Volatile Compounds of Roasted and Steamed Malaysian Tropical Almond Nut (Terminalia catappa L.)

Abstract

The flavor extract components contributing to the characteristic aroma notes of roasted and steamed tropical almond nuts were investigated by means of gas chromatography-olfactometry and mass spectrometry of solvent extracts. Roasting and steaming data revealed all of the main classes of compounds commonly listed as thermally generated flavors in oily seeds. A total of 72 volatile compounds were identified in the sample from roasted nuts; among these were 27 hydrocarbons, 12 aldehydes, 12 ketones, 8 acids, 4 esters, 3 alcohols, 3 furans, and a pyrazine. The steamed nuts, however, yielded 66 peaks from which 63 volatile compounds were identified (22 hydrocarbons, 9 aldehydes, 9 ketones, 9 esters, 4 acids, 9 alcohols, and a pyrazine). The small number of aldehydes, ketones, pyrazine, and alcohols identified are the ones most likely to contribute to the aroma of the nuts.

Keywords:

• Terminalia catappa
• Volatile compounds
• Roasted
• Steamed

INTRODUCTION

Tropical almond (Terminalia catappa L.) belongs to the family Combretaceae. It is commonly called tropical almond, wild almond, Indian almond, sea almond,[1] and ‘ketapang’ in Malaysia, respectively. It is typical of the South East Asia region and of Africa. The tree is among the most common trees in the tropics, growing wild as well as cultivated as a popular wayside tree. Tropical almond has a characteristic ‘pagoda’ shape because it sends out a single stem from the top centre.[2] The green almond-shaped fruit turns yellow when ripe. The fruit has an endocarp, which contains an edible oily seed that tastes like almond. The dried raw seeds of tropical almond are highly relished by children in India, Malaysia, and Nigeria.[3] Siderhurst and Jang[4] employed the gas chromatography-electroantennogram detection (GC-EAD) analysis of the volatiles from the fruits of tropical almond to elicit antennal responses from oriental fruit flies. The fruit revealed 22 compounds that served as host plant attractants. The edible seeds have been used as snacks at tea time in Jamaica and India.[5] Furthermore, some sections of Malaysia sprinkle the roasted or steamed nuts over cereal or yogurt for breakfast and the nuts are sometimes added to a soft cheese to make a tasty spread for crackers or bread. Considering the growing importance of the nut as a snack item in some regions of the world, most especially Asia, Africa, and the West Indies, there is the need to study the aroma constituents of this nut to have an insight into those compounds responsible for the characteristic aroma of the tropical almond nut. While many studies have been done on the medicinal and antioxidant activity of the leaf extracts of tropical almond,[2,6,7] there is little or no information on the aroma constituents of the edible nuts of the tropical almond. Therefore, the objective of this study was to identify and to compare the volatile constituents of roasted and steamed Terminalia catappa nuts.

MATERIALS AND METHODS

Extraction

Freshly harvested tropical almond (Terminalia catappa L.) fruits were obtained from the Department of Forestry, Universiti Putra Malaysia. Intact almonds (about 250 g for each treatment) were manually slit with a sharp knife and steamed for 30 min in an Oster food steamer (5712 Electronic 2-Tier-6-Quart, Oster Electronics, Halifax, Canada), or placed in a 200°C roaster (model Duetl-M, Probat, Emmerich, Germany) and roasted for 50 min. Samples were allowed to cool to room temperature before being manually ground with a household food processor (Kenwood FP 730, Watford, UK). Ground samples (100 g) were extracted with freshly distilled dichloromethane (100 ml) at room temperature, under agitation, for 3 h. At this point, the dichloromethane extract was separated from the slurry via filtration, replaced with fresh solvent, and the second extraction continued for another 3 h. The dichloromethane extracts were combined (200 ml) and after drying over Na₂SO₄, were concentrated using a rotary evaporator and a water bath set at 45°C.[8] The extracts were further concentrated at 45°C to 400 μl by using a small size Vigreux column (Sigma-Aldrich, St. Louis, MO, USA).[9]

