Flavor formation in frying process of green onion (Allium fistulosum L.) deep-fried oil

Food Research International 121 (2019) 296–306

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Flavor formation in frying process of green onion (Allium fistulosum L.) deepfried oil

T

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Ning Zhanga,b,c, Baoguo Suna,b,c, Xueying Maod, Haitao Chenb, , Yuyu Zhangc a

Beijing Advanced Innovation Center for Food Nutrition and Human Health (BTBU), Beijing 100048, China Beijing Key Laboratory of Flavor Chemistry，Beijing Technology and Business University (BTBU), Beijing 100048, China c Beijing Laboratory for Food Quality and Safety, Beijing Technology & Business University (BTBU), Beijing 100048, China d College of Food Science & Nutritional Engineering, China Agricultural University, Beijing 100083, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Green onion Deep-fried oil Aroma active compounds Flavor Formation mechanism Analysis

Fried allium oil has been widely used in traditional Chinese home cooking and recently has grown in popularity in the food manufacturing industry. Thus, physical and chemical changes during frying process were measured to investigate the flavor formation mechanism in green onion (Allium fistulosum L.) deep-fried oil. With the increase of the oil temperature, important variations took place when the temperature rose above 140 °C during the whole frying process. A detailed study of these changes was made from both macro and micro aspects. From a macro perspective, sensory attributes including burnt, fried, oily, cooked vegetable and salty were strengthened. Meanwhile, the reference points of the oil samples on the fingerprint chart were distinguishable from others by electronic nose. In addition, contents of furans and furanones, sulfur-containing compounds, aldehydes and alcohols increased sharply according to SAFE-GC-MS analysis from a microscopic point of view, and contents of unsaturated fatty acids dropped remarkably while the saturated ones increased. These changes were considered to be caused by interactions between carbohydrates, proteins and fats in the deep-fried system and thermo degradations of sugars, amino acids and fats. The results indicated that the stage, when frying at temperatures ranging from 140 °C to 165 °C, was the most significant period for the flavor formation of the deepfried oil.

1. Introduction The flavor of fried allium oil has been widely used in Chinese cuisine, and recently has gained popularity in the food manufacturing industry. In the preparation of many Chinese dishes, small amounts of those seasonings, like green onions, garlics, shallots, et al., are fried in vegetable oil before adding other ingredients at a high temperature until the special flavor obtained. In addition, this unique flavor can be used to cover the unpleasant flavors generated from other ingredients as well as to enhance the whole flavor profile of the dish. Among the allium spices, green onion (Allium fistulosum L.) is one of the most important seasonings in traditional Chinese home cooking as well as in this frying process. With the development of the food industry, this flavor has also been widely applied in many kinds of food products, such as instant noodles, potato chips and so on. However, compared to investigations on raw, unprocessed green onion, studies on its volatiles of thermally processed products are rather scarce. In general, the flavor of fried allium oil has an important impact on the industrialization of

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traditional Chinese cuisine. The flavor of fried food is the result of complex physical and chemical interactions between food ingredients and the oil medium during the heating process (Bansal et al., 2010; Saguy & Dana, 2003; Zhang et al., 2018). The physical reactions that occur during the frying process provide a material basis for the texture of fried food products. In addition, the chemical reactions in frying include the reactions between fat itself and other ingredients in the dish (Sumnu & Sahin, 2008; Zhang, Saleh, Chen, & Shen, 2012). Thermal degradation reactions of fat provide the material basis for the flavor of fried food, while Maillard reactions between proteins and sugars form the flavor profile and color of the fried food. During the frying process, the flavor is mainly determined by the oxidative degradation of fat and the Maillard reaction. The browning reaction and degradation of amino acids also contributed to the production of flavor compounds. The volatiles generated during the frying process include alkanes, alcohols, aldehydes, ketones, acids, esters, lactones, and heterocyclic compounds (Hammouda, Freitas, Ammar, Gomes Da Silva, & Bouaziz, 2017; Nieva-Echevarría,

Corresponding author. E-mail address: [email protected] (H. Chen).

https://doi.org/10.1016/j.foodres.2019.03.006 Received 1 October 2018; Received in revised form 2 March 2019; Accepted 6 March 2019 Available online 08 March 2019 0963-9969/ © 2019 Published by Elsevier Ltd.

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Thermo Fisher Scientific Inc. (Shanghai, China), and the internal standard was from Aladdin Industrial Corporation (Shanghai, China). Sodium chloride (AR) was from Sinopharm Chemical Reagent Co., Ltd. (Beijing China). To measure the retention indices (RI), a mixture of nalkanes ranging from hexane to triacontane was used (Sigma-Aldrich Co., St. Louis, Missouri, USA).

Goicoechea, Manzanos, & Guillén, 2016; Zhang et al., 2012). Although, the main components and the formation mechanism of green onion deep-fried oil flavor have not yet been reported, the volatiles of other alliums generated during heating processes have been studied. Volatile compounds from raw, baked and fried shallots have been previously studied, and sulfides, disulfides, trisulfides and thiophenes were found to be the major components (Wu, Chou, Chen, & Wu, 1982). A simultaneous distillation extraction (SDE) method was used to analyze the volatiles of fried shallot, and the results showed that dimethyl trisulfide, 2,4-dimethylthiophene, furfural, 2,5-dimethylpyrazine and 2-ethyl-3,6-dimethylpyrazine were the major factors affecting the flavor profile (Chyau, Lin, & Mau, 1997). Besides, two different heating treatments were applied to evaluate the qualities of fried shallots, and the main volatile substances were identified as acids (2butenoic acid and acetic acid), unsaturated aldehydes (2-propenal, 2,4heptadienal, and 3-methyl-2-butenal) and a thiophene (2,4-dimethyl thiophene) (Chu & Hsu, 2001). Garlic oil processed by heating has also been studied. A modified SDE apparatus was employed to identify the flavor of fried, oil-cooked, microwave-fried, baked and microwavebaked garlic samples (Yu, Wu, & Ho, 1993). The results showed that the flavor compounds varied when treated by different heating treatments: diallyl disulfide and diallyl trisulfide were the dominant compounds in baked and microwave-baked garlic samples; diallyl disulfide, methyl allyl disulfide and vinyldithiins were the dominant compounds in fried, oil-cooked and microwave-fried garlic samples. Oil treatments of garlic favored the formation of vinyldithiins, and the contents of diallyl trisulfide in oil-treated garlic samples were very low. In addition, significant amounts of nitrogen-containing volatile compounds were found in baked and oil-treated garlic samples. Using headspace gas analysis, some of the compounds, including dimethyl sulfide, allyl alcohol, diallyl sulfide, methyl allyl disulfide and diallyl disulfide, were considered as the major volatiles; however, the major compounds were diallyl disulfide, diallyl trisulfide and dithiins when separated by SDE (Kim, Wu, Kobayashi, Kubota, & Okumura, 1995). In addition, the device of solvent-assisted flavor evaporation (SAFE) was invented and designed in 1999 (Engel, Bahr, & Schieberle, 1999). Recently, this method has been widely used in the field of food flavor due to its high extraction efficiency (Buttara, Intarapichet, & Cadwallader, 2014; Liu et al., 2015; Zhao et al., 2017). Besides, apart from classic instrumental techniques (mainly chromatographic), another efficient method applied to food control is electronic nose analysis (Loutfi, Coradeschi, Mani, Shankar, & Rayappan, 2015; Majchrzak, Wojnowski, Dymerski, Gębicki, & Namieśnik, 2018; Röck, Barsan, & Weimar, 2008). In terms of the response values of diverse sensors, the samples' aromatic profiles can be distinguished by electronic nose with a short time of analysis, adequate sensitivity and relatively low cost. So far, there were few literatures on the flavor of green onion deep-fried oil using these novel methods above. In this study, green onion and soybean oil were both selected as the raw materials to prepare the oil. A systematic study, including sensory analysis, electronic nose and SAFE-GC-MS analysis, was conducted to identify the physical and chemical properties as well as the formation mechanism of the flavor components of green onion deep-fried oil. Moreover, variations in the flavor profile and fatty acid components generated during the preparation process were estimated to explore the mechanism by which the flavor components formed. Above all, it is of great importance to study the flavor of fried allium oil, and a theoretical basis can be developed for the standardization and industrialization of traditional Chinese dishes.

