# Tea Infusions as a Source of Phenolic Compounds in the Human Diet
## 3 Results and Discussion
According to technological assumptions, the leaves used most commonly for the production of black tea are older and harder than those used for the production of green tea, which is expected to result in an increased oxalic acid content [33; 35]. Some authors have confirmed the trend towards the highest oxalate content being found in black teas [13; 16], whereas others claim that green tea contains these compounds at higher levels [19; 33]. The source of the discrepancies in the results published by different authors may be the varying size and structure of tea leaves used as raw material, different conditions of tea plant cultivation, and different time and methods of harvesting leaves, as well as the duration, conditions, and method of storage [13; 14; 23]. There was also little variation in copper levels in the tea infusions under analysis. Depending on the type and method of fermentation of the analyzed tea leaves and the time of their extraction, copper content ranged from 2.30 ug/100 mL (determined in a red tea infusion prepared for 20 min) to 8.42 ug/100 mL (determined in black tea leaves extracted for 6 min). The low variation in copper levels in tea leaves is also indicated by a study by [PERSON] et al. [27], who detected no statistically significant differences in the levels  
Figure 2: Classification and the best sources of phenolic compounds, as well as the most important changes occurring in processed tea leaves. of this element among differently cultivated leaves of the same type of tea and between leaves processed by different methods. The analyzed tea infusions were a rich source of manganese, with its best source being the leaves of green tea, which contained the element at levels ranging from 370.8 to 560.2 ug/100 mL. They contained the largest quantity of manganese as compared to the infusions of the other types of tea for each of the analyzed brewing times; the differences found were, in certain cases, more than two-fold. This trend was also confirmed by a previous study conducted by the author of the current paper [5], which demonstrated that the average content of this element, determined for 16 different green tea types, amounted to 512 ug/100 mL, whereas for black teas (n = 11), its content was at a level of 286 ug/100 mL and, for red teas (n = 2), at a level of 219 ug/100 mL. Having analyzed the effect of the duration of tea infusion preparation on the content of analyzed compounds, it can be stated that as the leaf extraction time increased to 20 min, the total content of phenolic compounds in the green and red tea infusions increased, and the highest pace of this process was noted between 10 and 20 min (Tables 1 and 2). Over the next 10 min of the leaf contact with water, the level of these compounds in green tea infusions decreased by 30% and, for red tea, remained at the same level. The phenolic compound content in black tea decreased steadily with duration of infusion. Many authors reported changes in phenolics content under the influence of different methods of making infusions [1; 12; 24; 26; 46]. [PERSON] et al. [32] linked this to the change in the temperature of water used for phenolic compound extraction and reported that for green tea leaves, the largest increase in these components occurred when the water temperature dropped from 100 \({}^{\circ}\)C to 90 \({}^{\circ}\)C. These authors also observed a trend toward a steady decrease in the polyphenol levels in black tea infusions as the temperature of water used for the extraction decreased. [PERSON] et al. [4] linked the greater transfer of phenolic compounds to freshly prepared tea infusions to the fact that pouring boiling water over the leaves reduces viscosity and surface tension, which promotes their solubility and explains the reduction in their level with increased duration of leaf contact with water by the degradation of these compounds. Another factor that impedes the transfer of polyphenols to black tea infusions may be the structure of these compounds.