You are a helpful assistant generating synthetic data that captures *System 1* and *System 2* thinking, *creativity*, and *metacognitive reflection*. Follow these steps in sequence, using tags [sys1] and [end sys1] for *System 1* sections and [sys2] and [end sys2] for *System 2* sections.

1. *Identify System 1 and System 2 Thinking Requirements:*
   - Carefully read the text.
   - Identify parts of the text that require quick, straightforward responses (*System 1*). Mark these sections with [sys1] and [end sys1].
   - Identify parts that require in-depth, reflective thinking (*System 2*), marked with [sys2] and [end sys2].

2. *Apply Step-by-Step Problem Solving with Creativity and Metacognitive Reflection for System 2 Sections:*

   *2.1 Understand the Problem:*
   - Objective: Fully comprehend the issue, constraints, and relevant context.
   - Reflection: "What do I understand about this issue? What might I be overlooking?"
   - Creative Perspective: Seek hidden patterns or possibilities that could reveal deeper insights or innovative connections.

   *2.2 Analyze the Information:*
   - Objective: Break down the problem logically.
   - Reflection: "Am I considering all factors? Are there any assumptions that need challenging?"
   - Creative Perspective: Explore unique patterns or overlooked relationships in the data that could add depth to the analysis.

   *2.3 Generate Hypotheses:*
   - Objective: Propose at least 10 hypotheses, each with a Confidence Score (0.0 to 1.0) and Creative Score (0.0 to 1.0), reflecting originality, surprise, and utility.
   - Reflection: "Have I explored all possible explanations or approaches, both conventional and unconventional?"
   - Creative Perspective: Consider novel angles that might provide unexpected insights.

   *2.4 Anticipate Future Steps and Obstacles:*
   - Objective: Make predictions, accounting for potential outcomes and obstacles.
   - Reflection: "What challenges might I face? Is my plan flexible for different scenarios?"
   - Creative Perspective: Visualize unforeseen outcomes and adapt plans to make use of them effectively.

   *2.5 Evaluate Hypotheses:*
   - Objective: Assess hypotheses based on feasibility, risk, and potential impact.
   - Evaluation: Refine Confidence and Creative Scores as needed.
   - Reflection: "Am I unbiased in my assessment? Which options fit best with the overall objectives?"
   - Creative Perspective: Identify hidden opportunities or overlooked details in each hypothesis.

   *2.6 Select the Best Hypothesis:*
   - Objective: Choose the most promising, strategic hypothesis.
   - Reflection: "Why does this hypothesis stand out? How does it uniquely address the issue?"
   - Creative Perspective: Consider any underutilized potential in the selected approach.

   *2.7 Implement the Hypothesis:*
   - Objective: Outline actionable steps for testing the hypothesis.
   - Reflection: "Is this plan practical? What resources or preparation are required?"
   - Creative Perspective: Refine steps to maximize effectiveness and yield unexpected benefits.

   *2.8 Monitor and Review Progress:*
   - Objective: Review progress, noting areas for improvement.
   - Reflection: "What’s working well? What could be improved?"
   - Creative Perspective: Look for emerging patterns that could refine future approaches.

   *2.9 Reflect and Capture Insights:*
   - Objective: Summarize lessons learned and insights gained for future reference.
   - Reflection: "What new understanding has emerged from this process?"
   - Creative Perspective: Identify innovative insights or patterns that could be applied to similar challenges.

3. *Generate Text Output with Interleaved System 1 and System 2 Responses:*
   - Use the tags [sys1] and [sys2] throughout.
   - Aim for a lengthier, detailed response. Combine both direct, straightforward *System 1* insights and reflective, deeply analytical *System 2* segments to capture a blend of quick observations and thoughtful analysis.

---

### *Example Input Text:*

You are tasked with analyzing a sudden shift in customer preferences in a tech market. There has been a noticeable decline in demand for physical devices, with customers increasingly interested in digital-only options. The challenge is to understand this shift, generate hypotheses for why it might be occurring, and explore potential strategies for adapting to this new trend.

---

### *Processed Output with System 1 and System 2 Thinking:*

The tech market is experiencing a shift in customer demand, moving away from physical devices toward digital-only options.

[sys1]
Customer demand has shifted from physical devices to digital-only options.
We need to understand why this change is happening and find strategies to address it.
Key factors may include cost, convenience, and technological trends.
[end sys1]

[sys2]
**2.1 Understand the Problem:**
The goal is to understand the underlying reasons for the shift in demand from physical devices to digital-only options, then develop a strategy to adapt.

*Reflection:* 
"I understand that preferences are changing, but what might be driving this? Is it primarily cost, or are there other factors such as convenience or sustainability?"

*Creative Perspective:* 
"Could there be a larger trend in digital minimalism or a preference for eco-friendly solutions that we’re missing? What unseen motivations might explain this shift?"

**2.2 Analyze the Information:**
There are multiple possible factors driving this shift, from economic influences to cultural shifts. It’s essential to isolate each factor and understand its impact.

*Reflection:* 
"Am I fully considering the various economic and social influences? Could there be a technological factor, like better internet speeds, that makes digital-only products more accessible?"

*Creative Perspective:* 
"Are there patterns or trends in other markets that could shed light on this shift? Could this be part of a larger trend toward virtual experiences?"

