Characterization of Traditional Chinese Sesame Oil by Using Headspace Solid-Phase Microextraction/Gas Chromatography–Mass Spectrometry, Electronic Nose, Sensory Evaluation, and RapidOxy

Xiao Mo Xiang You (XMXY) is a traditional Chinese sesame oil variety that is obtained through a hot water flotation process. This unique process gives the oil a unique aroma, health benefits, and excellent product stability. Although XMXY is always the most expensive among all the sesame oil varieties, it is usually used as a flavoring in many traditional Chinese daily food products and is increasingly popular. In order to reveal the characteristics of the oil, the volatile components, sensory evaluation, and oxidation stability of five XMXY samples were, respectively, analyzed by using headspace solid-phase microextraction/gas chromatography–mass spectrometry, an electronic nose, sensory evaluation, and RapidOxy. Comparisons and multidimensional statistical analysis were also carried out to distinguish XMXY from roasted sesame oil (RSO) and cold-pressed sesame oil (CSO) samples. In total, 69 volatiles were identified from XMXY, RSO, and CSO samples. Some compounds possessed high odor activity value (OAV > 1) in XMXY, including heterocyclic compounds, phenols, and sulfur-containing compounds. Additionally, they were also the main volatile components that distinguish XMXY from RSO and CSO. Roasted and nutty aromas were the dominant aroma attributes of XMXY. XMXY had better flavor intensity and oxidation stability than the other two sesame oil samples. These results are very valuable for the quality control and product identification of traditional Chinese sesame oil.


Introduction
Sesame (Sesamum indicum L.) is considered an important oilseed crop that is widely cultivated in Asia, Africa, and Central and South America [1]. Sesame seeds can be processed in various ways to obtain sesame oil. Sesame oil contains many kinds of natural phytochemicals, such as sesamin, sesamol, and sesamolin, which are known to possess anti-inflammatory and anticancer activities [2][3][4][5]. Sesame oil varieties are usually divided into refined, roasted, and cold-pressed oil according to their processing methods [6]. The aroma of the sesame oil is a characteristic indicator for consumers to select it. Most aroma compounds of the oil are formed by various reactions during processing, for example, enzymatic reactions and thermal reactions such as Maillard reaction, Strecker degradation, caramelization, and lipid thermal reaction [7,8]. Many studies also showed that the roasting temperature of sesame seeds has a significant impact on the concentration of volatiles, and pyrazines are major volatile compounds in sesame oil [9][10][11]. As a result, the aroma of sesame oil is very different due to different processing techniques. In addition, sesame oil varieties are also obviously different in the aspect of product stability for the same reasons [12][13][14].

HS-SMPE/GC-MS Analysis of Sesame Oil Samples
The volatile components of sesame oil samples were analyzed using a reported HS-SMPE/GC-MS method [18], with slight modifications. A CAR/DVB/PDMS fiber was employed for the SPME of the samples, and the fiber was preconditioned for 30 min at 270 • C before the extraction of samples. Briefly, 1 g of each sesame oil sample was transferred into a 4 mL headspace sample vial. The vial was equilibrated at 60 • C for 20 min in a thermostatic water bath after tightening the cap. Then, the aged DVB/CAR/PDMS fiber head was inserted into the headspace of the sample vial. After 60 min, the fiber was instantly injected into the GC-MS injection port, which desorbed for 5 min at 250 • C.
GC-MS analysis was performed using an Agilent 7890B gas chromatograph coupled with a 5977B series mass spectrometer (Santa Clara, CA, USA). The volatile components were, respectively, separated in the column systems of two different polarities: HP-5MS (60 m × 250 µm × 0.25 µm) and DB-WAXETR (60 m × 250 µm × 0.25 µm). The operating conditions were modified appropriately according to the method of Jia et al. [18]. For the HP-5MS column, the oven temperature was set at 40 • C, increased to 160 • C with a temperature ramp of 2 • C/min, and finally increased to 240 • C with a temperature Foods 2022, 11, 3555 3 of 12 ramp of 10 • C/min. For the DB-WAXETR column, the oven temperature at 40 • C was increased to 170 • C with a temperature ramp of 2.5 • C/min, maintained for 3 min, and finally increased to 240 • C with a temperature ramp of 10 • C/min and maintained for 4 min. A splitless mode was used, and a flow rate of 1.0 mL/min was helium as the carrier gas. The temperature of the ion source was 250 • C, the ionization energy was 70 eV, and the scan mass range was 35-350 aum.

