## 3 Results
### Biosynthesis of Volatile Organic Compounds
When analysing the total concentration of ester compounds, it can be seen that the highest concentrations were found in the samples fermented by _H. uwarum_ D9, slightly lower concentrations (of approximately 100 mg/L) were found for the samples fermented by _K. barmettii_ D1, and the lowest for samples containing _W. anomalous_ D11 (below 25 mg/L). The level of inoculation did not significantly affect the biosynthesis of esters, and the concentration was higher only in the case where 5% of the inoculum _H. uwarum_ D9 and _W. anomalous_ D11 was used. The concentrations of carbonyl compounds formed during fermentation at 15 \({}^{\circ}\)C are shown in Tables 7 and 8. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Concentration**} & \multicolumn{3}{c}{**\(K\). barmettiti D1**} & \multicolumn{3}{c}{_H. uwarum_ D9**} & \multicolumn{3}{c}{_W. anomalous_ D11**} & \multirow{2}{*}{\(p\)**-Value**} \\ \cline{2-3} \cline{5-10}
**[sg/L] AP** & **1\%** & **5\%** & **10\%** & **1\%** & **5\%** & **10\%** & **1\%** & **5\%** & **10\%** \\ \hline Actaldehyde & 5357.17 a & 1632.13 ef & 831.84 g & 5471.13 a & 5275.34 a & 4048.57 c & 1556.93 f & 1980.04 de & 1551.23 f & \textless{}0.0001 \\ \hline Acetone & 73.10 de & 77.99 cd & 227.41 a & 102.33 b & 82.83 c & 66.09 e & 78.64 cd & 52.69 f & 65.67 e & \textless{}0.0001 \\ \hline Isobutyraldehyde & 36.43 c & 20.68 f & 14.45 g & 17.35 fg & 65.35 a & 47.74 b & 31.03 d & 26.61 de & 29.94 d & \textless{}0.0001 \\ \hline
3-methylbutanal & 29.71 b & 27.54 b & 23.33 c & 4.16 fg & 9.57 e & 3.17 g & 28.62 b & 38.85 a & 19.53 d & \textless{}0.0001 \\ \hline
2-methylbutanal & 22.10 a & 14.00 c & 9.27 de & 5.55 fg & 17.90 b & 8.34 e & 10.78 d & 13.89 c & 8.34 e & \textless{}0.0001 \\ \hline
2-3-Butanedione & 19.60 h & 24.21 g & 25.48 g & 22.58 gh & 20.11 h & 15.49 i & 47.02 e & 63.48 d & 37.12 f & \textless{}0.0001 \\ \hline \hline \end{tabular} a-i–mean values in columns denoted by different letters differ statistically significantly (one-way ANOVA, \(p<0.0001\)).  
\end{table} 
Table 7: SHS-GC-MS analysis of carbonyl compounds formed during fermentation of untreated apple pomace at 15 \({}^{\circ}\)C. Detailed report, including ion chromatograms after qualitative analysis, is shown in Supplementary Materials (“Supplementary reports S1-S3”). Carbonyl compounds include aldehydes and ketones. During fermentation, aldehydes are produced by the activity of microorganisms, and ketones by the metabolism of citrates and glucose. Most of them have a fruity or floral aroma [74; 75]. In all samples, after the fermentation of untreated apple pomace, four aldehydes and two ketones were detected. These carbonyl compounds were isobutyraldehyde, 3-methylbutanal, 2-methylbutanal, 2,3-butanedione, acetone, and acetaldehyde. In all analysed samples, the biosynthesis of acetaldehyde was the most effective. This compound occurs naturally in fruits and is also produced during fermentation by yeast. It gives a pungent and fruity aroma [76]. In this study, the highest concentration was obtained in the samples fermented by _H. uvarum_ D9. In the case of other yeasts, this level was similar, except for the test with the addition of 1% of _K. barnettii_ D1 inoculum. In this sample, the concentration exceeded 5 mg/L. Similarly to the esters, the results do not suggest any strong connection between the amounts of these volatile compounds formed and the percentage of inoculum used for the fermentation [45]. A slight decrease in the content of carbonyl compounds was observed in samples fermented by _H. uvarum_ D9 and _K. barnettii_ D1. In the samples containing 1% inoculum, the total amount of carbonyl compounds was the highest, with the samples containing 10% inoculum having the lowest total amount. The concentrations of higher alcohols formed during fermentation at 15 \({}^{\circ}\)C are shown in Tables 9 and 10. Higher alcohols, also called fused alcohols, are produced during fermentation, either directly from sugars or indirectly from amino acids in the Ehrlich reaction.