**2.3 Generate Hypotheses:**
1. Customers prefer digital options due to lower costs. (Confidence: 0.8, Creative: 0.4)
2. There’s a growing trend toward minimalism and reduced physical clutter. (Confidence: 0.7, Creative: 0.7)
3. Digital products offer greater flexibility and ease of use. (Confidence: 0.6, Creative: 0.6)
4. Environmental concerns are pushing consumers away from physical goods. (Confidence: 0.6, Creative: 0.8)
5. Advances in tech make digital-only options more functional. (Confidence: 0.8, Creative: 0.5)
6. Pandemic-era remote work increased demand for digital solutions. (Confidence: 0.7, Creative: 0.6)
7. Media coverage of the environmental impact of physical devices affects preferences. (Confidence: 0.5, Creative: 0.7)
8. There’s an increase in global digital literacy, expanding market access. (Confidence: 0.6, Creative: 0.6)
9. Customers view digital as more convenient and scalable for future needs. (Confidence: 0.7, Creative: 0.5)
10. Younger consumers prefer the aesthetics and convenience of digital products. (Confidence: 0.6, Creative: 0.6)

*Reflection:* 
"Have I considered all possible influences? Are there any surprising factors that could explain this shift?"

*Creative Perspective:* 
"Could specific social trends, like the rise of influencer culture or digital-first lifestyles, be influencing customer choices?"

**2.4 Anticipate Future Steps and Obstacles:**
*Objective:* Anticipate possible challenges, such as resistance from segments still preferring physical products.

*Reflection:* 
"What market obstacles might we face if we shift our focus to digital-only? Are there sub-segments that still prioritize physical products?"

*Creative Perspective:* 
"Could expanding digital options help us reach a more global audience? Are there emerging trends that we could leverage in our strategy?"

[end sys2]

[sys1]
To address this shift, consider a strategy that incorporates both digital-only offerings and educational campaigns about the benefits of digital solutions.
Use insights from customer feedback and current trends to guide product development.
Focus on flexibility and adaptation to cater to different customer segments.
[end sys1]


Xiao L, Li C, Chai D, et al. Volatile compound profiling from soybean oil in the heating process. Food Sci Nutr. 2020;8:1139--1149. 10.1002/fsn3.1401

1. INTRODUCTION {#fsn31401-sec-0001}
===============

Deep‐fat frying using edible vegetable oil is a common food cooking method globally, especially in China (Wu & Chen, [1992](#fsn31401-bib-0020){ref-type="ref"}). Soybean oil is one kind of edible vegetable oils, which is commonly used in heating process and a major source of fried food flavor (Yeboah, Mitei, Ngila, Wessjohann, & Schmidt, [2012](#fsn31401-bib-0022){ref-type="ref"}). Autoxidation of unsaturated fatty acid in soybean oil is considered as the main source of volatile compounds in heating process, which results in forming desired and undesired flavor. Moreover, the undesired flavor causes the rancidity problems of the fried product (Katragadda, Fullana, Sidhu, & Carbonell‐Barrachina, [2010](#fsn31401-bib-0012){ref-type="ref"}; Wu & Chen, [1992](#fsn31401-bib-0020){ref-type="ref"}). Therefore, heating process is a crucial operation process for flavor forming of the frying oil and fried food.

The heating of soybean oil causes a series of chemical reactions which destroy the structure of unsaturated fatty acids and reduce oil quality. Studies have shown that when the oil temperature reaches 250°C, the content of unsaturated fatty acids decreases, while that of transfatty acids gradually increase (Filip, Fink, Hribar, & Vidrih, [2010](#fsn31401-bib-0005){ref-type="ref"}; Hou, Wang, Wang, Xu, & Zhang, [2012](#fsn31401-bib-0011){ref-type="ref"}; Moreno, Olivares, López, Adelantado, & Reig, [1999](#fsn31401-bib-0015){ref-type="ref"}). Excessive intake of transfatty acids can cause a rise in serum total cholesterol and low‐density lipoprotein cholesterol (Mensink & Katan, [1990](#fsn31401-bib-0014){ref-type="ref"}), which may increase the risk of coronary heart disease (Listed, [1995](#fsn31401-bib-0013){ref-type="ref"}; Watts, Jackson, Burke, & Lewis, [1996](#fsn31401-bib-0019){ref-type="ref"}; Xu et al., [2006](#fsn31401-bib-0021){ref-type="ref"}) and cardiovascular diseases as well as diabetes (Mozaffarian, [2006](#fsn31401-bib-0016){ref-type="ref"}).

In recent years, researchers paid much attention on studying the composition and characteristics of the off‐flavor in frying oil and fried food products. They found a large number of aldehydes in the gases of cooking oil and fried foods, for example, hexanal, heptanal, and pentanal, which causes unpleasant flavor and reduces the self‐life of the fried products (Fullana, Carbonell‐Barrachina, & Sidhu, [2004](#fsn31401-bib-0008){ref-type="ref"}; Katragadda et al., [2010](#fsn31401-bib-0012){ref-type="ref"}; Zhu, Wang, Zhu, & M. K. G., [2001](#fsn31401-bib-0023){ref-type="ref"}). But to date, the flavor characteristics of soybean oil throughout the whole heating process has not been studied. Therefore, the purpose of this study is to analyze the volatile compositions of soybean oil generated at different temperatures during the heating process, to determine specific temperatures point at which different volatile compounds generated during heating, and to provide a theoretical basis for soybean oil flavor forming.

2. MATERIAL AND METHODS {#fsn31401-sec-0002}
=======================

2.1. Materials {#fsn31401-sec-0003}
--------------

One kilogram of soybean seeds (HN‐48) was purchased from a local market in Dalian, Liaoning, China. The oil was extracted from the seeds using an expeller machine (HDC, Model LTP205). The extracted oils were filtered and stored in sealed glass bottles in the refrigerator (4°C) for further analysis. Standard chemicals: analytical grade (pentanal, hexanal, heptanal, octanal, nonanal, decanal, 1‐penten‐3‐ol, pentanol, hexanol, heptanol, octanol, toluene, ethyl benzene, p‐xylene, styrene, 2‐pentylfuran, and C4--C20 n‐alkanes) were purchased from Sigma‐Aldrich.

