Investigation of the Volatile Profile of Red Jujube by Using GC-IMS, Multivariate Data Analysis, and Descriptive Sensory Analysis

Abstract

The aroma characteristics of six red jujube cultivars (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’, Qiyuexian—‘QYX’, Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’), cultivated in Xinjiang Province, China, were studied by E-nose and GC-IMS. The presence of acetoin, E-2-hexanol, hexanal, acetic acid, and ethyl acetate played an important role in the classification results. JC, JZ, HZ, and YZ were different from others, while QYX and HTDZ were similar to each other. HZ had the most abundant specific VOCs, including linalool, nonanoic acid, methyl myristoleate, 2-acetylfuran, 1-octen-3-one, E-2-heptenal, 2-heptenone, 7-octenoic acid, and 2-pentanone. HZ had higher intensity in jujube ID, floral, sweet, and fruity attributes. Correlation analysis showed that jujube ID (identity) might be related to phenylacetaldehyde and isobutanoic acid that formed by the transamination or dehydrogenation of amino acids; meanwhile, the sweet attribute was correlated with amino acids, including threonine, glutamic acid, glycine, alanine, valine, leucine, tyrosine, phenylalanine, lysine, histidine, and arginine.

Full text

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Citation: Qiao, Y.; Chen, Q.; Bi, J.;
Wu, X.; Jin, X.; Gou, M.; Yang, X.;
Purcaro, G. Investigation of the
Volatile Profile of Red Jujube by
Using GC-IMS, Multivariate Data
Analysis, and Descriptive Sensory
Analysis. Foods 2022,11, 421.
https://doi.org/10.3390/
foods11030421
Academic Editor:
Remedios Castro-Mejías
Received: 21 December 2021
Accepted: 24 January 2022
Published: 31 January 2022
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foods
Article
Investigation of the Volatile Profile of Red Jujube by Using
GC-IMS, Multivariate Data Analysis, and Descriptive
Sensory Analysis
Yening Qiao 1,2,† , Qinqin Chen 1 ,† , Jinfeng Bi 1, *, Xinye Wu 1,2, Xinwen Jin 3, Min Gou 1,2, Xinrui Yang 1
and Giorgia Purcaro 2, *
1
Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences (CAAS), Key Laboratory
of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing 100193, China;
yening.qiao@student.uliege.be (Y.Q.); celerylc@163.com (Q.C.); xinye.wu@student.uliege.be (X.W.);
Min.Gou@student.uliege.be (M.G.); 15003702066@163.com (X.Y.)
2Gembloux Agro-Bio Tech Department, University of Liége, 5030 Gembloux, Belgium
3Institute of Agro-Products Processing Science and Technology, XinJiang Academy of Agricultural and
Reclamation Science, Shihezi 832000, China; njs701022@163.com
*Correspondence: bjfcaas@126.com (J.B.); gpurcaro@uliege.be (G.P.)
† These authors contributed equally to this work.
Abstract:
The aroma characteristics of six red jujube cultivars (Jinchang—‘JC’, Junzao—‘JZ’,
Huizao—‘HZ’
,
Qiyuexian—‘QYX’, Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’), cultivated in Xinjiang Province, China,
were studied by E-nose and GC-IMS. The presence of acetoin, E-2-hexanol, hexanal, acetic acid, and
ethyl acetate played an important role in the classification results. JC, JZ, HZ, and YZ were different
from others, while QYX and HTDZ were similar to each other. HZ had the most abundant specific
VOCs, including linalool, nonanoic acid, methyl myristoleate, 2-acetylfuran, 1-octen-3-one, E-2-heptenal, 2-
heptenone, 7-octenoic acid, and 2-pentanone. HZ had higher intensity in jujube ID, floral, sweet, and fruity
attributes. Correlation analysis showed that jujube ID (identity) might be related to phenylacetaldehyde
and isobutanoic acid that formed by the transamination or dehydrogenation of amino acids; meanwhile,
the sweet attribute was correlated with amino acids, including threonine, glutamic acid, glycine, alanine,
valine, leucine, tyrosine, phenylalanine, lysine, histidine, and arginine.
Keywords: red jujube; aroma distinction; E-nose; GC-IMS; sensory attributes
1. Introduction
Jujube (Ziziphus Jujuba Mill.), belonging to the family Rhamnaceae, is a plant largely
distributed in tropical and sub-tropical regions. In particular, it is widely distributed
in China, where its fruits are regularly consumed for their good aroma, delicious taste,
and high nutraceutical value in the Chinese medicine tradition. Moreover, at present,
red jujubes are also widely used in the food industry (as an ingredient of tea, snacks,
bread, cakes, yogurt, etc.) [
1
]. There are over 1000 varieties of red jujube cultivated in
China, with distribution in Xinjiang, Gansu, Ningxia, Shaanxi, Shanxi, Shandong, Hebei,
and Henan Provinces [
2
], but Xinjiang Province (latitude: 34
◦
22
0
N~49
◦
10
0
N; longitude:
73◦400E
~96
◦
23
0
E; elevation: 967.2 m~1388.78 m) is by far the largest jujube-producing
region in China, with an annual output of more than 1.45 million tons in 2018 [
3
]. The
quality of red jujubes from Xinjiang is superior to other regions because of the low rainfall,
with periodic drought, abundant sunshine, and substantial differences between day and
night temperatures [
4
]. Different cultivars of red jujubes are present in China, among which
Huizao (HZ), Junzao (JZ), Yuanzao (YZ), Qiyuexian (QYX), Jinchang (JC), and Hetiandazao
(HTDZ) are the most widely cultivated in different regions of Xinjiang Province. Huizao
(HZ) alone accounts for 62.9% of the production from the region, followed by Junzao (JZ),
which accounts for 32.7%. Moreover, Charkhlik HZ and Khotan JZ are two cultivars of
Foods 2022,11, 421. https://doi.org/10.3390/foods11030421 https://www.mdpi.com/journal/foods

Foods 2022,11, 421 2 of 12
protected geographical indication [
3
]. Several studies on phytochemical profiling, bioactive
components, and pathogenic factor analysis have been carried out to better understand
the quality gained by red jujube cultivars planted in Xinjiang [
5
,
6
]. Nevertheless, no
studies have been conducted to investigate the particular aroma profile of the red jujube
cultivated in Xinjiang Province, and to correlate the sensory characteristics with a chemical
composition including VOCs, fatty acids, amino acids, organic acids, and sugars.
