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. 2020 May 12;8(7):3362–3379. doi: 10.1002/fsn3.1616

Changes in volatile compounds of fermented minced pepper during natural and inoculated fermentation process based on headspace–gas chromatography–ion mobility spectrometry

Yuyu Chen 1,2, Haishan Xu 1,2, Shenghua Ding 3, Hui Zhou 1,2, Dan Qin 1,2, Fangming Deng 1,2, Rongrong Wang 1,2,✉
PMCID: PMC7382115  PMID: 32724601

Abstract

Changes in volatile compounds of fermented minced pepper (FMP) during natural fermentation (NF) and inoculated fermentation (IF) process were analyzed by the headspace–gas chromatography–ion mobility spectrometry (HS‐GC‐IMS). A total of 53 volatile compounds were identified, including 12 esters, 17 aldehydes, 13 alcohols, four ketones, three furans, two acids, one pyrazine, and one ether. Generally, fermentation time played an important role in volatile compounds of FMP. It was found that most esters, aldehydes, and alcohols obviously decreased with the increase in fermentation time, including isoamyl hexanoate, methyl octanoate, gamma‐butyrolactone, phenylacetaldehyde, methional, and E‐2‐hexenol. Only a few volatile compounds increased, especially for 2‐methylbutanoic acid, 2‐methylpropionic acid, linalool, ethanol, and ethyl acetate. However, no significant difference in volatile compounds was found between NF and IF samples at the same fermentation time. In addition, the fermentation process in all samples was well discriminated as three stages (0 days; 6 day; and 12, 18, and 24 days), and all volatile compounds were divided into two categories (increase and decrease) based on principal component analysis and heat map.

Keywords: fermentation process, fermented minced pepper, headspace–gas chromatography–ion mobility spectrometry, volatile compounds

Changes in volatile compounds of FMP during NF and IF process were analyzed by HS‐GC‐IMS. Fermentation time played an important role in volatile compounds of FMP. Most esters, aldehydes, and alcohols obviously decreased, and only a few volatile compounds increased during NF and IF, especially for acids. NF and IF posed little effect on volatile compounds of FMP.

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1. INTRODUCTION

Pepper (Capsicum annuum L.), belonging to the Solanaceae family, is widely planted worldwide and covers approximately 1.99 million ha of harvested area and an annual production of 36.77 million tons in 2018 according to FAO (2018). Pepper fruits are rich in nutrients and bioactive compounds, including L‐ascorbic acid, phenolic compounds, carotenoids, and capsaicin, which exhibit antioxidant activities and anti‐inflammatory effects (Hernández‐Carrión et al., 2015; Ribes‐Moya, Raigón, Moreno‐Peris, Fita, & Rodríguez‐Burruezo, 2018). Nowadays, pepper fruits are an important ingredient in several fermented food, including kimchi, fermented pepper paste, and fermented minced pepper (FMP; Li et al., 2019; Wang, Wang, Xiao, Liu, Deng, et al., 2019; Wang, Wang, Xiao, Liu, Jiang, et al., 2019). FMP, as a traditional and local fermented vegetable in the southern regions of China, is widely consumed due to its nutritional and sensory properties (Li, Zhao, et al., 2016). It can be eaten directly or used as cooking ingredient. FMP can be prepared by natural fermentation (NF) and inoculated fermentation (IF). NF was a kind of fermentation methods in which microorganisms from natural environment were used for fermentation, and IF was a kind of fermentation methods in which inoculated microorganisms were used for fermentation. Whatever NF or IF, lactic acid bacteria (LAB) become the dominant microorganism when conditions are suitable for their growth (Sanlier, Gökcen, & Sezgin, 2017). Meanwhile, LAB fermentation involves the production of various metabolites in FMP such as alcohols, organic acids, and active metabolites that contribute to its nutrition, taste, flavor, and functionality (Wang, Wang, Xiao, Liu, Deng, et al., 2019; Wang, Wang, Xiao, Liu, Jiang, et al., 2019).

Flavor plays a key important role in defining sensory and consumer acceptance of FMP. The flavor of FMP is a complex trait, including hot taste from peppers, umami taste from amino acids, and salty taste from NaCl. Previous researchers have focused on flavor characteristics of fermented vegetables products, including fermented peppers, sauerkraut, and kimchi (Sanlier et al., 2017). Wang, Wang, Xiao, Liu, Deng, et al. (2019) showed that alcohols, esters, and ketones were the dominant volatile fractions in fermented/chopped pepper by solid‐phase microextraction and gas chromatography–mass spectrometry, and the fermentation stage was mainly affected by esters, alcohols, aldehydes, and terpenes. Liu et al. (2019) found that the flavor profiles of Sichuan pickle fermented in glass jars (GL), porcelain jars (PO), and plastic jars (PL) were different. The compound with the highest concentration in both PO and GL was the alkanes, while the highest concentration of compound was ester in the PL. Wu et al. (2015) measured changes of flavor compounds in suan cai during NF, and found that there were 17 varieties of volatile flavor components in the early fermentation time, but increased to 57 in the middle fermentation time. In addition, the result also showed that esters and aldehydes were in the greatest diversity and abundance, contributed most to the aroma of suan cai. Kang and Baek (2014) found that 19 aroma‐active compounds were detected by aroma extract dilution analysis in Korean fermented red pepper paste (gochujang), and 12 aroma‐active compounds were detected by headspace–solid‐phase microextraction–gas chromatography–olfactometry. Hence, the variety and content of volatile compounds in fermented vegetables could be affected by multiple factors, such as raw material, container, and fermentation time. For FMP, the previous research and industrial production mainly focused on the optimization of process parameters, including raw material, NaCl and CaCl2 content, LAB inoculum, fermentation temperature, and time based on the change of quality. However, knowledge about changes in flavor compounds of FMP at different stages with NF or IF is not available.

Ion mobility spectrometry (IMS), a rapid detection technique, was used to detect the gasified volatile compounds by ion separation based on their ion mobility velocity (Zhang et al., 2016). The detection technology presented many advantages, including easy operation, high analysis speed, high sensitivity, and no complex sample preparation steps. However, its analysis characteristics were often limited for complex samples, especially for complex systems in food and agricultural products (Arce et al., 2014). Combining IMS with other instruments is a more suitable and effective way to make better use of its advantages. Recently, headspace–gas chromatography–ion mobility spectrometry (HS‐GC‐IMS) has been applied in detecting volatile compounds of fruits and vegetables, such as candied kumquats (Hu et al., 2019), jujube fruits (Yang et al., 2019), and dried peppers (Ge et al., 2020). As a consequence, HS‐GC‐IMS is effective to identify the flavor characteristic of fruits and vegetables. However, the changes in characteristic compounds linked to the flavors of FMP during NF and IF process are still not available. Hence, HS‐GC‐IMS can be used to establish fingerprints of volatile compounds in FMP during NF and IF process.

