## 3 Results and Discussion  
### Acidity of the Analyzed juices and Tea Infusions  
The juices' acidity was measured using potentiometric titration at the initial research stage. Lemon juice had the highest tittatable acidity of 626.56 mmol \(\pm\) 14.85 H\({}^{+}\)/L. The tittatable acidity of Japanese quince juice was 583.47 \(\pm\) 12.49 mmol H\({}^{+}\)/L, while that of quince juice was only 41.43 \(\pm\) 2.57 mmol H\({}^{+}\)/L. When converting the titration acidity result into citric acid, the results were 40.1, 37.3, and 2.7 g/L for lemon juice, Japanese quince, and quince, respectively. Based on the result of determining the tittatable acidity of juices and converting the obtained value into the citric acid content, this acid's corresponding solutions were prepared. These solutions were used to prepare tea infusions to determine the aluminum content and compare its influence on this parameter with the impact of adding selected juices. When preparing infusions with the addition of citric acid solution, the steps described for preparing infusions with juices were followed. Higher tittatable acidity is usually associated with a lower pH, which was also observed in the results. Therefore, lemon juice had the lowest pH (pH = 2.35 \(\pm\) 0.03). Japanese quince juice had a pH value of 2.65 \(\pm\) 0.02, and quince juice had a pH of 3.8 \(\pm\) 0.02. According to the recommendations of the Association of the Industry of Juices and Nectars of the European Union, the tittatable acidity of lemon juice converted into citric acid should range from 44.8 to 62.0 g/L [33]. The result obtained is lower than these recommendations but consistent with the results of research published by [PERSON] et al., in which they tested 92 samples of fresh and 92 samples of reconstituted lemon juices, obtaining a range of tittatable acidity from 35.1 to 78.1 g/L [34]. The tested tittatable acidity of Japanese quince juice was within the range determined in published studies, which is 26-56 g/L [35]. Quince juice had the lowest tittatable acidity, which is confirmed in the literature. However, the obtained tittatable acidity value converted to citric acid was lower than the literature data, with it ranging from 3.8 to 7.6 g/L [36; 37; 38]. This may be because, in published works, fresh quince juice is most often tested, and the juice used in our research is commercially available. It was decided to use commercially available Japanese quince and quince juices due to the poor availability of fresh fruit. At the same time, it allowed us to reproduce the most likely scenario from the consumer's point of view. In the next stage, tests were conducted to determine the volume of juices added to the tea infusion. It was decided to use juice with the highest tittatable acidity added to the most frequently consumed black leaf tea. Figure 1 shows how the aluminum content changes depending on the amount of lemon juice added to black tea during brewing. The presented data indicate that 2 mL of added lemon juice results in an almost complete increase in the aluminum content in the infusion. Larger volumes of lemon juice added during brewing do not cause a statistically significant increase in the content of the analyzed element in the infusion. Therefore, it was determined that the volume of added juices used during further studies would be 2 mL. Figure 1: Mean and SD pH value and aluminum concentration (mg/L) in black leaf tea infusions depending on the volume of lemon juice added during brewing. Different letters (a, b, c, etc.) show a significant difference with \(p<0.05\). Table 3 presents the tea infusions' active acidity (pH) measurements with additives and infusions constituting control samples. Based on the measurements of infusions without additives, the teas can be ranked with increasing pH as follows: black granulated < black leaf < green < white < pu-erh. The pH values of these teas ranged from 5.01 to 5.65 and were consistent with the literature data [39, 40, 41].