2.2. Oil preparation process Fresh green onion samples were purchased from a local market in Beijing, China. The vegetable oil used was soybean oil produced in China. Green onion stalks were chosen for the experiment. The stalks were cut into sections approximately 1 cm long. A 12–18 cm diameter basin and an induction cooker were used for frying and an oil thermometer was used to measure the temperature. Through pre-experiment, preparation process of the green onion deep-fried oil was determined to gain a favorable flavor. The process conditions were as follows: soybean oil (300 g) was added into the basin and was heated to 140 °C. Green onion stalks were then added to the hot oil and fried until the oil reached the desired final temperature, i.e., 165 °C. The moisture in stalks caused the temperature to drop first and then rise during frying procedure, and the minimum temperature in this process was slightly lower than 100 °C. The fried stalks and oil were immediately separated once the oil temperature reached 165 °C, and the oil was cooled to room temperature as soon as possible. During this procedure, the mixture was stirred continuously to prevent local overheating. The preparation method of the oil samples mentioned above was based on the optimization results of the preliminary process conditions in order to gain a better flavor of the green onion deep-fried oil. To explore formation mechanism of the flavor of the oil, changes in the flavor composition during the above procedure were investigated. Oils of different final temperatures (100 °C, 110 °C, 120 °C, 130 °C, 140 °C and 165 °C) were tested while the other conditions remained unchanged. Each of the oil samples were prepared twice and mixed together for the subsequent analyses. 2.3. Sensory analysis A descriptive sensory analysis was carried out to profile the main odor constituents of the green onion deep-fried oil, and the experiment was performed in a sensory room with single cubicles at ambient temperature and relative humidity. The sensory analysis was conducted by a panel consisting of ten panelists (six females and 4 males, age: 23 to 30 years). They were recruited and trained for descriptive analysis and had rich experience in sensory profiling of diverse food samples. A 10-g oil sample was put in a 30-mL transparent PET bottle, and the samples were randomly arranged. During the training sessions, the panelists smelled the samples and discussed the odor attributes. They determined the final list of 7 descriptive terms by consensus, which were considered as the most important contributors to the whole flavor profile of the deep-fried green onion oil. Consequently, odor attributes of salty, fried, oily, burnt, cooked vegetable, green grass and pungent were evaluated. A descriptive sensory analysis was performed along a scoring linein duplicate with a scent intensity ranging from 0 (none) to 9 (very strong). The results obtained were averaged for each odor attribute and were analyzed by principal component analysis (PCA). 2.4. Electronic nose analysis An electronic nose device, PEN 3 E-Nose (Winmuster Airsense Analytic Inc., Schwerin, Germany) was used. The device was equipped with a sampling apparatus, a detector unit containing the sensor array, and pattern recognition software for data recording and analysis. The sensor array system consisted of 10 metal oxide semiconductors (MOS) of different chemical compositions and thicknesses to provide selectivity for volatile compounds of various classes, including W1C

2. Materials and methods 2.1. Chemicals The solvent, dichloromethane, and the internal standard, 1,2-dichlorobenzene, were of HPLC grade. The dichloromethane was from 297

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250 °C; mass range, m/z 35–550 amu; injection volume, 1 μL; injector temperature, 250 °C; and a split ratio of 5:1. The volatile compounds were tentatively identified by GC-MS by comparing their mass spectra with spectra from the NIST 2014 data base. A homologous series of n-alkanes (C6 ~ C30) were analyzed under the same chromatographic conditions to calculate the retention index (RI) of the detected compounds to compare them to the RIs in the NIST 2014 database using the same capillary column. Authentic compounds were used to confirm the identification of the volatile compounds. The following Eq. (1) was applied to calculate the RI of each compound:

(aromatic compounds), W5S (broad-range compounds), W3C (ammonia and aromatic compounds), W6S (hydrogen), W5C (aromaticaliphatic), W1S (methane, broad-range compounds), W1W (sulfur compounds), W2S (broad-alcohol compounds), W2W (sulfur-chlorine), and W3S (methane-aliphatic). Oil samples of different termination temperatures were analyzed by electronic nose. A 7-g sample of oil was placed in a 15-mL glass vial that was then sealed with a septa and screw cap. Each sample was then heated at 100 °C for 30 min to equilibrate for headspace generation above the sample. During the measurement time (60 s), the sampling unit withdrew the volatile gases present in the headspace at a constant rate (300 mL/min) causing changes in the sensor's conductance, and the sampling time was long enough for the sensor signals to reach a steady value. When a measurement was completed, a standby of 10 min was initiated with the circuit, and the chamber was flushed with clean air until the sensor signals returned to the baseline. The analysis was performed at least three times for each sample. Data interpretation was carried out by applying multivariate statistical techniques of PCA analysis on the obtained sensor responses.

RI = 100 × {n + [lg t ′ (i )‐lg t ′ (n)]/[lg t ′ (n + 1)‐lg t ′ (n)]}

(1)

where: n and (n + 1) represent the number of carbon atoms in the alkanes eluting before and after the compound, respectively, t’(n) and t’(n + 1) are the corresponding retention times of the alkanes, and t’(i) is the retention time of the compound to be identified (t’(n) < t’(i) < t’(n + 1)). Relative concentrations of the aroma compounds were calculated by relating the peak areas of the volatiles to the peak area of the internal standard (1,2-dichlorobenzene) (Bordiga et al., 2013). Content of each compound can be expressed in the Eq. (2).

2.5. Extraction of volatiles by solvent-assisted flavor evaporation (SAFE)

w (i) = f ′ (i) × [A (i)/ A (s )] × w (s )

2.5.1. Direct solvent extraction (DSE) of fresh green onion Direct solvent extraction (DSE) was applied to extract the volatile compounds of fresh green onion at ambient temperature (20 ± 2 °C). A 20-g sample of freshly chopped green onion was weighed and put into an Erlenmeyer flask. Distilled water (100 mL) was added to the flask, and the mixture was saturated with analytical-grade sodium chloride. The mixture was homogenized at 1500 rpm for 5 min using an SCILOGEX BlueSpin LED digital hotplate magnetic stirrer (MS-H280-Pro, Berlin, CT, USA). An internal standard stock solution (1000 μL, 26.0 μg·mL−1 of 1,2-dichlorobenzene) and dichloromethane were then added to the mixture for extraction. During extraction, the mixtures were agitated for 30 min at 1500 rpm at ambient temperature (20 ± 2 °C). Then, the solvent phase was separated by centrifugation at 3000 rpm for 10 min at 4 °C. The procedure was repeated twice more, and the solvent extract (100 mL in total) was used for the solvent-assisted flavor evaporation.

(2)

where: w(i) represents the volatile compound to be detected; w(s) is the content of the internal standard; A(i) and A(s) are the peak areas of the volatile compound and the internal standard, respectively, on the GC chromatogram; and the relative correction factor, f’(i), for each component to be measured was considered to be 1.0. 2.7. Determination of the fatty acids contents The contents of fatty acids were identified according to GB/T 17376-2008 & GB/T 17377-2008. 2.8. Statistical analysis Results are expressed as the mean ± standard deviation (SD) of at least three independent pretreatment experiments (extraction procedure) for each sample of different termination temperature. Principal component analysis (PCA) was also performed against the differences in the volatile compounds of different oil samples. All statistical analyses were performed using the SPSS software package and XL-stat-Pro (Addinsoft).

2.5.2. Solvent-assisted flavor evaporation (SAFE) For the oil samples, 50 g of oil together with extraction solvent (dichloromethane, m:V = 1:4) and internal standard stock solution (26.0 μg/mL of 1,2-dichlorobenzene) were mixed together for the SAFE procedure. The extracts were subjected to SAFE to remove any nonvolatile materials (Kreissl & Schieberle, 2017). The procedure was conducted by connecting a compact to a distillation vessel to rapidly achieve a high yield of the volatiles from the solvent extracts. A high vacuum was applied to the apparatus to separate the volatiles away from the organic phase. After removal of the non-volatile compounds, the SAFE distillate was dried over anhydrous Na2SO4 and then was slowly concentrated to 6–8 mL using a rotatory evaporator. The extracts were concentrated to 500 μL using a gentle stream of nitrogen gas. Samples were prepared in triplicate and stored in 2-mL glass vials at −80 °C for GC-MS analysis.

3. Results and discussion 3.1. Sensory analysis The oils from different termination temperatures were evaluated using sensory analysis. As shown in Fig. 1, the two principle components (PC1 and PC2) accounted for 94.94% of the variance (82.61% and 12.33%, respectively). According to the PCA results, those oil samples were separate on the PCA plot. Sensory attributes including burnt, fried, oily, cooked vegetable and salty increased with oil temperature whereas note of green grass and pungent decreased. When the temperature was increased from 140 °C to 165 °C, the notes of burnt, oily and fried increased significantly, which were the most closely related to the sample of 165 °C. Moreover, this phenomenon further indicated that the contents of the substances responsible for the related aroma attributes increased rapidly at this stage, which caused an overall change in the flavor profile of the oil.