In this study, the changes of volatile compounds in FMP during NF and IF process were analyzed, and several target volatile compounds in samples were detected using HS‐GC‐IMS, principal component analysis (PCA), and the heat map. The results confirmed the potential of HS‐GC‐IMS to identify the volatile compound characteristics and provided a rapid method to determine the flavor quality of FMP during fermentation process.

2. MATERIALS AND METHODS

2.1. Materials

Fresh peppers (Capsicum annuum L.) cv. “yanhong” were grown in an experimental field of Gaoqiao Town, Changsha County, Hunan Province, China (N28°28′38.08, E113°20′54.65″). All harvested peppers were up to commercial maturity. After harvesting, the peppers were delivered to the laboratory immediately. Peppers with uniformity of size, color, and weight, free from visible blemishes, disease, and/or physical damage, were selected as the experiment raw materials. W‐4 LAB was isolated, purified, and extended culture from our previous FMP obtained by NF, and it posed strong resistance to high salt and acid.

2.2. Preparation of FMP

Selected peppers were removed handle, washed, drained, and minced into small pieces (0.5–1 cm × 0.5–1 cm), and 10% (w/w) NaCl and 0.1% CaCl2 (w/w) were added to the minced peppers and then stirred for 5 min. Above samples were divided into two groups: One group inoculated with 5% (w/w) W‐4 LAB (107 CFU/ml) was regarded as IF, and another group without inoculated LAB was regarded as NF. Prepared samples were put into pickle jars, and the jars were sealed with water to exclude air and fermented in a 30°C incubator. FMPs were obtained on 0, 6, 12, 18, and 24 days, respectively. The collected samples were detected immediately after freeze‐drying and grinding.

2.3. HS‐GC‐IMS analysis

All the analyses were obtained by HS‐GC‐IMS instrument and related supplementary analysis software according to the method of Sun et al. (2019) with some modifications. Freeze‐dried (0.5 g) FMP was weighted and then transferred into a 20‐ml headspace bottle. The FMP was incubated at 80°C in the headspace bottle, with the speed of 500 rpm for 10 min. After incubation, 500 μl headspace was automatically injected using a heated syringe at 85°C into a FS‐SE‐54‐CB‐1 (15 m × 0.53 mm ID) capillary column. The carried gas during injection was nitrogen (99.99% purity), which was under the below programmed flow to carry samples: 2 ml/min held for 2 min, flowed ramp from 2 ml/min to 100 ml/min in 18 min, and then maintained 100 ml/min for 10 min until stopping. The analyses were separated in the column at 60°C and then ionized in the IMS ionization chamber at 45°C. The constant flow of drift gas flow was set up to 150 ml/min. The instrument was standardized by linear retention index (RI) of n‐ketones, for the reason that IMS had no response to alkanes. Calculation of RI of volatile compounds was based on the n‐ketones C4‐C9. Comparing the drift time and RI in the GC‐IMS library, volatile compounds in NF and IF samples were well identified. The qualitative analysis of volatile compounds was conducted based on the IMS and NIST database built in GC × IMS Library Search. The quantitative analysis for volatile compounds was mainly based on the peak intensity in HS‐GC‐IMS, and the peak intensity was proportional to the content of volatile compounds.

2.4. Data analysis

All the experiment was performed in triplicate. The spectra were analyzed with laboratory analytical viewer (LAV), and the difference profiles and fingerprints of volatile compounds were constructed with the Reporter and Gallery plug‐ins. The NIST and IMS database were built into the software for qualitative analysis of samples. The line and bar charts drawn by Origin 2018 were used to analyze change of volatile compounds in FMP during fermentation process. PCA obtained by the original date was used for clustering analysis of principal compounds in samples. The heat map was generated using the heat map plug‐in of origin 2018, and the methods used for clustering of original date were Ward minimum variance and Euclidean distance.

3. RESULTS AND DISCUSSION

3.1. Changes of HS‐GC‐IMS spectra in FMP

The volatile compounds of FMP during NF and IF were detected by HS‐GC‐IMS. The data were exhibited with the 3D spectrum, where the x‐axis represented the ion migration time, the y‐axis represented the retention time of the gas chromatograph, and the z‐axis represented the peak intensity. As shown in Figure 1, there was the similar peak signal distribution in all samples during fermentation process. This phenomenon suggested that all FMP posed the same volatile compounds during fermentation process. However, the peak signal intensity showed some differences in FMP during fermentation process. It indicated that content of volatile compounds changed (increasing or decreasing) with the prolonging of fermentation time. Yu et al. (2013) proved that the prominent microorganisms posed clear relationships with changes in flavor during fermentation. However, no obvious differences in volatile compounds were found between NF and IF samples. It was found that the signal intensities of almost all volatile compounds in IF samples were similar to those in NF ones. This phenomenon showed that two fermentation methods played minor effects on changes in volatile compounds during fermentation process.

FIGURE 1.

FIGURE 1

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Changes in 3D topography of volatile compounds during NF and IF process. (a) NF; (b) IF. IF: inoculated fermentation; NF: natural fermentation

The volatile compounds in FMP were not easy for analysis by 3D spectra. Hence, the 2D spectra were used for further comparison, as shown in Figure 2. The reactive ion peak (RIP) was exhibited with the red vertical line at the horizontal coordinate of 1.0. The normalized migration time, from 7.92 to 7.97 ms, was used to avoid the change of ion migration time caused by temperature and pressure deviation during detection process. For comparing with the differences in volatile compounds in all samples, fresh samples were taken as the reference, and the spectral background colors of other samples were white after deducting that of original samples. In 2D spectra, every dot on the right side of RIP represented the specific volatile compound. The color of dots represented the concentration of volatile compounds. The blue dots represented that the volatile compounds posed lower concentration comparing with those of the control, and the red dots represented that the volatile compounds posed higher concentration comparing with those of the control. As shown in Figure 2a,b, most dots were located in 2D spectra area from 0 to 400 s of retention time and from 1.0 to 1.5 of drift time, and few dots were located in 2D spectra area from 400 to 1,200 s of retention time.

FIGURE 2.

FIGURE 2

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Changes in 2D topography of volatile compounds during NF and IF process. (a) NF; (b) IF. IF: inoculated fermentation; NF: natural fermentation

As shown in Figure 2a,b, the total number of dots in 2D spectra hardly changed. However, the number of blue or red dots changed (increase or decrease). These results indicated that the varieties of volatile compounds in samples were the same, but the content of volatile compounds changed with the prolonging of fermentation time. The reason was that metabolism and decomposition of microorganisms caused volatile compounds to increase or decrease during fermentation process. Wu et al. (2015) showed that various bacteria and fungi, especially for LAB and yeast, possessed obvious correlation with the changes in volatile compounds during fermentation process. In addition, some pathways of biosynthesis and degradation also existed in FMP, including EMP pathway, Strecker pathway, and decarboxylation pathway, which could increase volatile compound content (Kang & Baek, 2014; Li, Zhao, et al., 2016; Li, Dong, Huang, & Wang, 2016). However, the color of red or blue dots in NF and IF samples was similar to each other. Therefore, the content of volatile compounds in NF and IF samples was almost the same. This result was because of the fact that LAB was the prominent microorganism under suitable conditions whatever NF or IF.