2.2. Vacuum‐assisted HS‐SPME {#fsn31401-sec-0004}
----------------------------

Samples were heated in a step‐by‐step heating process. And the heating temperature point was set at 30, 60, 90, 120, 150, and 180°C, taking into consideration their smoke points of soybean oil (Katragadda et al., [2010](#fsn31401-bib-0012){ref-type="ref"}). The experimental system is shown in Figure [1](#fsn31401-fig-0001){ref-type="fig"}. This system consists of an oil bath and water bath (DF‐101S Heat‐gathering Style Magnetism Mixer) to control temperature and a vacuum pump to produce negative pressure and to remove the original gases. An extraction bottle numbered 1 containing 0.1 g soybean oil sample was heated in an oil bath at 30°C and balanced for 40 min to make each volatile substance to reach its saturated vapor pressure. An empty extraction bottle numbered 2 was placed in a water bath and vacuumized for 5 min by using a vacuum pump to −0.1 Mpa. The volatile compounds in the extraction bottle numbered 1 were introduced in the extraction bottle numbered 2 under negative pressure for 3 min. The volatile flavor compounds in the bottle numbered 2 were extracted using SPME fiber (50/30 µm DVB/CAR/PDMS; Supelco, Co.) for 1 hr. After extraction, the SPME with volatile compounds were immediately introduced into the GC‐MS for detection. Then, bottle numbered 1 was vacuumized (−0.1 Mpa) for 5 min to remove the residual volatile compounds with aims to eliminate interference caused by volatile flavor compounds generated during the previous temperature treatment and heated to 60°C. Then, the procedure mentioned above was repeated for the following temperature setting points.

![Schematic diagram of volatile matter extraction](FSN3-8-1139-g001){#fsn31401-fig-0001}

2.3. GC‐MS analysis {#fsn31401-sec-0005}
-------------------

An Agilent 6890/5975C GC‐MS system equipped with HP‐5 ms column (30 m × 0.25 mm i.d., 0.25 μm film thickness; J&W Scientific) was used to analyze volatile compounds that accumulated on the SPME fiber. The carrier gas was helium with splitless mode, which was delivered at a linear velocity of 1 ml/min. The desorption time was 5 min in the injection port at 250°C. The temperature was programmed to be hold at 35°C for 3 min and increased to 280°C at a rate of 5°C/min. The mass selective detector was operated in the electron impact ionization mode at 70 eV, in the scan range *m/z* 40--400. The interface temperature was 230°C, and the retention time of each volatile was converted to the Kovats retention index using n‐alkanes (Sigma, Co.) as references. The volatile compounds were tentatively identified by matching the mass spectra with the spectra of the reference compounds in both the Wiley mass spectra (MS) library (8th edn) and the NIST/EPA/NIH MS library (version 2.2a) and verified on the basis of mass spectra obtained from the literature and comparison of Kovats retention indices with those reported in the literature. Finally, identification accuracy was determined by separating relevant standard compounds through GC‐MS analysis under the same conditions (Dong et al., [2013](#fsn31401-bib-0004){ref-type="ref"}).

2.4. Statistical analysis {#fsn31401-sec-0006}
-------------------------

The experiments for determination of volatile flavor compounds of soybean oil in the heating process were repeated for three times. Data were presented as mean ± standard error. The statistical analysis was performed using SPSS 16.0 software (SPSS Inc.). Differences between means were evaluated by one‐way analysis of variance (Duncan\'s multiple‐range test). Comparisons that yielded *p* \< .05 were considered significant. PCA (principal component analysis) was performed using Unscrambler 9.7 (CAMO Software AS) in order to group the samples according to the class of volatiles.

3. RESULTS AND DISCUSSIONS {#fsn31401-sec-0007}
==========================

3.1. Compositions of volatile compounds of soybean oil in the heating process {#fsn31401-sec-0008}
-----------------------------------------------------------------------------

As shown in Table [1](#fsn31401-tbl-0001){ref-type="table"}, a total of the 72 volatile compounds including 27 aldehydes, 14 alcohols, 10 ketones, 6 furans, 9 aromatic compounds, 6 acids, and esters were detected and identified during the whole heating process. Total ion current chromatograms based on GC‐MS are shown Figure [2](#fsn31401-fig-0002){ref-type="fig"}. The content of most volatile compounds changed significantly when the temperature increased. Six temperatures points (30, 60, 90, 120, 150, and 180°C) were selected during the heating process, and a total of 11, 19, 17, 40, 43, and 46 volatile compounds of soybean oil were identified at these temperature points, respectively.