In fact, the primary metabolite distribution in fruits (including sugars, organic acids,
and amino acids) may be associated with sensory traits such as sweetness and sourness,
both as precursors of VOCs related to aroma and taste, or as involved in the browning
reaction [
7
,
8
]. For example, the content of sucrose in baked potatoes had an influence on the
sweetness [
9
]. The composition of citric, quinic, and malic acids had a strong relationship
with sourness in blackcurrant juice [
10
]. Threonine, serine, and alanine were found to be
correlated with the sweet attribute, aspartate and glutamate were correlated with the sour
attribute, and valine, methionine, isoleucine, leucine, and arginine were found to contribute
to the bitter taste in table grape berries [11].
E-noses have been applied to successfully discriminate the aroma qualities of food,
such as the varieties of jujube and Lycium ruthenicum Murray from different provinces,
harvest years, and varieties. The sum of PC1 and PC2 was in the range of 80~98% for
the classifications [
12
–
14
]. The aim of this work is to investigate the aroma profile of red
jujube cultivars cultivated in Xinjiang Province by using gas chromatography–ion mobility
spectrometry (GC-IMS) and electronic noses (E-noses), and investigate the relationship
between the sensory perception and the characteristic VOCs and metabolite precursors.
The data from the volatile profile was elaborated for the following reasons: (i) to
discriminate among the different cultivars of red jujube (JC, JZ, HZ, QYX, HTDZ, and
YZ cultivated in Xinjiang Province; (ii) to investigate the correlation between sensory
attributes and chemical compositions including VOCs, fatty acids, amino acids, organic
acids, and sugars.
2. Materials and Methods
2.1. Plant Material Preparation
Six cultivars of red jujubes (Ziziphus Jujuba Mill., JC, JZ, HZ, QYX, HTDZ, and YZ,
each cultivar with three biological repeats), which were grown in the Xinjiang Province
of China, were used in this study (Table S1). The botanical identification was confirmed
by expert botanists. Fifty kilograms of each cultivar of red jujube were bought at mature
commercial stage in the Beiyuanchun Jujube Market (Urumqi, China, 2018). Red jujubes
without physical damage and/or infections were selected and stored at 4
◦
C before analysis
within 72 h.
2.2. Chemicals and Regents
The chemical standards of formic acid, hexanoic acid, propionic acid, acetic acid, isobu-
tyric acid, 3-methylbutanoic acid, pentanoic acid, 3-heptenoic acid, nonanoic acid, crotonic
acid, 7-octenoic acid, n-decanoic acid, heptanoic acid, 2-heptenoic acid, ethanol, linalool,
E-2-hexenol, 1-octen-3-ol, 5-methyl-2-furanmethanol, 6-methyl-5-hepten-2-ol, 1-nonen-4-ol,
butanal, 2-methylbutanal, 3-methylbutanal, hexanal, furfurol, benzaldehyde, E-2-heptenal,
E-2-octenal, n-nonanal, phenylacetaldehyde, pentanal, acetoin, 6-methyl-5-hepten-2-one,
1-octen-3-one, 2-pentanone, acetone, 3-octanone, 2-hexanone, 2-heptanone, methyl ac-
etate, ethyl acetate, ethyl propanoate, propyl acetate, ethyl 2-hydroxypropanoate, ethyl
3-methylbutyrate, butyl acetate, isoamyl acetate, ethyl pentanoate, butyrolactone, ethyl
hexanoate, methyl myristoleate, methyl hexanoate, ethyl benzoate, methyl benzoate, ethyl
heptanoate, hexyl butanoate, gamma-terpinene, alpha-phellandrene, myrcene, limonene,
2-acetylfuran, 2-pentyl furan, o-cymene, and 2-cyclohexenone were purchased from Milli-
poreSigma (St. Louis, MO, USA).
Malic acid, citric acid, quinic acid, lactic acid, tartaric acid, glucose, fructose, maltose,
sodium hydroxide, and potassium hydroxide were of HPLC grade (Sinopharm Chemical

Foods 2022,11, 421 3 of 12
Reagent Co., Ltd., Beijing, China). Glutamic acid (GLU) and aspartic acid (ASP) were
obtained from MilliporeSigma; serine (SER), threonine (THR), aspartic acid (ASP), glycine
(GLY), alanine (ALA), citrulline (CIT), valine (VAL), methionine (MET), isoleucine (ILE),
leucine (LEU), tyrosine (TYR), lysine (LYS), histidine (HIS), arginine (ARG), and proline
(PRO) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan).
2.3. Sample Preparation
The detailed procedure of sample preparation was referred to by Chen et al. (2018) [
12
].