3.2. Qualitative analysis of volatile compounds in FMP

The qualitative analysis of volatile compounds in FMP during fermentation process was represented by numbers, as shown in Figure 3 and Table 1. Each dot represented a type of volatile compound. The marked dots were identified volatile compounds, but unmarked dots were nonidentified volatile compounds. All FMP showed 53 identified dots by GC × IMS library analysis. Therefore, there were the same volatile compounds in all samples. This result indicated that fermentation methods hardly affected varieties of volatile compounds during fermentation process. In Figure 3 and Table 1, a total of 43 typical volatile compounds, including nine esters, 12 aldehydes, 11 alcohols, four ketones, three furans, two acids, one pyrazine, and one ether, were identified by NIST and IMS database in all samples. However, monomer and dimer of the same volatile compound exhibited similar retention time, but different migration time. The volatile compounds with high content or high proton affinity were beneficial for the production of new dimer (Arroyo‐Manzanaresa et al., 2018; Lantsuzskaya, Krisilov, & Levina, 2015). Arroyo‐Manzanaresa et al. (2018) found that several monomers with proton affinity absorbed some reactant protons, and then formed dimer by the combination of different monomers. Meanwhile, several volatile compounds could produce different signals, consisting of isoamyl hexanoate, methyl octanoate, nonanal, phenylacetaldehyde, heptadienal, benzaldehyde, gamma‐butyrolactone, methional, E‐2‐hexenol, hexanal, methylbutanal, methylbutanol, and ethanol. Li et al. (2010) showed that the same volatile compounds with different concentrations also might produce multiple signals in tricholoma matsutake. In addition, new formed dimer showed higher molecular weight than the monomer, and further generated multiple signals.

FIGURE 3.

FIGURE 3

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Ion migration spectra of volatile compounds identified by HS‐GC‐IMS during NF and IF process. IF: inoculated fermentation; NF: natural fermentation

TABLE 1.

Information of identified volatile compounds during NF and IF process

Count Compound CAS# Formula MW RI Rt (s) Dt (RIPrel) Comment Identification
1 Hexyl hexanoate C6378650 C12H24O2 200.3 1,558.7 1,170.184 1.5963   RI,Dt
2 Ethyl nonanoate C123295 C11H22O2 186.3 1,374.8 892.49 1.5408   RI,Dt
3 Isoamyl hexanoate C2198610 C11H22O2 186.3 1,260.8 720.352 1.5153 Monomer RI,Dt
4 Isoamyl hexanoate C2198610 C11H22O2 186.3 1,257.5 715.481 2.1399 Dimer RI,Dt
5 Methyl salicylate C119368 C8H8O3 152.1 1,220.3 659.302 1.2006   RI,Dt
6 Ethyl octanoate C106321 C10H20O2 172.3 1,185.2 606.274 1.4812   RI,Dt
7 Methyl octanoate C111115 C9H18O2 158.2 1,124.9 515.216 1.468 Monomer RI,Dt
8 Methyl octanoate C111115 C9H18O2 158.2 1,124.5 514.614 2.0618 Dimer RI,Dt
9 Nonanal C124196 C9H18O 142.2 1,108.9 491.123 1.4867 Monomer RI,Dt
10 Nonanal C124196 C9H18O 142.2 1,108.5 490.521 1.9397 Dimer RI,Dt
11 Methyl benzoate C93583 C8H8O2 136.1 1,092.5 466.428 1.2237   RI,Dt
12 Phenylacetaldehyde C122781 C8H8O 120.2 1,043.3 394.148 1.2576 Monomer RI,Dt
13 Phenylacetaldehyde C122781 C8H8O 120.2 1,042.9 393.546 1.5376 Dimer RI,Dt
14 E‐E‐2‐4‐Heptadienal C4313035 C7H10O 110.2 1,016.8 358.741 1.196   RI,Dt
15 Octanal C124130 C8H16O 128.2 1,008.3 348.385 1.4139   RI,Dt
16 E‐Z‐2‐4‐Heptadienal C4313024 C7H10O 110.2 1,002.6 341.766 1.2064   RI,Dt
17 6‐Methyl‐5‐hepten‐2‐one C110930 C8H14O 126.2 995.3 333.382 1.1743   RI,Dt
18 Benzaldehyde C100527 C7H6O 106.1 965.2 303.377 1.1489 Monomer RI,Dt
19 Benzaldehyde C100527 C7H6O 106.1 964.7 302.936 1.4701 Dimer RI,Dt
20 2‐Pentylfuran C3777693 C9H14O 138.2 994.0 332.058 1.2533   RI,Dt
21 1‐Octen‐3‐ol C3391864 C8H16O 128.2 987.5 324.998 1.1569   RI,Dt
22 Gamma‐butyrolactone C96480 C4H6O2 86.1 927.0 272.937 1.0809 Monomer RI,Dt
23 Gamma‐butyrolactone C96480 C4H6O2 86.1 924.0 270.819 1.2971 Dimer RI,Dt
24 Methional C3268493 C4H8OS 104.2 909.5 261.135 1.0882 Monomer RI,Dt
25 Methional C3268493 C4H8OS 104.2 910.5 261.74 1.4003 Dimer RI,Dt
26 2‐6‐Dimethylpyrazine C108509 C6H8N2 108.1 904.4 257.806 1.5315   RI,Dt
27 Cyclohexanone C108941 C6H10O 98.1 887.1 247.358 1.1514   RI,Dt
28 1‐Hexanol C111273 C6H14O 102.2 876.0 241.002 1.3241   RI,Dt
29 2‐Acetylfuran C1192627 C6H6O2 110.1 932.1 276.569 1.4392   RI,Dt
30 E‐2‐Hexenol C928950 C6H12O 100.2 852.7 228.451 1.1793 Monomer RI,Dt
31 E‐2‐Hexenol C928950 C6H12O 100.2 855.0 229.661 1.5157 Dimer RI,Dt
32 Dimethyl disulfide C624920 C2H6S2 94.2 743.1 178.214 0.9837   RI,Dt
33 Furfurol C98011 C5H4O2 96.1 831.3 217.693 1.0831   RI,Dt
34 Hexanal C66251 C6H12O 100.2 796.8 201.459 1.5618 Dimer RI,Dt
35 Hexanal C66251 C6H12O 100.2 797.2 201.665 1.2638 Monomer RI,Dt
36 3‐Methylpentanol C589355 C6H14O 102.2 833.4 218.721 1.5706   RI,Dt
37 1‐Pentanol C71410 C5H12O 88.1 770.0 189.54 1.2531   RI,Dt
38 2‐Methylbutanol C137326 C5H12O 88.1 754.2 182.759 1.4661   RI,Dt
39 Acetoin C513860 C4H8O2 88.1 722.3 170.019 1.3273   RI,Dt
40 2‐Ethylfuran C3208160 C6H8O 96.1 699.3 161.799 1.3107   RI,Dt
41 1‐Butanol C71363 C4H10O 74.1 671.5 153.168 1.3781   RI,Dt
42 3‐Methylbutanal C590863 C5H10O 86.1 653.7 148.27 1.4073   RI,Dt
43 2‐Methylpropanol C78831 C4H10O 74.1 642.2 145.31 1.3662   RI,Dt
44 Ethyl acetate C141786 C4H8O2 88.1 637.6 144.153 1.3364   RI,Dt
45 Butanal C123728 C4H8O 72.1 594.7 133.973 1.283   RI,Dt
46 Acetone C67641 C3H6O 58.1 524.9 117.893 1.1158   RI,Dt
47 Linalool C78706 C10H18O 154.3 1,103.6 483.186 1.218   RI,Dt
48 2‐Methylpropionic acid C79312 C4H8O2 88.1 798.3 202.172 1.363   RI,Dt
49 3‐Methylbutanol C123513 C5H12O 88.1 744.5 178.74 1.3325   RI,Dt
50 2‐Methylbutanoic acid C116530 C5H10O2 102.1 843.4 223.707 1.4662   RI,Dt
51 2‐Methylbutanal C96173 C5H10O 86.1 675.7 154.407 1.4087   RI,Dt
52 Ethanol C64175 C2H6O 46.1 458.1 102.494 1.0455 Monomer RI,Dt
53 Ethanol C64175 C2H6O 46.1 461.2 103.211 1.1372 Dimer RI,Dt
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The ordinal numbers in Table 1 corresponded to those in Figure 3.