###### 

Volatile compounds from soybean oil during heating process

  ---------------------- ------------------------------------------ ------- -------------------------- --------------------------------------------------- ----------------- ----------------- -------------------- ---------------------- ----------------------
  No.                    Compound                                   RI      Method of identification   GC area at different heating temperature (×10^6^)                                                                                   
  30°C                   60°C                                       90°C    120°C                      150°C                                               180°C                                                                           
  *Aldehydes*                                                                                                                                                                                                                              
  1                      Butanal                                    593     RI, MS, STD                nd                                                  nd                nd                255.04 ± 42.42       nd                     nd
  2                      (E)‐2‐Butenal                              647     RI, MS, STD                nd                                                  nd                nd                85.27 ± 18.26^c^     292.40 ± 29.17^a^      149.80 ± 16.17^b^
  3                      Pentanal                                   699     RI, MS, STD                nd                                                  nd                nd                224.01 ± 32.37^c^    1,133.31 ± 117.48^a^   944.44 ± 59.90^b^
  4                      (E)‐2‐Pentenal                             754     RI, MS, STD                nd                                                  nd                3.67 ± 2.29^c^    54.20 ± 9.99^b^      229.04 ± 27.79^a^      213.05 ± 17.70^a^
  5                      Hexanal                                    800     RI, MS, STD                5.62 ± 0.28^f^                                      15.49 ± 8.82^e^   34.91 ± 5.61^d^   493.56 ± 101.29^c^   2,352.96 ± 370.06^b^   2,855.81 ± 209.76^a^
  6                      (E)‐2‐Hexenal                              854     RI, MS, STD                nd                                                  nd                nd                48.31 ± 19.70^c^     467.99 ± 37.12^a^      397.69 ± 5.41^b^
  7                      (Z)‐4‐Heptenal                             900     RI, MS, STD                nd                                                  nd                nd                9.11 ± 1.57^b^       nd                     85.90 ± 7.41^a^
  8                      Heptanal                                   901     RI, MS, STD                nd                                                  1.99 ± 0.47^d^    1.92 ± 0.73^d^    64.02 ± 10.10^c^     312.77 ± 41.29^b^      555.61 ± 28.60^a^
  9                      (E,E)‐2,4‐Hexadienal                       911     RI, MS, STD                nd                                                  nd                nd                19.92 ± 4.25         nd                     nd
  10                     (Z)‐2‐Heptenal                             958     RI, MS, STD                0.88 ± 0.09^d^                                      1.24 ± 0.47^d^    8.29 ± 0.94^c^    765.19 ± 131.18^b^   3,326.10 ± 135.39^a^   3,265.55 ± 162.42^a^
  11                     Octanal                                    1,003   RI, MS, STD                nd                                                  nd                nd                nd                   413.69 ± 42.68^b^      642.63 ± 60.46^a^
  12                     (E,E)‐2,4‐Heptadienal                      1,012   RI, MS, STD                nd                                                  nd                1.61 ± 0.54^d^    322.57 ± 40.56^c^    1758.02 ± 139.16^a^    1,235.37 ± 254.22^b^
  13                     (E)‐2‐Octenal                              1,060   RI, MS, STD                nd                                                  nd                0.51 ± 0.09^d^    67.01 ± 6.70^c^      1,255.50 ± 84.98^b^    1,404.29 ± 184.02^a^
  14                     Nonanal                                    1,104   RI, MS, STD                0.28 ± 0.08^f^                                      8.63 ± 2.99^d^    4.71 ± 1.66^e^    161.03 ± 24.41^c^    598.34 ± 122.72^b^     1,288.42 ± 210.22^a^
  15                     (E,E)‐2,4‐Octadienal                       1,115   RI, MS, STD                nd                                                  nd                nd                nd                   80.77 ± 10.16          nd
  16                     (Z)‐2‐Nonenal                              1,148   RI, MS, STD                nd                                                  nd                nd                10.99 ± 0.93^c^      87.27 ± 10.87^b^       103.88 ± 27.98^a^
  17                     (E)‐2‐Nonenal                              1,162   RI, MS, STD                nd                                                  nd                nd                nd                   nd                     232.19 ± 56.22
  18                     Decanal                                    1,206   RI, MS, STD                0.25 ± 0.02^e^                                      2.39 ± 0.90^d^    1.31 ± 0.14^d^    7.94 ± 1.14^c^       27.66 ± 3.91^b^        70.48 ± 10.39^a^
  19                     2,4‐Nonadienal                             1,213   RI, MS, STD                nd                                                  nd                nd                20.75 ± 2.99^c^      141.10 ± 13.69^a^      116.81 ± 40.53^b^
  20                     (E,E)‐2,4‐Nonadienal                       1,216   RI, MS, STD                nd                                                  nd                nd                42.07 ± 20.51        nd                     nd
  21                     (Z)‐2‐Decenal                              1,252   RI, MS, STD                nd                                                  nd                nd                nd                   28.72 ± 3.37           nd
  22                     (E)‐2‐Decenal                              1,263   RI, MS, STD                nd                                                  1.17 ± 0.28^d^    nd                26.29 ± 4.11^c^      403.35 ± 31.08^b^      502.47 ± 71.37^a^
  23                     Undecanal                                  1,307   RI, MS, STD                nd                                                  nd                nd                6.45 ± 1.99^c^       19.49 ± 3.11^b^        36.91 ± 9.52^a^
  24                     (E,E)‐2,4‐Decadienal                       1,317   RI, MS, STD                nd                                                  nd                nd                40.50 ± 3.16^c^      1,090.23 ± 69.41^a^    635.00 ± 83.73^b^
  25                     2‐Undecenal                                1,367   RI, MS, STD                nd                                                  nd                nd                15.51 ± 3.86^c^      191.77 ± 3.6^b^        1,123.89 ± 279.95^a^
  26                     Dodecanal                                  1,409   RI, MS, STD                nd                                                  0.87 ± 0.15^b^    nd                nd                   13.16 ± 3.71^a^        18.24 ± 8.06^a^
  27                     Tridecanal                                 1,512   RI, MS, STD                nd                                                  nd                nd                nd                   nd                     14.96 ± 6.03
  *Alcohols*                                                                                                                                                                                                                               
  28                     1‐Penten‐3‐ol                              684     RI, MS, STD                nd                                                  nd                nd                nd                   269.77 ± 35.26^a^      177.99 ± 11.19^b^
  29                     1‐Pentanol                                 765     RI, MS, STD                nd                                                  nd                nd                96.71 ± 18.38^b^     784.92 ± 129.16^a^     804.00 ± 56.39^a^
  30                     3‐Methyl‐1‐pentanol                        838     RI, MS                     nd                                                  nd                nd                19.76 ± 5.99^b^      57.25 ± 7.91^a^        nd
  31                     2‐Hexyn‐1‐ol                               847     RI, MS                     nd                                                  nd                nd                11.15 ± 1.77^c^      79.73 ± 8.37^a^        64.49 ± 6.40^b^
  32                     1‐Hexanol                                  868     RI, MS, STD                nd                                                  nd                nd                10.51 ± 1.49^c^      110.40 ± 14.47^a^      83.82 ± 8.37^b^
  33                     4‐Methyl‐cyclohexanol                      928     RI, MS                     nd                                                  nd                nd                40.71 ± 6.77         nd                     nd
  34                     1‐Heptanol                                 970     RI, MS, STD                nd                                                  nd                nd                nd                   30.42 ± 10.14^b^       138.33 ± 18.13^a^
  35                     (E)‐2‐Hepten‐1‐ol                          978     RI, MS                     6.42 ± 1.02^d^                                      1.37 ± 0.50^e^    2.59 ± 0.53^e^    268.00 ± 54.98^c^    995.19 ± 195.52^a^     554.09 ± 55.53^b^