Approximately 100 g of red jujubes were washed with running water and the moisture on
the surface was wiped with filter papers. The kernel was removed, and the red jujubes
were sliced and ground for 90 s with a juicer (JYL-CO20, Joyoung Co., Ltd., Jinan, China).
A weight of 2.0 g red jujube pulp was placed into a 20 mL vial, which was sealed with a
magnetic screw cap and septum before E-nose or GC-IMS testing.
2.4. E-nose Analysis
A commercial PEN 3.5 E-nose (Airsense Analytics, GmBH, Schwerin, Germany) con-
taining ten metal oxide semiconductors (MOS) (Table S2) was used to distinguish among
the overall aroma perception of the six cultivars of red jujubes. The samples were firstly
equilibrated at room temperature (26
◦
C) for 30 min. The gaseous compounds in the
headspace were pumped through a Teflon tube into the sensor arrays at 400 mL/min using
reference air (filtered through charcoal). This clean air was also used to rinse the system
at a flow rate of 600 mL/min. The cleaning time, zero adjustment time, and detection
time were 180 s, 10 s, and 60 s, respectively. During the monitoring of the sample gas, the
ratio of G (the conductance of a sensor exposed to sample gas) to G0 (the conductance of a
sensor exposed to zero gas) for each sensor changed. WinMuster Software was used for
data processing. Five repeats from the same 100 g sample were prepared.
2.5. HS-GC-IMS Analysis
A GC-IMS (GC from Agilent Technologies, Palo Alto, CA, USA; IMS from FlavourSpec
®
,
Gesellschaft für Analytische Sensorsysteme mbH, Dortmund, Germany) system equipped
with an autosampler unit (CTC Analytics AG, Zwingen, Switzerland) was used in this
project. The vials prepared as in Section 2.3 were incubated at 50
◦
C for 20 min. Then a
headspace volume of 500
µ
L was collected from the vials with a heated syringe (85
◦
C) and
injected at 80
◦
C in splitless mode. GC conditions were as follows: the chromatographic
column was a FS-SE-54-CB-1 (15 m
×
0.53 mm ID
×
1.0
µ
m d
f
) column (60
◦
C isothermal
conditions). This is because the FS-SE-54-CB-1 column has also been used for the analysis of
the volatile organic compounds in winter jujube and avocado based on the comprehensive
separation effect (a better separation effect, a shorter separation time, and the boiling points,
polarities, and species of VOCs) [
15
,
16
]. The program for carrier gas (nitrogen, 99.999%)
was as follows: 2 mL/min for 2 min, ramped up to 10 mL/min for 8 min, then when
100 mL/min was reached for 10 min, finally altered to 150 mL/min for 5 min. All the
standards that are presented in Section 2.1 were run under the same test procedure to
support the GC-IMS Library Search for qualitative analysis. For the semi-quantitative
analysis, 2-cyclohexenone (2 mg/L) was selected as the internal standard.
IMS conditions were as follows: ion source was a tritium source (5.68 keV), positive
ion mode, drift tube length 9.8 cm, tube linear voltage 500 V/cm, and drift gas flow rate
150 mL/min (nitrogen, purity 99.999%). The temperature of the drift tube was 45 ◦C.
The spectrogram was elaborated using Laboratory Analytical Viewer (LAV). The
Reporter Plug-In software was used to compare spectrogram differences between samples.
Gallery Plot Plug-In was used to compare the differences in volatile fingerprints visually
and quantitatively. Three technical repeats were conducted for each cultivar.

Foods 2022,11, 421 4 of 12
2.6. Sensory Analysis
The sensory analysis was conducted according to the method set by Galindo et al.
(2015) [
17
]. The sensory analysis aimed to evaluate the following sensory attributes: sour,
sweet, bitter, astringent, jujube ID, fruity, and floral [
15
]. The intensities of the various
sensory attributes were evaluated using a numerical scale, where 0 represented none and
10 represented extremely strong, with 0.1 increments. A panel of twenty trained assessors
(aged 21–30, ten females and ten males) was invited to evaluate the flavor of red jujubes.
The assessors were recruited from fruit and vegetable processing teams and had extensive
experience in fruit sensory evaluation. For the evaluation, 15 red jujubes for each cultivar
per panelist were served in the testing room at 26
◦
C. There was a 10 min wait among the
sensory evaluation for different cultivars.
2.7. Analysis of Fatty Acids, Amino Acids, Monosaccharides, and Organic Acids
The determination of fatty acids used the method reported by Song et al. (2019),
using a GC–flame ionization detector (FID) (GC6890N, Agilent, Santa Clara, CA, USA).
Undecanoic acid (5 mg/mL) was used as internal standard and the results were expressed
in mg/g dry basis [5]. Analyses were carried out in triplicate.
The determination of free amino acids used the method reported by Song et al. (2019),
using a Hitachi model L-8900 amino acid analyzer (Hitachi Co. Ltd., Tokyo, Japan) with a
column packed with Hitachi custom ion-exchange resin 2622 (4.6 mm
×
60 mm, particle
size 5 µm).
Quantitative analyses of monosaccharides and organic acids in red jujubes were
performed by high-performance anion-exchange chromatography equipped with pulsed
amperometric detection (Dionex ICS-3000 system) according to Šimkovic et al. (2009) [
14
].
The standards of sugars and organic acids were tested under the same conditions for
quantitative analysis using external calibrations.
