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    Topic review

    Volatile Profile of Nuts

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    Definition

    Nuts are indehiscent dry fruits with one seed and a thick, hard pericarp. The presence of nuts in diets has notably increased due to their composition, and the presence of antioxidants and their unsaturated fatty acid profile has led to a considerable increase in their consumption. The volatile profile of nuts is important from different points of view. It affects consumer’s selection, influences raw material selection for the production of composite foods, dictates variety selection in breeding programs, and, from a quality perspective, its changes can indicate food degradation or alteration.

    1. Introduction

    Food odor and aroma have a great influence on consumers’ preference. These attributes are related to different volatile chemicals. Volatiles are a set of compounds with a relatively low molecular weight and high vapor pressure [1]. They include different classes of chemicals such as alcohols, hydrocarbons, esters, terpenes and aldehydes. The characterization of this complex and heterogeneous mix is important in food quality control and consequently, for the food industry [2].
    Molecules with low molecular weight are perceived in the nose and mouth sensors faster than others with higher molecular weight and, for this reason, they condition the final flavor of the product. These predominant compounds are known as odorants, and they are very important as some of them are associated with pleasant odors, and others with off-odors. Study of the chemical composition of the volatile profile of foods allows us to understand the optimal conditions for food production and contributes to obtaining valuable information about their composition. Odor molecules arrive into the nose by air, and they are perceived by the olfactory mucosa via orthonasal smell. Meanwhile, aromas are perceived in the nose by the olfactory receptors, in the olfactory epithelium via the retronasal pathway, when the food is put into the mouth and the cells are broken by chewing, thus releasing volatile compounds. Odor and aroma perception is the result of the activation of odorant receptors generating complex signals that are sent to the central nervous system [3]. The amount and type of volatiles released from the food highly depends on the nature of the sample matrix and the state of the food [4]. More specifically, the entire volatile profile reflects the phenotypic or metabolic state of foods. In plants, volatile compounds have diverse and important functions such as to attract pollinators to flower organs, to protect damage plants from the attack of herbivores and to exchange some low molecular weight terpenes during light changes, droughts, or other stress situations for the plant [4].
    Nuts are indehiscent dry fruits with one seed and a thick, hard pericarp. In the botanical sense, they are produced by some families of the order Fagales [5]. These families are the Juglandaceae (walnut and pecan nut), the Fagaceae (chestnut), and the Betulaceae (hazelnut). In the culinary sense, the term nut is applied to a wide variety of dried seeds and fruits and to any large, oily kernel found within a shell. These nuts belong to the Fabaceae (peanut), the Rosaceae (almond) and the Anacardiaceae (pistachio) families. From the food composition point of view, nuts are characterized by having a low water content, i.e., usually less than 50% weight, although some exceptions exist [6]. Due to the group’s diversity, nut volatile profiles are expected to be diverse.
    Based on data recorded by the Food and Agriculture Organization of the United Nations (FAOSTAT), worldwide nut production has increased significantly in recent years, from 12.9 Mtons in 2010 up to 17.5 Mtons in 2019 (Figure 1) [7]. In 2019, California was the top worldwide producer of nuts, with an estimated annual production of 27.0% of the total. The United States was the second largest producer at around 17.4%, followed by Turkey (7.5%), Iran (4.9%), Côte d’Ivoire (4.6%), India (4.3%) and Spain (3.3%).
    Figure 1. Worldwide nut production from 2010 to 2019.
    The majority of worldwide nut production in 2019 was registered as walnuts, followed by almonds, cashew, chestnut, hazelnuts, and finally, pistachio and Brazil nuts (Figure 2) [7]. Nuts are seeds showing a high sensory appeal and numerous health benefits due to their composition. In this sense, they present a high content in terms of protein (13–26% total weight) and lipids (43–76% total weight), carbohydrates (9–30% total weight) and fiber (7–12% total weight) with less than 6.4% water [6]. Nuts can be eaten raw, roasted, or salted as snacks. Thus, they are widely added to some important food formulations such as ice creams, chocolates, confectioneries, cookies, cereal bars and cakes. Due to their high level of dry matter and the possibility of undergoing technological processing, the volatile profile of nuts can undergo significant modifications compared to the profile of raw products.
    Figure 2. Worldwide nut production (Mtons) registered by FAOSTAT in 2019.

    2. 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 [8]. 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 [9].
    Figure 3. Scheme showing different pathways from the main precursors of nut volatiles [3][9].