Abbreviations: Dt: drift time; IF: inoculated fermentation; MW: molecular mass; NF: natural fermentation; RI: retention index; Rt: retention time.

3.3. Fingerprint analysis of volatile compounds in FMP

Although the 3D and 2D spectrum presented the change tendency of volatile compounds during NF and IF, the specific volatile compounds were not accurately judged. Hence, all signal peaks were used to make a detailed fingerprint analysis, as shown in Figure 4. In fingerprints, each column represented a kind of volatile compounds, each row represented the FMP samples, and volatile compound content was determined by the brightness degree of color.

FIGURE 4.

FIGURE 4

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Changes in fingerprint of volatile compounds during NF and IF process. D: dimer; IF: inoculated fermentation; IF‐0, IF‐6, IF‐12, IF‐18, IF‐24: IF samples were obtained on 0, 6, 12, 18, and 24 days; M: monomer; NF: natural fermentation; NF‐0, NF‐6, NF‐12, NF‐18, NF‐24: NF samples were obtained on 0, 6, 12, 18, and 24 days

In the raw materials (fermented on 0 days), high content of esters, aldehydes, and alcohols were found, especially for isoamyl hexanoate, gamma‐butyrolactone, phenylacetaldehyde, methional, E‐2‐hexanol, and 1‐butanol. The change in volatile compounds during NF and IF was presented in different color frames. In the red frame, the content of volatile compounds, such as hexyl hexanoate, ethyl nonanoate, isoamyl hexanoate, ethyl octanoate, and methyl octanoate, decreased to a low level. The decrease in above volatile compounds was related to the accumulation of acids. High acid condition induced by microorganisms could accelerate the hydrolysis of some volatile compounds (Bautista‐Expósito, Peñas, Silván, Frias, & Martínez‐Villaluenga, 2018; Tomita, Nakamura, & Okada, 2018). In addition, the content of several volatile compounds increased to a high level, such as ethyl acetate. However, the fingerprint brightness of above volatile compounds in NF samples was similar to those in IF samples during fermentation process. In the green frame, many volatile compounds sharply decreased with the prolonging of fermentation time, and showed low signal intensity and gray color in the late fermentation time. These volatile compounds were mainly aldehydes (nonanal, E‐Z‐2‐4‐heptadienal, hexanal, methylbutanal, etc.), ketones (acetone, acetoin, etc.), and alcohols (methylpentanol, 1‐butanol, etc.). This result was slightly different from the results of Wang, Wang, Xiao, Liu, Deng, et al. (2019), which might be related to pepper species, fermented methods, and the concentration of salt. Otherwise, the content of above volatile compounds showed similar fingerprint between NF and IF samples. In the yellow frame, the brightness degree of color in alcohols (linalool), furans (2‐ethylfuran, 2‐acetylfuran), and acids (2‐methylpropionic acid, 2‐methylbutanoic acid) increased with the prolonging of fermentation time, which indicated that these volatile compound content increased to a maximal level at the end of NF or IF. However, these volatile compounds in IF samples posed similar content with those in NF samples during fermentation process. Above result was accordance with the 3D and 2D spectra analysis. Moreover, the content of 2‐pentylfuran showed a fluctuate trend. To compare with volatile compound content in NF and IF samples during fermentation process, peak intensity values about volatile compounds needed further exploration.

3.4. Changes in volatile compounds of FMP

Seven kinds of volatile compounds, including eight esters, eight aldehydes, eight alcohols, and 11 other volatile compounds (four ketones, three furans, two acids, one ether, and one pyrazine), were used to explore the variation during NF and IF. Vegetable fermentation could be divided into aerobic fermentation and anaerobic fermentation (Gobbetti, Di Cagno, & De Angelis, 2010). The production of FMP was mainly anaerobic fermentation, such as alcohol and lactic acid. As shown in Figure 5a–d and Table 2, most of volatile compound content decreased with prolonging of fermentation time, only several volatile compounds increased. The reason for this phenomenon was that the growth of many microorganisms in FMP was inhibited by high concentration of salt, low pH, and oxygen‐deficient environment, so reduced the development of many volatile compounds. However, LAB posed a strong ability to resist adverse environment and produced volatile compounds by fermentation (homo‐ and heterofermentative LAB fermentation), especially for organic acids and alcohols (Esteban‐Torres et al., 2015; Yu et al., 2013). The followed analysis was used to further explore the specific change in various types of volatile compounds.

FIGURE 5.

