [4]. The health-promoting properties of teas result from the presence of these compounds. The most widely studied tea ingredient in the context of cardiovascular diseases is EGCG [1]. Many other health benefits resulting from tea consumption have been linked to the concentration of this compound [10]. It has been shown, among others, that EGCG can inhibit cancer cell proliferation in vitro and in vivo by regulating genes and proteins related to the cell cycle and proliferation. For example, this was proven in a nude mouse xenograft model, in which EGCG significantly inhibited the growth of a stomach tumour [9]. In the body, catechins act as antioxidants because they can donate hydrogen atoms, capture superoxide radicals, and thus inhibit chain reactions of radicals and terminate lipid peroxidation [5; 7]. In addition, catechins have been found to chelate transition metal ions, modulate enzyme activity, and affect gene expression [7; 9].