# Tea Infusions as a Source of Phenolic Compounds in the Human Diet
## 3 Results and Discussion

Table 1: The content of biologically active compounds in tea infusions extracted with water for 6, 10, 20, and 30 min. phenolics with the indication of their types that are most important in the case of tea leaves and which changes occurring during their processing determine the properties of tea infusions to the greatest extent. Such a high phenolic compound content in red tea leaves may be due to the considerable changes that occur in individual fractions of these compounds under the influence of the fermentation process conducted for this kind of tea. For most tea types, this process is carried out by polyphenol oxidase, which occurs naturally in tea plant leaves. However, during the production of red (pu-erh) tea, this enzyme is inactivated, and the fermentation process is carried out by a variety of microorganisms (e.g., _Apergillus niger_, _Penicillium_, _Rhizopus_, _Aspergillus glaucus_, _Sacharomyces_, and _Bacterium_). Depending on the fermentation conditions (how the leaves are arranged, temperature, time, moisture content, etc. ), the proportion of individual strains varies, which gives pu-erh tea specific sensory characteristics but also results in considerable changes in its chemical composition [48]. During such a fermentation process, catechins and their gallate derivatives are oxidized to complex phenolic tea pigments, including theaflavins, thearubigins, and theabrownins (Figure 2). Theaflavins undergo further oxidation to form more polymerized thearubigins, which are then condensed to theabrownins [44,49]. These changes result, e.g., in changes in the bonds formed by phenolic compounds with other components found in tea leaves, which leads to changes in their solubility and can result in greater quantities of these compounds being transferred to red tea water infusions. Pu-erh tea infusions were also found to be the best source of calcium and sodium, the contents of which, compared to the other tea types, were up to five times higher and, depending on the duration of infusion, ranged from 250.3 to 282.2 ug/100 mL (for calcium) and from 260.12 to 262.13 ug/100 mL (for sodium). The particularly high content of these elements in red tea infusions was confirmed by the author's previous study [5] and research by [PERSON] et al. [16], who reported that red tea infusions contain calcium levels three times higher than those found in green or black tea. A study by the same team also demonstrated a particularly high sodium level in red tea leaves, with levels 20 times higher than in other tea types, which authors attributed to the high salinity of the soil in which the tea plants were cultivated. In red tea infusions, oxalates were also found in rather large quantities, with the highest level determined in infusions prepared for 20 and 30 min (29.05 and 28.71 mg/100 mL). These values were not significantly different from those detected in green tea; infusions prepared for 30 min contained oxalates at the level of 30.88 mg/100 mL. A high oxalate content in red tea was also indicated by [PERSON] et al. [15] and [PERSON] et al. [16], who reported that its infusions contained these compounds at a level of 28 mg/100 mL. Figure 1: The total content of phenolic compounds in tea infusions extracted with water for 6, 10, 20, and 30 min. a–c—values denoted by the same letters are not statistically different in the analyzed type of tea at \(p<0.05\). The differences in oxalate content observed between the analyzed infusions were not as noticeable as those for phenolic compounds. In black tea infusions with the lowest oxalate content, they were present at concentrations of 17.19 to 19.93 mg/100 mL, which is in line with the results obtained by other authors [13; 33; 36]. What is puzzling is that the lowest oxalate content was found in black tea infusions, as compared to the other types of tea.