FIGURE 5

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Changes in peak volume of volatile compounds during NF and IF process. NF: natural fermentation; IF: inoculated fermentation; (a, a1) esters; (b, b1) aldehydes; (c, c1) alcohols; (d, d1, e, e1) other volatile compounds (ketones, furans, acids, pyrazine, and ether); a, b, c, d, and e showed changes in peak volume of volatile compounds during NF and IF process; a1, b1, c1, d1, and e1 showed peak volume differences in volatile compounds between NF and IF samples. 1‐BU: 1‐butanol; 2‐6‐DI: 2‐6‐dimethylpyrazine; 2‐AF: 2‐acetylfuran; 2‐EF: 2‐ethylfuran; 2‐MBA: 2‐methylbutanoic acid; 2‐ME: 2‐methylbutanal; 2‐ME: 2‐methylbutanol; 2‐MPA: 2‐methylpropionic acid; 2‐PF: 2‐pentylfuran; 3‐ME: 3‐methylbutanol; 6‐ME: 6‐methyl‐5‐hepten‐2‐one; BU: butanal; CY: cyclohexanone; DD: dimethyl disulfide; EA: ethyl acetate; EO: ethyl octanoate; ET‐D: ethanol‐D; ET‐M: ethanol‐M; GB‐D: gamma‐butyrolactone‐D; GB‐M: gamma‐butyrolactone‐M; HE‐D: E‐2‐hexenol‐D; HE‐M: E‐2‐hexenol‐M; HH: hexyl hexanoate; IH‐D: isoamyl hexanoate‐D; IH‐M: isoamyl hexanoate‐M; LI: linalool; MB: methyl benzoate; ME‐D: methional‐D; ME‐M: methional‐M; NO‐D: nonanal‐D; NO‐M: nonanal‐M; PH‐D: phenylacetaldehyde‐D; PH‐M: phenylacetaldehyde‐M

TABLE 2.