2.8. Statistical Analysis
MetaboAnalyst (version 5.0, Edmonton, AB, Canada) (www.metaboanalyst.ca) was
used for the calculation of VIP scores for VOCs. R (version 2.13.0) (Auckland, New Zealand)
was used for the loading plot of principle component analysis (PCA) and correlation
analysis between sensory attributes and chemical compositions. Each sample was analyzed
in triplicate except for E-nose analysis, which had five replicates.
3. Results and Discussion
3.1. E-nose Analysis
The averaged response values of different red jujube samples on ten MOS sensors
are reported in Figure 1. The response values showed a relative standard deviation (RSD)
lower than 5% (Table S2). The response signals for different red jujube samples increased at
different rates. The most significant increase and the highest level in response signals were
found in W1W sensitive to sulfides (minimum and maximum were 10.5~31.9), followed by
W5S sensitive to nitroxide (6.0~29.0), W2W sensitive to aromatic components and organic
sulfide (4.6~12.4), W2S (2.8~5.0) and W1S sensitive to methane (2.6~4.7). However, few
responses were shown in the sensors, including W6S, W3C, W5C, W3S, and W1C (around
1.0), sensitive to hydrogen, ammonia and aromatic components, alkanes and aromatics,
alkanes, and aromatics, respectively. The response values of W1W (31.9), W5S (29.0), and
W2W (12.4) were notably higher in HZ, followed by JZ (W1W, 20.2; W5S, 15.6; W2W, 8.1),
JC (W1W, 17.5; W5S, 10.1; W2W, 7.1) and YZ (W1W, 12.2; W5S, 8.1; W2W, 5.1). As shown in
Figure 1, QYX and HTDZ showed the same response level in the sensors of W1W, W2W,
W2S, W1S, W3S, and W5C, compared with other cultivars.

Foods 2022,11, 421 5 of 12
Foods 2022, 11, x FOR PEER REVIEW 5 of 14
and organic sulfide (4.6~12.4), W2S (2.8~5.0) and W1S sensitive to methane (2.6~4.7). How-
ever, few responses were shown in the sensors, including W6S, W3C, W5C, W3S, and
W1C (around 1.0), sensitive to hydrogen, ammonia and aromatic components, alkanes
and aromatics, alkanes, and aromatics, respectively. The response values of W1W (31.9),
W5S (29.0), and W2W (12.4) were notably higher in HZ, followed by JZ (W1W, 20.2; W5S,
15.6; W2W, 8.1), JC (W1W, 17.5; W5S, 10.1; W2W, 7.1) and YZ (W1W, 12.2; W5S, 8.1; W2W,
5.1). As shown in Figure 1, QYX and HTDZ showed the same response level in the sensors
of W1W, W2W, W2S, W1S, W3S, and W5C, compared with other cultivars.
Figure 1. The response data of sensors in E-noses on volatiles of the six red jujube cultivars in Xin-
jiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,Qiyuexian—‘QYX’, Heti-
andazao—‘HTDZ’, and Yuanzao—‘YZ’). Lower letters (a–f) correspond to significant differences at
p < 0.05.
The data were processed using principal component analysis (PCA) to visualize
whether the E-nose measurement was able to distinguish between the different red jujube
cultivars. The sum of PC1 and PC2 accounts for 71.7% of the E-nose results (Figure 2). The
technical repeats for the six cultivars were well clustered, except for a slight overlap of the
HTDZ and QYX cultivars. The response value on W1W played an important role in the
clustering of HTDZ and QYX (Figure 2). Meanwhile, the separation of JZ was mainly
caused by the response value of W1C.
Figure 1.
The response data of sensors in E-noses on volatiles of the six red jujube culti-
vars in Xinjiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,Qiyuexian—‘QYX’,
Hetiandazao—‘HTDZ’
, and Yuanzao—‘YZ’). Lower letters (a–f) correspond to significant differences
at p< 0.05.
The data were processed using principal component analysis (PCA) to visualize
whether the E-nose measurement was able to distinguish between the different red jujube
cultivars. The sum of PC1 and PC2 accounts for 71.7% of the E-nose results (Figure 2). The
technical repeats for the six cultivars were well clustered, except for a slight overlap of
the HTDZ and QYX cultivars. The response value on W1W played an important role in
the clustering of HTDZ and QYX (Figure 2). Meanwhile, the separation of JZ was mainly
caused by the response value of W1C.
Foods 2022, 11, x FOR PEER REVIEW 6 of 14
Figure 2. Principle component analysis (PCA) and loading plot of E- nose data for the six red jujube
cultivars in Xinjiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,Qiyuexian—
‘QYX’, Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’).
E-nose cannot provide qualitative information on the specific chemical compounds
responsible for the observed discrimination, but when properly trained and validated
with more information-rich techniques (e.g., GC-IMS), it could be used as a valid routine
method to discriminate the different jujube varieties.
3.2. GC-IMS Analysis
The chromatograms of GC-IMS are shown in Figure S1. Sixty-four compounds were
identified by GC-IMS (Table S3) in the six cultivars of red jujubes, including esters (17),
acids (14), aldehydes (11), ketones (8), alcohols (6), terpenoids (6), and furans (2). Sixteen
compounds, including hexanoic acid, isobutyric acid, 3-methylbutanoic acid, n-decanoic
acid, heptanoic acid, 1-octen-3-ol, hexanal, benzaldehyde, E-2-heptenal, n-nonanal, 6-me-
thyl-5-hepten-2-one, ethyl hexanoate, ethyl benzoate, methyl benzoate, 2-pentyl furan,
and o-cymene, were reported in previous studies [18].
