## 5 Aroma Profile of Nut Products After and Before Roasting Conditions  
As reported previously, fatty acids, carbohydrates, and free amino acids are the primary precursors of aroma compounds in nuts. Fatty acids, in particular, can undergo oxidation to form aldehydes and alcohols. Hexanal, a product of linoleic acid oxidation, is one of the most abundant aldehydes. Other important aldehydes, such as nonanal, are derived from oleic acid degradation. Additionally, linoleic and oleic acids can undergo decarboxylation to form \(\alpha\)-keto acids, which subsequently yield aldehydes like 3-methylbutanal and 2-methylbutanal, as well as various alcohols. Aromatic amino acids, such as phenylalanine, can undergo decarboxylation to form 2-phenylethanol and phenylateddehyde, contributing to floral aromas. Terpenoids, including limonene and linalool, contribute to the characteristic fresh fruit aroma of nuts [16]. Notably, lipids are responsible for the generation of flavor to enhance the taste of the roasted products. In general, the lipids' oxidation is attributed to the deterioration of the food matrix due to the generation of off-flavors. However, during roasting conditions, the oxidation of lipids can trigger the development of pleasant aromas. Lipid oxidation can proceed through multiple pathways, including autoxidation, photooxidation, thermal oxidation, and non-enzymatic oxidation. The major compounds resulting from lipid oxidation are aldehydes, characterizing the low odor threshold and higher concentration. In the raw food matrix and the presence of oxygen, the major reaction is autooxidation, which occurs in three different stages: initiation; propagation; and termination. In the initial stage, the triplet oxygen is activated to light, temperature, metal ions (Fe\({}^{2+}\) and Cu\({}^{2+}\)) in singlet oxygen, and produces reactive oxygen species (ROS), which are able to interact with unsaturated fatty acids. After the initial stage, the ROS start to oxidate several fatty acids and extend their degradation process; this phase is called propagation. Subsequently, the formation of primary oxidation products, such as lipid hydroperoxides, dienes, and trienes, occurs. These primary products can undergo further degradation to yield secondary oxidation products, including aldehydes, ketones, alcohols, hydrocarbons, epoxides, and organic acids. One example of oxidase processing starts from linoleic acids that produce 9- and 13-hydroperoxides and various volatile compounds such as 2,4-decadienal, 2-octenal, and hexanal [53]. Several studies reported that linoleic (18:2) and oleic acids (18:1) account for approximately 97% of the total fatty acids in Iranian bitter almond oil, and, in general, in other nut families. These fatty acids are likely precursors to hexanal and (E)-2-hexenal, which contribute to the green note aroma through the action of lipoxygenase and hydroperoxide lyase enzymes [60]. The reaction between Maillard reaction intermediates and lipid oxidation products, particularly aldehydes, can lead to the formation of a diverse range of aroma compounds, including pyrazines, thiophenes, pyridines, oxazoles, and thiazoles with various alkyl side chains. However, these interactions often result in compounds with higher odor thresholds, leading to a weaker overall aroma intensity [53]. A comparison of the major aromatic molecules present in raw and roasted hazelnuts, almonds, and pistachios is presented below (Table 5).