# Volatile Profile of Nuts, Key Odorants and Analytical Methods for Quantification
## 5 Volatile Profile of Nuts after Thermal Treatments
### Roasting Thermal Processing  
The most popular thermal processing method of nuts is hot air roasting. During this process, samples are heated to temperatures of 130-200 \({}^{\circ}\)C from 5 to 60 min [69]. As a result, different reactions develop, which affects the nut's volatile profile. These main reactions are the Maillard reaction, caramelization, fatty acid autoxidation, lipid degradation and the degradation of sulfur-containing amino acids. The roasting process is associated with non-enzymatic darkening reactions such as, for example, the Maillard and caramelization reactions. The Maillard reaction initiates when amino acids and certain reducing sugars react and produce diverse kinds of compounds depending on the reactants, but also on the pH and the temperature reached during the roasting process. In the Maillard reaction, some heterocyclic volatile compounds such as furans, ketones, pyrazines, pyrroles, aldehydes and pyridines are formed and some of them show an increase compared to raw homologue nuts [62]. As shown in Table 3, some specific \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Thermal** & **Nut** & **Processing Conditions** & **Main VOCs** & **Ref.** \\ \hline \multirow{4}{*}{Hot-air roasting} & \multirow{4}{*}{Hazelnut} & 34, 18, 13 min at & 2,3-pentanedione, 2-acetyl-1-pyrroline, dimethyl & \multirow{4}{*}{[59]} \\  & & 130, 140 and 150 \({}^{\circ}\)C & 2-monenal, 2-decneal, & \\  & & 20, 25 and 30 min & 2-methylpropanal, 2-methylbutanal, & 2-methylbutanal, \\  & & at 160 \({}^{\circ}\)C & 3-methylbutanal, 2,5-methylpyrazine & \\  & & & 2-methylbutanal, 3-methylbutanal, & \\  & & & 2,5-methylpyrazine, furfuryl alcohol, & \\  & & 40 min at 140 \({}^{\circ}\)C & 2-methylpropanal, ethyl acetate, 2,3-pentanedione, & \\  & & & 2-methylpyrazine, 2,5-methylpyrazine, furfural, & \\  & & & 1-hydroxy-2-propananone & \\  & & & & 1-hydroxy-2,5-dimethylbutanal, & \\  & & & & 2-methylbutanal, 2-methyl-pyrazine, & \\  & & & 170–190 \({}^{\circ}\)C & 2,5-dimethyl-pyrazine, furfural & \\  & & 10 min at 190 \({}^{\circ}\)C & 2,5-dimethyl-pyrazine, trimethylpyrazine & \\  & & & 33 min at 138 \({}^{\circ}\)C & 2,5-dimethyl-pyrazine, nonanal & \\  & & & 2-methylbutanal, 3-methylbutanal, hexanal, & \\  & & & 28 and 38 min at 138 \({}^{\circ}\)C & benzaldehyde, furfural, 2-phenyl acetaldehyde & \\  & & & 28, 33 and 38 min & 2-methylbutanal, 3-methylbutanal, hexanal, & \\  & & at 138 \({}^{\circ}\)C & benzaldehyde, furfural, 2-phenyl acetaldehyde & \\  & Chestnut & 25 min at 200 \({}^{\circ}\)C & Hexanal, butylacetate, ethylbenzene, & \\  & & & 2-hydroxy-2-cyclopenten-1-one & \\  & Pistachio with & & 90 min at 120 \({}^{\circ}\)C & \(\alpha\)-pinene, limonene, 3-carene & \\  & and without salt & & & \\  & Cashew & 3 and 9 min & Methylbutanal, hexanal, acetaldehyde, heptane, & \\  & & at 143 \({}^{\circ}\)C & ethanol, pentane, acetone & \\  & & & Benzyl alcohol, methional, benzaldehyde, & \\  & & & dimethylpyrazine, nonanal, undecane, 1-octen-3-ol, & \\  & & & 1,4-butylrolactone & \\  & Pistachio & & 480 or 640 W for & \\  & & & 2, 3 and 4 min & \(\alpha\)-pinene, limonene, nonanal & \\ Hot air-assisted & & & & 2,5-dimethyl-pyrazine, & \\ radio frequency & & Almond & 15 min at 120–130 \({}^{\circ}\)C & toluene, hexanal and heptane & \\  & Deep-frying & Almond & 5 min at 135 \({}^{\circ}\)C & Benzyl alcohol, methional, benzaldehyde, & \\  & & & 10–15 min at & 1-butanol, 1-octen-3-ol & \\  & & & 160–200 \({}^{\circ}\)C & Hexanal, 2-methylbutanal, & 3-methyl-butanol, & \\  & & & 160–200 \({}^{\circ}\)C & 2,5-dimethyl-pyrazine, 1-pentanol & \\  & Chestnut & & & Hexanal, octanal, nonanal, furfural, 3-heptanone, & \\  & & & 4-hydroxy-2-butanone & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Most popular thermal processing methods of nuts and main VOCs reported in the literature.