Identification, Quantification, and Odor Activity Value (OAV) Calculation of the Volatile Components
The tentative identification of the volatiles in sesame oil samples was based on the mass spectra in the NIST 17.0 database. The retention indices (RIs) were obtained to further confirm each compound by comparing its values with those reported in the literature [27]. The RIs of the compounds were calculated according to the formula: where n is the number of carbon atoms in a series of normal alkanes, and t Ru is the retention time of the volatile compound. t Rn and t R(n+1) are the retention times of the normal alkanes. The volatile compounds in sesame oil samples were quantified by using a semi-quantitative method [28]. The relative concentration of volatile compounds was obtained by comparing the peak areas of the compounds and the internal standard (2-methyl-3-heptanone).
The OAVs of all volatile compounds were calculated according to the following formula: where Ci is the relative concentration of the volatile compound calculated by using semi-quantitation. OTi is its corresponding order threshold by adopting the method of Tian et al. [29].

Electronic Nose Analysis
An electronic nose (PEN 3, Airsense Analytics Co., Ltd., Schwerin, Germany) was used to analyze the sensory profile of the sesame oil samples, which had ten aroma receptors. These sensors were named W1C, W5S, W3C, W6S, W5C, W1S, W1W, W2S, W2W, and W3S, were, respectively, sensitive to substances such as aromatic hydrocarbons, nitrogen oxides, aromatic ammonia, hydrogen compounds, short-chain alkane aromatic compounds, methylcontaining compounds, sulfides, alcohols, and carbonyl compounds, organic sulfides and aromatic components, long chain alkanes. Briefly, 5 g of sesame oil was transferred into a 25 mL headspace bottle, covered and sealed, and allowed to stand at 26 • C for 30 min before testing. In terms of electronic nose measurement parameters, cleaning time was 120 s; zero return time was 5 s; pre-injection time was 8 s; measurement time was 60 s, and carrier gas flow rate was 400 mL/min.

Sensory Analysis
Sensory analysis was performed by ten panelists (five males and five females, aged 20-30) from the Flavor Research Center at Zhengzhou University, China. The panelists were recruited based on their availability and experience, and they were trained at least three times according to ISO 8586. The quantitative descriptive analysis (QDA) was carried out in standard sensory booths with temperature control (22-24 • C) and air circulation. The sesame oil samples stored in the refrigerator were taken out and kept at room temperature for subsequent sensory evaluation. Each sample (5.0 mL) was loaded into glass bottles (10.0 mL) coded with 3-digit numbers and then presented to the panelists randomly. To evaluate the overall flavor profile of sesame oil samples, the panelists discussed and selected six sensory attributes of sesame oil samples, including roasted, nutty, sweet, smoky, pungent, and green. A 5-point scale ranging from 0 to 5 was applied to quantitatively evaluate the contribution of each sensory attribute to the overall flavor profile of the sesame oil. Additionally, 0 meant no odor, and 5 meant extremely strong odor. The results were expressed as the average score of 3 times in the spider diagram.

Oxidation Stability Analysis
The oxidation stability analysis of sesame oil samples was evaluated using a RapidOxy reactor (Anton Paar, Blankenfelde-Mahlow, Germany). A previously reported method was adopted [30], with some modifications. In each experiment, 3 g of one of the sesame oil samples was accurately weighed under the conditions of a temperature of 150 • C and pressure of 700 kpa, and the time of reaching a pressure drop of 10% was measured, which is the induction period (IP). The test of each sample was replicated six times.