    3. Analysis of VOCs Present in Nuts

    The large number of volatile compounds, usually up to 200, that can be identified in nuts, make their identification and quantification difficult [10]. In this case, a separation technique such chromatography is usually employed. The volatile nature of the compounds determined makes gas chromatography (CG) coupled to mass spectrometry (MS) the preferred chromatographic type utilized for this analysis [11]. By using this technique, volatile compounds are commonly introduced in the chromatograph by using the split or splitless mode but if a higher sample capacity is needed due to a low volatile concentration in the sample, a programmed temperature vaporizer (PTV) is employed as an injection port [12]. In order to identify the main VOCs, the most extended stationary phases are nonpolar phases such as polydimethylsiloxane, columns with 5% phenyl groups or moderately polar phases such as polyethylene glycols. The length of the employed columns can vary between 25 and 30 m, but if complex mixtures of volatile compounds need to be separated, the length of the column must be increased up to 200 m [12]. Finally, the detector selected is usually a mass spectrometer that allows spectral information of the peaks to be obtained for identification of the target analytes. In routine analysis, the flame ionization detector (FID) can also be employed [12].
    Prior to the identification and quantification of volatiles in nuts, the extraction of the target compounds must be optimized. A reference method for the extraction of VOCs in food matrices is liquid extraction (LE) with organic solvents. This methodology implies the employment of great amounts of organic solvents and, in many cases, low recoveries are obtained [13]. To solve this problem, liquid–liquid microextraction (LLME), dispersive liquid–liquid extraction (DLLE), stir bar sorptive extraction (SBSE) and solid-phase microextraction (SPME) are also employed. All of these analytical methodologies are commercially available, and extensively used.
    SPME is nowadays the most commonly used extraction methodology for the analysis of volatile compounds in food samples, either in direct or in headspace (HS) mode, since it is a solvent-free extraction process that employs lower amounts of sample [11]. The analytical methodology consists of a fiber holder, in which a fiber coated with polymeric material is employed to retain the volatiles. After this, the analytes can be directly desorbed on the injection port of a chromatograph [14]. SPME methodology involves equilibration and extraction steps. Firstly, the sample inside a sealed vial is exposed to a selected temperature for a specific period of time in order to promote analyte volatilization into the HS in the equilibration step. Secondly, the coated fiber is immersed in the HS of the sample, maintaining a constant temperature, for a selected period of time to extract the target compounds. Experimental conditions (time and temperature values) of both steps, equilibration and extraction, commonly employed in VOCs analysis in nut samples are depicted in Table 1.
    Table 1. Experimental SPME conditions (time and temperature values) of both steps, equilibration and extraction, commonly employed in VOCs analysis in nut samples.
           

    Equilibration Conditions

    Extraction Conditions

         

    Type of Nut

    Sample Amount (g)

    Fiber

    Agitation

    Time (min)

    Temperature (°C)

    Time (min)

    Temperature (°C)

    Quantification

    Column

    Ref.

    Almond

    5.0 *

    1-cm 50/30 µm DVB/CAR/PDMS

    No

    40

    24

    30

    24

    (1)

    DB-Wax

    (30 m × 0.25 mm × 0.25 μm)

    [15]

    Almond

    5.0 *

    1-cm 50/30 µm DVB/CAR/PDMS

    Yes

    45

    40

    45

    40

    (3)

    DB-Wax

    (30 m × 0.25 mm × 0.25 μm)

    [16]

    Almond

    0.250 *

    1-cm 50/30 µm DVB/CAR/PDMS

    No

    15

    25

    30

    25

    (2)

    DB-Wax

    (30 m × 0.25 mm × 0.25 μm)

    [17]

    Almond

    5.0 *

    1-cm 50/30 µm DVB/CAR/PDMS

    Yes

    45

    40

    45

    40

    (3)

    DB-Wax

    (30 m × 0.25 mm × 0.25 μm)

    [10]

    Almond

    3.0 *

    1-cm 50/30 µm DVB/CAR/PDMS

    No

    10

    40

    30

    40

    (1)

    TRB-5MS

    (30 m × 0.25 mm × 0.25 μm)

    [18]

    Beechnut, hazelnut, pistachio and walnut

    10.0 **

    1-cm 50/30 µm DVB/CAR/PDMS

    Yes

    60

    25

    60

    25

    No

    RTx-5 (60 m × 0.25 mm × 0.25 μm)

    [19]

    Hazelnut

    0.1 *

    1-cm 75 µm

    CAR/PDMS

    No

    10

    60

    10

    60

    (3)

    DB-Wax

    (30 m × 0.25 mm × 0.5 μm)

    [20]

    Hazelnut

    1.5 *

    2-cm 50/30 µm DVB/CAR/PDMS

    No

    20

    50

    20

    50

    (3) º

    MEGA-WAX™ (30 m × 0.20 mm × 0.20 μm)

    [21]

    Peanut

    5.0 *

    PDMS/DVB

    No

    30

    60

    15

    60

    (3)

    DB-5

    [22]

    Peanut

    5.0 *

    50/30 µm

    DVB/CAR/PDMS

    Yes

    1440

    25

    20

    21

    (1)

    SUPELCOWAX™ 10 (30 m, 0.25 mm, 0.25 mm)

    [23]

    Peanut

    3.0 **

    1-cm 65 µm

    PDMS/DVB

    Yes

    10

    50

    40

    50

    (1)

    DB-Wax

    (30 m × 0.25 mm × 0.25 μm)

    [24]

