## 3 Results
### Biosynthesis of Volatile Organic Compounds  
Volatile organic compounds are a huge group of by-products of yeast's primary and secondary metabolism [24; 25]. They also naturally occur in plant biomass [62]. They are mainly composed of esters, organic acids, higher alcohols, aldehydes, and ketones. Their biosynthesis occurs mostly in the stationary phase of yeast growth, but some of them, such as acetaldehyde, are produced in the logarithmic growth phase as an intermediate compound after glycolysis [63]. VOCs can beneficially contribute to the sensory profiles of fermented foods and beverages due to their interesting aromas [26]. However, at concentrations that are too high, VOCs can spoil the quality of products [64]. Depending on the concentrations and compositions of VOCs, their impact can either be desired, improving the overall characteristics of the end product, or undesired, decreasing its quality [26]. As temperature heavily influences yeast's metabolism, it also has an impact on the biosynthesis of VOCs. This means that the fermentation temperature could affect the composition of key aroma compounds [65]. The SHS-GC-MS method was used for the purpose of quantifying VOCs [66]. The analysed samples were collected after 5 days of fermentation of the untreated and hydrolysed apple pomace at 15 \({}^{\circ}\)C. The concentrations of esters formed during fermentation at 15 \({}^{\circ}\)C are shown in Tables 5 and 6. \begin{table}
\begin{tabular}{l c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Concentration**} & \multicolumn{3}{c}{_K. barnettii_ D1} & \multicolumn{3}{c}{_H. uwarum_ D9} & \multicolumn{3}{c}{_W. anomalous_ D11} & \multirow{2}{*}{_p-Value_} \\ \cline{2-3} \cline{5-11}
**[\%/L] AP** & **1\%** & **5\%** & **10\%** & **1\%** & **5\%** & **10\%** & **1\%** & **5\%** & **10\%** & **\(p\)-Value** \\ \hline Ethyl acetate & 237,107.53 c & 230,338.25 c & 237,367.87 c & 31,640.17 b & 366,183.91 a & 330,358.62 b & 10,035.32 g & 20,129.12 fg & 8515.68 g & \textless{}0.0001 \\ Isobutyl acetate & 138.47 cd & 114.43 cd & 148.51 c & 59.94 e & 958.83 a & 271.32 b & 4.74 f & 120.23 cd & 3.70 f & \textless{}0.0001 \\ \hline \begin{tabular}{l} 3-Methylbutyl \\ acetate \\ \end{tabular} & 1411.28 c & 837.60 e & 1190.81 d & 1158.22 d & 2119.26 a & 1133.25 d & 119.27 h & 1828.91 b & 78.04 h & \textless{}0.0001 \\ \hline \
\begin{tabular}{l} 2-Methylbutyl \\ acetate \\ \end{tabular} & 76.14 cd & 63.60 de & 90.83 c & 45.43 ef & 334.31 a & 126.64 b & 6.08 hi & 142.31 b & 4.02 i & \textless{}0.0001 \\ \hline
\begin{tabular}{l} 2-Phenethyl \\ acetate \\ \end{tabular} & 181.67 e & 90.58 e & 120.27 e & 514.84 d & 1927.60 a & 863.74 c & 42.56 e & 90.76 e & 24.02 e & \textless{}0.0001 \\ \hline Methyl acetate & 926.72 e & 797.21 f & 847.14 ef & 774.96 f & 1066.15 d & 561.58 g & 106.21 h & 1063.40 d & 84.16 h & \textless{}0.0001 \\ \hline Proyl acetate & 254.94 c & 232.05 c & 251.63 c & 318.00 b & 588.73 a & 338.23 b & 5.41 e & 15.81 de & 4.14 e & \textless{}0.0001 \\ \hline Butyl acetate & 28.61 a & 21.52 b & 24.26 b & 13.53 c & 24.66 b & 10.70 c & - & - & - & \textless{}0.0001 \\ \hline Ethyl formate & 5.13 d & 4.14 def & 4.30 de & 46.83 a & 11.54 c & 20.31 b & - & - & - & \textless{}0.0001 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: SHS-GC-MS analysis of esters formed during fermentation of untreated apple pomace at 15 \({}^{\circ}\)C. Detailed report, including total chromatograms after qualitative analysis, is shown in Supplementary Materials (“Supplementary reports S1–S3”).