2.6. Tentative identification and relative quantification of volatile compounds by GC-MS analysis The GC-MS analysis was performed using an ISQ LT-TRACE 1310 (Thermo Scientific, USA) system. Separations were performed on a TGWAX column (30 m × 0.25 mm i.d.; 0.25 μm film; Thermo fisher, USA). Helium was used as the carrier gas at a constant flow rate of 1.0 mL/ min. The GC oven temperature was programmed as follows: initial temperature 40 °C, held for 1 min, ramped at 4 °C/min to 220 °C, and held for 8 min. MS conditions were as follows: EI ionization source; ionization energy, 70 eV; ion source temperature, 250 °C; transfer line,

3.2. Electronic nose analysis An electronic nose was used to investigate the flavor variations in the oil preparation process. PCA results based on electronic nose 298

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immediately distinguishable from the others, and the distance between the dimensions increased noticeably. In general, the flavor of the oil samples changed significantly in the third stage, which was considered as the most important period to achieve the distinctive overall oil flavor profile during the whole preparation process. Meanwhile, the diversification presented by the electronic nose was consistent with the results of sensory analysis. Additionally, concerning the sensors of electronic nose, sensors of W5S, W6S, W1W and W2W were the most correlated to the oil sample of 165 °C (Fig. 2). In terms of the sensors' own characters, it can be concluded that sulfur-containing compounds have an important contribution to the flavor profile of the green onion deep-fried sample. 3.3. DSE-SAFE-GC-MS analysis 3.3.1. Changes in the aroma compositions during the preparation of green onion deep-fried oil Total ion chromatograms of different termination temperatures used during the preparation process as determined by SAFE-GC-MS are shown in Fig. S1 (Appendix A), and the corresponding analysis results are shown in Table 1. PCA analysis was carried out based on GC-MS data and the fingerprint result was shown in Fig. 3. As shown in Fig. 3, the frying procedure can be generally divided into three stages in the two–dimensional map based on the different terminal oil temperatures, namely, 100 °C~110 °C as the first stage located in the third quadrant, 120 °C~140 °C as the second stage mainly located in the second quadrant and 165 °C as the final stage located in the fourth quadrant. This distribution was in agreement with the result obtained with the electronic nose. When the terminal temperature was between 100 °C and 140 °C, the oil samples were mostly scattered on the negative side of PC1. However, when the terminal temperature was 165 °C, the oil samples were on the positive side of PC1, and the samples were well-separated from the other samples on the PCA plot. This phenomenon indicated that the overall flavor profile changed significantly when the oil was heated to 165 °C, which made this phase of frying the critical period for flavor formation. As shown in Table 1, a total of 103 kinds of volatile components were detected among all the oil samples from the different termination temperatures. The compounds were classified according to their functional groups, and they comprised 2 alkanes, 18 alcohols and phenols, 16 aldehydes, 12 ketones, 5 acids, 3 esters, 17 furans and furanones, 11 nitrogen-containing compounds and 19 sulfur-containing compounds. The content variation curves of each chemical group are shown in Fig. 4. It can be seen that with increasing oil temperature, the total contents of some of the chemical classes increased by a larger amount than others. The chemical class increased the most was furans and furanones, followed by sulfur-containing compounds, aldehydes and alcohols. The contents of each groups gradually increased during the three stages, and the generation rate accelerated when the oil temperature reached 165 °C. In addition, the variations in contents of nitrogen-containing compounds and ketones were very small in the first stage of the process, but they increased significantly in the third stage. This indicated that these components were mainly generated in the third period. However, the amount by which the content of acids increased was very small, and the content of alkanes did not noticeably change. Moreover, the loading diagrams of volatiles identified by GCMS were shown in Fig. S2 (Appendix A). A total of 68 components were detected in samples from all the termination temperatures, while 35 kinds of compounds were only identified in some of them. As shown in Fig. S2 (a) (Appendix A), components contributed the most to positive side of PC1 included A2 (dimethyl disulfide), A53 (furfural), A58 (2acetylfuran), A75 ((E)-methyl propenyl trisulfide), A91 (2-hydroxy-2cyclopenten-1-one), A73 (5-methyl-furfural) and A129 (5-hydroxymethylfurfural). Besides, the major volatiles related to the positive side of PC1 were A1 (2,3-pentanedione), A74 (4-cyclopentene-1,3dione), A87 (2(5H)-furanone), A105 (2,5-furandicarboxaldehyde), A68

Fig. 1. PCA plot of the oils from different termination temperatures during the preparation process by sensory analysis.

Fig. 2. PCA plot of the oils from different termination temperatures during the preparation process by electronic nose analysis.

analyses are shown in Fig. 2 and Table S1 (Appendix A). The two principle components (PC1 and PC2) accounted for 95.54% of the variance (76.46% and 19.08%, respectively). Based on the PCA results, the oil samples could be separated on the PCA plot. The samples can be divided into three stages based on the oil temperature, namely, 100 °C~120 °C as the first stage, 130 °C~140 °C as the second stage, and 165 °C as the final stage. The oil samples in the first stage were distributed on the positive side of PC1 dimension, and the sample of 100 °C were differentiated from that of 110 °C~120 °C on PC2 dimension. This indicated that oil samples in the first stage had changed a little in the whole flavor. The oil samples in the second stage scattered on the negative side of PC1dimension, and it was obvious that samples from the above two stages were completely separated. As the oil temperature increased to the third stage, the fingerprint of the sample was 299

RT (min)

5.28 5.52 5.71 5.98 6.57 6.82 6.85 6.92 7.57 7.58 7.87 7.98 8.19 8.28 8.53 8.99 9.11 9.20 9.32 9.55 9.59 9.89 9.94 10.12 10.19 10.48 10.50 11.18 11.25 11.29 11.47 11.89 12.03 12.16 12.37 12.51 12.57 12.80 13.26 13.96 14.02 14.10 14.51 14.70 14.89 15.00 15.10 15.57 15.72 16.30

No.

A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15 A16 A17 A18 A19 A20 A21 A22 A23 A24 A25 A26 A27 A28 A29 A30 A31 A32 A33 A34 A35 A36 A37 A38 A39 A40 A41 A42 A43 A44 A45 A46 A47 A48 A49 A50

2,3-Pentanedione Dimethyl disulfide Hexanal 2-Methyl-2-butenal 2-Methyl-thiophene (E)-2-Pentenal 1-(Ethylthio)-3-methyl-1,3-butadiene n-Propyl allyl sulfide 2-Methyl-2-pentenal 1-Penten-3-ol 3-Penten-2-ol Acetic acid pentyl ester 2-Heptanone Heptanal (−)-Limonene 2,3-Dimethyl-2-butanol 3-Hexen-2-one (E)-2-Hexenal S-oxide propanethial Methyl propyl disulfide 2-Pentyl-furan 3-Hydroxy-3-methyl-2-butanone γ-Terpinene 1-Pentanol 3,4-Dimethyl-thiophene (E)-Methyl 1-propenyl disulfide, 2-Methyl-pyrazine Acetoin Methyl 1-propenyl disulfide Octanal Ethyl n-propyl disulfide 2-Heptanol (E)-2-Penten-1-ol Prenol (E)-2-Heptenal 2,3-Octanedione 2-Ethylpyrazine 1,4-Pentadien-3-ol 2-Cyclopenten-1-one Dimethyl trisulfide Dipropyl disulfide 2-Ethyl-5-methyl-pyrazine Nonanal Isoxazole Propyl sulfide (E)-1-Propenyl propyl disulfide 5-Ethylcyclopentene-1-carbaldehyde (E)-2-Octenal 1-Allyl-2-isopropyldisulfane 1-Octen-3-ol

Compound name

600-14-6 624-92-0 66-25-1 1115-11-3 554-14-3 1576-87-0 49,563-09-9 27,817-67-0 623-36-9 616-25-1 1569-50-2 628-63-7 110-43-0 111-71-7 5989-54-8 594-60-5 763-93-9 6728-26-3 32,157-29-2 2179-60-4 3777-69-3 115-22-0 99-85-4 71-41-0 632-15-5 23,838-19-9 109-08-0 513-86-0 5905-47-5 124-13-0 30,453-31-7 543-49-7 1576-96-1 556-82-1 18,829-55-5 585-25-1 13,925-00-3 922-65-6 930-30-3 3658-80-8 629-19-6 13,360-64-0 124-19-6 288-14-2 111-47-7 23,838-21-3 36,431-60-4 2548-87-0 67,421-85-6 3391-86-4

CAS No.