Reagents

Volatile compound standards, such as 1-octanol, decanal, octanal, nonanal, hept-2-enal, (E, E)-2, 4-nonadienal, (E, E)-2, 4-decadienal, hexadecanoic acid, octadecanoic acid, decanoic acid, octanoic acid, heptanone, and furfuryl octanoate were obtained from Aldrich (Steinheim, Germany). Gamma-dodecalactone and 2-ethyl-3, 6-dimethylpyrazine were obtained from Acros Organics (Morris Plains, NJ, USA). Stock standard solutions of 10³ or 10⁴ mg L⁻¹ of each component was prepared by dissolving the pure standard in 40% (v/v) ethanol. The samples were stored at 4°C. Working standard solutions were prepared daily by mixing an aliquot of each individual solution and diluting with ultra pure water (Millipore Co., Bedford, MA, USA) to obtain a final ethanol content of 10% (v/v).

GC-MS Analysis

The extracts were analyzed using a Shimadzu QP-5050A GC-MS instrument (Shimadzu, Kyoto, Japan) equipped with a GC-17A Ver. 3 gas chromatograph with a flame ionization detector (FID). The column was a non-polar BP X 5 (5% phenylpolysilphenylenesiloxane) capillary column (30 m × 0.25 mm i.d., film thickness 0.25 μm (Scientific Instrument Services, Inc., Ringoes, NJ, USA). Helium was used as the carrier gas at a flow rate of 1.5 ml.min⁻¹; injection temperature, 280°C; detector temperature, 320°C; temperature program commenced at 50°C and held for 3 min, then raised to 280°C at a rate of 15°C min⁻¹, held for 10 min and then increased to 320°C at a rate of 10°C min⁻¹, with a final hold time of 5 min. The effluent from the capillary column was split into 2:1 (by vol.) onto two uncoated but deactivated fused silica capillaries (50 cm × 0.32 mm) leading to a FID and a sniffing port.

The mass spectrometer was operated in electron impact mode with the following conditions. The source temperature was 320°C; the quadruple temperature selected was 280°C; and the relative electron multiplier voltage applied was 400 V with a resulting voltage of 1553 V. In order to improve the detection limits, the scan and selected ion monitoring (SIM) mode was used. For scan mode, the scan range in mass-to-charge ratio (m/z) was 15–450 solvent cut time 5 min. Mass spectrometer (MS) start time 5.2 min. For SIM mode, the following m/z was selected: 56, 57, 58, 67, 80, 95, and 121. The solvent cut time was 4.5 min, and MS start time was 4.6 min for SIM mode. All sample groups were run in triplicate. The data acquisition was carried out with the HP-Chemstation software (Agilent, Palo Alto, CA, USA) and identified using the NIST/NBS75K database.

Gas Chromatography-Olfactometry

The samples were applied by the ‘cool’-on-column injection technique at 40°C. The odorants were screened in parallel by three panelists who sniffed the effluent on a Gerstel olfactory detector port (ODP 2, Gerstel, Mulheim, Germany). Prior to sniffing the samples, the subjects were screened for olfactory acuity according to a training procedure reported by Ong and Acree.[10] The sniffers were able to detect 0.82 ng of ethylbutyrate, 0.99 ng of ethylhexanoate, and 0.046 ng of β-damascenone eluting from the GC/O. The sniffers were asked to press the Gerstel olfactory pad. The resulting ‘olfactogram’ with voice comments were saved in the same file as the GC chromatogram. Spoken comments were converted into text using Dragon Naturally Speaking Software ver. 10.10 (Nuance Communications, Burlington, MA, USA). The sniffing analysis was repeated twice by each panelist. The compounds were tentatively identified by comparing their retention times with those of the available standards, odor quality perceived at the sniffing port, and on the basis of matches with literature mass spectra (NIST/NBS75K library).[22] GC-MS conditions were as follows: oven temperature was raised at 50°C min⁻¹ to 60°C min⁻¹ (BP X 5), held for 3 min isothermally, raised at 10°C min⁻¹ to 280°C, then raised at 15°C min⁻¹ to 320°C and held for 10 min. The flow rate of the carrier gas helium was 1.5 ml min⁻¹.