HS-SPME/GC-MS Analysis of Sesame Oil Samples
Firstly, some main parameters of HS-SPME were optimized according to the total peak areas, such as the tape of SPME fiber, extraction, and desorption conditions. The results (Figures S1 and S2) showed that a CAR/PDMS/DVB fiber, extraction at 60 • C for 60 min, and desorption at 250 • C for 5 min were suitable experimental conditions for the HS-SPME analysis of volatile components in sesame oil. In order to study the unique flavor of XMXY, in addition to the five XMXY samples, we also analyzed RSO and CSO samples.
As shown in Table S1 and Figure 1, 69 compounds were identified from the seven samples by comparing their RI and mass spectrum information, with the compounds referenced in the NIST17 mass spectra, comprising 22 N-heterocyclic compounds, 9 Oheterocyclic compounds, 9 S-containing volatile compounds, 8 phenols, 7 acids, 4 ketones, 4 aldehydes, 4 alcohols, 1 ester, and 1 alkene. The heterocyclic compound was the most abundant group in XMXY and RSO samples, while the main compounds in CSO were acids and aldehydes. More kinds of volatile components were identified from the sesame oil obtained by roasting sesame seeds, which was consistent with previous studies [8][9][10]18].
The content of N-heterocyclic compounds (118. .91 µg/g) was the highest in the volatile components of all the XMXY samples. Pyrazines mainly existed in the form of alkylated pyrazines, and generally had a strong aroma when roasted and nutty [31]. Additionally, pyrazines were mostly derived from the Strecker degradation pathway of the seeds during high-temperature roasting according to previous studies [7,32]. No pyrazine compound was found in the CSO sample, the reason for which may be that CSO was produced at lower temperatures.
Nine O-heterocyclic compounds were found in the XMXY samples, mainly including furans, furanones, and their derivatives. The total content of O-heterocyclic compounds (32.00-68.26 µg/g) was relatively high in the volatile components of the XMXY samples. O-heterocyclic compounds are usually produced through the caramelization reaction that occurs during the roasting of oil seeds [8]. The thresholds of the compounds were low, giving the XMXY sample its sweet, nutty, fruity, and caramel-like characteristics [33].
Eleven S-containing compounds were identified in this study, mainly including thioethers, thiols, and others. Dimethyl disulfide (found in onion, cabbage, etc.) may be produced by the Stray degradable reaction, especially in some vegetable oil varieties rich in sulfur-containing amino acids [34]. Methanethiol (putrid, cabbage-like, sulfury) was identified in almost all of the XMXY samples, which had a low threshold and played an important role in the flavor profile of the XMXY samples [35]. The content of N-heterocyclic compounds (118. .91 μg/g) was the highest in the volatile components of all the XMXY samples. Pyrazines mainly existed in the form of alkylated pyrazines, and generally had a strong aroma when roasted and nutty [31]. Additionally, pyrazines were mostly derived from the Strecker degradation pathway of the seeds during high-temperature roasting according to previous studies [7,32]. No pyrazine compound was found in the CSO sample, the reason for which may be that CSO was produced at lower temperatures. Eight phenols were identified in the volatile compounds of the XMXY samples. 2-Methoxy-phenol (14.84-85.49 µg/g) and 2-Methoxy-4-vinylphenol (4.08-23.78 µg/g) were the components with higher content in the XMXY samples. Phenols may play an important role in the formation of the XMXY aroma profile and are known to significantly contribute to the smoky note because of their low odor thresholds [36].
Other compounds were found in low concentrations in the volatile compounds of XMXY, but they were the main components of cold-pressed sesame oil, namely short-chain aldehydes and pelargonic acid, which gave it an associative and fruity aroma [37].
Orthogonal partial least-squares discriminant analysis (OPLS-DA) is a supervised statistical method of discriminant analysis, which is different from the principal component analysis [38]. In order to explore the aroma compounds profile of XMXY, an orthogonal  Figure 2B,C) were also adopted to confirm the key aroma compounds in XMXY. As shown in Figure 2C, variables with a VIP value greater than 1 (p < 0.05) were considered the volatile compounds that caused a significant difference [39]. The variables with significant differences mainly included heterocyclic compounds, phenols, and sulfur-containing compounds.
the components with higher content in the XMXY samples. Phenols may play an important role in the formation of the XMXY aroma profile and are known to significantly contribute to the smoky note because of their low odor thresholds [36].
Other compounds were found in low concentrations in the volatile compounds of XMXY, but they were the main components of cold-pressed sesame oil, namely shortchain aldehydes and pelargonic acid, which gave it an associative and fruity aroma [37].
Orthogonal partial least-squares discriminant analysis (OPLS-DA) is a supervised statistical method of discriminant analysis, which is different from the principal component analysis [38]. In order to explore the aroma compounds profile of XMXY, an orthogonal partial least-squares discriminant analysis (OPLS-DA) model (Figure 2A) was established based on the volatile compounds in XMXY and RSO samples. A loading S-plot and variable importance in the projection (VIP) plot ( Figure 2B,C) were also adopted to confirm the key aroma compounds in XMXY. As shown in Figure 2C, variables with a VIP value greater than 1 (p < 0.05) were considered the volatile compounds that caused a significant difference [39]. The variables with significant differences mainly included heterocyclic compounds, phenols, and sulfur-containing compounds.