    Peanut

    5.0 **

    2-cm 50/30 µm DVB/CAR/PDMS

    No

    30

    80

    10

    80

    (1)

    RTX-5MS

    (30 m × 0.25 mm × 0.25 μm)

    [25]

    Peanut

    5.0 **

    1-cm 50/30 µm DVB/CAR/PDMS

    Yes

    30

    50

    30

    50

    No

    DB-17MS

    (60 m × 0.25 mm × 0.25 μm)

    [26]

    Peanut

    0.2 *

    2-cm

    DVB/CAR/PDMS

    Yes

    8 h

    20

    50

    60

    (1)

    dB5-MS semi-polar (60 m × 0.32 mm × 1 μm)

    [27]

    Peanut

    5.0 **

    1-cm 65 µm

    DVB/CAR/PDMS

    No

    20

    80

    60

    80

    No

    HP-5

    (30 m × 0.25 mm × 0.25 μm)

    [28]

    Pistachio

    15.0 *

    50/30 µm

    DVB/CAR/PDMS

    Yes

    15

    50

    120

    50

    No

    HP-5

    (30 m × 0.32 mm × 0.25 μm)

    [13]

    Pistachio

    8.0 *

    50/30 µm

    DVB/CAR/PDMS

    No

    -

    -

    60

    83

    No

    HP-5MS (30 m × 0.25 mm × 0.25 μm)

    [29]

    Pistachio

    10.0 *

    50/30 µm

    DVB/CAR/PDMS

     

    15

    50

    120

    50

    (1)

    Equity-5 (30 m × 0.25 mm × 0.25 μm)

    [30]

    Pistachio

    24.5 *

    PDMS-DVB

    No

    30

    30

    20

    30

    No

    Agilent DB-1

    (60 m × 0.320 mm × 0.25 μm)

    [31]

    Pistachio

    1.5 **

    2-cm 50/30 µm DVB/CAR/PDMS

    Yes

    -

    -

    30

    40

    (1)

    DB-Wax

    (30 m × 0.25 mm × 0.25 μm)

    [2]

    Walnut

    0.5 *

    50/30 µm

    DVB/CAR/PDMS

    Yes

    15

    50

    30

    60

    (1)

    RTX-5MS

    (30 m × 0.25 mm × 0.25 μm)

    [32]

    Walnut

    3.0 ** (mL)

    1-cm 50/30 µm DVB/CAR/PDMS

    No

    10

    50

    30

    50

    No

    HP-INNOWAX

    (30 m × 0.25 mm × 0.25 μm)

    [33]

    Walnut

    1.0 **

    65 µm

    PDMS/DVB

    No

    -

    -

    30

    50

    No

    CP-Wax52CB

    (30 m × 0.25 mm × 0.25 μm)

    [14]

    Pecan

    2.0 *

    50/30 µm

    DVB/CAR/PDMS

    Yes

    30

    25

    30

    65

    (1)

    HP-5

    (30 m × 0.25 mm × 0.25 μm)

    [34]

    Sample pre-treatment: * Only grinding ** Grinding and oil extraction. Quantification: 1: Semi-quantification with IS, 2: (1) corrected by sample amount, 3: quantification. º: IS was employed. DVB: Divinylbenzene, CAR: Carboxen, PDMS: Polydimethylsiloxane.

    4. Effect of Harvesting Conditions and Healthy State of Nuts on Volatile Profile

    The main volatile compounds reported in raw nuts are reviewed in Table 2. As expected, several differences are detected that could be related to nut composition. Additionally, different factors could change the characteristic volatile profile of raw nuts. In this section, the effect of harvesting conditions is reviewed as well as the healthy state of nuts.
    Table 2. Main VOCs of raw nuts.

    Nut

    Main VOCs

    Ref.

    Almond

    1-Hexanol, 3-methyl-1-butanol, nonanal, 2-methyl-1-propanol, 1-propanol

    [17]

     

    Benzaldehyde, hexanal, 1,2-propanediol, 1-chloro-2-propanol, 3-methyl-1-butanol, pentanal, 2-heptanone, 1-hexanol.

    [15]

     

    Hexanal, 3-methyl-1-butanol, benzaldehyde, heptanal, nonanal, 1-octanol, 2-octanone

    [18]

    Chestnut

    ϒ-terpinene, phenylaldehyde, hexanal, furfural, α-terpinene

    [35]

    Hazelnut

    α-tujone, β-tujone, 2-pentanone, acetic acid, 3-methyl-2- butanol, n-decane

    [21]

    Peanut

    Hexanoic acid, 2-ethyl-1-hexanol, 1-hexanol, pentanal, hexanal, palmitic acid, 2-ethyl-5-methylpyrazine, heptanal

    [24]

     

    2-Propanone, α-pinene, benzene, α-terpinolene, hexanal, d-limonene

    [36]

     

    Toluene, α-limonene, γ-terpinene, p-cymene, nonanal, β, pinene, hexanoic acid

    [25]

     

    2,5-Dimethylpyrazine, nonanal, hexanal, 2-ethyl-5-methylpyrazine, octanal, 2,5-dimethyl-3-ethylpyrazine