The overall distribution of the different chemical classes according to the cultivar is
reported in Figure 3. The 64 compounds identified were used to perform a hierarchical
cluster analysis and the results were visualized using a heatmap (Figure 4). All the culti-
vars were well discriminated. The proximity of QYX and HTDZ was confirmed, although
they were separated in the hierarchical cluster analysis. Overall, acids (formic acid, hexa-
noic acid, propionic acid, acetic acid, isobutyric acid, 3-methyl butanoic acid, pentanoic
acid, 3-heptenoic acid, crotonic acid, nonanoic acid, 7-octenoic acid, n-decanoic acid, hep-
tanoic acid, and 2-heptenoic acid), esters (isoamyl acetate, ethyl acetate, ethyl propanoate,
ethyl pentanoate, ethyl hexanoate, butyl acetate, ethyl 3-methylbutyrate, ethyl heptano-
ate, hexyl butanoate, propyl acetate, and ethyl 2-hydroxypropanoate), and aldehydes (bu-
tanal, 2-methylbutanal, 3-methylbutanal, hexanal, furfurol, benzaldehyde, E-2-heptenal,
E-2-octenal, n-nonanal, phenylacetaldehyde, and pentanal) were the more predominant
VOCs of the six red jujube cultivars (Figure 3, Table S3).
Figure 2.
Principle component analysis (PCA) and loading plot of E- nose data for the six red jujube
cultivars in Xinjiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,Qiyuexian—‘QYX’,
Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’).
E-nose cannot provide qualitative information on the specific chemical compounds
responsible for the observed discrimination, but when properly trained and validated with

Foods 2022,11, 421 6 of 12
more information-rich techniques (e.g., GC-IMS), it could be used as a valid routine method
to discriminate the different jujube varieties.
3.2. GC-IMS Analysis
The chromatograms of GC-IMS are shown in Figure S1. Sixty-four compounds were
identified by GC-IMS (Table S3) in the six cultivars of red jujubes, including esters (17),
acids (14), aldehydes (11), ketones (8), alcohols (6), terpenoids (6), and furans (2). Sixteen
compounds, including hexanoic acid, isobutyric acid, 3-methylbutanoic acid, n-decanoic
acid, heptanoic acid, 1-octen-3-ol, hexanal, benzaldehyde, E-2-heptenal, n-nonanal, 6-
methyl-5-hepten-2-one, ethyl hexanoate, ethyl benzoate, methyl benzoate, 2-pentyl furan,
and o-cymene, were reported in previous studies [18].
The overall distribution of the different chemical classes according to the cultivar is re-
ported in Figure 3. The 64 compounds identified were used to perform a hierarchical cluster
analysis and the results were visualized using a heatmap (Figure 4). All the cultivars were
well discriminated. The proximity of QYX and HTDZ was confirmed, although they were
separated in the hierarchical cluster analysis. Overall, acids (formic acid, hexanoic acid, pro-
pionic acid, acetic acid, isobutyric acid, 3-methyl butanoic acid, pentanoic acid, 3-heptenoic
acid, crotonic acid, nonanoic acid, 7-octenoic acid, n-decanoic acid, heptanoic acid, and
2-heptenoic acid), esters (isoamyl acetate, ethyl acetate, ethyl propanoate, ethyl pentanoate,
ethyl hexanoate, butyl acetate, ethyl 3-methylbutyrate, ethyl heptanoate, hexyl butanoate,
propyl acetate, and ethyl 2-hydroxypropanoate), and aldehydes (butanal, 2-methylbutanal,
3-methylbutanal, hexanal, furfurol, benzaldehyde, E-2-heptenal, E-2-octenal, n-nonanal,
phenylacetaldehyde, and pentanal) were the more predominant VOCs of the six red jujube
cultivars (Figure 3, Table S3).
Foods 2022, 11, x FOR PEER REVIEW 7 of 14
Figure 3. Semi-quantitative results of GC-IMS analysis of the volatiles in six red jujube cultivars
taken from Xinjiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,Qiyuexian—
‘QYX’, Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’). Lower letters (a–f) correspond to significant
differences at p < 0.05.
Figure 3.
Semi-quantitative results of GC-IMS analysis of the volatiles in six red jujube cultivars
taken from Xinjiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,Qiyuexian—‘QYX’,
Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’). Lower letters (a–f) correspond to significant differences
at p< 0.05.
HZ had the highest content of ketones (3.96
µ
g/g), including acetoin, 1-octen-3-one, 6-
methyl-5-hepten-2-one, acetone, 2-hexanone, 2-pentanone, and 3-octanone. Acetoin acts as
an aglycone and exists in a glycosidically bound form in some fruits and vegetables (banana,
lychee, durian and kiwi fruit flowers, etc.), with a pleasant yogurt creamy odor [
19
]. Further,
1-octen-3-one was a result of the degradation of fatty acids, such as linoleic and linolenic
acids, which also existed in red jujube [
5
,
20
]. It was noticed that 6-methyl-5-hepten-2-one,
with a fruity and apple-like odor, also existed in the odor profile of tea [
21
]. Furthermore,
the concentrations of linalool (0.16
µ
g/g), nonanoic acid (1.24
µ
g/g), methyl myristoleate
(0.58
µ
g/g), 2-acetylfuran (0.54
µ
g/g), 1-octen-3-one (0.08
µ
g/g), (E)-2-heptenal (0.44
µ
g/g),
2-heptenone (1.00
µ
g/g), 7-octenoic acid (0.16
µ
g/g), and 2-pentanone (0.84
µ
g/g) were
the highest in HZ. Linalool (3, 7-dimethyl-1, 6-octadien-3-ol) widely existed in plants,
presenting a floral scent [
22
]. Furans were labeled by the caramel-like, sweet, fruity, and
nutty odor descriptions [
23
]. In particular, 2-acetylfuran had a sweet, almond, and cream
odor [
23
]. The (E)-2-alkenals, including (E)-2-heptenal, have been reported to contribute to

Foods 2022,11, 421 7 of 12
the oxidative off flavor [
24
]. Further, 2-heptanone was described as one of the characteristic
aroma compounds of ripe fruit, with a cinnamon and sweet odor [
25
]. JC was more
abundant in the content of acetone (1.16
µ
g/g), methyl acetate (2.20
µ
g/g), 3-methyl
butanal (0.77
µ
g/g), methyl benzoate (0.12
µ
g/g), and propyl acetate (2.42
µ
g/g). Acids
were dominant in both JZ (22.22 µg/g) and YZ (13.27 µg/g).