Quantification

Semi-quantitative analysis was made by the internal standard method, using decanal as a reference substance without the use of response factors for all compounds. The calibration curves of amount ratios (compound/internal standard) versus peak area ratio (compound/internal standard) were used to quantify positively identified compounds. The concentration of a compound in the sample was calculated as:[11]

(1)

RESULTS AND DISCUSSION

Calibration, Linearity, and Analytical Sensitivity

Five levels of concentration were tested in triplicate; these concentrations covered the concentration ranges expected for the various aroma compounds in the tropical almond nuts. The aroma compound/internal standard peak area ratio for the identified aroma compounds was used for each compound. The range of linearity studied for each aroma compound is shown in Table 1. The correlation coefficients were good (r ² > 0.99). This was followed by an excellent linearity in all cases. The linearity, which is the ‘on-line linearity’ (LOL), was determined by the following equation in which RSD is the relative standard deviation of the slope (expressed as a percentage):[12]

Table 1 Characteristics of the calibration curves

Compound	Linear range (mg/l)	Regression coefficient	Linearity (LOL, %)	Slope ±SD	Intercept ±SD
(Z)-Hept-2-enal	0.024–3.6	0.999	99.2	0.16 ± 0.00	0.013 ± 0.00
Octanal	0.007–0.61	0.997	99.4	4.40 ± 0.07	0.123 ± 0.02
Nonanal	0.006–1.0	0.996	98.6	0.89 ± 0.01	0.021 ± 0.00
Octanoic acid	0.027–1.5	0.999	99.0	1.46 ± 0.02	0.080 ± 0.01
Decanal	0.042–11.0	0.999	99.1	0.49 ± 0.01	0.013 ± 0.02
(E,E)-2, 4-Nonedienal	0.019–3.3	0.997	98.2	3.72 ± 0.06	0.39 ± 0.05
(E,E)-2, 4-Decadienal	0.002–0.32	1.00	99.5	2.21 ± 0.01	0.11 ± 0.00
2-Ethyl-3, 6-dimethylpyrazine	0.002–0.09	0.998	98.1	5.36 ± 0.1	0.06 ± 0.00
Decanoic acid	0.002–0.30	0.996	98.8	18.72 ± 0.33	0.073 ± 0.05
Gamma dodecalactone	0.003–0.065	0.995	97.8	18.39 ± 0.01	0.09 ± 0.01
Furfuryl octanoate	0.003–0.065	0.997	98.3	43.73 ± 0.74	0.25 ± 0.03
Hexadecanoic acid	0.054–2.0	0.994	97.9	0.389 ± 0.02	0.05 ± 0.07

(2)

The analytical sensitivity, detection, and quantitation limits were calculated from the curves constructed for each compound. Analytical sensitivity is defined by the quotient Sr/m, in which Sr is the residual standard deviation and m is the slope of the calibration curve. Moreover, the limits of detection (LOD) (3 × the relative standard deviation of the analytical blank values) and quantitation (LOQ) (10 × the relative standard deviation of the analytical blank values) obtained (Table 2) were low enough to determine the aroma compounds in the roasted and steamed tropical almond nut extracts. In addition, good recoveries of compounds were also obtained (Table 2).

