## 2 Results and Discussions
### Volatile Compound Composition  
The volatile compounds detected in HOC and FHOC are presented in Table 6. Thirteen compounds were detected in HOC and nineteen compounds in FHOC. The total determined volatile compound content in HOC was 6992.9 \(\pm\) 3.0 \(\upmu\)g/kg DM, while it was 2953.4 \(\pm\) 11,359.1 \(\pm\) 111.2 \(\upmu\)g/kg DM in FHOC. There were significant differences in the volatile composition of the headspace of the samples after fermentation. The only common compound detected among the samples was 2-ethyl-1-hexanol. This compound was also identified as a lipid oxidation product in hazelnut samples by [PERSON] et al. (2018) [87]. The most predominant compound identified in the headspace of HOC was acetic acid (37.2%), an organic acid, followed by limonene (27.8%), a member of the terpene class, and hexanal (8.7%), a straight-chain aldehyde. The literature on the volatile compounds of HOC is limited. [PERSON] and [PERSON] (2012) reported that the highest amount of aroma active compound in raw hazelnuts was hexanal, followed by acetic acid [88]. In the same study, hexanal and acetic acid were also detected in roasted hazelnut paste samples. In another study, the same \begin{table}
\begin{tabular}{c c c} \hline \hline
**Amino Acid (\%)** & **HOC** & **FHOC** \\ \hline Aspartic acid (Asp) \({}^{\star}\) & 4.19 \(\pm\) 0.18 & 5.45 \(\pm\) 0.29 \\ Glutamic acid(Glu) \({}^{\star}\) & 11.46 \(\pm\) 0.35 & 12.53 \(\pm\) 0.32 \\ Serine (Ser) & 1.44 \(\pm\) 0.06 & 2.29 \(\pm\) 0.13 \\ Histidine (His) & 1.01 \(\pm\) 0.19 & 0.98 \(\pm\) 0.18 \\ Glycine (Gly) & 2.06 \(\pm\) 0.12 & 2.29 \(\pm\) 0.28 \\ Threonine (Thr) & 1.22 \(\pm\) 0.07 & 1.49 \(\pm\) 0.05 \\ Arginine (Arg) \({}^{\star}\) & 3.30 \(\pm\) 0.40 & 8.21 \(\pm\) 0.65 \\ Alanine (Ala) & 2.17 \(\pm\) 0.14 & 2.62 \(\pm\) 0.17 \\ Tyrosine (Tyr) & 1.53 \(\pm\) 0.06 & 1.65 \(\pm\) 0.17 \\ Valine (Val) & 3.15 \(\pm\) 0.15 & 3.47 \(\pm\) 0.12 \\ Phenylalanine (Phe) \({}^{\star}\) & 1.76 \(\pm\) 0.05 & 2.08 \(\pm\) 0.04 \\ Isoleucine (Ile) & 1.49 \(\pm\) 0.10 & 1.46 \(\pm\) 0.08 \\ Leucine (Leu) & 2.94 \(\pm\) 0.08 & 3.33 \(\pm\) 0.22 \\ lysine (Lys) & 1.80 \(\pm\) 0.17 & 1.34 \(\pm\) 0.28 \\ Total \({}^{\star}\) & 39.55 \(\pm\) 1.48 & 49.26\(\pm\) 1.10 \\ \hline \hline \end{tabular} Values are given as mean \(\pm\) standard deviation. \({}^{\star}\) There is a statistical difference between the results (\(p\leq 0.05\)).  
\end{table} 
Table 5: Amino acid composition of HOC and FHOC (% amino acid/g cake). authors also detected these compounds in hazelnut and hazelnut paste [88]. [PERSON] (2009) examined the presence of hexanal and octanal in hazelnut flour and determined that the amount of hexanal was high in the samples [89]. The author explained this situation by noting that hexanal is formed as a result of oxidation of linoleic acid, octanal is formed as a result of oxidation of oleic acid, and linoleic acid is oxidized faster than oleic acid [89]. n-Alcohols and fatty acids were also determined in the air space of HOC. These compounds were identified in hazelnut samples in previous studies [90]. Benzaldehyde, an aromatic aldehyde and a metabolite of phenylalanine, constituted 48.8% of the headspace of FHOC samples. This compound might be produced during fermentation because [PERSON] et al. (2018) determined the benzaldehyde increment with fermentation in hazelnut-based food [91]. In addition to these compounds, the formation of 3-methylbutanal, 3-methylbutanoic acid, and 2-methylpropanoic acid was determined in FHOC. These compounds are formed by catabolism of branched chain amino acids [92]. Another abundant compound in the headspace was 2-nonanol (17.6%), which is a methyl ketone group. Methyl ketones (2-alkanones) are derived from medium length fatty acids [93]. 2-heptanone from the methyl ketone family was also detected in the headspace of the FHOC. Another group of compounds detected in fermented samples were furans, pyrazines and pyrroles. These compounds are the dominant compounds in heat-treated foods.