    [28]

     

    Hexanal, benzaldehyde, benzenacetaldehyde, 2,5-dimethylpyrazine, 2-heptenal, 2-ethyl-5-methylpyrazine, trimethylpyrazine, 3-ethyl-2,5-dimethylpyrazine

    [27]

    Pistachio

    9-Octadecenoic acid, α-pinene, 1-methyl-1H-pyrrole, α-terpinolene, limonene, dimethyl-2H-pyran-2-one, 2-octenal, 2-hexenal

    [29]

     

    α-Pinene, α-terpinolene, 1H-pyrrole, ethyl-alcohol, limonene, hexane

    [30]

     

    α-Pinene, β-pinene, 2-ethyl-1-hexanol, α-terpineol, camphene, hexanoic acid

    [2]

    Walnut

    Hexanal, hexanoic acid, 1-pentanol, 2-octenal, pentanal, 2-pentylfuran, propanoic acid

    [33]

     

    2-Octenal, hexanoic acid, hexanal, 2-decenal, 1-octen-3-ol, nonanal

    [14]

    The current limitation of water resources is threatening nut productivity, so new sustainable agronomic practices, such as regulated deficit irrigation (RDI), have been implemented. These irrigation strategies do not affect fruit quality [37][38]; however, water stress during the growing of nuts can modify its volatile profile [39]. For example, almonds produced under an RDI strategy presented higher total volatile content. In a study with pistachios, it was found lower water stress conditions produced nuts with higher terpene content such as α-pinene [40][41]. This compound is the most important volatile compound in pistachio samples, and it was observed that weather can influence the biosynthesis of VOCs [42] and also the harvesting time [43][23]. In another study, it was also evidenced that the flavor of pistachios was minimized by irrigation [40].
    Moreover, the volatile organic profile of nuts could be modified by the state of health of the nut. The navel orangeworm (NOW) is among the major concerns in the almond industry, due to it being able to cause fungal infection. VOCs emission of almonds and their relationship with NOW have been investigated [44]. Although differences in the volatile profile have been found, the identification of particular compounds and their relationship to NOW have not been addressed. Mature almonds from the Monterey variety were evaluated for their volatile composition after mechanical damage and compared with the volatile composition of undamaged almonds. 3-pentanol and two isomers of a spiroketal chalcogran were found in the damaged almonds. Moreover, the concentration of some compounds such as a spiroketal conophthorin, numerous four-carbon esters and ketones as well as alcohol derivatives, in addition to two eight-carbon chain compounds, increased in the damaged almonds [44][45].
    Changes in volatile organic compounds have been evaluated as an indicator of aflatoxin contamination [31][46][47]. Beck et al. studied the volatile emissions of whole and blanched almonds naturally contaminated with aflatoxins. Volatiles indicative of fatty acid decomposition were predominant in the samples that underwent some form of blanching. Moreover, they found an increase in the concentration of some aldehydes (e.g., hexanal, heptanal, octanal) and hexanoic acid [48]. A similar study was carried out with pistachios. A comparison of volatile compounds in healthy and naturally or artificially aflatoxin-contaminated pistachios was carried out by Georgiadou et al. They found some differences in specific compounds such as C-8 alcohols and aldehydes, sesquiterpenes, and monoterpenes, among others. These compounds allowed differentiation among contaminated and healthy pistachios by applying principal component analysis [30].

    5. Volatile Profile of Nuts after Thermal Treatments

    Processed nuts are consumed as a snack or added to confectionary and bakery products. For this purpose, heat treatments such as roasting and frying are necessary during nut processing to improve their sensory quality, digestibility, and microbiological safety. These heat treatments may significantly affect their properties and quality attributes, obtaining appreciable and desired changes in their texture, color, flavor and taste [49]. Interestingly, when nuts are thermally treated, new VOCs are formed from different reactions that are produced, others disappear, and others increase. Knowledge of the key odorants in thermal treatments of nuts can help to select the best preservation and thermal processing conditions. In this section, the most popular thermal processing methods of nuts and their influence on volatile profile are reviewed (Table 3).
    Table 3. Most popular thermal processing methods of nuts and main VOCs reported in the literature.

    Thermal Processing

    Nut

    Processing Conditions

    Main VOCs

    Ref.