C5H8O2 C2H6S2 C6H12O C5H8O C5H6S C5H8O C7H12S C6H12S C6H10O C5H10O C5H10O C7H14O2 C7H14O C7H14O C10H16 C6H14O C6H10O C6H10O C3H6OS C4H10S2 C9H14O C5H10O2 C10H16 C5H12O C6H8S C4H8S2 C5H6N2 C4H8O2 C4H8S2 C8H16O C5H12S2 C7H16O C5H10O C5H10O C7H12O C8H14O2 C6H8N2 C5H8O C5H6O C2H6S3 C6H14S2 C7H10N2 C9H18O C3H3NO C6H14S C6H12S2 C8H12O C8H14O C6H12S2 C8H16O

Formula

– 0.01 ± 0.003 0.06 ± 0.03 – 0.02 ± 0.004 – 0.01 ± 0.003 0.06 ± 0.02 0.34 ± 0.04 – 0.61 ± 0.15 – – 0.002 ± 0.001 0.06 ± 0.03 – – – 2.69 ± 0.94 0.18 ± 0.04 – – 0.06 ± 0.02 – 1.3 ± 0.31 0.03 ± 0.01 – 0.04 ± 0.03 0.17 ± 0.04 – 0.02 ± 0.01 0.05 ± 0.01 – 0.07 ± 0.02 – – – 0.02 ± 0.01 0.01 ± 0.004 – 2.19 ± 0.44 – 0.1 ± 0.04 – – 0.48 ± 0.11 – – 2.17 ± 0.48 –

– 0.002 ± 0 0.25 ± 0.08 0.02 ± 0 – 0.04 ± 0.01 – – – 0.07 ± 0.02 0.5 ± 0.1 – 0.003 ± 0 0.03 ± 0.01 0.002 ± 0 0.03 ± 0.01 0.003 ± 0 0.04 ± 0.01 – 0.01 ± 0 0.01 ± 0 0.01 ± 0 – 0.06 ± 0.01 0.01 ± 0 0.001 ± 0 0.02 ± 0 – 0.06 ± 0.01 0.01 ± 0 – 0.03 ± 0.01 0.01 ± 0 0.003 ± 0 0.69 ± 0.04 0.08 ± 0.08 – 0.01 ± 0 0.02 ± 0 0.26 ± 0.02 – – 0.07 ± 0.01 0.004 ± 0 0.01 ± 0 0.004 ± 0 0.004 ± 0 0.06 ± 0.01 0.05 ± 0.02 0.19 ± 0

100 °C – 0.01 ± 0 0.07 ± 0 0.02 ± 0 – 0.02 ± 0 – – – 0.02 ± 0 0.49 ± 0.07 – 0.002 ± 0 0.002 ± 0 0.003 ± 0 0.04 ± 0.01 0.002 ± 0 0.03 ± 0 – 0.02 ± 0 0.03 ± 0.01 0.01 ± 0 – 0.02 ± 0.01 0.02 ± 0 0.005 ± 0 0.02 ± 0.01 – 0.09 ± 0.01 0.01 ± 0 – 0.07 ± 0.01 0.003 ± 0 0.004 ± 0 0.43 ± 0.15 0.09 ± 0.08 – 0.01 ± 0 0.02 ± 0 0.52 ± 0.05 – – 0.07 ± 0.02 0.002 ± 0 0.02 ± 0 0.01 ± 0 0.02 ± 0 0.07 ± 0.03 0.1 ± 0.02 0.13 ± 0.04

110 °C – 0.01 ± 0 0.06 ± 0 0.02 ± 0 0.002 ± 0 0.01 ± 0 – – – 0.01 ± 0 0.41 ± 0.03 – 0.001 ± 0 0.002 ± 0 0.002 ± 0 0.03 ± 0 0.001 ± 0 0.03 ± 0 – 0.01 ± 0 0.03 ± 0 0.01 ± 0 – 0.01 ± 0 0.03 ± 0 0.04 ± 0 0.02 ± 0 – 0.14 ± 0 0.01 ± 0 – 0.03 ± 0 0.003 ± 0 0.01 ± 0 0.41 ± 0.08 0.04 ± 0.02 0.001 ± 0 0.01 ± 0 0.01 ± 0 0.43 ± 0.07 – 0.001 ± 0 0.08 ± 0 0.01 ± 0.01 0.06 ± 0 0.01 ± 0 0.02 ± 0 0.09 ± 0.01 0.04 ± 0.01 0.1 ± 0

120 °C – 0.04 ± 0.01 0.06 ± 0.01 0.02 ± 0 0.001 ± 0 0.01 ± 0 – – – 0.02 ± 0 0.38 ± 0.01 – 0.001 ± 0 0.01 ± 0 0.002 ± 0 0.03 ± 0 0.002 ± 0 0.04 ± 0 – 0.03 ± 0 0.04 ± 0 0.01 ± 0 – 0.01 ± 0 0.04 ± 0 0.07 ± 0.01 0.01 ± 0 – 0.27 ± 0.05 0.01 ± 0 – 0.03 ± 0 0.003 ± 0 0.002 ± 0 0.48 ± 0.02 0.09 ± 0 0.001 ± 0 0.01 ± 0 0.01 ± 0 0.64 ± 0.04 – 0.001 ± 0 0.07 ± 0 0.02 ± 0 0.08 ± 0.02 0.02 ± 0.01 0.02 ± 0 0.12 ± 0.01 0.1 ± 0.04 0.09 ± 0

130 °C

140 °C

165 °C

– 0.03 ± 0 0.1 ± 0.01 0.02 ± 0 0.003 ± 0 0.02 ± 0 – – – 0.03 ± 0 0.47 ± 0.06 – 0.001 ± 0 0.01 ± 0 0.002 ± 0 0.04 ± 0 0.004 ± 0 0.07 ± 0.01 – 0.02 ± 0 0.04 ± 0 0.01 ± 0 – 0.02 ± 0.01 0.1 ± 0.01 0.08 ± 0.01 0.02 ± 0 – 0.31 ± 0.02 0.01 ± 0 – 0.04 ± 0 0.003 ± 0 0.01 ± 0 0.58 ± 0.02 0.13 ± 0.01 0.003 ± 0 0.01 ± 0 0.01 ± 0 0.53 ± 0.01 – 0.002 ± 0 0.08 ± 0 0.02 ± 0 0.16 ± 0.01 0.02 ± 0 0.01 ± 0 0.11 ± 0 0.08 ± 0 0.1 ± 0

300

1068/1053 1080/1071 1089/1097 1102/1093 1124/1093 1134/1147 1135/— 1137/1137 1162/1160 1162/1176 1173/1178 1177/1179 1185/1180 1189/1188 1198/1204 1214/1082 1218/1209 1221/1243 1225/— 1232/1242 1234/1249 1244/1247 1245/1240 1252/1274 1254/1253 1264/1322 1264/1264 1287/1282 1289/1292 1291/1291 1297/1289 1310/1318 1315/1318 1319/1324 1326/1334 1330/1335 1332/1330 1340/— 1355/1369.2 1377/1421 1379/1387 1382/1376.4 1395/1398 1401/— 1407/1069 1411/— 1414/1416 1429/1434 1434/— 1453/1476

(continued on next page)

0.01 ± 0 0.11 ± 0.02 0.27 ± 0.06 0.02 ± 0 0.003 ± 0 0.04 ± 0.01 – – – 0.08 ± 0.01 0.49 ± 0.08 0.004 ± 0 0.002 ± 0 0.03 ± 0 0.003 ± 0 0.03 ± 0 0.01 ± 0 0.07 ± 0.01 – 0.05 ± 0.01 0.03 ± 0 0.01 ± 0 – 0.05 ± 0 0.12 ± 0.01 0.09 ± 0.02 0.01 ± 0 – 0.45 ± 0.04 0.01 ± 0 – 0.03 ± 0 0.01 ± 0 0.002 ± 0 0.99 ± 0.08 0.38 ± 0.02 – 0.02 ± 0 0.01 ± 0 0.62 ± 0.05 – – 0.11 ± 0 0.01 ± 0 0.04 ± 0.01 0.003 ± 0 0.01 ± 0 0.15 ± 0 0.05 ± 0 0.21 ± 0

Exp/Lit

Fresh sample

Deep-fried oils of different termination temperatures

RI

Content (ug/g)

Table 1 Comparative analysis of the results of the volatile compounds detected in fresh green onion and its deep-fried oils with different preparation processes.

N. Zhang, et al.

Food Research International 121 (2019) 296–306

RT (min)

16.39 16.52 16.75 17.35 17.55 17.68 17.78 17.91 18.05 18.25 18.31 18.42 18.76 18.85 18.97 19.15 19.16 19.36 19.47 19.48 19.55 19.70 19.95 20.29 20.66 21.08 21.17 21.43 21.90 22.47 22.52 22.55 23.77 24.21 24.51 24.88 24.97 25.29 25.32 25.37 25.48 25.84 26.01 26.50 26.57 27.00 27.16 27.19 27.41 27.62 27.92

No.