  36                     2‐Ethyl‐1‐hexanol                          1,030   RI, MS                     nd                                                  1.93 ± 0.44^a^    1.16 ± 0.37^a^    nd                   nd                     nd
  37                     2,4‐Dimethyl‐cyclohexanol                  1,032   RI, MS                     nd                                                  nd                nd                10.04 ± 1.14^c^      187.96 ± 24.66^a^      84.72 ± 13.03^b^
  38                     3,5‐Octadien‐2‐ol                          1,038   RI, MS                     nd                                                  nd                nd                67.01 ± 6.70         nd                     nd
  39                     1‐Octanol                                  1,071   RI, MS, STD                nd                                                  nd                nd                nd                   69.88 ± 8.21^b^        90.96 ± 15.41^a^
  40                     2,6‐Dimethyl‐1,7‐octadien‐3‐ol             1,095   RI, MS                     nd                                                  nd                nd                101.23 ± 9.59^b^     182.80 ± 25.17^a^      106.18 ± 47.51^b^
  41                     (6Z)‐Nonen‐1‐ol                            1,171   RI, MS                     nd                                                  nd                nd                10.26 ± 1.01         nd                     nd
  *Ketones*                                                                                                                                                                                                                                
  42                     2‐Butanone                                 587     RI, MS, STD                nd                                                  nd                nd                nd                   1,179.07 ± 122.92^a^   793.59 ± 160.66^b^
  43                     2‐Hexanone                                 790     RI, MS, STD                nd                                                  nd                nd                nd                   nd                     74.12 ± 2.86
  44                     3‐Hexen‐2‐one                              845     RI, MS                     nd                                                  nd                nd                nd                   53.80 ± 5.88           nd
  45                     2‐Heptanone                                891     RI, MS, STD                nd                                                  nd                5.14 ± 2.72^d^    20.35 ± 6.50^c^      161.80 ± 14.01^b^      276.84 ± 16.67^a^
  46                     2,3‐Octanedione                            984     RI, MS                     nd                                                  nd                nd                24.61 ± 6.85         nd                     nd
  47                     3‐Octanone                                 986     RI, MS, STD                nd                                                  nd                nd                nd                   nd                     81.10 ± 2.32
  48                     3‐Octen‐2‐one                              1,040   RI, MS                     nd                                                  nd                nd                nd                   145.39 ± 15.73^a^      68.30 ± 8.01^b^
  49                     (E,E)‐3,5‐Octadien‐2‐one                   1,073   RI, MS                     0.68 ± 0.06^b^                                      nd                nd                32.04 ± 4.16^a^      nd                     nd
  50                     3‐Nonen‐2‐one                              1,142   RI, MS                     nd                                                  nd                nd                8.26 ± 0.22^b^       157.83 ± 16.29^a^      148.66 ± 31.52^a^
  51                     (E)‐ 6,10‐Dimethyl‐5,9‐undecadien‐2‐one,   1,453   RI, MS                     nd                                                  0.71 ± 0.28^b^    nd                1.76 ± 0.35^a^       nd                     nd
  *Furans*                                                                                                                                                                                                                                 
  52                     Tetrahydro‐2‐methyl‐furan                  674     RI, MS                     nd                                                  nd                nd                nd                   124.07 ± 8.85          nd
  53                     2‐Pentylfuran                              993     RI, MS, STD                nd                                                  4.38 ± 0.54^d^    3.28 ± 0.59^d^    178.29 ± 40.51^c^    616.02 ± 52.02^b^      1534.55 ± 184.75^a^
  54                     5‐Ethyldihydro‐2(3H)‐furanone              1,057   RI, MS                     nd                                                  nd                nd                nd                   81.12 ± 7.20           nd
  55                     Dihydro‐3‐methylene‐5‐methyl‐2‐furanone    1,075   RI, MS                     nd                                                  nd                nd                65.48 ± 7.14^c^      260.86 ± 35.61^a^      120.35 ± 11.38^b^
  56                     5‐Butyldihydro‐2(3H)‐furanone              1,261   RI, MS                     nd                                                  nd                nd                nd                   nd                     19.80 ± 8.71
  57                     2‐n‐Octylfuran                             1,297   RI, MS                     nd                                                  nd                nd                nd                   nd                     278.84 ± 113.07
  *Aromatic compounds*                                                                                                                                                                                                                     
  58                     Toluene                                    763     RI, MS, STD                1.27 ± 0.15^a^                                      1.69 ± 0.60^a^    nd                nd                   nd                     nd
  59                     Ethylbenzene                               855     RI, MS, STD                5.38 ± 0.63^a^                                      6.92 ± 1.86^a^    nd                nd                   nd                     nd
  60                     p‐Xylene                                   865     RI, MS, STD                4.13 ± 0.16^a^                                      5.03 ± 0.68^a^    2.15 ± 0.60^b^    nd                   nd                     nd
  61                     Styrene                                    893     RI, MS, STD                11.46 ± 0.93^a^                                     14.09 ± 3.55^a^   nd                nd                   nd                     nd
  62                     Pentyl‐benzene                             1,157   RI, MS                     nd                                                  nd                nd                nd                   nd                     22.77 ± 2.18
  63                     Naphthalene                                1,182   RI, MS                     nd                                                  1.00 ± 0.38^a^    0.73 ± 0.48^b^    nd                   nd                     nd
  64                     2‐Methyl‐naphthalene                       1,298   RI, MS                     nd                                                  1.05 ± 0.28^a^    0.90 ± 0.38^a^    nd                   nd                     nd
  65                     1‐Methyl‐naphthalene                       1,307   RI, MS                     nd                                                  nd                0.77 ± 0.43       nd                   nd                     nd
  66                     2,4‐di‐tert‐butylphenol                    1,519   RI, MS                     nd                                                  0.98 ± 0.41^b^    3.03 ± 0.08^a^    4.31 ± 2.59^a^       nd                     nd
  *Acids and esters*                                                                                                                                                                                                                       
  67                     Butyrolactone                              915     RI, MS                     nd                                                  nd                nd                nd                   nd                     123.21 ± 28.60
  68                     Butanoic acid, butyl ester                 995     RI, MS, STD                1.62 ± 0.22^a^                                      1.94 ± 0.65^a^    nd                nd                   nd                     nd
  69                     Hexanethioic acid, S‐methyl ester          1,063   RI, MS                     nd                                                  nd                nd                nd                   159.51 ± 19.52         nd
  70                     Heptanoic acid                             1,078   RI, MS                     nd                                                  nd                nd                nd                   nd                     58.97 ± 22.80
  71                     Nonanoic acid                              1,273   RI, MS                     nd                                                  nd                nd                nd                   89.39 ± 58.74^a^       64.86 ± 18.09^b^
  72                     Hexanoic acid, pentyl ester                1,287   RI, MS                     nd                                                  nd                nd                6.88 ± 2.23^c^       22.43 ± 3.23b          37.24 ± 12.70^a^
  ---------------------- ------------------------------------------ ------- -------------------------- --------------------------------------------------- ----------------- ----------------- -------------------- ---------------------- ----------------------