Foods 2022, 11, x FOR PEER REVIEW 8 of 14
Figure 4.
Heatmap analysis of volatiles taken from GC-IMS of the six red jujube culti-
vars in Xinjiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,Qiyuexian—‘QYX’,
Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’).

Foods 2022,11, 421 8 of 12
QYX and HTDZ share a number of key features in the composition of VOCs. Acids
(10.02 and 11.71
µ
g/g) accounted for the highest proportion in the aroma composition,
followed by aldehydes (7.93 and 9.83
µ
g/g), esters (7.51 and 8.45
µ
g/g), alcohols (
3.66 and
4.45 µg/g
), ketones (2.14 and 2.75
µ
g/g), terpenoids (1.54 and 1.60
µ
g/g), and furans
(
0.31 and
0.40
µ
g/g) in QYX and HTDZ, respectively (Figure 3, Table S3). The con-
centrations of thirteen kinds of common VOCs in QYX and HTDZ were also compa-
rable, including pentanoic acid (2.16 and 2.83
µ
g/g), ethanol (2.31 and 3.00
µ
g/g), isobu-
tyric acid (
0.58 and
0.55
µ
g/g), butyl acetate (0.54 and 0.61
µ
g/g), hexanoic acid (both
0.26 µg/g
), 3-methyl butanoic acid (0.14 and 0.13
µ
g/g), hexanal (0.19 and 0.25
µ
g/g),
ethyl 3-methylbutyrate (both 0.03
µ
g/g), n-nonanal (0.41 and 0.44
µ
g/g), furfurol (
0.19 and
0.22 µg/g
), 2-methyl butanal (0.53 and 0.61
µ
g/g), 3-octanone (both 0.03
µ
g/g), and pen-
tanal (0.02 and
0.03 µg/g
). Among these common VOCs, hexanoic acid and pentanoic
acid were also found in grape and were perceived as having a sweat odor, n-nonanal as
citrus with a green odor, and furfurol as a sweet odor [
26
]. Further, 3-methylbutanoic acid
was identified as one of the most important odorants in chocolate, with fruity, sweaty,
rancid, and cheesy aromas [
27
,
28
]. Isobutyric acid is always detected in milk products, with
rancid, butter, cheese-like odors [
29
]. Ethanol was presented in food products with odor
descriptions such as alcohol, floral, ripe apple, and sweet [
30
]. The presence of esters such
as ethyl 3-methylbutyrate and butyl acetate always contributed to the fruity notes [31].
The VIP scores were used to highlight the most discriminatory features that contribute
to the clustering of the different cultivars. Acetoin, E-2-hexanol, hexanal, acetic acid,
and ethyl acetate were the components with VIP score > 1, indicating their role in the
discrimination of the different cultivars (Figure 5).
Foods 2022, 11, x FOR PEER REVIEW 10 of 14
Figure 5. Volatiles with variable importance in projection (VIP) scores over 1 in the six red jujube
cultivars from Xinjiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,Qiyuexian—
‘QYX’, Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’).
3.3. Fatty Acids, Amino Acids, Organic Acids, and Sugars
The composition in fatty acids, amino acids, organic acids, and sugars are reported
in Supplementary Table S4. The content of organic acids, including malic acid, citric acid,
quinic acid, lactic acid, and tartaric acid, in the six cultivars of red jujube was in the range
of 22.87~159.01 mg/g. The amount of three kinds of sugars (glucose, fructose, and maltose)
was 20.59~126.02 mg/g. The concentration of fatty acids and amino acids was in trace
amounts (0~0.51 mg/g), except for proline (1.55~2.25 mg/g).
3.4. Correlation Analysis between Sensory Attributes and Chemical Composition
The sensory profiles of the six red jujube cultivars are displayed in Figure S2. All the
six red jujube cultivars were characterized by higher values on jujube ID (a sweet and
fruity flavor associated with jujube) (7.0~8.0), sweet (6.4~7.8), fruity attributes (5.8~6.4).
The sour (1.9~3.2), floral (0.6~1.5), bitter (0.4~0.8), and astringent attributes (0.3~1.0) were
relatively lower in the red jujubes, compared with the aforementioned three attributes.
Additionally, HZ was more prominent in fruity (6.4) and floral (1.5) attributes. JZ had a
higher value in sweet (7.8), sour (3.2), and jujube ID (8.0) attributes. HTDZ obtained higher
scores in the bitter (0.8) and astringent (1.0) attributes. Furthermore, JC, QYX, and YZ
showed higher values in jujube ID (8.0), sweet (7.2), and sour (3.0), respectively.
Figure 5.