A51 A52 A53 A54 A55 A56 A57 A58 A59 A60 A61 A62 A63 A64 A65 A66 A67 A68 A69 A70 A71 A72 A73 A74 A75 A76 A77 A78 A79 A80 A81 A82 A83 A84 A85 A86 A87 A88 A89 A90 A91 A92 A93 A94 A95 A96 A97 A98 A99 A100 A101

Table 1 (continued)

Methional Acetic acid Furfural 2-Ethenyl-6-methyl-pyrazine (E,E)-2,4-Heptadienal 1-(Ethylthio)-2-methyl-1-propene Furfuryl formate 2-Acetylfuran 3-Nonen-2-one 3-Methoxy-1-butanol 3,5-Octadien-2-one Methyl propyl trisulfide (E)-2-Nonenal 4-Ethylcyclohexanol 2,4-Dimethyl-cyclohexanol 2-Pyridinecarboxaldehyde (E)-Prop-1-en-1-yl propane dithioate 6-Methyl-3(2H)-pyridazinone Propanoic acid Linalyl acetate 1H-1,2,4-Triazole Dimethyl sulfoxide 5-Methyl-furfural 4-Cyclopentene-1,3-dione (E)-methyl propenyl trisulfide 2-Octen-1-ol (E)-2-Octen-1-ol γ-Butyrolactone Benzeneacetaldehyde Dipropyl trisulfide 2-Furanmethanol Dihydro-5-methyl-5-vinyl-2(3H)-furanone 3,4-Dihydro-2H-pyran-2-carboxaldehyde 5-Methyl-2-furanmethanol (Z)-1-Propenyl propyl trisulfide 2-Undecenal 2(5H)-Furanone 3,5-Diethyl-1,2,4-trithiolane 2,4-Decadienal 1,2,5-Trithiepane 2-Hydroxy-2-cyclopenten-1-one (E)-1-propenyl propyl trisulfide, 1-(2-Butoxyethoxy)-ethanol (E,E)-2,4-Decadienal Dipropyl sulfoxide 3-Methyl-1,2-cyclopentane-dione 2-Hydroxyethyl ethyl sulfide 2-Ethyltetrahydro-2H-thiopyran 1-Ethylidene-1H-indene 5,6-Dihydro-2H-pyran-2-one Hexanoic acid

Compound name

3268-49-3 64-19-7 98-01-1 13,925-09-2 4313-03-5 27,482-14-0 13,493-97-5 1192-62-7 14,309-57-0 2517-43-3 38,284-27-4 17,619-36-2 18,829-56-6 4534-74-1 69,542-91-2 1121-60-4 67,269-06-1 13,327-27-0 79-09-4 115-95-7 288-88-0 67-68-5 620-02-0 930-60-9 23,838-25-7 22,104-78-5 18,409-17-1 96-48-0 122-78-1 6028-61-1 98-00-0 1073-11-6 100-73-2 3857-25-8 23,838-26-8 2463-77-6 497-23-4 54,644-28-9 2363-88-4 6576-93-8 10,493-98-8 23,838-27-9 54,446-78-5 25,152-84-5 4253-91-2 765-70-8 110-77-0 1613-52-1 2471-83-2 3393-45-1 142-62-1

CAS No.

C4H8OS C2H4O2 C5H4O2 C7H8N2 C7H10O C6H12S C6H6O3 C6H6O2 C9H16O C5H12O2 C8H12O C4H10S3 C9H16O C8H16O C8H16O C6H5NO C6H10S2 C5H6N2O C3H6O2 C12H20O2 C2H3N3 C2H6OS C6H6O2 C5H4O2 C4H8S3 C8H16O C8H16O C4H6O2 C8H8O C6H14S3 C5H6O2 C7H10O2 C6H8O2 C6H8O2 C6H12S3 C11H20O C4H4O2 C6H12S3 C10H16O C4H8S3 C5H6O2 C6H12S3 C8H18O3 C10H16O C6H14OS C6H8O2 C4H10OS C7H14S C11H10 C5H6O2 C6H12O2

Formula

– – 0.05 – – – – – – – – 0.16 – – – – 0.15 – – 0.04 – – – – – – – – – 0.94 – – – – 0.62 – – 0.36 – 0.09 – 0.78 0.73 – 0.22 – – 0.43 – – –

301 ± 0.11

± 0.09

± 0.35 ± 0.24

± 0.01

± 0.13

± 0.15

± 0.22

± 0.01

± 0.07

± 0.05

± 0.02

0.01 ± 0 – 0.29 ± 0.01 0.001 ± 0 0.54 ± 0.04 – 0.03 ± 0.01 0.003 ± 0 0.004 ± 0 0.03 ± 0 0.01 ± 0 0.002 ± 0 0.01 ± 0 0.03 ± 0 0.03 ± 0 – – – – – – 0.03 ± 0 0.001 ± 0 – 0.01 ± 0 0.01 ± 0 0.002 ± 0 – 0.03 ± 0.01 0.01 ± 0 – 0.01 ± 0 – 0.001 ± 0 – 0.01 ± 0 – – 0.09 ± 0.02 – 0.01 ± 0 0.002 ± 0 0.01 ± 0 0.19 ± 0.05 – – 0.01 ± 0 – 0.004 ± 0 – 0.03 ± 0

100 °C 0.01 ± 0 – 0.27 ± 0.04 0.001 ± 0 0.44 ± 0.14 0.01 ± 0 0.04 ± 0.01 0.004 ± 0 0.01 ± 0 0.03 ± 0 0.01 ± 0 0.02 ± 0 0.01 ± 0 0.03 ± 0.01 0.03 ± 0.01 0.01 ± 0 – – – – – 0.04 ± 0.01 0.01 ± 0 – 0.004 ± 0 0.01 ± 0 0.003 ± 0 – 0.15 ± 0.02 0.04 ± 0 – 0.004 ± 0 – 0.003 ± 0 – 0.01 ± 0 – – 0.14 ± 0.05 – 0.01 ± 0 0.002 ± 0 0.02 ± 0 0.3 ± 0.12 – – 0.01 ± 0 – 0.003 ± 0 – 0.04 ± 0.01

110 °C 0.01 ± 0 – 0.34 ± 0.01 0.001 ± 0 0.36 ± 0.01 0.02 ± 0 0.05 ± 0.01 0.01 ± 0 0.01 ± 0 0.03 ± 0 0.004 ± 0 0.03 ± 0 0.01 ± 0 0.03 ± 0 0.03 ± 0 0.02 ± 0 – – – – – 0.03 ± 0 0.04 ± 0.01 0.01 ± 0 0.04 ± 0.01 0.01 ± 0 0.004 ± 0 – 0.42 ± 0.03 0.005 ± 0 0.02 ± 0.01 – – 0.02 ± 0.01 – 0.02 ± 0 – – 0.26 ± 0.01 – 0.02 ± 0 0.004 ± 0 0.01 ± 0 0.53 ± 0.03 – 0.003 ± 0 0.01 ± 0 – 0.002 ± 0 – 0.02 ± 0

120 °C 0.01 ± 0 – 0.45 ± 0.04 0.001 ± 0 0.31 ± 0.02 0.02 ± 0 0.03 ± 0.01 0.04 ± 0.01 0.01 ± 0 0.03 ± 0 – 0.06 ± 0 0.01 ± 0 0.03 ± 0 0.03 ± 0 0.01 ± 0 – 0.003 ± 0 – – – 0.03 ± 0 0.07 ± 0 0.02 ± 0 0.18 ± 0.02 0.004 ± 0 0.01 ± 0 – 0.45 ± 0.04 0.01 ± 0 0.03 ± 0 – – 0.02 ± 0 – 0.02 ± 0 0.01 ± 0 – 0.3 ± 0.06 – 0.03 ± 0 0.005 ± 0 0.02 ± 0 0.61 ± 0.12 – 0.01 ± 0 0.04 ± 0.01 – 0.003 ± 0 – 0.04 ± 0

130 °C

140 °C

165 °C

0.01 ± 0 – 0.74 ± 0.05 0.001 ± 0 0.34 ± 0.01 0.03 ± 0 0.03 ± 0 0.07 ± 0.01 0.01 ± 0 0.03 ± 0 – 0.05 ± 0 0.06 ± 0.04 0.03 ± 0 0.04 ± 0.01 0.02 ± 0 – 0.004 ± 0 – – – 0.03 ± 0 0.13 ± 0.01 0.04 ± 0 0.18 ± 0.07 0.02 ± 0 0.01 ± 0 – 0.47 ± 0.01 0.01 ± 0 0.05 ± 0 – – 0.03 ± 0 – 0.02 ± 0 0.01 ± 0 – 0.22 ± 0.02 – 0.05 ± 0.01 0.002 ± 0 0.04 ± 0.01 0.47 ± 0.06 – 0.01 ± 0 0.03 ± 0 – 0.002 ± 0 – 0.04 ± 0

1456/1474 1460/1484 1468/1469 1488/1490 1494/1490 1498/— 1502/1504 1506/1512 1511/1508 1518/— 1520/1521 1523/1531 1535/1535 1538/— 1542/— 1548/1570 1548/— 1555/— 1559/1564 1561/1556 1562/— 1567/1620 1575/1610 1587/1605 1599/1588.7 1614/1647 1617/1649 1626/1626 1643/1638 1663/1703.2 1665/1649 1666/1683 1709/— 1725/1722 1736/— 1749/1755 1753/1745 1764/1751 1766/1767 1767/— 1771/1769 1785/1765 1791/1800 1809/1807 1812/— 1828/1800.1 1834/— 1835/— 1844/1867 1852/1838 1863/1854

(continued on next page)

0.01 ± 0 0.09 ± 0.01 1.49 ± 0.16 0.001 ± 0 0.62 ± 0 0.01 ± 0 0.02 ± 0 0.19 ± 0.01 0.03 ± 0 0.03 ± 0 – 0.08 ± 0 0.02 ± 0 0.06 ± 0 0.06 ± 0 – – 0.06 ± 0.02 0.01 ± 0 – 0.03 ± 0.01 0.06 ± 0 0.52 ± 0.01 0.17 ± 0 0.72 ± 0.24 0.03 ± 0 0.01 ± 0 0.06 ± 0 0.5 ± 0.02 – 0.73 ± 0.05 – 0.01 ± 0 0.05 ± 0 – 0.02 ± 0 0.09 ± 0.01 – 0.32 ± 0.04 – 0.11 ± 0.01 0.001 ± 0 0.02 ± 0 0.72 ± 0.09 – 0.03 ± 0 0.05 ± 0 – 0.004 ± 0 0.01 ± 0 0.07 ± 0.01

Exp/Lit

Fresh sample

Deep-fried oils of different termination temperatures

RI

Content (ug/g)

N. Zhang, et al.

Food Research International 121 (2019) 296–306

RT (min)

30.09 30.63 30.77 30.94 31.42 31.93 32.03 32.18 32.75 33.05 33.35 33.61 33.88 34.01 34.89 35.73 36.32 36.56 37.23 37.36 37.52 38.40 38.58 38.66 38.74 40.16 40.36 42.54 43.74 44.40 44.5 44.86 45.15 48.64 52.01 53.1

No.