The peak volume values of volatile compounds during NF and IF process

Compound NF IF
0 days 6 days 12 days 18 days 24 days 0 days 6 days 12 days 18 days 24 days
Esters
Hexyl hexanoate 696.30 ± 20.34a 559.14 ± 33.24b 434.00 ± 12.77c 408.59 ± 3.46c 436.06 ± 41.10c 779.21 ± 95.42a 542.20 ± 28.10bc 579.30 ± 51.81b 448.62 ± 14.80c 443.13 ± 69.57c
Ethyl nonanoate 600.66 ± 41.68a 407.66 ± 5.54b 425.48 ± 24.07b 387.19 ± 31.80b 427.15 ± 85.05b 628.67 ± 121.40a 349.95 ± 96.91b 423.90 ± 35.68b 393.83 ± 64.64b 406.79 ± 34.57b
Isoamyl hexanoate‐M 1,891.90 ± 76.32a 906.95 ± 24.72b 502.06 ± 12.55c 277.58 ± 37.58d 289.29 ± 20.52d 2,308.16 ± 63.08a 817.42 ± 38.20b 744.71 ± 31.23c 430.00 ± 12.28d 246.64 ± 27.50e
Isoamyl hexanoate‐D 406.63 ± 21.23b 477.36 ± 42.08a 391.57 ± 30.30b 242.91 ± 7.21c 245.05 ± 12.14c 515.36 ± 69.63a 360.54 ± 77.93bc 414.34 ± 11.48b 291.81 ± 16.11cd 216.06 ± 28.54d
Methyl salicylate 184.20 ± 9.83b 179.81 ± 13.57b 213.58 ± 15.25a 172.11 ± 21.31b 182.27 ± 17.92b 181.85 ± 24.87b 166.66 ± 12.82b 243.66 ± 6.86a 190.73 ± 14.17b 194.66 ± 17.17b
Ethyl octanoate 309.48 ± 14.18a 236.60 ± 18.51b 243.39 ± 29.63b 237.88 ± 19.48b 224.92 ± 32.42b 317.71 ± 48.60a 204.26 ± 50.24b 225.48 ± 23.26b 225.25 ± 6.30b 227.38 ± 23.10b
Methyl octanoate‐M 832.48 ± 8.71a 157.87 ± 6.11b 131.33 ± 12.74cd 117.48 ± 7.20d 136.08 ± 12.27c 977.04 ± 11.39a 159.16 ± 17.56b 133.20 ± 7.43b 138.95 ± 18.19b 138.54 ± 5.71b
Methyl octanoate‐D 166.44 ± 10.03a 108.47 ± 10.16b 94.36 ± 1.57bc 83.73 ± 2.62c 100.26 ± 8.83b 204.37 ± 40.59a 111.87 ± 1.52b 103.89 ± 2.62b 90.41 ± 9.75b 96.38 ± 17.43b
Gamma‐butyrolactone‐M 825.36 ± 2.72a 164.00 ± 2.65b 75.09 ± 6.86c 61.10 ± 3.34d 54.93 ± 2.55d 839.73 ± 18.28a 160.32 ± 5.11b 140.78 ± 11.56b 91.15 ± 9.38c 43.18 ± 1.68d
Gamma‐butyrolactone‐D 2,129.95 ± 31.33b 2,946.32 ± 19.31a 2,071.85 ± 144.52b 2,165.19 ± 82.49b 1,725.34 ± 58.30c 2,117.70 ± 70.64c 3,250.77 ± 62.68a 3,309.24 ± 86.57a 2,646.14 ± 37.57b 1,598.90 ± 174.77d
Methyl benzoate 211.83 ± 7.43d 421.04 ± 17.80c 566.03 ± 23.24a 570.57 ± 9.50a 534.35 ± 9.30b 276.18 ± 12.41c 413.70 ± 5.22b 575.99 ± 14.03a 590.11 ± 33.00a 608.06 ± 8.19a
Ethyl Acetate 81.35 ± 2.18d 238.55 ± 5.48c 656.58 ± 52.67ab 689.45 ± 32.42a 606.97 ± 3.23b 75.79 ± 5.42d 306.21 ± 8.91c 485.49 ± 39.70b 455.22 ± 44.40b 772.66 ± 31.42a
Aldehydes
Nonanal‐M 901.72 ± 35.72a 235.38 ± 6.27b 99.42 ± 8.88c 79.25 ± 1.18c 75.29 ± 7.40c 749.11 ± 13.39a 256.23 ± 14.35b 114.30 ± 6.70c 87.70 ± 7.88d 68.69 ± 7.37d
Nonanal‐D 413.59 ± 41.93a 130.30 ± 14.13b 78.61 ± 4.86c 79.17 ± 6.79c 85.60 ± 6.99c 277.11 ± 25.45a 129.45 ± 15.12b 91.67 ± 11.58c 84.51 ± 16.67c 79.35 ± 15.62c
Phenylacetaldehyde‐M 2,049.89 ± 18.72a 581.59 ± 14.96b 533.98 ± 21.69c 407.69 ± 28.34d 377.50 ± 10.83d 2,033.72 ± 20.30a 541.22 ± 42.13c 667.36 ± 20.72b 374.30 ± 3.80d 328.87 ± 25.21d
Phenylacetaldehyde‐D 1,640.78 ± 5.51a 253.47 ± 1.56b 217.62 ± 26.72c 160.80 ± 11.20d 137.49 ± 5.31d 1,559.10 ± 10.51a 244.21 ± 9.30c 302.99 ± 20.48b 182.33 ± 8.99d 116.76 ± 8.39e
E‐E‐2‐4‐Heptadienal 166.42 ± 8.89a 37.81 ± 2.19b 36.48 ± 1.42b 31.07 ± 5.03bc 24.95 ± 2.46c 234.56 ± 2.22a 29.63 ± 7.76c 42.18 ± 3.35b 33.16 ± 2.43c 24.95 ± 4.99c
E‐Z‐2‐4‐Heptadienal 207.79 ± 18.55a 153.22 ± 5.68b 159.16 ± 7.36b 156.15 ± 2.40b 113.49 ± 7.36c 286.46 ± 0.66a 142.60 ± 4.82d 192.44 ± 3.53b 170.30 ± 13.49c 138.35 ± 1.19d
Benzaldehyde‐M 228.13 ± 4.24a 114.96 ± 11.48b 99.94 ± 3.19c 95.88 ± 10.29c 101.64 ± 4.45bc 236.69 ± 11.75a 100.88 ± 9.11c 137.76 ± 8.25b 143.83 ± 3.41b 65.41 ± 2.06d
Benzaldehyde‐D 109.47 ± 8.82a 95.06 ± 4.54b 64.24 ± 5.37c 61.27 ± 2.75c 57.42 ± 2.08c 138.22 ± 3.63a 85.08 ± 2.85c 104.97 ± 6.17b 74.21 ± 5.06d 57.76 ± 3.35e
Methional‐M 372.66 ± 4.09a 90.70 ± 4.38b 50.42 ± 2.66c 40.37 ± 5.61d 37.83 ± 5.15d 374.08 ± 8.34a 82.70 ± 2.29b 86.40 ± 5.44b 60.24 ± 2.73c 30.72 ± 1.28d
Methional‐D 1,745.87 ± 5.70a 532.18 ± 8.10b 503.26 ± 36.28b 477.18 ± 26.30c 351.82 ± 32.74d 1,652.81 ± 11.12a 536.69 ± 4.06c 697.63 ± 21.16b 506.88 ± 2.91d 454.11 ± 6.13e
Furfurol 151.51 ± 4.35a 24.53 ± 1.35b 21.30 ± 2.53bc 20.33 ± 2.50bc 19.44 ± 6.30c 124.95 ± 7.69a 21.36 ± 4.67b 23.43 ± 2.09b 18.71 ± 1.83b 18.68 ± 4.29b
Hexanal‐M 157.50 ± 4.51a 20.09 ± 1.86b 19.06 ± 1.90b 17.76 ± 1.49b 16.75 ± 1.68b 139.62 ± 2.30a 19.68 ± 4.41b 22.23 ± 2.61b 20.10 ± 1.77b 17.51 ± 0.66b
Hexanal‐D 480.58 ± 36.06a 19.36 ± 2.35b 13.80 ± 1.57b 16.27 ± 1.63b 17.79 ± 3.39b 537.34 ± 63.31a 15.68 ± 0.013b 15.88 ± 3.34b 17.27 ± 2.20b 20.44 ± 4.49b
Butanal 385.21 ± 3.31a 72.40 ± 2.48b 72.51 ± 0.81b 58.45 ± 3.46c 65.88 ± 6.31b 350.60 ± 10.21a 70.63 ± 5.74b 63.33 ± 6.08b 33.20 ± 1.02c 62.34 ± 6.78b
Octanal 87.73 ± 6.93b 90.67 ± 1.82b 114.84 ± 13.25a 97.02 ± 11.21b 81.40 ± 2.17b 81.33 ± 6.15b 85.09 ± 4.12b 63.37 ± 2.66c 68.87 ± 9.45c 111.55 ± 7.24a