Table 2 Performance characteristics of the calibration curves

Compound	Analytical sensitivity	Detection limit (LOD, mg/l)	Quantitation limit (LOQ, mg/l)	Recovery (%)
(Z)-Hept-2-enal	0.038	0.100	0.355	93.3
Octanal	0.004	0.011	0.035	90.2
Nonanal	0.022	0.063	0.209	99.7
Octanoic acid	0.021	0.059	0.185	90.6
Decanal	0.133	0.382	1.270	93.1
(E, E)-2, 4-Nonedienal	0.027	0.076	0.245	90.1
(E,E)-2, 4-Decadienal	0.002	0.006	0.020	93.1
2-Ethyl-3, 6-dimethylpyrazine	0.002	0.007	0.023	90.6
Decanoic acid	0.004	0.010	0.038	89.7
Gamma-dodecalactone	0.002	0.005	0.018	99.1
Furfuryl octanoate	0.002	0.004	0.014	91.9
Hexadecanoic acid	0.028	0.076	0.274	90.1

Volatile Compounds in Roasted and Steamed Tropical Almond

The volatile compounds of roasted and steamed tropical almond nuts are listed, in order of increasing retention time, in Tables 3 and 4, respectively. A total of 72 compounds were identified in the roasted nuts, while the steamed nuts yielded 65 volatile compounds, respectively. The volatile compounds of the roasted nuts included 27 hydrocarbons, 12 aldehydes, 12 ketones, 8 acids, 4 esters, 3 alcohols, 3 furans, and 1 pyrazine. The carboxylic acids were the most abundant of these volatile compounds. They constituted approximately 33% of the total compounds. The carboxylic acids were closely followed by the aldehydes (22.08%), hydrocarbons (16.31%), and 2-ethyl-3, 6-dimethylpyrazine (15.39%). The alcohols contributed the least percentage (0.57%) to the total volatile compounds of the roasted tropical almond nuts. A similar trend was noticed in the steamed nuts. The carboxylic acids constituted 46% of the total volatile compounds identified in the steamed nuts. This was followed by aldehydes (37.6%), hydrocarbons (6%), ketones (4%), esters (3%), alcohols (2.3%), and others (2%).

Table 3 Volatile constituents of solvent extracted roasted (200°C) tropical almond nuts