A102 A103 A104 A105 A106 A107 A108 A109 A110 A111 A112 A113 A114 A115 A116 A117 A118 A119 A120 A121 A122 A123 A124 A125 A126 A127 A128 A129 A130 A131 A132 A133 A134 A135 A136 A137

Table 1 (continued)

Methyl 1-(propylthio)propyl disulfide 1-(1H-Pyrrol-2-yl)-ethanone S-Methyl methane-thiosulphonate 2,5-Furandicarboxaldehyde 2-Furyl hydroxymethyl ketone 1H-Pyrrole-2-carboxaldehyde Methyl 1-(1-propenylthio) propyl disulfide Furaneol 2,3-Bis(methylthio)bicycle [2.2.1]hept-2-ene 1-(1-(Methylthio)propyl)-2-propyldisulfane Dihydroxyacetone 1-(1-Propenylthio)propyl propyl disulfide 5-Methyl-1H-pyrrole-2-carboxaldehyde S-Propyl propane-1-sulfonothioate 1-Methyl-2-(1-(propyl disulfanyl) propyl) disulfane Nonanoic acid Dihydro-6-methyl-2H-Pyran-3(4H)-one 3,4-Dimethyl-2-(propyl disulfanyl) thiophene Dibenzofuran 6-Ethyl-4,5,7,8-tetrathiaundecane Pyranone 3,5-Dihydroxy-2-methyl-4H-pyran-4-one (Z)-1-(1-Propenyldithio) propyl propyl disulfide 2,4-Di-tert-butylphenol (E)-1-(1-propenyldithio) propyl propyl disulfide 1-Allyl-3-(2-(allylthio)propyl) trisulfane 2,3-Dihydro-benzofuran 5-Hydroxymethylfurfural Vanillin Dihydro-4-hydroxy-2(3H)-furanone Methyl vanillate Benzyl benzoate Apocynin Vanillyl alcohol n-Hexadecanoic acid 4-Hydroxy-benzaldehyde

Compound name

126,876-21-9 1072-83-9 2949-92-0 823-82-5 17,678-19-2 1003-29-8 126,876-23-1 3658-77-3 Nist:186616 126,876-22-0 96-26-4 143,193-11-7 1192-79-6 Nist: 414096 126,876-30-0 112-05-0 43,152-89-2 126,876-33-3 132-64-9 126,876-35-5 28,564-83-2 1073-96-7 126,876-37-7 96-76-4 126,876-36-6 193,625-59-1 496-16-2 67-47-0 121-33-5 5469-16-9 3943-74-6 120-51-4 498-02-2 498-00-0 57-10-3 123-08-0

CAS No.

C7H16S3 C6H7NO C2H6O2S2 C6H4O3 C6H6O3 C5H5NO C7H14S3 C6H8O3 C9H14S2 C7H16S3 C3H6O3 C9H18S3 C6H7NO C6H14O2S2 C7H16S4 C9H18O2 C6H10O2 C9H14S3 C12H8O C9H20S4 C6H8O4 C6H6O4 C9H18S4 C14H22O C9H18S4 C9H16S4 C8H8O C6H6O3 C8H8O3 C4H6O3 C9H10O4 C14H12O2 C9H10O3 C8H10O3 C16H32O2 C7H6O2

Formula

0.28 ± 0.07 – – – – – 0.55 ± 0.13 – 0.05 ± 0.02 0.14 ± 0.05 – 0.27 ± 0.06 – 0.07 ± 0.04 0.09 ± 0.02 – – 0.06 ± 0.03 0.01 ± 0.01 0.33 ± 0.07 – – 0.04 ± 0.02 0.39 ± 0.13 0.08 ± 0.003 0.05 ± 0.02 0.12 ± 0.03 – 0.32 ± 0.13 – 1.08 ± 0.38 0.41 ± 0.09 1.36 ± 0.42 0.15 ± 0.04 0.3 ± 0.21 0.24 ± 0.08

– – – – – – – – – – – – – – – – – – – – 0.003 ± 0 – – 0.02 ± 0 – – – 0.03 ± 0.01 – – – 0.02 ± 0 – – 0.02 ± 0 –

– – – – – – – – – – – – – – – – – – – – – – – 0.02 – – – 0.02 – – – 0.03 – – 0.03 –

302 ± 0.02

±0

± 0.01

±0

110 °C

100 °C – 0.03 0.01 – – 0.02 – – – – – – – – – – – – – – 0.14 0.03 – 0.02 – – – 0.18 – – – 0.02 – – 0.05 – ± 0.02

±0

± 0.01

±0

± 0.05 ± 0.01

±0

±0 ±0

120 °C – 0.08 ± 0.02 0.02 ± 0.01 0.002 ± 0 0.02 ± 0 0.03 ± 0 – – – – – – – – – – – – – – 0.41 ± 0.04 0.37 ± 0.09 – 0.59 ± 0.12 – – – 0.61 ± 0.12 – 0.003 ± 0 – 0.02 ± 0 – – 0.04 ± 0.01 –

130 °C

140 °C

165 °C

– 0.1 ± 0 0.01 ± 0 0.004 ± 0 0.06 ± 0.02 0.03 ± 0.01 – – – – – – – – – 0.12 ± 0.03 – – – – 0.44 ± 0.03 0.62 ± 0.06 – 0.02 ± 0.01 – – – 0.8 ± 0.08 – 0.01 ± 0.01 – 0.02 ± 0.01 – – 0.06 ± 0.01 –

– 0.31 ± 0.03 0.02 ± 0 0.021 ± 0 0.24 ± 0.04 0.04 ± 0.01 – 0.04 ± 0.01 – – 0.04 ± 0.01 – 0.04 ± 0.01 – – 0.04 ± 0.01 0.02 ± 0 – – – 0.51 ± 0.15 0.86 ± 0.19 – – – – – 1.74 ± 0.07 – 0.04 ± 0.01 – 0.04 ± 0 – – 0.06 ± 0 –

1948/— 1970/1969 1975/— 1982/1996 2001/1989 2023/2032 2027/— 2033/2031 2057/— 2069/— 2082/2075 2093/— 2104/2088.1 2110/— 2148/— 2184/2164 2210/— 2221/— 2251/2270 2257/— 2264/2267 2304/2309 2313/— 2316/2330 2320/— 2387/— 2396/2389 2502/2512 2563/2568 2596/2457 2601/2600 2620/2636 2635/2623 2800/2787 > 2900/2910 > 2900/2958

Exp/Lit

Fresh sample

Deep-fried oils of different termination temperatures

RI

Content (ug/g)