Volatiles with variable importance in projection (VIP) scores over 1 in the six red jujube culti-
vars from Xinjiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,
Qiyuexian—‘QYX
’,
Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’).

Foods 2022,11, 421 9 of 12
3.3. Fatty Acids, Amino Acids, Organic Acids, and Sugars
The composition in fatty acids, amino acids, organic acids, and sugars are reported
in Supplementary Table S4. The content of organic acids, including malic acid, citric acid,
quinic acid, lactic acid, and tartaric acid, in the six cultivars of red jujube was in the range
of 22.87~159.01 mg/g. The amount of three kinds of sugars (glucose, fructose, and maltose)
was 20.59~126.02 mg/g. The concentration of fatty acids and amino acids was in trace
amounts (0~0.51 mg/g), except for proline (1.55~2.25 mg/g).
3.4. Correlation Analysis between Sensory Attributes and Chemical Composition
The sensory profiles of the six red jujube cultivars are displayed in Figure S2. All the
six red jujube cultivars were characterized by higher values on jujube ID (a sweet and fruity
flavor associated with jujube) (7.0~8.0), sweet (6.4~7.8), fruity attributes (5.8~6.4). The sour
(1.9~3.2), floral (0.6~1.5), bitter (0.4~0.8), and astringent attributes (0.3~1.0) were relatively
lower in the red jujubes, compared with the aforementioned three attributes. Additionally,
HZ was more prominent in fruity (6.4) and floral (1.5) attributes. JZ had a higher value in
sweet (7.8), sour (3.2), and jujube ID (8.0) attributes. HTDZ obtained higher scores in the
bitter (0.8) and astringent (1.0) attributes. Furthermore, JC, QYX, and YZ showed higher
values in jujube ID (8.0), sweet (7.2), and sour (3.0), respectively.
A correlation analysis between sensory characteristics and all VOCs, fatty acids, amino
acids, organic acids, and sugars was also carried out. A redundancy analysis (RDA) was
carried out to analyze the relationship between chemistry composition (factors) and sensory
values. The correlation between the sensory profile and the chemical composition (limited
to VOCs with VIP score > 1, as they played an important role in the differentiation of the
cultivars) is reported in Figure 6, while the correlation with all the VOCs is reported in
Supplementary Figure S3. The floral attribute was highly correlated with VOCs (acetoin,
E-2-hexenol, and acetic acid). The fruity attribute was correlated with organic acids (malic
acid, quinic acid, lactic acid, and tarctic acid). The jujube ID was mainly correlated with
the amino acids (THR, GLU, GLY, ALA, VAL, LEU, TYR, PHE, LYS, HIS, and ARG). This
finding could be correlated with the phenomenon that free amino acids may transaminate
or dehydrogenate into aldehydes or acids [
32
]; for example, phenylacetaldehyde and
isobutanoic acid could be issued from the oxidation of PHE and VAL, separately [
33
].
Thus, phenylacetaldehyde and isobutanoic acid may be contributors to the jujube ID.
The concentration of MET and SER in fresh jujube (Junzao cultivar) at red maturity were
0.58 mg/100 g and 36.91 mg/100 g, respectively [
5
]. However, MET and SER were not
detected in the six cultivars of red jujubes, suggesting that they might gradually decompose
completely and form sulfur-containing substances, amine compounds, aldehydes, and
ketones during the natural drying process (from fresh jujubes to red jujubes), through
deamination and decarboxylation reactions. The astringency attribute was only correlated
with hexanal and ethyl acetate. Astringency was perceived as a comprehensive feeling of
roughing, drying, shrinking or drawing, which could increase and last for a longer time after
swallowing [
34
]. Phenolic components, multivalent salts, such as alum, organic acids, and
charged polysaccharides, such as chitosan, can stimulate astringency [
35
,
36
]. The astringent
attribute is, thus, a complex physicochemical reaction among volatile chemicals, non-
volatile chemicals, and chewing, thus explaining the low correlation with the compounds
examined here. The bitter attribute was correlated with hexanal and margaric acid. The
sweet attribute covaried with amino acids (THR, GLU, GLY, ALA, VAL, LEU, TYR, PHE,
LYS, HIS, and ARG). Amino acids were perceived as one or more tastes according to their
structures. In the cases of L-/D-ARG and L-/D-HIS, the main taste was evaluated as sweet
and/or bitter. The cases of GLY, THR, and ALA were expected to be sweet in red jujubes.
VAL, LEU, and PHE were evaluated as sweet and bitter tastes in D-form [
37
]. The sour
attribute had a close relationship with organic acids (malic acid, citric acid, quinic acid,
lactic acid, and tartaric acid), amino acids (ILE and PRO), and some fatty acids. Among the
five correlated organic acids, citric acid could contribute to a sour gustatory flavor quality,
as in lemon [
38
]. However, L-ILE and D-PRO were evaluated as a bitter taste [
37
]. As a

Foods 2022,11, 421 10 of 12
result, the taste was a complex effect of the structural properties of amino acids, which
could be explained by the fact that different structures of amino acids were perceived by
multiple taste receptors [39,40].
Foods 2022, 11, x FOR PEER REVIEW 12 of 14
Figure 6. Correlation analysis among sensory attributes and fatty acids, amino acids,
organic acids, sugars and VOCs (VIP score > 1) of the six red jujube cultivars from
Xinjiang Province, China.