    Hot-air roasting

    Hazelnut

    34, 18, 13 min at 130, 140 and 150 °C

    2,3-pentanedione, 2-acetyl-1-pyrroline, dimethyl sulphide, 2-furfurylthiol, 3-methylbutanal, 2-nonenal, 2-decenal, hydroxy-2,5-dimethyl-3(2H)-furanone

    [50]

       

    20, 25 and 30 min

    at 160 °C

    2-methylpropanal, 2-methylbutanal, 3-methylbutanal, 2,5-methylpyrazine

    [51]

       

    40 min at 140 °C

    2-methylbutanal, 3-methylbutanal, 2,5-methylpyrazine, furfuryl alcohol, 2-methylpropanal, ehtyl acetate, 2,3-pentanedione, 2-methylpyrazine, 2,5-methylpyrazine, furfural, 1-hydroxy-2-propanone

    [21]

     

    Almond

    5 min at 177 °C

    Benzyl alcohol, benzaldehyde, 1-octen-3-ol, toluene, dimethylpyrazine, 1-butanol, hexanal

    [52]

       

    5–10 min at

    170–190 °C

    Hexanal, 2-methyl-butanal, 2-methyl-pyrazine, 2,5-dimethyl-pyrazine, furfural

    [53]

       

    10 min at 190 °C

    2,5-dimethyl-pyrazine, trimethylpyrazine

    [39]

       

    33 min at 138 °C

    Hexanal, benzeneacetaldehyde, 2,5-dimethyl-pyrazine, nonanal

    [15]

       

    28 and 38 min at 138 °C

    2-methylbutanal, 3-methylbutanal, hexanal, benzaldehyde, furfural, 2-phenyl acetaldehyde

    [15]

       

    28, 33 and 38 min

    at 138 °C

    2-methylbutanal, 3-methylbutanal, hexanal, benzaldehyde, furfural, 2-phenyl acetaldehyde

    [54]

     

    Chestnut

    25 min at 200 °C

    Hexanal, butylacetate, ethylbenzene, 2-hydroxy-2-cyclopenten-1-one

    [54]

     

    Pistachio with and without salt

    90 min at 120 °C

    α-pinene, limonene, 3-carene

    [55]

     

    Cashew

    3 and 9 min

    at 143 °C

    Methylbutanal, hexanal, acetaldehyde, heptane, ethanol, pentane, acetone

    [56]

    Microwave roasting

    Almond

    120 V for 2 min

    Benzyl alcohol, methional, benzaldehyde, dimethylpyrazine, nonanal, undecane, 1-octen-3-ol, 1,4-butyrolactone

    [52]

     

    Pistachio

    480 or 640 W for 2, 3 and 4 min

    α-pinene, limonene, nonanal

    [57]

    Hot air-assisted radio frequency

    Almond

    15 min at 120–130 °C

    2,5-dimethyl-pyrazine, toluene, hexanal and heptane

    [58]

    Deep-frying

    Almond

    5 min at 135 °C

    Benzyl alcohol, methional, benzaldehyde, 1-butanol, 1-octen-3-ol

    [52]

       

    10–15 min at 160–200 °C

    Hexanal, 2-methyl-butanal, 3-methyl-butanal, 2,5-dimethyl-pyrazine, 1-pentanol

    [53]

     

    Chestnut

    15 min at 240 °C

    Hexanal, octanal, nonanal, furfural, 3-heptanone, 4-hydroxy-2-butanone

    [59]