N. Zhang, et al.

Food Research International 121 (2019) 296–306

Food Research International 121 (2019) 296–306

N. Zhang, et al.

Hsu, 2001; Chyau et al., 1997; Chyau & Mau, 2001; Wu et al., 1982) and fried garlic (Kim et al., 1995; Yu et al., 1993), the similarity between them lay in their chemical groups of the identified compounds. Among these chemical classes, sulfur-containing compounds, pyrazines and unsaturated aldehydes were all considered as the most significant flavor contributors. However, when it came to a certain compound, differences existed in the types and contents. For example, major sulfurcontaining compounds in fried garlic were allyl sulfides, while methyl sulfides were more remarkable in green onions. 3.3.2. Comparative analysis of the volatile compounds in fresh green onion and its deep-fried oil DSE-SAFE-GC-MS was conducted to analyze the volatile compounds present in fresh green onion. The chromatogram is shown in Fig. S3 (Appendix A), and the related analysis results are shown in Table 1. The results showed that a total of 59 volatile compounds were identified, namely, 2 alkanes, 7 alcohols, 6 aldehydes, 3 ketones, 1 acid, 3 esters, 3 furans and furanones and 34 sulfur-containing compounds. Among them, the substances present in the highest concentrations were propanethial S-oxide (2.69 μg/g), 1-allyl-2-isopropyldisulfane (2.17 μg/g), dipropyl disulfide (2.19 μg/g), apocynin (1.36 μg/g), 3,4-dimethylthiophene (1.3 μg/g) and methyl vanillate (1.08 μg/g). Some other compounds were also present at relatively high contents, including dipropyl trisulfide (0.94 μg/g), (E)-1-propenyl propyl trisulfide (0.78 μg/g), (Z)-1-propenyl propyl trisulfide (0.62 μg/g), 3-penten-2-ol (0.61 μg/g) and 1-(2-butoxyethoxy)-ethanol (0.73 μg/g). Generally, sulfur-containing compounds have the lowest threshold values of all kinds of aroma-active substances. They were typically present in low concentrations but contributed substantially to the aroma; thus, they played an essential role in the overall food flavor profile. Sulfur-containing compounds generally have unpleasant aromas; however, when diluted to ppb or ppm concentrations, their aromas change dramatically to generate the aromas of food such as fresh onion, garlic, meat, and tropical fruit (Liu & Sun, 2003; McGorrin, 2011). In accordance with the analysis results, sulfur-containing compounds dominated the profile of fresh green onion, and these compounds account for 70% of the total content of odor-contributing compounds. There were 34 sulfur-containing compounds detected in fresh green onion, namely, 7 kinds of monosulfides (4.73 μg/g), mainly propanethial S-oxide (2.69 μg/g) and 3,4-dimethyl-thiophene (1.3 μg/ g); 11 kinds of disulfides (5.52 μg/g), mainly dipropyl disulfide (2.19 μg/g) and 1-allyl-2-isopropyldisulfane (2.17 μg/g); 11 kinds of trisulfides (4.25 μg/g), mainly dipropyl trisulfide (0.94 μg/g), (E)-1propenyl propyl trisulfide (0.78 μg/g) and (Z)-1-propenyl propyl trisulfide (0.62 μg/g); and 5 kinds of tetrasulfides (0.59 μg/g), mainly 6ethyl-4,5,7,8-tetrathiaundecane (0.33 μg/g). The kinds of compounds identified were similar to those in previous studies (Huang, 2004; Zhang, Wu, & Li, 2006), however, their contents differed. In addition, this phenomenon was most related to factors including the place of origin and the growth environment. Among the sulfur-containing compounds, sulfides (up to 21 kinds) were the major components, and they accounted for 69.5% of the total content. Obviously, the sulfides were the greatest contributors to the characteristic profile of fresh green onion. As shown in Table 1, a total of 25 compounds were identified in both fresh green onion and its deep-fried oil. The most common substances were 11 sulfur-containing compounds (9 sulfides and 2 thiophenes), 3 saturated aldehydes (hexanal, heptanal, and nonanal), 3 furans (furfural, dibenzofuran, and 2,3-dihydro-benzofuran) and other compounds. Moreover, thermal treatment led to variations in the composition of volatile compounds compared with the composition found in raw green onion. For the compounds mentioned above, the variation trend chart showing the components that increased with increasing oil temperature is shown in Fig. S4 (Appendix A), while chart showing the opposite trend is shown in Fig. S5 (Appendix A). It can be seen that some compounds increased in concentration

Fig. 3. PCA plot of the volatile compounds from oils of different termination temperatures during the preparation process as determined by SAFE-GC-MS.

Fig. 4. Trends in the contents of compounds detected in the oils from different termination temperatures during the preparation process classified by chemical families.

(6-methyl-3(2H)-pyridazinone), A81 (2-furanmethanol), and A106 (2furyl hydroxymethyl ketone) as presented in Fig. S2 (b) (Appendix A). These above specific compounds had a significant effect on the general flavor of the oil sample of 165 °C, among which were mainly furans and furanones and sulfur-containing compounds. Consequently, we can draw a conclusion that the classes, which increased the most including furans and furanones, sulfur-containing compounds, aldehydes and alcohols, were the most important contributors to the flavor of green onion deep-fired oil. In other words, they were closely related to sensory attributes like burnt, fried, oily, cooked vegetable and salty. Among them, furans and furanones played the most significant part. Additionally, compared with the previous study on fried shallot (Chu & 303

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compound with notes of caramel as well as pineapple and strawberry. The aroma intensity of DMHF was 12.5 times higher than that of maltol and 2.5 times higher than that of ethyl maltol (Schwab & Roscher, 1997). The planar keto enol structure of DMHF can form five-membered rings through hydrogen bonding, and the common features of these structures are the cyclic enol structure at the α position, and these cyclic compounds were the degradation products of non-enzymatic browning under base catalysis from the heating of carbohydrates. The ring was found to be necessary for it to impart its abovementioned characteristic aroma. In other words, DMHF played a significant role in the flavor profile of the oil. Additionally, nitrogen-containing compounds were only identified in the oil and not in the raw materials, and they were generated through the Maillard reaction during the preparation process. Among these compounds, some of them, such as 2-methyl-pyrazine, 2-ethylpyrazine and 2-ethyl-5-methyl-pyrazine, which often have baking and nut notes, also contributed substantially to the flavor profile of the oil. (2) Thermo degradations between basic components. These compounds can be divided into three categories; thermal degradation of sugars, thermal degradation of amino acids, and thermal oxidative degradation of fats. In this study, the major product of the thermal degradation of sugars was 2-acetylfuran. The more important compounds in this study were the compounds generated by the thermal degradation of amino acids. The products mainly included dimethyl trisulfide, propyl sulfide, and methional. Most of them have low threshold values and high aroma intensity, and they are the major components contributing to the aroma of meat in Chinese cuisine. Dimethyl trisulfide has a characteristic odor of onion and garlic, and its threshold value (0.005–0.01 ppb) was even lower than that of DMHF, which was deemed one of the most important volatiles in the oil profile. During the frying process, its content increased as the oil temperature increased, and it may be produced through the degradation of sulfur-containing substance like S-methyl-L-cysteine sulfoxide in the green onion. Methional was produced by coming from methionine through an enzyme-catalysed process (Koutidou, Grauwet, Loey, & Acharya, 2017) and has salty and cooked vegetable notes.

including dimethyl disulfide, methyl 1-propenyl disulfide, (E)-methyl 1propenyl disulfide and furfural. In contrast, compounds that decreased in concentration including 2-methyl-thiophene, methyl propyl disulfide, 3,4-dimethyl- thiophene, (E)-1-propenyl propyl disulfide, 1-allyl2-isopropyldisulfane, methyl propyl trisulfide, dipropyl trisulfide, (E)-1propenyl propyl trisulfide, (-)-limonene, prenol, 1-(2-butoxyethoxy)ethanol, benzyl benzoate and hexadecanoic acid. In addition, the contents of some compounds, including saturated aldehydes (hexanal, heptanal, and nonanal) and alcohols (3-penten-2-ol, 2-heptanol, and 1,4-pentadien-3-ol), remained generally unchanged. In addition to the common features, there were many differences observed between fresh green onion and its deep-fried oil. As shown in Tables 1, 34 volatile compounds were only identified in the fresh sample, while 78 compounds were only identified in the fried samples. Components identified only in fresh green onion samples were 1 alcohol, 3 aldehydes, 1 alkane, 2 ketones, 2 esters, 2 furans and 23 sulfurcontaining compounds. The sulfur-containing compounds contributed the most to the profile of the raw sample. In addition, sulfides (12 in total) account for up to 49.6% of the total content of these compounds. This result was consistent with the analysis of electronic nose (Fig. 2). Other compounds that also played an important role were methyl vanillate, 2-methyl-2-pentenal, vanillin, 4-hydroxy-benzaldehyde, vanillyl alcohol, et al. The compounds identified only in the raw samples were likely either removed by evaporation or converted into other substances as the temperature of the oil increased. Moreover, the compounds detected only in the oil included 1 alkane, 10 alcohols, 11 ketones, 13 aldehydes (10 unsaturated ones), 4 acids, 2 esters, 18 furans and furanones, 11 nitrogen-containing compounds and 8 sulfur-containing compounds. In general, the number of different sulfur-containing compounds present decreased dramatically, but some new sulfur-containing substances were generated, and dimethyl trisulfide was the major component. The number of different furans and furanones together with unsaturated aldehydes increased significantly. In addition, nitrogen-containing compounds were only found in the oil and not in the raw sample, which indicated that the main flavor of the sample was significantly changed by the frying procedure, and the process caused a substantial number of sensory differences. In addition to carbohydrates and essential oils, green onion was rich in proteins and amino acids, including S-methyl-L-cysteine sulfoxide (MCSO), S-propenyl-L-cysteine sulfoxide (PeCSO), S-propyl-L-cysteine sulfoxide (PCSO), besides, some free amino acids and polysaccharides were also considered as the primary components (Block, 1992; Fenwick, Hanley, & Whitaker, 1985; Golovchenko, Khramova, Ovodova, Shashkov, & Ovodov, 2012; Kyung, 2012; Lanzotti, 2006; Zhang, Zhang, Lu, Luo, & Zha, 2016). Moreover, soybean oil served as the heat-conducting medium in the oil preparation process. Considering the reasons mentioned above, the 78 compounds identified only in the oil can be divided into two categories in terms of their possible formation pathways:

Thermal oxidative degradation of fats can be divided into two categories, one is the olefine aldehydes generated from unsaturated fatty acids, and the other is the methyl ketone, lactones and acids that produced by saturated fatty acids. These products are considered to be the characteristic aroma compounds of fried foods, and they often have fried and fatty notes. For the first category, (E)-2-heptenal and 2,4decadienal were present in the highest concentrations. In addition, 2,4decadienal (0.07 ppb) was deemed the characteristic aroma compound of fried food, and it contributed the most of all the carbonyl compound generated by the thermal decomposition of fats. For the second category, the main products were 2-heptanone, γ-butyrolactone and lowcarbon acids like propanoic acid. Although they were not the main aroma compounds of the overall profile, they can modify and even out the overall flavor.