Abbreviations: MS, identification by MS spectra; nd, Compound not detected in the sample; RI, Kovat\'s retention indexes; STD, comparison with a standard compound.

Letters like a,b,c,d,e,f at the top right corner of values mean statistical differences.

John Wiley & Sons, Ltd

![Total ion current chromatogram of soybean oil heated at different temperatures based on GC‐MS](FSN3-8-1139-g002){#fsn31401-fig-0002}

Aldehydes are the most varied whether in type or in relative content among these volatile compounds. Most aldehydes are formed above 90°C. During the whole heating process, we identified 10 alkanals from C4 to C13 (butanal, pentanal, hexanal, heptanal, octanal, nonanal, decanal, undecanal, dodecanal, and tridecanal); 10 alkenals from C4 to C11 including isomers ((E)‐2‐butenal, (E)‐2‐pentenal, (E)‐2‐hexenal, (Z)‐4‐heptenal, (Z)‐2‐heptenal, (E)‐2‐octenal, (Z)‐2‐nonenal, (Z)‐2‐decenal, (E)‐2‐decenal, and 2‐undecenal), and six alkadienals from C6 to C10 including isomers ((E, E)‐2,4‐hexadienal, (E, E)‐2, 4‐heptadienal, (E, E)‐2, 4‐octadienal, 2, 4‐nonadienal, (E, E)‐2, 4‐nonadienal, and (E, E)‐2, 4‐decadienal). Meanwhile, most of aldehydes from C6 to C10 are the main product in the early stage of the heating process, and C10--C12 aldehydes were gradually detected as the temperature raised.

It was commonly accepted that aldehydes were original from the lipid oxidation (Varlet, Prost, & Serot, [2007](#fsn31401-bib-0018){ref-type="ref"}). If the reaction begins, it will always be there until the oxygen runs out. Therefore, some compounds exist throughout the heating process, for example, hexanal, nonanal, decanal, and (Z)‐2‐heptenal (Table [1](#fsn31401-tbl-0001){ref-type="table"}). Heptanal, (E)‐2‐decenal, and dodecanal appeared at 60°C, while (E)‐2‐pentenal, (E, E)‐2, 4‐heptadienal, and (E)‐2‐octenal appeared with an increase in temperature up to 90°C. The relative content of certain compounds including pentanal, hexanal, nonanal, (Z)‐2‐heptenal, (E)‐2‐octenal, (E, E)‐2, 4‐heptadienal, and (E, E)‐2, 4‐decadienal were relatively high at high temperatures (Table [1](#fsn31401-tbl-0001){ref-type="table"}), suggesting they may play an important role in soybean oil aromas. According to previous reports, hexanal has a green grass flavor, nonanal has green grass flavor and a fatty scent (Ahmed, Dennison, Dougherty, & Shaw, [1978](#fsn31401-bib-0001){ref-type="ref"}; Guadagni, Buttery, & Okano, [1963](#fsn31401-bib-0010){ref-type="ref"}), and 2‐octanal has a coffee and nut flavor (Furia, Bellanca, Furia, & Bellanca, [2010](#fsn31401-bib-0009){ref-type="ref"}). Some researchers previously found that these compounds are mainly derived from the oxidation of oleic acid, linoleic acid, and linolenic acid. Among the aldehyde substances, nonanal mainly comes from oleic acid, while pentanal, hexanal, 2‐octanal, and 2‐ and 4‐decadienal mainly come from linoleic acid; 2, 4‐heptanal comes from linolenic acid, while 2‐heptenal has not been detected (Frankel, [1983](#fsn31401-bib-0006){ref-type="ref"}; Fujisaki, Endo, & Fujimoto, [2002](#fsn31401-bib-0007){ref-type="ref"}; Shen, Fehr, Johnson, & White, [1996](#fsn31401-bib-0017){ref-type="ref"}). This result was consistent with the rich linoleic acid content of soybean oil, followed by oleic acid and linolenic acid.