2‐Methylbutanal 395.26 ± 2.48b 761.38 ± 7.42a 362.78 ± 6.04c 359.56 ± 8.58c 355.68 ± 7.67c 399.05 ± 26.24d 800.30 ± 3.72a 569.86 ± 13.43b 481.04 ± 14.80c 185.03 ± 15.50d
3‐Methylbutanal 466.59 ± 1.29b 499.93 ± 4.13a 214.44 ± 6.35c 168.68 ± 13.57d 183.05 ± 9.27d 443.43 ± 8.87b 520.31 ± 29.67a 346.11 ± 18.29c 218.82 ± 18.43d 91.93 ± 4.49e
Alcohols
1‐Hexanol 248.44 ± 3.18a 109.37 ± 7.07b 78.36 ± 7.53c 88.10 ± 2.52c 85.23 ± 5.52c 268.05 ± 5.30a 99.23 ± 8.09b 105.23 ± 3.03b 98.31 ± 1.83b 72.78 ± 4.52c
E‐2‐Hexenol‐M 259.58 ± 3.61a 78.81 ± 4.40b 64.97 ± 3.95c 53.88 ± 5.48d 80.76 ± 5.34b 281.19 ± 4.04a 63.70 ± 6.22c 81.93 ± 2.09b 52.92 ± 4.23d 48.09 ± 2.45d
E‐2‐Hexenol‐D 2,867.64 ± 36.09a 216.76 ± 2.47d 314.55 ± 10.02c 401.14 ± 12.39b 392.21 ± 12.87b 2,604.23 ± 27.02a 232.49 ± 7.89e 472.15 ± 28.12b 382.60 ± 13.48c 313.76 ± 7.33d
3‐Methylpentanol 217.96 ± 10.10a 8.72 ± 2.41b 7.78 ± 1.00b 8.30 ± 0.28b 9.15 ± 0.36b 239.95 ± 5.68a 6.57 ± 0.92c 12.59 ± 2.11b 8.38 ± 1.95bc 8.42 ± 0.98bc
1‐Pentanol 115.87 ± 3.14a 72.42 ± 3.47b 45.23 ± 0.77c 45.69 ± 1.92c 74.00 ± 5.20b 108.39 ± 4.93a 73.84 ± 5.51c 41.61 ± 3.03e 53.25 ± 7.89d 98.81 ± 2.81b
2‐Methylbutanol 726.89 ± 10.43a 71.20 ± 0.46c 94.15 ± 3.98b 54.59 ± 0.72d 68.58 ± 1.73c 769.92 ± 10.83a 61.32 ± 9.62c 71.37 ± 7.79c 61.53 ± 4.59c 120.63 ± 2.73b
3‐Methylbutanol 82.91 ± 2.44c 881.67 ± 75.86b 1,678.44 ± 33.45a 1,732.73 ± 60.22a 1,642.38 ± 51.01a 83.97 ± 4.37d 701.33 ± 8.50c 1,320.41 ± 121.77b 1,414.22 ± 122.67ab 1,478.50 ± 36.83a
1‐Butanol 2,057.89 ± 22.54a 159.23 ± 13.94c 213.62 ± 2.98b 215.31 ± 8.76b 204.30 ± 5.35b 2,003.53 ± 35.67a 163.69 ± 5.56c 256.78 ± 4.76b 183.54 ± 12.67c 157.86 ± 6.32c
2‐Methylpropanol 74.90 ± 1.80abc 83.12 ± 6.58a 64.10 ± 6.23c 71.23 ± 5.42bc 81.87 ± 7.78ab 67.38 ± 3.10b 80.79 ± 12.07a 78.07 ± 3.61a 79.10 ± 3.85a 54.66 ± 0.20c
Ethanol‐M 773.99 ± 13.26b 906.03 ± 11.51a 384.08 ± 13.28c 370.67 ± 19.75c 380.81 ± 2.68c 1,123.28 ± 51.23a 1,030.45 ± 104.34a 624.72 ± 33.53b 542.54 ± 16.16b 244.02 ± 16.41c
Ethanol‐D 88.59 ± 1.29d 544.47 ± 13.18c 1,885.69 ± 70.41a 1,831.05 ± 43.84a 1,573.56 ± 40.76c 136.04 ± 7.10d 667.01 ± 37.36c 1,457.70 ± 91.41b 1,389.34 ± 40.02b 1,779.19 ± 39.47a
Linalool 307.34 ± 11.22d 1,540.96 ± 30.63c 2,312.47 ± 38.35b 2,315.23 ± 21.20b 2,423. 01 ± 47.84a 339.16 ± 17.03c 1,549.49 ± 8.65b 2,416.06 ± 23.38a 2,490.05 ± 8.40a 2,503.09 ± 105.21a
1‐Octen‐3‐ol 88.72 ± 1.04b 126.64 ± 4.72a 69.07 ± 4.56c 58.37 ± 5.21d 71.75 ± 3.36c 79.98 ± 8.15c 132.44 ± 4.42a 107.83 ± 4.79b 85.97 ± 2.42c 49.27 ± 9.97d
Ketones
Acetoin 1,068.65 ± 16.72d 1,247.25 ± 3.54c 1,471.69 ± 23.17b 1,556.51 ± 13.99a 1,458.45 ± 17.31b 1,052.56 ± 2.79d 1,229.49 ± 31.47c 1,539.82 ± 10.37a 1,540.08 ± 26.82a 1,405.61 ± 37.57b
Acetone 4,182.73 ± 43.16a 1,782.04 ± 28.14b 1,291.06 ± 19.61c 1,098.89 ± 39.74d 1,036.14 ± 68.25d 3,835.80 ± 34.11a 1,740.47 ± 24.98c 1,977.81 ± 136.24b 1,539.08 ± 28.29d 386.61 ± 4.58e
Cyclohexanone 301.37 ± 11.62d 425.41 ± 3.79a 377.72 ± 6.84b 289.24 ± 3.70e 318.22 ± 1.86c 306.27 ± 8.33a 327.62 ± 2.33b 380.17 ± 5.76a 281.61 ± 12.24c 209.76 ± 19.91d
6‐Methyl‐5‐hepten‐2‐one 237.37 ± 7.95a 112.55 ± 6.67b 66.75 ± 4.31c 66.60 ± 8.27c 67.71 ± 4.94c 275.64 ± 10.79a 89.15 ± 3.22c 100.01 ± 4.38b 82.24 ± 2.84c 53.16 ± 1.72d
Furans
2‐Ethylfuran 151.09 ± 5.03d 174.04 ± 5.90d 231.90 ± 8.35c 443.32 ± 24.77b 484.44 ± 24.55a 232.79 ± 3.40d 160.65 ± 6.18e 431.02 ± 31.97a 351.12 ± 32.36b 299.73 ± 3.15c
2‐Pentylfuran 83.89 ± 6.23a 47.01 ± 6.85c 59.54 ± 1.53b 50.80 ± 5.36c 48.02 ± 3.19c 92.62 ± 9.18a 57.73 ± 6.06c 78.26 ± 3.88b 47.28 ± 4.18cd 42.22 ± 3.59d
2‐Acetylfuran 116.14 ± 2.52d 143.32 ± 7.09c 139.18 ± 2.80c 161.21 ± 10.90b 278.84 ± 14.15a 109.20 ± 6.03d 142.58 ± 4.92c 178.39 ± 3.22b 143.48 ± 6.29c 205.64 ± 20.76a
Acids
2‐Methylpropionic acid 980.12 ± 30.62c 9,593.35 ± 149.27a 9,592.00 ± 30.64a 9,576.79 ± 72.2a 8,732.55 ± 186.26b 918.27 ± 39.00e 10,156.44 ± 82.03a 8,986.20 ± 110.58d 9,678.86 ± 74.85b 9,319.22 ± 82.58c
2‐Methylbutanoic acid 68.92 ± 2.18e 134.76 ± 3.51d 525.19 ± 12.09a 352.67 ± 2.62c 404.33 ± 8.15b 70.03 ± 6.08c 145.99 ± 18.00b 153.33 ± 22.79b 197.27 ± 28.50b 808.58 ± 53.04a
Pyrazine
2‐6‐Dimethylpyrazine 249.48 ± 7.16a 100.61 ± 2.53b 84.91 ± 4.95c 63.36 ± 6.12d 68.36 ± 3.92d 237.88 ± 7.17a 93.85 ± 1.40c 115.23 ± 5.90b 61.01 ± 2.88e 77.49 ± 3.93d
Ether
Dimethyl disulfide 1,684.55 ± 14.51a 286.21 ± 2.08b 58.01 ± 1.31d 66.06 ± 1.91d 90.91 ± 3.98c 1,623.68 ± 28.53a 271.58 ± 5.36b 105.17 ± 9.91c 88.40 ± 7.71c 98.90 ± 1.83c
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The significance analysis results were based on the peak volume value of FMP during NF and IF process, and different letters indicated significant differences (p < .05).