Peak No.	Retention time (min)	Compound	Odour description ^a	Conc., mg/kg	% Total	Identification methods
1	5.76	3-Ethyl-2-methyl-1-heptene		3.68	0.49	MS
2	5.82	3-Acetyl-2, 5-dimethylfuran	Musty-nutty	4.85	0.65	MS, Odour
3	6.21	3, 4, 4-Trimethyl-2-hexene	Fresh citrus	2.25	0.30	MS, Odour
4	6.52	(Z)-Hept-2-enal	Almond-like	24.05	3.21	MS, Rt, Odour
5	6.90	2-Pentylfuran	Beany-like	13.95	1.86	MS, Odour
6	7.13	Octanal	Citrus-like	3.75	0.05	MS, Rt, Odour
7	7.51	Eucalyptol	Camphoraceous	2.63	0.35	MS, Odour
8	7.56	1-Chloro-2, 3-dihydro-1H-Indene		1.95	0.26	MS
9	7.82	n-Butylbenzene	Sweet pungent	1.20	0.16	MS, Odour
10	7.86	(E)-2-Octenal	Fresh cucumber	3.98	0.53	MS, Odour
11	7.94	2, 4-Dimethylheptane		2.78	0.37	MS
12	8.09	Hexachloroethane		2.63	0.35	MS
13	8.18	4-Methyl-3-heptanone		2.48	0.33	MS
14	8.28	(Z)-4-Tridecene	Waxy	14.70	1.96	MS, Odour
15	8.37	Nonanal	Tallow fruity	18.08	2.41	MS, Rt, Odour
16	8.51	1, 4-Dimethyl-5-oxabicyclo [2, 1, 0] pentane		1.80	0.24	MS
17	8.88	(E)-2-Nonenal	Fatty tallow	7.35	0.98	MS, Odour
18	8.98	Pentylbenzene	Sweet balsam	1.43	0.19	MS, Odour
19	9.03	(E)-2-Decenal	Orange-sweaty	2.03	0.27	MS, Odour
20	9.12	Octanoic acid	Sweaty	1.05	0.14	MS, Rt, Odour
21	9.29	n-Udecane		1.73	0.23	MS
22	9.49	Decanal	Green soapy	2.93	0.39	MS, Rt, Odour
23	9.69	(E, E)-2, 4-Nonadienal	Fatty	1.35	0.18	MS, Rt, Odour
24	9.87	1-Hexylcyclopentene		1.80	0.24	MS
25	9.95	2-Chloro-octane		1.88	0.25	MS
26	10.01	1-Undecyne	Vinegar-like	4.28	0.57	MS, Odour
27	10.03	Dodecamethyl-cyclo Hexasiloxane		4.50	0.60	MS
28	10.10	(Z)-2-Decenal	Green	32.18	4.29	MS, Odour
29	10.32	Tridecane	Waxy	1.88	0.25	MS, Odour
30	10.45	(E, E)-2, 4-Decadienal	Fatty, waxy	41.55	5.54	MS, Rt, Odour
31	10.71	2-Ethyl-3, 6-dimethylpyrazine	Chocolate	115.43	15.39	MS, Rt, Odour
32	10.94	8-Methyl-1-decene		1.13	0.15	MS
33	10.98	3-Nonen-2-one	Fruity	5.33	0.71	MS, Odour
34	11.05	n-Decanoic acid	Sweaty	13.5	1.80	MS, Rt, Odour
35	11.09	(E)-2-Dodecen-1-al	Citrus-metallic	15.65	2.09	MS, Odour
36	11.18	Gamma Dodecalactone	Fruity	0.83	0.11	MS, Rt, Odour
37	11.21	1-Dodecanol	Fatty, coconut	0.98	0.13	MS, Odour
38	11.24	2, 3-Nonadiene		1.05	0.14	MS