4. Conclusions
Volatile profiles of six red jujube cultivars, which originated from Xinjiang Province,
China, were analyzed by E-nose and GC-IMS. The chemical compositions of the non-vol-
atile metabolites, i.e., fatty acids, amino acids, organic acids, and sugars, were assessed,
and a sensory evaluation was also performed. Descriptive sensory analysis and multivar-
iate data analysis were carried out to correlate the sensory perception with the chemical
fingerprinting. A total of sixty-four kinds of VOCs have been identified in the six red ju-
jube cultivars (HZ, JZ, YZ, JC, HTDZ, and QYX). Esters and acids were the more predom-
inant VOCs. Acetoin, E-2-hexanol, hexanal, acetic acid, and ethyl acetate played an im-
portant role for the discrimination of the six red jujube cultivars. The differences in sen-
sory attributes of the red jujubes could be explained by the composition of VOCs, amino
acids, fatty acids, and organic acids, to some extent.
Supplementary Materials: The following supporting information can be downloaded at:
www.mdpi.com/xxx/s1: Figure S1: the chromatograms of the six red jujube cultivars in Xinjiang
Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,Qiyuexian—‘QYX’, Hetiandazao—
‘HTDZ’, and Yuanzao—‘YZ’); Figure S2: sensory profiles of the six red jujube cultivars in Xinjiang
Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,Qiyuexian—‘QYX’, Hetiandazao—
‘HTDZ’, and Yuanzao—‘YZ’); Figure S3: correlation analysis among sensory attributes and VOCs
of the six red jujube cultivars from Xinjiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—
‘HZ’,Qiyuexian—‘QYX’, Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’); Table S1: information about
six cultivars of red jujube in Xinjiang Province, China; Table S2: response characteristics and repeat-
ability of response values of ten sensors in E-nose; Table S3: composition identification of volatiles
in six red jujube cultivars from Xinjiang Province, China, via GC-IMS; Table S4: the composition of
fatty acids, amino acids, organic acids, and sugars of six red jujube cultivars from Xinjiang Province,
China.
Author Contributions: Y.Q. and Q.C.: methodology, investigation, data curation, writing original
draft; J.B.: conceptualization, funding acquisition; X.W.: software, formal analysis; X.J.: funding ac-
quisition; M.G.: software; X.Y.: software; G.P.: formal analysis, review and editing. All authors have
read and agreed to the published version of the manuscript.
Funding: The funding support of the Science and Technology Cooperation Project of Xinjiang Pro-
duction and Construction Corps (2021BC007), Financial Science and Technology Project of Xinjiang
production and construction Corps (2020CB008), and Agricultural Science and Technology Innova-
tion Project (CAAS-ASTIP-2020-IFST-04) are greatly appreciated by the authors.
Figure 6.
Correlation analysis among sensory attributes and fatty acids, amino acids, organic acids,
sugars and VOCs (VIP score > 1) of the six red jujube cultivars from Xinjiang Province, China.
4. Conclusions
Volatile profiles of six red jujube cultivars, which originated from Xinjiang Province,
China, were analyzed by E-nose and GC-IMS. The chemical compositions of the non-volatile
metabolites, i.e., fatty acids, amino acids, organic acids, and sugars, were assessed, and a
sensory evaluation was also performed. Descriptive sensory analysis and multivariate data
analysis were carried out to correlate the sensory perception with the chemical fingerprint-
ing. A total of sixty-four kinds of VOCs have been identified in the six red jujube cultivars
(HZ, JZ, YZ, JC, HTDZ, and QYX). Esters and acids were the more predominant VOCs.
Acetoin, E-2-hexanol, hexanal, acetic acid, and ethyl acetate played an important role for
the discrimination of the six red jujube cultivars. The differences in sensory attributes of
the red jujubes could be explained by the composition of VOCs, amino acids, fatty acids,
and organic acids, to some extent.
Supplementary Materials:
The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/foods11030421/s1: Figure S1: the chromatograms of the six red
jujube cultivars in Xinjiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,Qiyuexian—
‘QYX’, Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’); Figure S2: sensory profiles of the six red jujube
cultivars in Xinjiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’, Huizao—‘HZ’,Qiyuexian—‘QYX’,
Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’); Figure S3: correlation analysis among sensory attributes
and VOCs of the six red jujube cultivars from Xinjiang Province, China (Jinchang—‘JC’, Junzao—‘JZ’,
Huizao—‘HZ’,Qiyuexian—‘QYX’, Hetiandazao—‘HTDZ’, and Yuanzao—‘YZ’); Table S1: information
about six cultivars of red jujube in Xinjiang Province, China; Table S2: response characteristics
and repeatability of response values of ten sensors in E-nose; Table S3: composition identification
of volatiles in six red jujube cultivars from Xinjiang Province, China, via GC-IMS; Table S4: the
composition of fatty acids, amino acids, organic acids, and sugars of six red jujube cultivars from
Xinjiang Province, China.
Author Contributions:
Y.Q. and Q.C.: methodology, investigation, data curation, writing original
draft; J.B.: conceptualization, funding acquisition; X.W.: software, formal analysis; X.J.: funding
acquisition; M.G.: software; X.Y.: software; G.P.: formal analysis, review and editing. All authors
have read and agreed to the published version of the manuscript.

Foods 2022,11, 421 11 of 12
Funding:
The funding support of the Science and Technology Cooperation Project of Xinjiang
Production and Construction Corps (2021BC007), Financial Science and Technology Project of Xinjiang
production and construction Corps (2020CB008), and Agricultural Science and Technology Innovation
Project (CAAS-ASTIP-2020-IFST-04) are greatly appreciated by the authors.
Institutional Review Board Statement: Not applicable.
Data Availability Statement: Data is available upon request.
Conflicts of Interest: The authors declare no conflict of interest.
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