(1) Interactions between basic components. The basic components refer to the carbohydrates, proteins and fats in the system. These substances interacted during the heating process and the Maillard reaction (browning reaction) between sugars and amino acids was the most important process. Generally, due to the short heating time and low temperature, the main Maillard reaction products during the initial stage of were Strecker aldehydes. These products can further interact to form the characteristic aroma compounds such as lactones and furans. As the heating time increased and the temperature rose, some substances with baking aromas, such as pyrazine, pyrrole and pyridine, were generated. In the oil, the major substances were furans, furanones and nitrogen-containing compounds. Based on both the number and contents of all the identified compositions, furans and furanones were the major components, and the most abundant compound was a furaneol (DMHF, 2,5-dimethyl-4-hydroxy-3(2H)-furanone). DMHF was an essential flavor

3.4. Determination of fatty acid contents Contents of fatty acids from the different termination temperatures during frying process were measured. According to the results shown in Table 2, the most important fatty acids detected in the oils were linoleic acid, oleic acid, palmitic acid, linolenic acid and stearic acid. These 5 fatty acids were identified in the oils of all different termination temperatures, and their contents were higher than 1.0 g/100 g. Of these compounds, linoleic acid and oleic acid were present in the highest concentrations. In terms of the saturation level, a total of 10 saturated fatty acids (SFA) were identified, while 7 unsaturated fatty acids (UFA) were detected. The variation curves of both the SFA and UFA during the 304

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Table 2 Variations in the contents of fatty acids and moisture in oils from different stages during the preparation process. No

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Compound name

C12:0 dodecanoic acid (lauric acid) C14:0 tetradecanoic acid/myristic acid C15:0 pentadecanoic acid C16:0 hexadecanoic acid/palmitic acid C17:0 heptadecanoic acid/margaric acid C18:0 octadecanoic acid /stearic acid C20:0 eicosanoic acid /arachidic acid C21:0 heneicosanoic acid C23:0 tricosanoic acid C24:0 tetracosanoic acid /lignoceric acid C16:1-9c cis-9-hexadecenoic acid/palmitoleic acid C18:1 cis-9-octadecenoic acid/oleic acid C18:2 cis-9,12-octadecadienoic acid/linoleic acid C18:3 cis-9,12,15-octadecatrienoic acid/linolenic acid C20:1-11c cis-11-eicosenoic acid C20:2-11c,14c cis-11,14-eicosadienoic acid C20:5-5c,8c,11c,14c,17c cis-5,8,11,14,17-eicosapentaenoic acid (EPA) saturated fatty acid (SFA) monounsaturated fatty acid (MUFA) polyunsaturated fatty acid (PUFA) unsaturated fatty acid(UFA)

Content(g/100 g) Unfried oil

100 °C

110 °C

120 °C

130 °C

140 °C

165 °C

0.01 0.07 0.02 10.13 0.12 3.92 0.36 0.03 – 0.13 0.08 21.51 52.47 7.34 0.18 0.04 0.41 14.79 21.77 60.26 82.03

0.01 0.07 0.02 9.88 0.12 3.79 0.31 0.03 – 0.12 0.08 20.86 50.83 7.09 0.17 0.04 0.38 14.35 21.11 58.34 79.45

– 0.07 0.02 10.04 0.12 3.87 0.32 0.03 – 0.12 0.08 21.23 51.83 7.23 0.18 0.04 0.39 14.59 21.49 59.49 80.98

– 0.07 0.02 9.88 0.12 3.8 0.31 0.02 – 0.12 0.08 20.91 51.03 7.13 0.17 0.04 0.38 14.34 21.16 58.58 79.74

– 0.07 0.02 9.91 0.12 3.83 0.31 – – 0.12 0.08 21.03 51.39 7.16 0.17 – 0.38 14.38 21.28 58.93 80.21

– 0.07 0.01 9.96 0.12 3.85 0.32 0.03 – 0.12 0.08 21.43 51.66 7.2 0.17 0.04 0.39 14.48 21.68 59.29 80.97

– 0.07 0.02 10.32 0.12 3.99 0.32 – 0.04 0.13 0.07 21.01 48.58 6.56 0.17 0.21 0.4 15.01 21.25 55.75 77

Note: “—”meant not detected；the detection limit of fatty acid was 0.01 g/100 g.

note of oily in the overall flavor profile.

preparation process are shown in Fig. S6 (Appendix A). Their variation trends were identical up to 140 °C, but as the oil temperature increased to 165 °C, the content of SFA increased rapidly while that of UFA did not. To further explore the decrease in the content of UFA, changes in the monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) contents were also studied. As shown in Fig. S7 (Appendix A), the decrease in the content of UFA was mainly due to variations in the content of PUFA. As shown in Table 2, dodecanoic acid (lauric acid) was only identified in reference soybean oil and oil heated to 100 °C. In addition, the contents of oleic acid, linoleic acid and linolenic acid all decreased as the oil temperature increased, but the change was small. The amount of cis-11,14-eicosadienoic acid increased noticeably when the oil was heated to 165 °C. The fatty acid composition in fried oil can affect the flavor of fried products. Due to their double bond, unsaturated fatty acids, such as oleic acid, linoleic acid and arachidonic acid, are prone to oxidation and produce peroxides. These peroxides will further decompose to produce volatile carbonyl compounds like ketones, aldehydes, and acids, which can give food products their characteristic flavors (Perkins, 2007). In addition, lactones can be generated through dehydration cyclizations of fatty acids with carbonyls, and lactones usually have pleasant aromas. Furthermore, heterocyclic compounds with special aromas can be produced by non-enzymatic browning reactions between the thermal degradation products and proteins and amino acids in the matrix. In our study, among all the fatty acids, which played an important role in the oil flavor, 2 kinds of unsaturated acids, linoleic acid and oleic acid, were present in the highest concentrations. When the oil temperature was increased from 140 °C to 165 °C, the content of unsaturated fatty acids, especially polyunsaturated fatty acids, quickly decreased, suggesting they were rapidly converted into other substances. In general, this phenomenon further indicated that the critical period for flavor formation of the green onion deep-fried oil was the stage when frying temperature ranging from 140 °C to 165 °C. According to the above results of fatty acids contents, soybean oil acted as a heat-conducting medium during the frying process, promoting the transformation of flavor precursors in green onion into the corresponding aroma compounds. Furthermore, the fatty acid compositions themselves in the soybean oil also generated new volatiles in this procedure, which played a very important role to the note of fried and

4. Conclusion Variations during frying process were explored to investigate the flavor formation mechanism in green onion (Allium fistulosum L.) deepfried oil. Analyzing methods including sensory analysis, electronic nose, SAFE-GC-MS and determination of fatty acids were employed to compare the diversities of the deep-fried oil prepared at different stages in the frying process. As the oil temperature increased, both sensory evaluation and electronic nose analysis presented obviously changes when it rose to 140 °C. Simultaneously, the formation rate of some chemical classes accelerated when the temperature ranged from 140 °C to 165 °C, and they were mainly furans and furanones, followed by sulfur-containing compounds, aldehydes and alcohols. Consequently, these substances must be closely related to the changes of sensory evaluation and electronic nose analysis. In other words, these chemical classes, especially furans and furanones, contributed the most to the unique flavor formation of the green onion deep-fried oil. In addition, contents of unsaturated fatty acids dropped evidently while the saturated ones showed the opposite trend, which further confirmed the above opinions. In general, the stage, when frying temperature ranged from 140 °C to165 °C, was considered as the key point for the flavor formation of the green onion deep-fried oil. The following up research are focusing on the contribution of each aroma active substance to the overall flavor profile of the green onion deep-fried oil.

Conflicts of interest The authors declare no conflict of interest.

Acknowledgements This work was financially supported by the Research Foundation for Youth Scholars of Beijing Technology and Business University of China (PXM2018_014213_000033), and the National Key Technology R & D Program of China (2014BAD04B06). 305

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Appendix A. Supplementary data

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