39	11.27	2, 4, 6-Trimethyl-decane		1.05	0.14	MS
40	11.31	4-Heptenal	Cream-like	5.78	0.77	MS, Odour
41	11.56	11-Dodecamethyl-hexasiloxane		8.93	1.19	MS
42	11.61	Vanillin	Vanilla-like	6.90	0.92	MS, Rt, Odour
43	12.06	(Z)-3-Nonen-1-ol	Waxy-green	0.68	0.09	MS, Odour
44	12.17	Pentadecane	Woody	8.03	1.09	MS, Odour
45	12.20	4-Hexyl-2, 5-dihydro-2, 5-Dioxo-3-furanacetic acid		3.08	0.41	MS
46	12.92	2, 4-Bis[(trimethylsily)oxy] benzoic acid		6.90	0.92	MS
47	12.98	3, 8-Dihydroxy-3, 4-Dihydronaphthalen-1-(2H)-one		1.28	0.17	MS
48	13.03	Unknown	Solvent-like	2.48	0.33	MS, Odour
49	13.83	Heptadecane	Fresh green	4.13	0.33	MS, Odour
50	13.90	2-Tridecanone	Slightly citrus	1.95	0.26	MS, Odour
51	14.03	2, 6-Dimethyl-2-octene		1.95	0.26	MS
52	14.08	Unknown		4.65	0.62	MS
53	14.48	Furfuryl octanoate	Fruity	3.30	0.44	MS, Rt, Odour
54	14.59	2, 7, 19-Trimethyldodecane		2.33	0.31	MS
55	14.96	6, 10, 14-Trimethyl-2-Pentadecanone	Celery-like	1.95	0.26	MS
56	15.13	Hexadecamethyl-octasiloxane		4.20	0.56	MS
57	15.22	Butyl-2-ethylhexyl ester	Fruity	6.60	0.88	MS, Odour
58	15.33	3, 6-Dimethyldecane		2.25	0.30	MS
59	15.43	2-Nonadecanone		2.03	0.27	MS
60	15.57	Nonadecanoic acid, methylester	Rancid sweaty	1.28	0.17	MS, Odour
61	15.74	Tetrahydro-6-tridecyl-2H-pyran-2-one	Oily-fruity	1.35	0.18	MS, Odour
62	15.88	Hexadecanoic acid	Waxy	157.80	21.04	MS, Rt, Odour
63	16.82	2-Pentadecanone	Fresh jasmine	2.55	0.34	MS, Odour
64	16.95	Dihydro-5-tetradecyl-2(3H)-furanone	Fruity	4.13	0.55	MS, Odour
65	17.07	(Z, Z)-9, 12-Octadecadienoic acid	Sweaty	19.95	2.66	MS, Odour
66	17.09	(E)-9-Octadecenoic acid	Sweaty	31.50	4.20	MS,Rt,Odour
67	17.18	6-Heptyltetrahydro-2H Pyran-2-one	Fruity, oily	27.08	3.61	MS, Odour
68	17.09	Octadecanoic acid		13.88	1.85	MS, Odour
69	19.44	Di-n-octylphthalate		3.98	0.53	MS
70	19.81	Octadecamethyl-Cyclonosiloxane		2.93	0.39	MS
71	20.22	11-(1-Ethylpropyl)heneicosane		2.48	0.33	MS
72	20.88	Squalene	Fish-like	15.38	2.05	MS, Odour
73	21.22	2, 6, 10, 14-Tetramethyl-Hexadecane		5.48	0.73	MS
74	22.41	6-Hydroxy-7-n-pentadecyl-Mercapto-5, 8-quinolindione		15.65	2.09	MS
^aOdour quality perceived at the sniffing port.

