## 3 Results
### Biosynthesis of Volatile Organic Compounds
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Concentration**} & \multicolumn{3}{c}{_K. bametelli_**D1**} & \multicolumn{3}{c}{_H. warrant_**D9**} & \multicolumn{3}{c}{_W. anomalus_**D11**} & \multirow{2}{*}{_p_**-Value**} \\ \cline{2-2} \cline{4-11}  & **1\%** & **5\%** & **10\%** & **1\%** & **5\%** & **10\%** & **1\%** & **5\%** & **10\%** \\ \hline Ethyl propionate & 376.45 d & 329.63 d & 359.18 d & 639.58 c & 845.25 a & 709.76 b & 77.64 gh & 136.45 eg & 61.08 h & \textless{}0.0001 \\ \hline Isoamyl propionate & 3.68 d & 0.83 g & 1.03 g & 4.70 c & 1.45 fg & 1.94 ef & 4.61 c & 16.25 a & 2.35 e & \textless{}0.0001 \\ \hline Ethyl butyrate & 2.25 ab & 2.02 b & 2.40 a & 0.34 cd & 0.12 d & 0.15 d & 0.49 c & 0.23 cd & 0.31 cd & \textless{}0.0001 \\ \hline Ethyl isothirate & 1.84 de & 1.48 ef & 1.57 de & 9.57 c & 15.78 a & 12.02 b & 2.58 d & 2.05 de & 1.22 ef & \textless{}0.0001 \\ \hline Ethyl hexanoate & 0.04 bc & 0.04 bc & 0.04 bc & 0.23 a & 0.04 bc & 0.06 b & 0.03 c & 0.02 c & 0.02 c & \textless{}0.0001 \\ \hline Ethyl octanoate & 0.10 b & 0.08 bc & 0.07 bc & 0.15 a & 0.06 bc & 0.07 bc & 0.06 bc & 0.05 c & 0.04 c & \textless{}0.0001 \\ \hline Ethyl nonanoate & 0.04 a & 0.04 ab & 0.03 abc & 0.03 bed & 0.02 bed & 0.02 bed & 0.03 abd & 0.02 cd & 0.02 d & \textless{}0.0001 \\ \hline Ethyl decanoate & 0.05 a & 0.04 ab & 0.04 ab & 0.04 ab & 0.03 bc & 0.02 cd & 0.02 cd & 0.02 cd & 0.02 d & \textless{}0.0001 \\ \hline Ethyl dodecanoate & 0.05 a & 0.04 ab & 0.04 abc & 0.04 abc & 0.03 cd & 0.02 de & 0.02 e & 0.02 de & 0.01 e & \textless{}0.0001 \\ \hline \hline \end{tabular} a-h—mean values in columns denoted by different letters differ statistically significantly (one-way ANOVA, \(p<0.0001\)).  
\end{table} 
Table 5: _Cont._From the data obtained after the fermentation of untreated apple pomace, it can be observed that esters were the biggest group of VOCs produced during the fermentation of untreated apple pomace. During fermentation, the biosynthesis of two main groups of esters occurs--acetate esters and ethyl esters. The first are made from acetic acid and ethyl alcohol, or alcohol produced when yeast metabolises amino acids. Ethyl esters are formed from ethyl alcohol and medium-chain fatty acids (MCFAs) [67; 68]. In this study, esters formed during fermentation included ethyl acetate, ethyl formate, ethyl propionate, isobutyl acetate, ethyl butyrate, 3-methylbutyl acetate, 2-methylbutyl acetate, ethyl hexanoate, ethyl octanoate, 2-phenethyl acetate, ethyl nonanoate, ethyl decanoate, ethyl dodecanoate, methyl acetate, propyl acetate, ethyl isobutyrate, butyl acetate, and isoamyl propionate. Among them, ethyl acetate was formed in large quantities during fermentation, exceeding the threshold reported in the literature [64]. This compound is characterised by a fruity aroma. The highest concentration of ethyl acetate was obtained in the samples fermented by _H. uwarum_ D9, and the lowest concentration was obtained in samples fermented by _W. anomalous_ D11. This compound was the main product among biosynthesised acetate esters. The average odour threshold of ethyl acetate in the air is 18 ppm [69]; however, it depends on the matrix in which this ester occurs, the presence of other substances, as well as individual sensory sensitivity. For instance, the odour threshold of ethyl acetate in apple juice varies between 2 and 31 mg/kg [70]; in wine, it varies between 60 and 328 mg/L [71]; in beer, it is 16.8 mg/L [72]; and in plum distillates, it is 34 mg/L of ethyl alcohol 40% (\(v/v\)) [73]. In the group of ethyl esters, the highest concentration was obtained for ethyl propionate. The highest amount was determined in the samples fermented by _H. uwarum_ D9, and the lowest in samples fermented by _W. anomalous_ D11. Other esters were not formed in very large quantities; for example, ethyl hexanoate or ethyl octanoate were only present in trace amounts. Butyl acetate and ethyl formate were not detected at all in the samples fermented by _W. anomalous_ D11.