## 3 Results and Discussion
### Selected Organic Acids in Banana Peel Hydrolysates Before and After Yeast Cultivation  
Organic acids are important indicators of metabolic activity, the condition of the raw material, and the hydrolysis process, so we determined the concentrations of selected organic acids in the hydrolysates before and after yeast cultivation. Galacturonic acid reached the highest concentration among the acids determined in the hydrolysates (0.93 \(\pm\) 0.081 g/L) (Table 5) for the sample subjected to the pressure-thermal-enzymatic (PTE) treatment sequence. [PERSON] et al. [48] hydrolysed banana peels with a mixture of Celluclast 1.5 L, Novozyme-188 and Pectinase P-2611. After converting their results to the same ratio of dry peel mass to total sample mass, they determined the following in the hydrolysate: 0.9 to 5.1 g/L of galacturonic acid formed from the breakdown of pectin. Slightly lower concentrations of this acid were observed in hydrolysates obtained using microwaves (0.62 \(\pm\) 0.025 g/L) and ultrasound (0.73 \(\pm\) 0.051 g/L). It can be assumed that the decomposition of pectin during the acid and enzymatic hydrolysis contributed to the increase in the concentration of this acid in the analysed liquids. In the case of acetic acid, concentrations about 50% lower than those of galacturonic acid were \begin{table}
\begin{tabular}{c c c c c} \hline  & **Cellobiose** & **Glucose** & **Xylose** & **Arabinose** \\ \hline
**Sample** & \multicolumn{4}{c}{**Concentration [g/L]**} \\ \hline PT + G & 0.08 \(\pm\) 0.006 & 0.02 \(\pm\) 0.001 & 0.36 \(\pm\) 0.026 & 0.24 \(\pm\) 0.012 \\ PT + S & 0.05 \(\pm\) 0.005 & 0.02 \(\pm\) 0.001 & 0.29 \(\pm\) 0.019 & 0.27 \(\pm\) 0.017 \\ ME + G & 0.05 \(\pm\) 0.005 & 0.01 \(\pm\) 0.001 & 0.2 \(\pm\) 0.019 & 0.1 \(\pm\) 0.007 \\ ME + S & 0.07 \(\pm\) 0.006 & n.d. & 0.03 \(\pm\) 0.002 & 0.06 \(\pm\) 0.004 \\ PTE + G & 0.02 \(\pm\) 0.002 & n.d. & 0.03 \(\pm\) 0.001 & 0.1 \(\pm\) 0.005 \\ PTE + S & 0.02 \(\pm\) 0.002 & n.d. & 0.62 \(\pm\) 0.056 & 0.12 \(\pm\) 0.01 \\ UE + G & 0.06 \(\pm\) 0.003 & n.d. & 0.12 \(\pm\) 0.01 & 0.09 \(\pm\) 0.007 \\ UE + S & 0.05 \(\pm\) 0.005 & n.d. & 0.1 \(\pm\) 0.008 & 0.09 \(\pm\) 0.003 \\ \hline \end{tabular} n.d.—not detectable.  
\end{table} 
Table 4: Concentration of selected sugars after yeast cultivation in banana peel hydrolysate. observed. The concentration of acetic acid in the hydrolysates before cultivation ranged from 0.35 \(\pm\) 0.001 g/L to 0.43 \(\pm\) 0.0035 g/L for samples PT and UE, respectively. It can be seen that enzymatic treatment (ME, PTE and UE samples) caused a slight increase in concentration compared to the PT reference sample. Succinic acid was another organic acid detected, with a maximum concentration of 0.29 \(\pm\) 0.017 g/L in the case of ME hydrolysate (Table 5). Slightly lower concentrations of this acid were recorded for PTE and UE samples, while a significantly lower concentration of 0.05 \(\pm\) 0.003 g/L was observed for the sample subjected only to pressure-thermal treatment. The last acid to be determined was formic acid, with a concentration of 0.03 \(\pm\) 0.001 g/L in the reference PT hydrolysate. In the case of enzymatically treated hydrolysates, the concentration of this compound was approximately 25% higher. The concentrations of the same acids initially determined in the banana peel hydrolysates were also determined in the effluent after the tested yeast strains had been cultivated (Table 6). The concentration of galacturonic acid ranged from 0.6 to 0.9 g/L. It should be emphasised that these concentrations were close to the values assayed in hydrolysates before cultivation (where the concentration range of this compound was between 0.62 and 0.93 g/L) The concentration of galacturonic acid after yeast cultivation indicates that it was not assimilated by the test strains, which is confirmed in the literature [66; 67].