Table 4 Volatile compounds of solvent extracted steamed tropical almond nuts

Peak No.	Retention time (min)	Compound	Odour description ^a	Conc. mg.kg^−1	% Total	Identification method
1	5.53	2-Heptanone	Fruity, spicy	6.15	0.82	MS, RT, Odour
2	6.23	6-Methyl-1-octene	Herbaceous	1.58	0.21	MS, Odour
3	6.55	(Z)-Hept-2-enal	Almond-like	7.35	0.98	MS, RT, Odour
4	6.85	1, 5-Anhydro-1-rhamnitol		0.68	0.09	MS
5	6.90	(E, E)-2, 4-Nonadienal	Green, fatty	3.38	0.45	MS, Odour
6	6.93	2-Octanone	Apple-like	8.78	1.17	MS, Odour
7	7.12	1-Octanol	Green-orange	3.60	0.48	MS, RT, Odour
8	7.50	Eucalypol	Camphoraceous	2.18	0.29	MS, Odour
9	7.78	1-chloro-2, 3-dihydro-1H-indene		0.15	0.02	MS
10	7.80	Butylbenzene	Sweet	1.65	0.22	MS, Odour
11	7.86	(E)-2-Octenal	Fresh cucumber	0.60	0.08	MS, Odour
12	8.08	Hexachloroethane		1,28	0.17	MS
13	8.19	2-Nonanone	Fruity, soapy	1,73	0.23	MS, Odour
14	8.28	5-Methyl-2-hexen-4-ol		2.03	0.27	MS
15	8.36	Nonanal	Tallow, fruity	7.88	1.05	MS, RT, Odour
16	8.52	4-Hydroxyl-4-methyl-2-pentanone		1.50	0.20	MS
17	8.98	Pentyl-benzene	Sweet floral	2.25	0.30	MS, Odour
18	9.29	2, 4-Dimethyl-3-hexanone		0.98	0.13	MS
19	9.32	Unknown		1.05	0.14
20	9.48	Decanal	Green, soapy	1.88	0.25	MS, RT, Odour
21	9.95	(Z)-3-Methyl-5-undecene	Leafy	1.73	0.23	MS, Odour
22	10.03	2, 4-Dimethyl-1-decene	Tallow, fresh	3.30	0.44	MS, Odour
23	10.10	(Z)-2-Decenal	Green	16.50	2.20	MS, Odour
24	10.17	Nonanoic acid	Sweaty	1.88	0.25	MS, Odour
25	10.32	Tridecane	Waxy	1.13	0.15	MS, Odour
26	10.46	(E, E)-2, 4-Decadienal	Fatty, waxy	2.85	0.38	MS, RT, Odour
27	10.71	2-Ethyl-3, 6-dimethylpyrazine	Chocolate	6.30	0.84	MS, RT, Odour
28	10.94	1, 2-Cyclohexanediol		1.43	0.19	MS
29	10.99	3-Nonen-2-one	Fruity	3.38	0.45	MS, Odour
30	11.05	n-Decanoic acid	Sweaty	9.15	1.22	MS, RT, Odour
31	11.09	2-Undecenal		11.40	1.52	MS
32	11.18	gamma-n-Amylbutyrolactone	Creamy	4.8	0.64	MS, Odour
33	11.22	1-Tetradecanol		2.18	0.29	MS
34	11.27	n-Nonane		2.93	0.39	MS
35	11.56	1, 1, 1, 5, 5, 5, -Hexamethyl-3,3-bis (trimethylsily)oxy-trisiloxane		1.73	0.23	MS
36	12.07	3-Decen-1-ol		0.60	0.08	MS
37	12.18	Unknown		0.60	0.80	MS
38	12.34	Butylatedhydroxytoluene		3.00	0.40	MS
39	12.39	9-Dodecyltetradecahydro-anthracene		0.45	0.06	MS
40	12.92	Hexadecamethyl-cyclooctasiloxane		0.90	0.12	MS
41	12.99	3,4-Dihydro-8-hydroxy-3-methyl-1H-Benzopyran-1-one		0.90	0.12	MS
42	13.03	Pentadecane	Woody	0.90	0.12	MS, Odour
43	13.32	2-Chloroethylcaprate		0.90	0.12	MS
44	13.63	(Z)-4-Decen-1-ol		0.83	0.11	MS
45	13.68	(E)-5-Octadecene		0.68	0.09	MS
46	13.83	Heptadecane	Fresh, green	2.33	0.31	MS, Odour
47	13.91	2-Tridecanone	Citrus	0.98	0.13	MS, odour
48	14.48	Furfuryl octanoate	Fruity	1.20	0.16	MS, RT, Odour
49	14.60	3, 7-Dimethylnonane		1.50	0.16	MS
50	15.23	1, 2-Benzenedicarboxylic acid bis (2-methylpropyl)ester		2.10	0.28	MS
51	15.34	Eicosane		0.68	0.09	MS
52	15.58	Methylheptacosanoate		1.43	0.09	MS
53	15.90	n-Hexadecanoic acid	Waxy	235.7	31.4	MS, RT, Odour
54	16.96	Hexyloctylether		1.13	0.15	MS
55	17.11	(Z)-9-Octadecenal		230.0	30.7	MS,
56	17.23	Octadecanoic acid	Sweaty	100.05	13.3	MS, Odour
57	17.65	2-Chloroethylpalmitate		3.08	0.41	MS
58	17.97	Nonadecane		1.05	0.14	MS
59	18.55	Furfuryl hexanoate	Sweet,fruity	1.95	0.26	MS, Odour
60	18.65	Hexanedioc acid, bis(2-ethyl hexyl)ester	Fruity	1.58	0.21	MS, Odour
61	19.43	Di-n-Octylphthalate		7.43	0.99	MS
62	20.20	2, 3, 5, 8-Tetramethyldecane		0.75	0.01	MS
63	20.82	Squalene	Fish-like	4.43	0.59	MS, Odour
64	21.21	Tritetracontane		2.18	0.29	MS
65	22.39	gamma-Tocopherol		5.25	0.70	MS
66	22.73	Unknown		2.48	0.33
^aOdour quality perceived at the sniffing port.

