# Volatile Profile of Nuts, Key Odorants and Analytical Methods for Quantification
## Main Volatile Organic Compounds (VOCs) Present in Nuts  
The final flavor of nuts is conditioned by the volatile compounds generated during fruit growth and maturation. Further changes in the volatile profile can occur during the storage after the harvesting of fruits and in the processing and cooking of nuts, thus affecting their sensorial quality [9]. The precursors of the VOCs are mainly fatty acids, carbohydrates and amino acids present in the plants and fruits (Figure 3). Among the VOCs in nuts, it is possible to find saturated and unsaturated molecules with straight, branched or cyclic structures including different functional groups such as alcohols, aldehydes, ketones, esters and ethers and also nitrogen and sulfur [10].  
The most representative volatile compounds derived from fatty acid oxidation are aldehydes and alcohols [11]. The concentration level of some of these compounds can be employed to indicate the degree of oxidative deterioration of a food sample. In this sense, in walnut oils, the main ones associated with rancid flavor were 2-octenal, hexanal, 2-heptenal, 1-octen-3-ol, hexanoic acid and nonanal, whereas in almond oils, the total oxidation of lipids was mainly related to 1-pentanol, hexanal, and hexanoic acid [12]. In relation to peanuts, it was found that the compounds that were directly linked to an oxidized flavor were octanal, nonanal, hexanal and 2-pentylpyridine [13]. Hexanal is the most characteristic volatile product formed as a consequence of nut oxidation due to the high concentration of linoleic acid in the fat of the nuts. Another important aldehyde is nonanal, which appears as a by-product of oleic acid degradation [14]. Moreover, the most relevant volatile compounds present in the aroma notes of hazelnuts are in the group of  
Figure 2: Worldwide nut production (Mtons) registered by FAOSTAT in 2019.  
aldehydes such as octanal (soapy), hexanal (grassy or green) and 2-methylbutananal (malty). As alcohols, linalool is representative of flowery odor and ketones such as 2-hepten-4-one and 5-methyl-(Z)-hepten-4-one are responsible for nutty and fruity odors and 2,3-butanedione of buttery. Other important volatiles in hazelnuts are pyrazines such as 3,6-dimethyl-2-ethyl pyrazine (roasted and earthy) among other compounds [8].  
Some odor notes in nuts are associated with unripe fruits, which is mainly related to hexane derived molecules such as hexanal, cis-3-hexanal, and hexanol, among others, their precursors being linolenic and linoleic fatty acids [15]. The same fatty acids are also precursors of other volatile compounds formed after decarboxylation of the alfa-keto acids to generate aldehydes such as 3-methylbutananal and 2-methylbutananal and other respective alcohols. Additionally, free aromatic amino acids such as phenylalanine can suffer decarboxylation processes and some precursors of floral aromas are formed such as 2-phenylethanolanol and phenylacetaldehyde [16].  
Meanwhile, terpenoids are also present in the volatile profile of nuts, providing the characteristic aroma of fresh nuts. These compounds are molecules with an isopentane skeleton such as valencene, limonene, and linalool. In this sense, linalool is also responsible for the flavor of a lot of fruits such as tomatoes and oranges and some herbs such as lavender and basil [3]. Finally, other components such as carotenoids are also precursors of some fruity and floral smells formed during the maturity stage. Some examples of these compounds are ketones such as 6-methyl-5-hepten-2-one and \(\beta\)-ionone.