## 3 Results and Discussion
### Optimizing Initial Glucose Concentration for Enhanced Selectivity in Erythririrol Production  
In the last experiment, the possibility of reducing the initial glucose concentration was tested to improve the selectivity of erythroid production. At the start of the cultivation, the carbon source in all the processes described above was glucose (25.0 g/L) and the compounds contained in the crystallization liquid of erythroid, which was added to the medium in an amount of 130 mL. This amount of liquid resulted in initial concentrations of erythroid (approximately 20.0-22.0 g/L), as well as mannitol, arabitol, and citric acid at 8.5 g/L, 5.8 g/L, and 5.4 g/L, respectively. In subsequent cultivations, 250 mL of the crystallization liquid was used, generating approximately double the initial concentrations of erythroid, mannitol, arabitol, and CA. Four cultivations containing 250 mL of crystallization liquid were carried out, with initial glucose concentrations ranging from 0.0 to 15.0 g/L (Figure 4). The final concentration of erythroid in these cultures ranged from 156.6 g/L to 174.8 g/L (Figure 4A). The erythroid production yield (0.55 and 0.58 g/g) was highest in the process without glucose or with 5.0 g/L of glucose, while the Q parameter showed similar values of 0.90 \(\pm\) 0.03 g/Lh at glucose concentrations ranging from 5.0 to 15.0 g/L. The conducted cultivations demonstrated the highest production selectivity in cultures with the lowest glucose concentration or no glucose in the medium, at 89% and 90%, respectively (Figure 4B). Thus, it can be concluded that the crystallization liquid of erythroid and a minimal initial glucose concentration of 5.0 g/L can be effectively used for the erythroid production. One of the key objectives of this study was to assess the potential of organic compounds in the post-crystallization erythroid filtrate as alternative carbon sources. To achieve this goal, a detailed analysis was performed to monitor changes in the concentrations of mannitol, arabitol, and citric acid during cultivation. This analysis was carried out both in the absence of glucose and in the presence of 15.0 g/L glucose in the medium. The results of these experiments are presented in Figure 5A,B. However, it should be noted that the erythroid present in the filtrate was not utilized, as seen in Figure 4A, because the K1 UV15 strain, like the Wratislavia K1 strain, does not utilize this polyol [16]. Nevertheless, in the case of many other strains, erythroid is utilized [26], which increases the pool of available carbon sources. In these processes, the yeast utilized the compounds in the crystallization liquid; however, the use of these compounds as a carbon source for yeast growth was strongly dependent on the presence of glucose in the medium, as shown in Figure 5. In the absence of sugar in the medium, the yeast utilized the available citric acid and nearly 50% of mannitol within the first 24 h. The concentration of arabitol also decreased by approximately 3.0 g/L (Figure 5A). In the next 24 h, further reductions in mannitol concentration were observed, down to 5.5 g/L. Subsequently, a slight increase in the amount of mannitol was noted, as well as citric acid and arabitol, whose final concentration was the highest among all the cultivations conducted, reaching 10.5 g/L. In the presence of sugar, however, glucose was the preferred carbon source. Nonetheless, a significant decrease in citric acid concentration, to below 1.0 g/L, was observed during the first 24 h. Mannitol and arabitol remained at unchanged levels during this period (Figure 5B). In this cultivation, during the following hours, the concentrations of mannitol and arabitol stabilized and remained within the range of 6.0-9.6 g/L, while the concentration of citric acid increased to 9.1 g/L by the end of the process. Figure 4: The increase in erythroid concentration at different initial glucose concentrations (**A**).