Identification of Aroma Active Compounds by OAVs
Among the volatile components of vegetable oil, only a small number of components directly affect the organoleptic properties of vegetable oil. Abundant studies showed that the concentration and odor threshold of volatile compounds were the key factors that determined the contribution of volatile compounds to the overall aroma characteristics [8].
OAV is the ratio of the concentration to the odor threshold of oil. It was possible to compare the contribution of each compound to the aroma characteristics in the sesame oil samples by the calculation of OAV [40]. The volatile compounds with an OAV greater than 1 were considered the key aroma-active compounds [41]. As shown in Table 1, the OAVs of the volatile components in XMXY were generally higher than that of the RSO and CSO. The volatile components with larger OAVs in XMXY mainly included methanethiol, dimethyl disulfide, methyl-pyrazine, 2-ethyl-6-methyl-pyrazine, 3-ethyl-2,5-dimethyl-pyrazine, 2-furanmethanol, 2-methoxy-4-vinylphenol, 2-methoxy-phenol, etc. These compounds were important contributors to the aroma profile of XMXY samples. In addition, N-heterocyclic compounds, S-containing compounds, and phenols were also the components with high OAVs in RSO. Hexanal and hexanoic acid were relatively high-

Identification of Aroma Active Compounds by OAVs
Among the volatile components of vegetable oil, only a small number of components directly affect the organoleptic properties of vegetable oil. Abundant studies showed that the concentration and odor threshold of volatile compounds were the key factors that determined the contribution of volatile compounds to the overall aroma characteristics [8].
OAV is the ratio of the concentration to the odor threshold of oil. It was possible to compare the contribution of each compound to the aroma characteristics in the sesame oil samples by the calculation of OAV [40]. The volatile compounds with an OAV greater than 1 were considered the key aroma-active compounds [41]. As shown in Table 1, the OAVs of the volatile components in XMXY were generally higher than that of the RSO and CSO. The volatile components with larger OAVs in XMXY mainly included methanethiol, dimethyl disulfide, methyl-pyrazine, 2-ethyl-6-methyl-pyrazine, 3-ethyl-2,5-dimethyl-pyrazine, 2-furanmethanol, 2-methoxy-4-vinylphenol, 2-methoxy-phenol, etc. These compounds were important contributors to the aroma profile of XMXY samples. In addition, N-heterocyclic compounds, S-containing compounds, and phenols were also the components with high OAVs in RSO. Hexanal and hexanoic acid were relatively high-value compounds in CSO but were not found in XMXY and RSO.

Electronic Nose Analysis of Sesame oil Samples
An electronic nose can detect simple or complex smells by simulating the human smelling process [42]. With the advantages of high speed and low cost, electronic noses have been widely used to study the volatile components of many agricultural products [43]. In this study, an electronic nose was used to analyze the volatile components of XMXY, RSO, and CSO samples. The radar chart drawn from the electronic nose data is shown in Figure 3A. The ten sensors of the electronic nose had different responses to the volatile components in the sesame oil samples, which was similar to the results reported by Hai et al. [44]. The aroma profile of the RSO sample was similar to that of the XMXY samples, but the overall aroma intensity of almost all XMXY samples was stronger than that of RSO. The response intensity of the volatile compounds in CSO was weak on all sensors. In order to evaluate the flavor difference between XMXY and the other two sesame oil samples, XMXY3 with the most intense flavor was used as the representative, and the results are shown in Figure 3B. There were significant differences (p < 0.05) between XMXY3 and CSO on all 10 sensors. Except for the two sensors W1C and W3S, the responses of XMXY3 and RSO samples to the other eight sensors were significantly different (p < 0.05). In addition, the response of XMXY3 was significantly higher on W1W, W2W, and W5S sensors, indicating that XMXY3 may contain a higher concentration of organic sulfides and nitrogen oxides. The results of the electronic nose were consistent with those obtained through GC-MS analysis.

