## 3 Results
### Fermentation Activity of Yeast  
The fermentation activity of the examined yeast isolates was assessed in terms of their ability to produce ethanol during alcoholic fermentation, which can be oxidised into acetic acid. Under aerobic conditions, yeast can oxidise ethanol to acetaldehyde and then to acetic acid [60]. The reaction involves enzymes such as alcohol dehydrogenase and aldehyde dehydrogenase, which catalyse the relevant chemical reactions. As a result, acetic acid is produced. Known for their preservative properties, organic acids can be used for natural food preservation and as ingredients in various beverages. They also contribute flavour and aroma [52]. The concentrations of ethyl alcohol and acetic acid produced during fermentation are presented in Table 4.  
After the fermentation of apple pomace treated with pectinolytic and cellulolytic enzymes, the observed concentrations of acetic acid were higher than those after the fermentation of untreated apple pomace. For _H. warum_ D9 and _W. anomalous_ D11, the concentration of acetic acid did not exceed 0.2 g/L. The highest concentration was found in the sample collected after fermentation by _K. barnettii_ D1 with 1% and 10% inoculum, while _W. anomalous_ D11 with 5% inoculum yielded a very low concentration in comparison. Acetic acid was formed in all samples fermented with untreated apple pomace. _Hanseniosprau warum_ D9 produced the highest concentration, especially with 1% and 5% inoculum, while 10% inoculum give a lower concentration. _Wickerhamomyces anomalus_ D11 produced the smallest amount of acetic acid, with the highest concentration being obtained \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Inoculum [\%]** & \multicolumn{3}{c}{**Acetic Acid [g/L]**} & \multicolumn{3}{c}{**Ethanol [g/L]**} \\ \cline{3-6}  & **Untreated AP** & **Hydolysed AP** & **Untreated AP** & **Hydolysed AP** \\ \hline \multirow{3}{*}{_K. barnettitii_ D1} & 1\% & 0.18 \(\pm\) 0.01 & 0.24 \(\pm\) 0.01 & 1.19 \(\pm\) 0.04 & 1.06 \(\pm\) 0.03 \\ \cline{2-6}  & 5\% & 0.20 \(\pm\) 0.01 & 0.18 \(\pm\) 0.01 & 0.92 \(\pm\) 0.03 & 1.06 \(\pm\) 0.04 \\ \cline{2-6}  & 10\% & 0.06 \(\pm\) 0.00 & 0.21 \(\pm\) 0.01 & 0.89 \(\pm\) 0.03 & 0.93 \(\pm\) 0.03 \\ \hline \multirow{3}{*}{_H. uvarum_ D9} & 1\% & 0.54 \(\pm\) 0.02 & 0.20 \(\pm\) 0.01 & 5.79 \(\pm\) 0.21 & 3.48 \(\pm\) 0.12 \\ \cline{2-6}  & 5\% & 0.51 \(\pm\) 0.02 & 0.13 \(\pm\) 0.00 & 0.79 \(\pm\) 0.03 & 1.81 \(\pm\) 0.05 \\ \cline{2-6}  & 10\% & 0.44 \(\pm\) 0.02 & 0.12 \(\pm\) 0.00 & 2.02 \(\pm\) 0.07 & 1.04 \(\pm\) 0.04 \\ \hline \multirow{3}{*}{_W. anomalous_ D11} & 1\% & 0.04 \(\pm\) 0.00 & 0.03 \(\pm\) 0.01 & 0.59 \(\pm\) 0.02 & 2.88 \(\pm\) 0.10 \\ \cline{2-6}  & 5\% & 0.10 \(\pm\) 0.00 & 0.02 \(\pm\) 0.00 & 0.64 \(\pm\) 0.02 & 3.71 \(\pm\) 0.13 \\ \cline{1-1} \cline{2-6}  & 10\% & 0.04 \(\pm\) 0.00 & 0.04 \(\pm\) 0.00 & 0.48 \(\pm\) 0.02 & 6.05 \(\pm\) 0.16 \\ \hline \end{tabular}
\end{table} 
Table 4: Acetic acid and ethanol contents after yeast fermentation at 15 °C in untreated and hydrolys angle pomace. Abbreviations: “Hydolysed AP” refers to apple pomace treated with 0.25 mL of Viscozyme® and 0.25 mL of Rohament® CL enzymes; “Untreated AP” refers to apple pomace untreated with enzymes. with 5% inoculum. Additionally, _Kazachstania barnettii_ D1 produced no more than 0.2 g/L of acetic acid at all inoculum levels. However, such amounts do not produce excessively pungent odours [61]. Overall, a higher level of inoculum does not correspond to a higher level of obtained organic compounds. When it comes to ethanol production during fermentation, the highest concentrations were obtained in hydrolysed apple pomace, in particular for the samples fermented by _W. anomalous_ D11. The lowest values were reached for _K. barnettii_ D1. Regarding the fermentation of untreated apple pomace, the ethanol content was remarkably lower in all samples. The highest concentration of ethanol was observed for the samples fermented by _H. uwarum_ with 1% and 10% inoculum.