Expression of Multidisciplinary Flavour Science
CHANGES IN THE AROMA COMPONENTS OF PECANS DURING
ROASTING
K.R. CADWALLADER, H. Kim, S. Puangpraphant, and Y. Lorjaroenphon
Department of Food Science and Human Nutrition, University of Illinois at UrbanaChampaign, 1302 W. Pennsylvania Avenue, Urbana, IL 61801, USA
Abstract
Volatile components formed during the roasting of pecan kernels originate mainly via
lipid oxidation/degradation and Maillard/Strecker reactions. Of particular importance
to the sweet and nutty aroma of roasted pecans are 2-acetyl-1-pyrroline, 2-propionyl1-pyrroline, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, 3-methylbutanal, 3-ethyl-2,5dimethylpyrazine, 2-ethyl-3,5-dimethyl-pyrazine, 2,3-diethyl-5-methylpyrazine, 2pentylpyridine, and 2-acetyltetrahydropyridine. Results from this study provide useful
information to producers and end-users of pecan on the components that have the
greatest impact on the aroma quality of this important tree nut.
Introduction
Tree nuts are appreciated worldwide and are used extensively in confectionary,
bakery, culinary and other food product applications. Recent studies have
demonstrated various nutritional and health benefits associated with consumption of
tree nuts (1). Pecans (Carya illinoinensis) are the only native tree nuts grown for
commercial purposes in the US and rank number three worldwide behind almonds
and walnuts, with about 90M metric tons of pecans produced annually. Only a few
studies have been published on the volatile constituents of pecans (2-4). Our
previous studies indicated the involvement of numerous thermally generated volatile
compounds in the characteristic aroma of roasted pecan (5). The objective of the
present study was to monitor the generation of selected potent odorants in pecan
kernels during roasting.
Experimental
Roasting of pecans. Freshly shelled pecan kernels (halves; Stewart variety, 2007
season, Vienna, GA, USA) were roasted for 0, 10, 20 or 30 min) at 170 °C in a
forced-air oven.
Extraction of volatile compounds. Ground sample (50 g), plus 12.5 g of NaCl and
10 μL of internal standard solution containing 2-methyl-3-heptanone (4.31 mg/mL of
methanol), α-6-amylpyrone (0.81 mg/mL), 2,4,6-trimethylpyridine (1.21 mg/mL), 2ethylbutanoic acid (1.50 mg/mL) and ethyl maltol (2.08 mg/mL) was extracted with
diethyl ether (1 x 100 mL; 2 x 100 mL. The ether extract was subjected to solventassisted flavour evaporation (SAFE) and then separated into neutral, acidic and
basic components. Extracts were concentrated to 200 μL prior to GC-MS analysis
(6). Duplicate extractions were performed for each treatment.
GC-MS analysis. Analyses were performed using an Agilent 6890/5973N
GC/MSD system (Palo Alto, CA, USA) in EI mode (70 eV). Each extract (1 μL) was
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Expression of Multidisciplinary Flavour Science
injected in the cool on-column mode. Neutral and acidic compounds were separated
on a Stabilwax column (30 m length x 0.25 mm I.D x 0.25 um film, Restek, Bellefonte,
PA, USA). Oven temperature was programmed from 40 to 225 °C at a rate of 4
°C/min with initial and final hold times of 5 and 20 min, respectively. Basic
compounds were separated using a SAC-5 column (30 m length x 0.25 mm I.D x
0.25 um film, Supelco, Bellefonte, PA, USA). The GC oven was held at 40 for 5 min,
ramped to 240 °C at 6 °C/min, and held at 240 °C for 20 min. Helium was used as a
carrier gas at a constant flow of 1.0 mL/min.
Calibration procedures. For accurate quantification, response factors were
determined using five-point standard curves. Recovery factors were determined
using soybean oil (Crisco, Orrville, OH, USA) as a mimic matrix, which was then
subjected to the above mentioned extraction procedures and GC-MS analyses.
Enantiomeric composition. Enantiomeric composition of selected lactones (nos. 911) were determined by GC-MS using an Inertcap Chiramix column (30 m length x
0.32 mm I.D x 0.32 um film, GL Science Inc., Tokyo, Japan). The oven was held at
120 °C for 2 min, ramped to 165 °C (held for 20 min) at a rate of 5 °C/min. Carrier
gas (He) was set at a constant flow of 1.5 mL/min. MSD conditions were same as
above, except SIM mode was used for detection of no. 9 (ions 97 and 68) and nos.
10 and 11 (ion 85).
Odour threshold determination. Orthonasal odour detection threshold for 6Smassoialactone (Fleyrchem Inc., Middletown, NY, USA) was determined in fresh
canola oil using ASTM procedure E679-91 (7).
Results
Concentrations for selected volatile constituents are given in (Table 1). Lipid
(thermal) oxidation and Maillard/Strecker degradation reactions accounted for the
formation of most of the predominant odorants in roasted pecan. Highest rate of
formation for lipid-derived compounds occurred between 20 to 30 min, while Strecker
aldehydes formed more rapidly between 10 and 20 min. Most of the other Maillard
reaction volatiles formed at a steady rate between 10 and 30 min. Based on their
relatively high odour-activity values (OAVs), odorants with green/rancid/fatty (e.g nos.
4-8, 20), coconut-like (no. 9), caramel (no. 13) and nutty/roasty (nos. 1, 14-19) notes
were the main contributors to the overall aroma of roasted pecan (Figure 1).