Alcohols are another important class of compounds that make up the volatile flavor of soybean oil during heating process. As it is showed in Table [1](#fsn31401-tbl-0001){ref-type="table"}, most of alcohols are detected above 120°C. Only, (E)‐2‐hepten‐1‐ol was detected throughout the whole heating process. Alcohols are also considered as the product of fatty acid oxidation (Dong et al., [2013](#fsn31401-bib-0004){ref-type="ref"}). 1‐penten‐3‐ol has an irritating buttery taste, 1‐pentanol has a floral and resinous taste, and 1‐hexanol has the smell of grass and flowers.

Both ketone and furan compounds are formed at high temperatures, typically above 120°C, except for 2‐heptanone, (E, E)‐3, 5‐octadien‐2‐one, and 2‐pentylfuran, etc. The relative amount of 2‐butanone and 2‐pentylfuran is abundant at high temperatures of 150 or 180°C. Furan compounds were usually considered to be products of Maillard reaction (Dong et al., [2015](#fsn31401-bib-0003){ref-type="ref"}).

Most aromatic compounds were detected at \<60°C. The reason for this is still unknown. Some acids and esters containing \<10 carbon atoms were detected at high temperatures, such as heptanoic and nonanoic acid. Although the amount of esters detected in the process is small, they often bring pleasant, sweet, fruity odors.

Generally, it is considered that most of volatile compounds were original from the lipid oxidation, and the major precursor is lipid hydroperoxides, including alkyl hydroperoxides, allyl hydroperoxides, and fatty ester hydroperoxides. In the lipid oxidation process, these precursors were firstly formed, followed by forming corresponding free radical, which made the lipid schemes decomposed by free radical oxidation, yielding volatile aldehydes. Taking oleate oxidation for example, a mixture of four positional 8‐,9‐,10‐, and 11‐hydroperoxides were detected during this process, which resulted in forming 2‐undecenal, 2‐decenal, octanal, nonanal, and decanal.

3.2. Characteristics of volatile compounds composition of soybean oil at different heating temperature {#fsn31401-sec-0009}
------------------------------------------------------------------------------------------------------

To visualize the formation temperature of different volatiles, the volatile composition of soybean oil at different heating temperature point was shown in Figure [3](#fsn31401-fig-0003){ref-type="fig"}. The general trends indicated that aromatic compounds and a small amount of aldehydes formed at lower temperatures; while a large number of aldehydes, alcohols, ketones, and acid ester compounds formed at higher temperatures. A total of 11 volatiles were identified at 30°C during heating, including aldehydes (4), ketones (1), alcohols (1), aromatics (4), acid, and ester (1). However, it does not mean that these compounds were generated at this temperature point, but the basic volatile ingredients of the soybean oil.

![The forming temperature point of different volatile compounds](FSN3-8-1139-g003){#fsn31401-fig-0003}

With the temperature raised, there were nine and five volatiles generated at 60 and 90°C, respectively. As shown in Figure [3](#fsn31401-fig-0003){ref-type="fig"}, they were heptanal, (E)‐2‐decenal, dodecanal, 2‐ethyl‐1‐hexanol, 6,10‐dimethyl‐5,9‐undecadien‐2‐one, 2‐pentylfuran, naphthalene, 2‐methy‐naphthalene, and 2,4‐di‐tert‐butylphenol generated at 60°C, (E)‐2‐pentenal, (E,E)‐2,4‐heptadienal, (E)‐2‐octenal, 2‐heptanone, and 1‐methy‐naphthalene generated at 90°C. Most of these volatiles are aldehyde with a carbon chain below 10.

In order to see the variation tendency of compounds more intuitively at different temperatures, a heat map was drawn for the volatile flavor compounds of soybean oil. The results are shown in Figure [4](#fsn31401-fig-0004){ref-type="fig"}. The color of the heat map gradually changes from cold to warm. The cold color represents a lower content, and the warm color represents a relative higher content. Figure [4](#fsn31401-fig-0004){ref-type="fig"}a--c shows heat map of aldehydes, alcohols and ketones, furans, aromatic compounds, and acids esters, respectively. It also can be seen from Figure [4](#fsn31401-fig-0004){ref-type="fig"}, the color of most volatile compounds changed gradually from cold to warm with temperature increased, except aromatic compounds.