    This entry is adapted from 10.3390/foods10071611

    References

    1. Rowan, D.D. Volatile Metabolites. Metabolites 2011, 1, 41–63.
    2. Ojeda-Amador, R.M.; Fregapane, G.; Salvador, M.D. Influence of Cultivar and Technological Conditions on the Volatile Profile of Virgin Pistachio Oils. Food Chem. 2020, 311.
    3. Sud Ali, N.; Cano-Lamadrid, M.; Noguera-Artiaga, L.; Lipan, L.; Carbonell-Barrachina, Á.A.; Sendra, E. Flavors and Aromas. In Postharvest Physiology and Biochemistry of Fruits and Vegetables; Yahia, E.M., Carrillo-López, A., Eds.; Elsevier: London, UK, 2019; pp. 385–404.
    4. Mansurova, M.; Ebert, B.E.; Blank, L.M.; Ibáñez, A.J. A Breath of Information: The Volatilome. Curr. Genet. 2018, 64, 959–964.
    5. Phenol-Explorer Database Phenol-Explorer Database. Available online: http://phenol-explorer.eu/ (accessed on 15 January 2021).
    6. USDA Database. Available online: Https://Fdc.Nal.Usda.Gov/ (accessed on 20 February 2021).
    7. FAOSTAT Food and Agriculture Organization of the United States. FAOSTAT Database. Available online: http://www.fao.org/faostat/en/#data (accessed on 20 February 2021).
    8. Beltrán, A.; Ramos, M.; Grané, N.; Martín, M.L.; Garrigós, M.C. Monitoring the Oxidation of Almond Oils by HS-SPME–GC–MS and ATR-FTIR: Application of Volatile Compounds Determination to Cultivar Authenticity. Food Chem. 2011, 126, 603–609.
    9. Schwab, W.; Davidovich-Rikanati, R.; Lewinsohn, E. Biosynthesis of Plant-Derived Flavor Compounds. Plant. J. 2008, 54, 712–732.
    10. Franklin, L.M.; King, E.S.; Chapman, D.; Byrnes, N.; Huang, G.; Mitchell, A.E. Flavor and Acceptance of Roasted California Almonds during Accelerated Storage. J. Agric. Food Chem. 2018, 66, 1222–1232.
    11. Beltrán, A.; Maestre, S.E.; Grané, N.; Valdés, A.; Prats, M.S. Variability of Chemical Profile in Almonds (Prunus dulcis) of Different Cultivars and Origins. Foods 2021, 10, 153.
    12. Lubes, G.; Goodarzi, M. Analysis of Volatile Compounds by Advanced Analytical Techniques and Multivariate Chemometrics. Chem. Rev. 2017, 117, 6399–6422.
    13. Aceña, L.; Vera, L.; Guasch, J.; Busto, O.; Mestres, M. Comparative Study of Two Extraction Techniques to Obtain Representative Aroma Extracts for Being Analysed by Gas Chromatography-Olfactometry: Application to Roasted Pistachio Aroma. J. Chromatogr. A 2010, 1217, 7781–7787.
    14. Mu, H.; Gao, H.; Chen, H.; Fang, X.; Zhou, Y.; Wu, W.; Han, Q. Study on the Volatile Oxidation Compounds and Quantitative Prediction of Oxidation Parameters in Walnut (Carya cathayensis Sarg.) Oil. Eur. J. Lipid Sci. Technol. 2019, 121, 1–9.
    15. Lee, J.; Xiao, L.; Zhang, G.; Ebeler, S.E.; Mitchell, A.E. Influence of Storage on Volatile Profiles in Roasted Almonds (Prunus dulcis). J. Agric. Food Chem. 2014, 62, 11236–11245.
    16. Franklin, L.M.; Chapman, D.M.; King, E.S.; Mau, M.; Huang, G.; Mitchell, A.E. Chemical and Sensory Characterization of Oxidative Changes In Roasted Almonds Undergoing Accelerated Shelf Life. J. Agric. Food Chem. 2017, 65, 2549–2563.
    17. Rogel-Castillo, C.; Luo, K.; Huang, G.; Mitchell, A.E. Effect of Drying Moisture Exposed Almonds on the Development of the Quality Defect Concealed Damage. J. Agric. Food Chem. 2017, 65, 8948–8956.
    18. Oliveira, I.; Malheiro, R.; Meyer, A.S.; Pereira, J.A.; Gonçalves, B. Application of Chemometric Tools for the Comparison of Volatile Profile from Raw and Roasted Regional and Foreign Almond Cultivars (Prunus dulcis). J. Food Sci. Technol. 2019, 56, 3764–3776.
    19. Stuebiger, G.; Buchbauer, G.; Krist, S.; Bail, S.; Unterweger, H. Characterization of Volatile Compounds and Triacylglycerol Profiles of Nut Oils Using SPME-GC-MS and MALDI-TOF-MS. Eur. J. Lipid Sci. Technol. 2009, 111, 170–182.
    20. Pastorelli, S.; Valzacchi, S.; Rodriguez, A.; Simoneau, C. Solid-Phase Microextraction Method for the Determination of Hexanal in Hazelnuts as an Indicator of the Interaction of Active Packaging Materials with Food Aroma Compounds. Food Addit. Contam. 2006, 23, 1236–1241.
    21. Nicolotti, L.; Cordero, C.; Bicchi, C.; Rubiolo, P.; Sgorbini, B.; Liberto, E. Volatile Profiling of High Quality Hazelnuts (Corylus avellana, L.): Chemical Indices of Roasting. Food Chem. 2013, 138, 1723–1733.