Roasting caused great increases in some lipid-derived odorants, e.g, (E,E)-2,4decadienal (no. 8) increased by > 20,000 fold after roasting for 30 min. Despite the
large increases in the lipid-derived aldehydes, the OAVs for these compounds were
only moderately high due to their relatively high odour detection thresholds in the
lipid-based medium (pecan contains over 70% unsaturated lipid). The aroma of
pecan, especially in its roasted form, might be influenced by presence of lactones
(especially no. 9). Lactones have not been previously reported pecan and have been
only rarely reported in other tree nuts (9). Among the lactones, the coconut-smelling
massoialactone increased by ∼75 fold as a result of roasting (170 °C; 30 min).
Surprisingly, its enantiomeric ratio (85% 6S: 15% 6R) was not affected by roasting
(data not shown).
A number of N-containing heterocyclic volatiles were formed during roasting of
pecan. The harsh/green smelling 2-pentyl pyridine (no. 20) may have been formed by
the reaction of 2,4-decadienal (no. 8) with free ammonia liberated from amide groups
of amino acids during roasting (10). Roasty/nutty odorants (nos. 14-19) appear to
very important to the overall aroma of roasted pecan. In particular, 2-acetyl-1302
Expression of Multidisciplinary Flavour Science
pyrroline (no. 14) and 2-propionyl-1-pyrroline (no. 15) had by far the highest OAVs
among the compounds listed in Table 1. Both of these compounds have been
previously reported as key odorants in various heated foods (11).
Table 1. Concentrations of selected volatile components of natural (raw) and
roasted pecan kernels.
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Roasting period (170 °C) (ng/g) b
raw
10 min 20 min 30 min
Compound a
Neutral compounds c
2-methylbutanal [57]
3-methylbutanal [58]
Phenylacetaldehyde [120]
Hexanal [56]
(E)-2-Nonenal [70]
(E)-2-Decenal [70]
(E)-2-Undecenal [70]
(E,E)-2,4-Decadienal [81]
6-Pentyl-5,6-dihydro-2H-pyran-2-one
(massoialactone) (85:15 6S/6R ratio)
[97] d
γ-Nonalactone (racemic) [85] d
γ-Decalactone (racemic) [85] d
Acidic compounds e
3-Hydroxy-2-methyl-4-pyrone (maltol)
[126]
4-Hydroxy-2,5-dimethyl-3(2H)furanone (HDMF) [128]
Basic compounds g
2-Acetyl-1-pyrroline [83]
2-Propionyl-1-pyrroline [97]
2-Acetyltetrahydropyridine (sum of
tautomers) [125]
3-Ethyl-2,5-dimethylpyrazine [135]
2-Ethyl-3,5-dimethylpyrazine [135]
2,3-Diethyl-5-methylpyrzine [150]
2-Pentylpyridine [93]
a
15
7.4
19
335
13
12
3.3
0.4
94
99
82
117
605
26
110
132
124
598
833
419
309
2630
113
742
991
1390
4460
1240
474
336
7100
342
3080
3880
8150
6960
43
122
50
116
80
118
215
139
- -f
208
211
500
--
874
933
2450
----
----
80
40
6.9
146
66
81
-----
21
0.5
---
444
9.8
28
29.3
992
20
100
113
Numbers in brackets indicate mass ion used for quantitative analysis. b Average of
duplicate determinations. c Determined against 2-methyl-3-heptanone (i.s.) unless
otherwise noted. d Determined against 6-α-amylpyrone (i.s.). e Determined against
ethyl maltol (i.s.). f Not detected. g Determined against 2,4,6-trimethylpyridine (i.s.).
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Expression of Multidisciplinary Flavour Science
O
O
O
2
3
4
OAV = 36
OAV = 15
OAV = 59
O
O
O
O
O
8
OAV = 45
O
N
N
O
HO
13
OAV = 98
9
a
OAV = 6
15
14
b
OAV = 1460
OAV = 660
O
N
16
b
OAV = 50
N
N
N
N
N
17
19
20
OAV = 41
OAV = 200
OAV = 23
Figure 1. Structures and odour-activity values (OAVs) for selected volatile
components of pecan kernels roasted at 170 °C for 30 min. [aOAVs were
calculated from odour detection threshold determined in canola oil (1100
ng/g for no. 9 in present study) or from published odour detection
thresholds in oil or bwater (from 10).]
References
1.
Mukuddem-Peterson J., Oosthuizen W., Jerling J.C. (2005) J. Nutr. 135: 20822089.
2. Horvat R.J., Senter S.D. (1980) J. Am. Oil Chem. Soc. 57: 111.
3. Mody N.V., Hedin P.A., Neel W.W. (1976) J. Agric. Food Chem. 24: 175-177.
4. Wang P.S., Odell G.V. (1972) J. Agric. Food Chem. 20: 206-210.
5. Puangpraphant S. (2007) MS Thesis, University of Illinois, December 2007.
6. Rotsatchakul P., Chaiseri S., Cadwallader, K.R. (2008) J. Agric. Food Chem. 56:
528-536.
7. ASTM. (1992) Standard practice E 679-91, Determination of Odor and Taste
Thresholds by a Forced-Choice Ascending Concentration Series Method of
Limits; American Society for Testing and Materials, Philadelphia, PA.
8. Rychlik M., Schieberle P., Grosch W. (1998). Compilation of Odor Thresholds,
Odor Qualities and Retention Indices of Key Food Odorants; Deutsche
Forschungsanstalt für Lebensmittelchemie and Institut für Lebensmittelchemie
der Technischen Universität München, Garching, Germany.
9. Cadwallader K.R., Puangpraphant S. (2008) In Tree Nut Neutraceuticals and
Pytochemicals. (Alasalvar C., Shahidi F., eds.); Taylor and Francis, LLC (in
press).
10. Kim Y-S, Hartman T.G., Ho C.-T. (1996) J. Agric. Food Chem. 44: 396-3908.
11. Hofmann T., Schieberle P. (1998) J. Agric. Food Chem. 46: 2721-2726.
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