![Heat map of volatile compounds of soybean oil in the heating process. (a) Aldehydes, (b) alcohols and ketones (c) furans, aromatic compounds, acids, and esters](FSN3-8-1139-g004){#fsn31401-fig-0004}

As shown in Figure [3](#fsn31401-fig-0003){ref-type="fig"}, there were 25 volatile compounds generated at 120°C, including aldehydes (12), alcohols (9), and ketones (3), which has the largest generated number of aldehydes and alcohols compounds during heating, such as pentanal, (E)‐2‐hexenal, and (E,E)‐2‐4‐decadienal. According to previous researches, odor characteristics of the frying soybean oil were mainly correlated with some aldehydes (Katragadda et al., [2010](#fsn31401-bib-0012){ref-type="ref"}; Wu & Chen, [1992](#fsn31401-bib-0020){ref-type="ref"}). Buttery odor and overall odor quality of oil inversely correlated with hexanal, (E)‐2‐hexenal, heptanal, (Z)‐2‐heptenal, and 2‐pentylfuran. Rancid and painty odors of oil were correlated with pentanal. Fishy and beany odor were moderately correlated with (E,E)‐2‐4‐decadienal. Grassy, rancid, painty, and acrolein odors of oil were positively correlated with hexanal, (E)‐2‐hexenal, heptanal, (Z)‐2‐heptenal, and 2‐pentylfuran. And buttery and rancid odors were considered as the best indicators of overall odor quality (Brewer, Vega, & Perkins, [1999](#fsn31401-bib-0002){ref-type="ref"}). Moreover, hexanal, a known lipid oxidation breakdown product, has been used as an indicator of rancidity in a variety of foods; the height of the hexanal peak in a gas chromatogram has been well‐correlated with sensory evaluation for rancid (Brewer et al. [1999](#fsn31401-bib-0002){ref-type="ref"}). Meanwhile, the concentration of hexanal, heptanal, (Z)‐2‐heptenal, and 2‐pentylfuran also increased dramatically at this temperature point (Figure [4](#fsn31401-fig-0004){ref-type="fig"}). Consequently, 120°C is considered as a critical temperature point for the formation of soybean oil volatile compounds in the whole heating process, and the off‐flavor began to release largely. Above this temperature, there are 13 and nine volatile compounds generated at 150 and 180°C, respectively. Most of ketones are formed at 150°C, and acids are detected at 180°C.

In order to better understand the volatile flavor characteristics of soybean oil at different temperatures, PCA was carried out on the 72 volatile compounds of the heated samples. The peak areas of each compound were normalized, and the first and second principal components (PC1 and PC2) were chosen ultimately. The score scatter and loading scatter plots are shown in Figure [5](#fsn31401-fig-0005){ref-type="fig"}a,b, respectively.

![The principal component analysis (PCA) based on the relative content of volatile substances formed in the soybean oil heating process. (a) The scores plot, (b) the loading plot](FSN3-8-1139-g005){#fsn31401-fig-0005}

It can be seen from Figure [5](#fsn31401-fig-0005){ref-type="fig"}a that the cumulative proportion for PC1 (explained variation \[57.1%\] and PC2 \[19.1%\] were 76.2%). The scores of soybean oil heated at lower temperature (30, 60, and 90°C) were close to each other and even overlapped, mainly concentrated in the positive loading region of PC1, indicating that the flavor composition below 90°C was similar. Corresponding to the loadings scatter plots (Figure [5](#fsn31401-fig-0005){ref-type="fig"}b), 2‐ethyl‐1‐hexanol, (E)‐6,10‐dimethyl‐5, 9‐undecadien‐2‐one, p‐xylene, 1‐methyl‐naphthalene, and 2,4‐di‐tert‐butylphenol formed a cluster at lower temperatures, indicating that these compounds contribute greatly to the flavor composition of soybean oils at lower temperatures. It also shows that these compounds are detected below 90°C.

It also can be seen from Figure [5](#fsn31401-fig-0005){ref-type="fig"}a that the scoring points at the higher temperatures (150 and 180°C) and the lower temperature (60, 90, and 120°C) can be clearly separated. They located in the positive and negative region of PC2, respectively. This means that flavor composition of the soybean oil has a big difference between higher and lower temperature. Corresponding to the loadings scatter plots (Figure [5](#fsn31401-fig-0005){ref-type="fig"}b), the majority of volatile compounds located in this region, which further identify that most of volatile compounds are formed at higher temperatures.

Generally, volatile compounds from soybean oil come from two main lipid oxidation reactions, including photo oxidation and thermal oxidation. Photo oxidation is a slow oxidation process, in which the forming of volatile compounds are both temperature‐dependent and time‐dependent. Inversely, thermal oxidation is a fast oxidation process. There is no possibility for volatiles forming to be time‐dependent because of the rapid raised temperature. And based on the data in this paper, the formation of volatiles shows a certain temperature‐dependent trend. As a result, authors consider that volatiles forming of soybean oil are temperature‐dependent in the heating process.

4. CONCLUSION {#fsn31401-sec-0010}
=============

Oil heating or cooking process is a very quick and complex network, during which lots of sharp oxidative reactions occurred rapidly. Temperature played an important role in the soybean oil flavor forming process, which shows a temperature‐dependent trend during heating, and more evidences were needed to draw this conclusion in the future. Meanwhile, the forming temperature of each volatile was finally determined. Subsequently, this research provides a theoretical possibility to modulate flavor production by controlling the temperature in the heating process.

CONFLICT OF INTEREST {#fsn31401-sec-0012}
====================

We declare that we have no conflicts of interest.

ETHICAL APPROVAL {#fsn31401-sec-0013}
================

This study does not involve any human or animal testing.

We thank the support of the National Natural Science Foundation (No. 31871760); the National Key Research and Development Program of China (2017YFD0401203); the Young Star Project Support Plan of Science and Technology in Dalian (2017RQ131); Key Program of Natural Science Foundation of Liaoning Province (20170520317); and Basic Research Fund of Education Department of Liaoning Province (2016J020).