## 3 Results and Discussion
### Selected Organic Acids in Banana Peel Hydrolysates Before and After Yeast Cultivation
In the case of succinic acid, the concentration in the case of EU, ME and PTE hydrolysates remained at the same level, while in the case of cultivation in the PT hydrolysate, a slight increase in succinic acid from the initial level of 0.05 g/L to 0.14 g/L (PTG sample) and 0.13 g/L (PTS sample) was observed. In the case of formic acid, an increase in concentration was observed in all samples from 0.03-0.04 g/L to 0.1-0.36 g/L. The highest growth of this compound was observed for the cultivation of both yeast strains in ME and EU hydrolysates. In the case of acetic acid, no major trends in the change in concentration of this compound in the liquid waste after cultivation could be observed. The changes in the concentrations of organic acids could result from the metabolic activity of yeast and potential contaminating microflora present during the process. Microbiological analysis of the raw material and assessment of changes in microflora during hydrolysis and cultivation were not included in the research plan. It may be assumed that the cultivation process was predominantly influenced by the selected yeast strains due to the addition of the inoculum. However, the possibility that other microorganisms were active in minor quantities during the process cannot be entirely excluded. They may have originated from the raw material, particularly in the absence of thermal treatment exceeding 100 \({}^{\circ}\)C, or may have been introduced through subsequent contamination. It should be noted that routine microscopic examination of the post-cultivation liquid (using an optical microscope at 480\(\times\) magnification) did not reveal the presence of bacteria or yeast morphologically distinct from the intentionally applied strains. \begin{table}
\begin{tabular}{c c c c c} \hline
**Sample** & **Galacturonic Acid** & **Succinic Acid** & **Formic Acid** & **Acetic Acid** \\ \hline  & & \multicolumn{3}{c}{**Concentration [g/L]**} \\ \hline PT & 0.64 \(\pm\) 0.02 & 0.05 \(\pm\) 0.003 & 0.03 \(\pm\) 0.001 & 0.35 \(\pm\) 0.01 \\ \hline ME & 0.62 \(\pm\) 0.025 & 0.29 \(\pm\) 0.017 & 0.04 \(\pm\) 0.002 & 0.39 \(\pm\) 0.029 \\ \hline PTE & 0.93 \(\pm\) 0.081 & 0.26 \(\pm\) 0.015 & 0.04 \(\pm\) 0.001 & 0.42 \(\pm\) 0.027 \\ \hline UE & 0.73 \(\pm\) 0.051 & 0.27 \(\pm\) 0.01 & 0.04 \(\pm\) 0.004 & 0.43 \(\pm\) 0.035 \\ \hline \end{tabular}
\end{table} 
Table 5: Concentration of selected organic acids before yeast cultivation in banana peel hydrolysate. The succinic and acetic (together with pyruvate) acids are predominantly synthesised by yeast through the tricarboxylic acid cycle, crucial to generating various intermediate carboxylic acids, including succinic acid [68]. Additionally, the glycolytic pathway, which facilitates the conversion of glucose to pyruvate, and the glyoxylate cycle, crucial for growth on two-carbon substrates such as ethanol and acetate, contribute significantly by providing precursors for biosynthesis through an anaperiodic mechanism [69]. Furthermore, the production of acetic acid under anaerobic conditions is associated with glycerol synthesis as a means of maintaining redox balance [70; 71].