## 3 Results and Discussion
### Acids  
Organic acids were present in extremely small quantities in WDF_t0, as the overall TIC area was only 33.5 \(\times\) 10\({}^{6}\). They increased in both number and abundance in the aged samples, and were among the species that most characterized the aroma profile of WDF_t2. All data relating to the acid class are shown in Table 4.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & **WDF\_t0** & **WDF\_t1** & **WDF\_t2** & \\ \hline
**Analyte** & **Aroma** & **LRI** & **Area (\(\times\)10\({}^{6}\))** & **Area (\(\times\)10\({}^{6}\))** & **Area (\(\times\)10\({}^{6}\))** & **Ref** \\ \hline Acetic acid & Vinegar, acid, pungent, strong & 598 & 21.8 \(\pm\) 0.1 & 210 \(\pm\) 0.5 & 283 \(\pm\) 0.4 & [10] \\ \hline Propanoic acid & Vinegar, cheese, acid, pungent & 680 & - & 13.8 \(\pm\) 0.1 & - & - \\ \hline Propanoic anhydride & - & 688 & - & - & 113 \(\pm\) 0.2 & - \\ \hline
3-Methyl-butanoic acid & - & 817 & - & 9.2 \(\pm\) 0.06 & 21.8 \(\pm\) 0.1 & - \\ \hline
2-Methyl-butanoic acid & - & 824 & - & 2.40 a  & 7.22 \(\pm\) 0.07 & - \\ \hline Butanoic acid & Bitter, milk, buttery, fruity & 849 & - & 21.1 \(\pm\) 0.1 & 38.2 \(\pm\) 0.1 & - \\ \hline \hline \end{tabular}
\end{table}
Table 4: Composition of the VOC fraction of Gavina® watermelon pomace WDF identified using HS-SPME-GC-MS analysis: acids class.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & & **WDF\_t0** & **WDF\_t1** & **WDF\_t2** & \\ \hline
**Analyte** & **Aroma** & **LRI** & **Area (\(\times\)10\({}^{6}\))** & **Area (\(\times\)10\({}^{6}\))** & **Area (\(\times\)10\({}^{6}\))** & **Ref** \\ \hline
2,3-Butandiol & Fruity, creamy, buttery & 777 & 4.21 a  & 2.80 a  & - & - \\ \hline
2,5-Dimethyl-1,5-hexadien-3-ol & - & 839 & 11.2 \(\pm\) 0.3 & 7.40 a  & 6.89 \(\pm\) 0.06 & - \\ \hline
1-Hexanol &
\begin{tabular}{c} Pungent, oleic, fruity, \\ alcoholic \\ \end{tabular} & 851 & 3.67 a  & 2.17 a  & - & [2,6,23,32] \\ \hline
1-Heptanol &
\begin{tabular}{c} Must, pungent, green, \\ fruity \\ \end{tabular} & 926 & - & 10.1 \(\pm\) 0.1 & - & [2,23,26] \\ \hline
1-Octen-3-ol &
\begin{tabular}{c} Mushroom, earthy, green \\ \end{tabular} & 936 & 23.6 \(\pm\) 0.2 & 21.5 \(\pm\) 0.1 & 19.2 \(\pm\) 0.1 & [23,29,31,32] \\ \hline
2-Methyl-6-hepten-1-ol & - & 944 & - & - & 11.1 \(\pm\) 0.1 & - \\ \hline Benzyl alcohol & Sweet, floral, fruity & 983 & - & 20.2 \(\pm\) 0.1 & 5.97 a  & 23,26,28,32] \\ \hline
1-Octanol &
\begin{tabular}{c} Waxy, green, citrus, floral \\ \end{tabular} & 997 & 15.2 \(\pm\) 0.1 & 32.3 \(\pm\) 0.3 & - & [23,27,29,31] \\ \hline
2,6-Dimethylcyclohexanol & - & 1044 & 132 \(\pm\) 0.5 & 37.3 \(\pm\) 0.2 & 28.2 \(\pm\) 0.1 & - \\ \hline
2,6-Nonadien-1-ol &
\begin{tabular}{c} Cucumber, green \\ \end{tabular} & 1051 & 10.3 \(\pm\) 0.2 & - & - & [29,31,32,33] \\ \hline
6-Nonen-1-ol & Green, melon, cucumber & 1056 & 11.2 \(\pm\) 0.1 & - & - & [23,27,32] \\ \hline
3,4-Dimethylcyclohexanol & - & 1129 & 22.0 \(\pm\) 0.3 & 6.11 a  & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 3: _Cont._The amount of acetic acid progressively increased over time, probably because of the combined effects of degradation and oxidation of molecules of different kinds originally present in the initial matrix, such as sugars. The 3-methyl-butanoic and 2-methyl butanoic acids could reasonably originate from the oxidation of the respective aldehydes, 3-methyl-butanal, and 2-methyl-butanal [38]. In general, it can be assumed that all acids formed over time are derived from chemical and biochemical processes similar to those indicated for acetic acid. In conclusion, acids were among the predominant aromas formed during WDF aging, giving the sample a more pungent aroma.