All the main classes of compounds commonly listed as thermally generated flavors in oily seeds were identified in the roasted and steamed tropical almond nuts, with the most important, from a flavor standpoint, being furans and volatile heterocyclic compounds. Four furans were identified in both the roasted and steamed nuts. The furans were the musty-nutty, 3-acetyl-2, 5-dimethylfuran, furfuryl octanoate, and the beany-flavored 2-pentylfuran identified in the roasted nuts. On the other hand, furfuryl hexanoate and furfuryl octanoate were identified in the steamed nuts, respectively. 2-Pentyl furan was previously identified as a compound of the volatile decomposition products of slightly autoxidized soybean and cottonseed oils and those of thermal oxidation of corn oil and hydrogenated cottonseed oil, respectively.[13] Furans are products of carbohydrate thermal degradation and rearrangement.[14] The heating process catalyzes the reaction between amino acids and sugars via the Maillard reaction, which is responsible for the development of color and volatile heterocyclic compounds. In the present study, it is worthy to note that only one pyrazine derivative (2 ethyl-3, 6-dimethylpyrazine) was detected in the roasted and steamed almond nuts. Interestingly, this chocolate-like volatile compound yielded a significantly high concentration (115.43 mg kg⁻¹) in the roasted nuts than the steamed sample. The low number of pyrazine derivatives in tropical almond nuts might be due to the low lipid content. This is in agreement with previous studies that focused on the interaction of lipid content in the Maillard reaction.[15,16] Saittagaroon et al.[15] reported that defatting coconuts before roasting entirely modified their pleasant characteristic aroma, while Mottram and Edwards[16] reported that defatting meat before cooking strongly modified the aroma of the final product.

Although a significant number of hydrocarbons were identified in both roasted and steamed almond nuts, they are not likely to play a significant role since they possess a relatively weak aroma. However, a relatively small number of aldehydes, ketones, and alcohols were identified in this study (Tables 3 and 4). (Z)-Hept-2-enal, (E,E)2,4-nonadienal, (Z)-9-octadecenal, octanal, (E)2-octenal, nonanal, (E,E)2,4-decadienal, 3-nonen-2-one, 2,heptanone, 2-octanone, 2-nananone, 2-tridecanone, 6,10,14-trimethyl-2-pentadecanone, 1-octanol, 5, methyl-2-hexen-4-ol, 3-decen-1-ol, 1-dodecanol, and (Z)3-nonen-1-ol might play an important role in the sweet aroma of both roasted and steamed tropical almond nuts. Moreover, most of these volatile compounds have been identified in some oily seeds. For instance, (Z)-hept-2-enal, (E, E) 2, 4-nonadienal, nonanal, (Z)-2-decenal, (E, E) 2, 4-decadienal, and 3-nonen-2-one were reported in roasted sesame seeds,[17] roasted peanuts,[18] and as an off flavor in tuna fish oil.[19] (Z)-9-octadecenal and (E, E) 2, 4-decadienal were the most abundant aldehydes in the steamed and roasted nuts, respectively. (E, E) 2,4-Decadienal has been identified in the dried leaves of tropical almond[7] and it is formed by the oxidative degradation of linoleic and linolenic acids.[11]

The compound 6, 10, 14-trimethyl-2-pentadecanone, identified only in the roasted nuts, has a weak celery note and has been reported to contribute to the characteristic aroma of Terminalia catappa leaves.[7] Other compounds identified and not previously reported in other oily seeds are eucalyptol that has a characteristic camphoraceous pungent odor and squalene with a fish-like odor. Squalene has been reported in shark liver oil.[20] Tridacane with a characteristic waxy note was recently reported in a traditional Turkish dry fermented sausage.[21] Overall, the roasted almond nuts yielded more aldehydes, ketones, and hydrocarbons than the steamed nuts.

CONCLUSION

Roasting and steaming data revealed all the main classes of compounds commonly listed as thermally generated flavors in oily seeds. While 72 volatile compounds were identified in the roasted nuts, only 63 compounds were identified in the steamed nuts. Although, a significant number of hydrocarbons were identified in both roasted and steamed nuts, they are not likely to contribute to the characteristic aroma note of the nut. The small number of aldehydes, ketones, pyrazine, and alcohols identified are the ones most likely to contribute to the aroma of the nut. However, a detailed study of the potent odorants of the nut would be necessary to provide an insight into the role of the individual aroma compound already identified.