    22. Baker, G.L.; Cornell, J.A.; Gorbet, D.W.; O’Keefe, S.F.; Sims, C.A.; Talcott, S.T. Determination of Pyrazine and Flavor Variations in Peanut Genotypes during Roasting. J. Food Sci. 2003, 68, 394–400.
    23. Abegaz, E.G.; Kerr, W.L.; Koehler, P.E. The Role of Moisture in Flavor Changes of Model Peanut Confections during Storage. LWT Food Sci. Technol. 2004, 37, 215–225.
    24. Liu, X.J.; Jin, Q.Z.; Liu, Y.F.; Huang, J.H.; Wang, X.G.; Mao, W.Y.; Wang, S.S. Changes in Volatile Compounds of Peanut Oil during the Roasting Process for Production of Aromatic Roasted Peanut Oil. J. Food Sci. 2011, 76, 404–412.
    25. Costa De Camargo, A.; Aparecida Bismara Regitano-d’Arce, M.; Matias De Alencar, S.; Guidolin Canniatti-Brazaca, S.; Ferreira de Souza Vieira, T.M.; Shahidi, F. Chemical Changes and Oxidative Stability of Peanuts as Affected by the Dry-Blanching. J. Am. Oil Chem. Soc. 2016, 93, 1101–1109.
    26. Xu, L.; Yu, X.; Li, M.; Chen, J.; Wang, X. Monitoring Oxidative Stability and Changes in Key Volatile Compounds in Edible Oils during Ambient Storage through HS-SPME/GC–MS. Int. J. Food Prop. 2018, 20, S2926–S2938.
    27. Lykomitros, D.; Fogliano, V.; Capuano, E. Drivers of Preference and Perception of Freshness in Roasted Peanuts (Arachis spp.) for European Consumers. J. Food Sci. 2018, 83, 1103–1115.
    28. Dun, Q.; Yao, L.; Deng, Z.; Li, H.; Li, J.; Fan, Y.; Zhang, B. Effects of Hot and Cold-Pressed Processes on Volatile Compounds of Peanut Oil and Corresponding Analysis of Characteristic Flavor Components. LWT Food Sci. Technol. 2019, 112, 107648.
    29. Kendirci, P.; Onoǧur, T.A. Investigation of Volatile Compounds and Characterization of Flavor Profiles of Fresh Pistachio Nuts (Pistacia vera, L.). Int. J. Food Prop. 2011, 14, 319–330.
    30. Georgiadou, M.; Gardeli, C.; Komaitis, M.; Tsitsigiannis, D.I.; Paplomatas, E.J.; Sotirakoglou, K.; Yanniotis, S. Volatile Profiles of Healthy and Aflatoxin Contaminated Pistachios. Food Res. Int. 2015, 74, 89–96.
    31. Beck, J.J.; Willett, D.S.; Mahoney, N.E.; Gee, W.S. Silo-Stored Pistachios at Varying Humidity Levels Produce Distinct Volatile Biomarkers. J. Agric. Food Chem. 2017, 65, 551–556.
    32. Lee, J.; Vázquez-Araújo, L.; Adhikari, K.; Warmund, M.; Elmore, J. Volatile Compounds in Light, Medium, and Dark Black Walnut and Their Influence on the Sensory Aromatic Profile. J. Food Sci. 2011, 76, C199–C204.
    33. Zhou, Y.; Fan, W.; Chu, F.; Wang, C.; Pei, D. Identification of Volatile Oxidation Compounds as Potential Markers of Walnut Oil Quality. J. Food Sci. 2018, 83, 2745–2752.
    34. Gong, Y.; Kerrihard, A.L.; Pegg, R.B. Characterization of the Volatile Compounds in Raw and Roasted Georgia Pecans by HS-SPME-GC-MS. J. Food Sci. 2018, 83, 2753–2760.
    35. Krist, S.; Unterweger, H.; Bandion, F.; Buchbauer, G. Volatile Compound Analysis of SPME Headspace and Extract Samples from Roasted Italian Chestnuts (Castanea sativa Mill.) Using GC-MS. Eur. Food Res. Technol. 2004, 219, 470–473.
    36. Mexis, S.F.; Kontominas, M.G. Effect of Gamma Irradiation on the Physico-Chemical and Sensory Properties of Raw Shelled Peanuts (Arachis hypogaea, L.) and Pistachio Nuts (Pistacia vera, L.). J. Sci. Food Agric. 2009, 89, 867–875.
    37. Lipan, L.; Martín-Palomo, M.J.; Sánchez-Rodríguez, L.; Cano-Lamadrid, M.; Sendra, E.; Hernández, F.; Burló, F.; Vázquez-Araújo, L.; Andreu, L.; Carbonell-Barrachina, Á.A. Almond Fruit Quality Can Be Improved by Means of Deficit Irrigation Strategies. Agric. Water Manag. 2019, 217.
    38. Lipan, L.; García-Tejero, I.F.; Gutiérrez-Gordillo, S.; Demirbaş, N.; Sendra, E.; Hernández, F.; Durán-Zuazo, V.H.; Carbonell-Barrachina, A.A. Enhancing Nut Quality Parameters and Sensory Profiles in Three Almond Cultivars by Different Irrigation Regimes. J. Agric. Food Chem. 2020, 68.
    39. Lipan, L.; Cano-Lamadrid, M.; Vázquez-Araújo, L.; Łyczko, J.; Moriana, A.; Hernández, F.; García-García, E.; Carbonell-Barrachina, Á.A. Optimization of Roasting Conditions in HydroSOStainable Almonds Using Volatile and Descriptive Sensory Profiles and Consumer Acceptance. J. Food Sci. 2020, 85, 3969–3980.
    40. Şahan, A.; Bozkurt, H. Effects of Harvesting Time and Irrigation on Aroma Active Compounds and Quality Parameters of Pistachio. Sci. Hortic. 2020, 261, 108905.