Sensory Analysis of Sesame Oil Samples
Six odor attributes were chosen for the evaluation of sesame oil samples, and the results are shown in Figure 4, which reveal that roasted and nutty were the dominant aroma attributes of XMXY, followed by smoky, pungent, sweet, and green. The evaluation of RSO was similar to XMXY, but its aroma intensity was lower than that of XMXY, and the roasted and nutty aromas of XMXY were more intense. Furthermore, a one-way ANOVA was used to distinguish the differences (p < 0.05) in sensory evaluation scores, and it was found that the aroma profile of CSO was significantly different from that of XMXY and RSO, green was the dominant aroma attributor, and the intensity of other aroma characteristics was weak. The sensory analysis results were basically consistent with the results obtained through gas chromatography analysis and the use of E-nose, further confirming the accuracy of the previous experimental results.

Oxidation Stability Analysis of Sesame Oil Samples
In this study, the oxidation stability of the sesame oil samples was evaluated under accelerated oxidation conditions by using a RapidOxy reactor. The results are shown in Table 2, which reflects the induction time of XMXY1-5, RSO, and CSO samples under different pressure drops. Compared with CSO (32.11 min), XMXY and RSO samples had longer induction time (48.43-61.52 min) and better oxidative stability. CSO is produced from unheated sesame seeds, whereas antioxidant active components such as sesamol and Maillard reaction products are produced during sesame roasting, which may be the reason for the higher oxidative stability of XMXY and RSO samples [45][46][47]. This conclusion could be verified in the analysis of GC-MS.  the results are shown in Figure 3B. There were significant differences (p < 0.05) between XMXY3 and CSO on all 10 sensors. Except for the two sensors W1C and W3S, the responses of XMXY3 and RSO samples to the other eight sensors were significantly different (p < 0.05). In addition, the response of XMXY3 was significantly higher on W1W, W2W, and W5S sensors, indicating that XMXY3 may contain a higher concentration of organic sulfides and nitrogen oxides. The results of the electronic nose were consistent with those obtained through GC-MS analysis.

Sensory Analysis of Sesame Oil Samples
Six odor attributes were chosen for the evaluation of sesame oil samples, and the results are shown in Figure 4, which reveal that roasted and nutty were the dominant aroma attributes of XMXY, followed by smoky, pungent, sweet, and green. The evaluation of RSO was similar to XMXY, but its aroma intensity was lower than that of XMXY, and the roasted and nutty aromas of XMXY were more intense. Furthermore, a one-way ANOVA was used to distinguish the differences (p < 0.05) in sensory evaluation scores, and it was found that the aroma profile of CSO was significantly different from that of XMXY and RSO, green was the dominant aroma attributor, and the intensity of other aroma characteristics was weak. The sensory analysis results were basically consistent with the results obtained through gas chromatography analysis and the use of E-nose, further confirming the accuracy of the previous experimental results.

Oxidation Stability Analysis of Sesame Oil Samples
In this study, the oxidation stability of the sesame oil samples was evaluated under accelerated oxidation conditions by using a RapidOxy reactor. The results are shown in Table 2, which reflects the induction time of XMXY1-5, RSO, and CSO samples under different pressure drops. Compared with CSO (32.11 min), XMXY and RSO samples had longer induction time (48.43-61.52 min) and better oxidative stability. CSO is produced from unheated sesame seeds, whereas antioxidant active components such as sesamol and Maillard reaction products are produced during sesame roasting, which may be the reason for the higher oxidative stability of XMXY and RSO samples [45][46][47]. This conclusion could be verified in the analysis of GC-MS.

Conclusions
In this study, traditional Chinese sesame oil (XMXY) was analyzed using HS-SPME/GC-MS, an electronic nose, sensory evaluation, and RapidOxy. In total, 69 volatile compounds were identified from XMXY, RSO, and CSO samples. In terms of the identified volatile components, multidimensional statistical analysis and OAV value analysis revealed that heterocyclic compounds, phenols, and sulfur-containing compounds were the key volatile compounds in XMXY and contributed more to the overall aroma. Roasted and nutty aromas were the dominant aroma attributes of XMXY. The XMXY sample obtained through hot water flotation had better flavor and oxidation stability than the other two sesame oil (RSO and CSO) samples. This study provides some information on the quality control of traditional sesame oil. Future research will focus on the formation mechanism of the traditional sesame oil flavor, the precise control of product quality, etc.

Conflicts of Interest:
The authors and company both declare no conflict of interest.