## 3 Results and Discussion
### Characterization of AW Media
The protein nitrogen content in NDAW and deproteinized whey media is shown in Figure 6.  
The deproteinization of AW had little impact on the content of non-protein nitrogen (NPN). The highest initial NPN content was observed in the NDAW medium (0.047%) and the lowest in the D7.5 medium (0.038%). After culturing _K. xylinus_, a significant difference between the content of non-protein nitrogen was observed only in the medium of NDAW. Before the culture, the content of NPN in this medium was 0.047% and decreased after culturing to 0.044%, which corresponded to the consumption of 40 mg\(\cdot\)dm\({}^{-3}\) and the production of 1.57 g\(\cdot\)dm\({}^{-3}\) BC in this medium. In other media, the consumption of NPN was statistically insignificant, which in turn did not affect the amount of BC obtained. For comparative purposes, in the study by [PERSON] et al. [50] involving BC biosynthesis by _A. xylinum_ DSM 15973, 0.58 g\(\cdot\)dm\({}^{-3}\) of biopolymer was obtained in the medium of deproteinized whey using 19 mg\(\cdot\)dm\({}^{-3}\) free amino nitrogen (FAN) and a total consumption of about 50 g\(\cdot\)dm\({}^{-3}\) sugar. However, in that study, the initial FAN content was 56 mg\(\cdot\)dm\({}^{-3}\)[50]. It is presumed that free amino acids may have had the greatest impact on the production of BC from among the low molecular weight compounds included in whey NPN, such as urea, ammonia, uric acid, free amino acids, creatine, creatinine, and biogenic amines [51]. Aspartic acid, phenylalanine, and serine have recently been recognized as indispensable amino acids for BC biosynthesis [52]. On the other hand, in other studies, methionine is also considered an important amino acid for BC biosynthesis [53]. According to [PERSON] and [PERSON], [54] the addition of arginine and threonine to the medium only slightly increased the yield from BC biosynthesis [54]. It is worth noting that all the amino acids mentioned above were present in AW at concentrations from 20 g\(\cdot\)kg\({}^{-1}\) total protein in the case of methionine to even 100 g\(\cdot\)kg\({}^{-1}\) total protein for aspartic acid [55]. The non-protein nitrogen content in the NDAW and deproteinized whey media is presented in Figure 7.  
Figure 6: The content of protein nitrogen in NDAW and deproteinized AW media during the culture of _K. xylinus_ K2C30. * significant difference in the medium component before and after culture, _p_-value 0.05. * non-significant difference. The total acidity and content of lactic acid in the NDAW and deproteinized AW media are presented in Table 3. For all media, an initial pH value of 5.0 was established. Subsequently, culturing was conducted, after which the change in the active acidity and titratable acidity of the media were assessed. The pH of the deproteinized AW media after culturing ranged from 5.79 \(\pm\) 0.037 to 7.33 \(\pm\) 0.421, compared with the pH of the of NDAW medium, amounting to 6.38 \(\pm\) 0.065 (Table 3). These values indicate that the pH increased due to the conducted culturing of _K. xylimus_ K2G30 and BC biosynthesis. During the cultivation of AAB in order to produce the BC, as a consequence of the conversion of glucose into gluconic acid, the pH of the medium decreased [30]. However, according to the data derived from the literature, the pH of the medium could increase during the production of BC in whey. In the pre-culture [PERSON] et al. [45] study, the pH of whey was about 5.0 and increased to 8.0 after the end of culturing. The authors suggested that this is related to the consumption of the organic acids from the medium by the bacteria producing BC. This may be confirmed by the titratable acidity values of the media under study. The titratable acidity is expressed in \({}^{\circ}\)SH and 1 \({}^{\circ}\)SH corresponds to 22.5 mg of lactic acid. In the conducted studies, the concentration of lactic acid decreased during the culture in all of the analyzed media (Table 3).