    41. Carbonell-Barrachina, Á.A.; Memmi, H.; Noguera-Artiaga, L.; del Carmen Gijón-López, M.; Ciapa, R.; Pérez-López, D. Quality Attributes of Pistachio Nuts as Affected by Rootstock and Deficit Irrigation. J. Sci. Food Agric. 2015, 95.
    42. Polari, J.J.; Zhang, L.; Ferguson, L.; Maness, N.O.; Wang, S.C. Impact of Microclimate on Fatty Acids and Volatile Terpenes in “Kerman” and “Golden Hills” Pistachio (Pistacia vera) Kernels. J. Food Sci. 2019, 84, 1937–1942.
    43. Vas, G.; Vékey, K. Solid-Phase Microextraction: A Powerful Sample Preparation Tool Prior to Mass Spectrometric Analysis. J. Mass Spectrom. 2004, 39, 233–254.
    44. Beck, J.J.; Higbee, B.S.; Merrill, G.B.; Roitman, J.N. Comparison of Volatile Emissions from Undamaged and Mechanically Damaged Almonds. J. Sci. Food Agric. 2008, 88.
    45. Beck, J.J.; Merrill, G.B.; Higbee, B.S.; Light, D.M.; Gee, W.S. In Situ Seasonal Study of the Volatile Production of Almonds (Prunus dulcis) Var. ‘Nonpareil’ and Relationship to Navel Orangeworm. J. Agric. Food Chem. 2009, 57.
    46. Scott-Thomas, A.; Chambers, S.T. Volatile Organic Compounds: Upcoming Role in Diagnosis of Invasive Mould Infections. Curr. Fungal Infect. Rep. 2017, 11.
    47. Beck, J.J.; Willett, D.S.; Gee, W.S.; Mahoney, N.E.; Higbee, B.S. Differentiation of Volatile Profiles from Stockpiled Almonds at Varying Relative Humidity Levels Using Benchtop and Portable GC-MS. J. Agric. Food Chem. 2016, 64.
    48. Beck, J.J.; Mahoney, N.E.; Cook, D.; Gee, W.S. Volatile Analysis of Ground Almonds Contaminated with Naturally Occurring Fungi. J. Agric. Food Chem. 2011, 59.
    49. Valdés, A.; Beltrán, A.; Karabagias, I.K.; Badeka, A.; Kontominas, M.G.; Garrigós, M.C. Effect of Frying and Roasting Processes on the Oxidative Stability of Sunflower Seeds (Helianthus annuus) under Normal and Accelerated Storage Conditions. Foods 2021, 10, 944.
    50. Yang, J.; Pan, Z.; Takeoka, G.; MacKey, B.; Bingol, G.; Brandl, M.T.; Garcin, K.; McHugh, T.H.; Wang, H. Shelf-Life of Infrared Dry-Roasted Almonds. Food Chem. 2013, 138, 671–678.
    51. Marzocchi, S.; Pasini, F.; Verardo, V.; Ciemniewska-Żytkiewicz, H.; Caboni, M.F.; Romani, S. Effects of Different Roasting Conditions on Physical-Chemical Properties of Polish Hazelnuts (Corylus avellana, L. Var. Kataloński). LWT Food Sci. Technol. 2017, 77, 440–448.
    52. Agila, A.; Barringer, S. Effect of Roasting Conditions on Color and Volatile Profile Including HMF Level in Sweet Almonds (Prunus dulcis). J. Food Sci. 2012, 77, 1–8.
    53. Valdés, A.; Beltrán, A.; Karabagias, I.; Badeka, A.; Kontominas, M.G.; Garrigós, M.C. Monitoring the Oxidative Stability and Volatiles in Blanched, Roasted and Fried Almonds under Normal and Accelerated Storage Conditions by DSC, Thermogravimetric Analysis and ATR-FTIR. Eur. J. Lipid Sci. Technol. 2015, 117, 1199–1213.
    54. Xiao, L.; Lee, J.; Zhang, G.; Ebeler, S.E.; Wickramasinghe, N.; Seiber, J.; Mitchell, A.E. HS-SPME GC/MS Characterization of Volatiles in Raw and Dry-Roasted Almonds (Prunus dulcis). Food Chem. 2014, 151, 31–39.
    55. Penci, M.C.; Martinez, M.L.; Fabani, M.P.; Feresin, G.E.; Tapia, A.; Ighani, M.; Ribotta, P.D.; Wunderlin, D.A. Matching Changes in Sensory Evaluation with Physical and Chemical Parameters: A Case Study: Argentinean Pistachio Nuts (Pistachia vera, L. Cv Kerman). Food Bioprocess. Technol. 2013, 6, 3305–3316.
    56. Agila, A.; Barringer, S.A. Volatile Profile of Cashews (Anacardium occidentale, L.) from Different Geographical Origins during Roasting. J. Food Sci. 2011, 76.
    57. Hojjati, M.; Noguera-Artiaga, L.; Wojdyło, A.; Carbonell-Barrachina, Á.A. Effects of Microwave Roasting on Physicochemical Properties of Pistachios (Pistaciavera, L.). Food Sci. Biotechnol. 2015, 24, 1995–2001.
    58. Xu, Y.; Liao, M.; Wang, D.; Jiao, S. Physicochemical Quality and Volatile Flavor Compounds of Hot Air-Assisted Radio Frequency Roasted Almonds. J. Food Process. Preserv. 2020, 44.
    59. Li, Q.; Shi, X.; Zhao, Q.; Cui, Y.; Ouyang, J.; Xu, F. Effect of Cooking Methods on Nutritional Quality and Volatile Compounds of Chinese Chestnut (Castanea mollissima Blume). Food Chem. 2016, 201, 80–86.
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