Abbreviations: D: dimer; IF: inoculated fermentation; IF‐0, IF‐6, IF‐12, IF‐18, IF‐24: IF samples were obtained on 0, 6, 12, 18, and 24 days; M: monomer; NF: natural fermentation; NF‐0, NF‐6, NF‐12, NF‐18, NF‐24: NF samples were obtained on 0, 6, 12, 18, and 24 days.

Esters, closely related to the fruity and sweet flavor, played an important role in FMP flavor. The change of eight esters during fermentation process is shown in Figure 5a and Table 2. Most of esters decreased with the increasing in fermentation time, including hexyl hexanoate, ethyl nonanoate, methyl octanoate, isoamyl hexanoate, and gamma‐butyrolactone‐M. High‐acid environment might promote the hydrolysis of above esters (Tomita et al., 2018). In the early fermentation, isoamyl hexanoate‐M sharply deceased, but the isoamyl hexanoate‐D sharply increased. This phenomenon indicated that high content of isoamyl hexanoate‐M turned into the isoamyl hexanoate‐D, which was in accordance with the result of fingerprint analysis. In addition, isoamyl hexanoate posed an acid stability and was not easy to hydrolyze at low pH. Hence, isoamyl hexanoate content was the highest in all the eaters during fermentation process. Wang, Wang, Xiao, Liu, Deng, et al. (2019) found that ethyl salicylate, ethyl palmitate, methyl linoleate, ethyl linoleate, and ethyl stearate were the main esters in FMP during fermentation process, but no many isoamyl hexanoate was produced. The reason for the difference in two experiments was in accordance with fermentation conditions, salt content, and pepper varieties. According to these results, it was speculated that isoamyl hexanoate played an important role in the aroma quality of FMP. Ethyl acetate, with pineapple flavor, showed an increased tendency during fermentation process. Microorganisms turned the fermented carbohydrates into the end products during fermentation process, including organic acids and alcohols (Kim et al., 2016). The organic acids and alcohols could combine each other to form several esters, such as ethyl acetate (Wang, Wang, Xiao, Liu, Deng, et al., 2019). It was found that more ethyl acetate was produced during liquor fermentation, and became a key volatile compound of liquor during liquor brewing process (Cai et al., 2019). However, little research reported that ethyl acetate could increase to high level during vegetable fermentation. Hence, ethyl acetate and isoamyl hexanoate were the two main esters at the end of fermentation, and attributed to the FMP flavor.

Aldehydes showed lower threshold values and strong fragrance‐giving ability. For FMP, aldehydes with suitable content were beneficial for the maintenance of pungency flavor. Eight kinds of aldehydes were selected to explore the change in aldehydes during fermentation process, as shown in Figure 5b and Table 2. Most aldehydes decreased with the prolonging of fermentation time, including methional, phenylacetaldehyde, and furfurol, especially for methional and phenylacetaldehyde. During fermentation process, the aldehydes turned into the acids and alcohols under the action of microorganisms, which was an important reason for aldehyde degradation. Meanwhile, condensation could be occurred among aldehydes (Sukharev, Mariychuk, Onysko, Sukhareva, & Delegan‐Kokaiko, 2019), which accelerated aldehyde degradation. The decrease in above aldehydes weakened the pungency of FMP at some extent. Zhao et al. (2015) found that almost all aldehydes, such as butanal, octanal, and others, decreased with the prolonging of pumpkin juice fermentation time. For 2‐methylbutanal, the content increased in the first 6 days, then decreased in the late 18 days. Li, Dong, et al. (2016) explored the relationship between bacterial communities and volatile compounds in red pepper paste, and found that Pseudomonas posed an obvious correlation with 2‐methylbutanal. Hence, it was speculated that the change in 2‐methylbutanal content might be related to Pseudomonas during fermentation process. Hazelwood, Daran, van Maris, Pronk, and Dickinson (2008) found that some aldehydes, such as methylbutanal, could be generated by Strecker degradation of methionine and leucine or the Ehrlich pathway. Although octanal content slightly increased during fermentation process, the content was still lower than most other aldehydes. At the end of fermentation, the content of all aldehydes was low, which weakened the pungency of FMP.

Alcohols, with high threshold values, were mainly generated by alcohol and lactic acid fermentation (Wang, Wang, Xiao, Liu, Deng, et al., 2019). However, the threshold of unsaturated aldehyde was low, which made many contributions to food flavor (Lorenzo, Carballo, & Franco, 2013). As shown in Figure 5c and Table 2, the content of some alcohols increased sharply in the early fermentation time, and then maintained a stable level in the late fermentation time, including linalool, ethanol‐D, and 3‐methylbutanol. It indicated that the ability in early fermentation was stronger than that in late fermentation. In early fermentation time, LAB became the prominent microorganism quickly comparing with other microorganisms, promoted LAB fermentation, and produced some acids and alcohols. Nguyen et al. (2013) also proved that LAB in traditional fermented vegetables of Vietnam could grow rapidly and became the dominant bacteria under suitable conditions during fermentation process. In addition, alcohol fermentation also existed in early fermentation process, and produced some alcohols, especially for ethanol. In late fermentation time, the fermentation ability of microorganism was weakened because of the worst living conditions, such as less nutrient substance (Beganovi et al., 2014). However, some alcohols showed obvious decrease in tendency in early fermentation time, then hardly changed, especially for E‐2‐hexenol‐D and 1‐butanol. E‐2‐hexenol‐D was sensitive to the acids, and easily decomposed or turned into other compounds at low pH. The reason for 1‐butanol degradation might be that 1‐butanol turned into butyric acid at the action of microbial metabolism and oxidation. The increase in 2‐methylbutanoic acid was probably related to above change. At the end of fermentation, linalool content was the highest in all alcohols, and gave flower‐like aroma, which posed an important contribution to the FMP flavor. Linalool was also existed in some fermented vegetable products, such as fermented red pepper pastes and cucumber (Li, Dong, Zhao, & Zhu, 2020; Zhou & McFeeters, 1998). Zhou and McFeeters (1998) found that the content of linalool in fermented cucumber increased to several times than its odor threshold during fermentation.

The other volatile compounds, including 4 ketones, 3 furans, 2 acids, 1 pyrazine, and 1 ether, are shown in Figure 5d,e and Table 2. The content of 2‐methylpropionic acid and 2‐methylbutanoic acid increased after fermentation. In particular for 2‐methylpropionic acid, the content sharply increased to a high level. Whatever NF or IF, lactic acid fermentation was the main fermentation pathway, and involved the oxidation of carbohydrates to other compounds, which caused the production of lots of acids. Li et al. (2020) explored the effect of different salt content (10%, 15%, 20%, and 25%) on microbial categories and related qualities, and found that LAB became the prominent population at high salt content. At the end of fermentation, it was found that the pH value was up to about 4.10, which was similar to our experiment. Ketones possessed a lower threshold value and more contributed to food flavor. Acetone, cyclohexanone, and 6‐methyl‐5‐hepten‐2‐one decreased with the prolonging of fermentation time. In particular for acetone, the peak value decreased from 4,009.27 to 711.38 in the whole fermentation. Sukharev et al. (2019) found that ketone could condensate with aldehydes or other ketones, and condensation reaction might be the main reason for the decrease in ketones during fermentation process. The final peak values of 2‐ethylfuran, 2‐pentyfuran, and 2‐acetylfuran were about 2 times than those of the initial ones. It indicated that some furans were generated during fermentation process. Furans were hardly detected in many fermented foods (Kim et al., 2019). It is indicated that 2‐ethylfuran, 2‐pentyfuran, and 2‐acetylfuran were probably the characteristic volatile compounds in FMP. Only 1 pyrazine and ether were detected. Hence, it was difficult to analyze the change in pyrazine and ether during fermentation process.

As shown in Figure 5a1–d1, it was found that there was similar content in volatile compounds between NF and IF samples. Under suitable fermentation conditions, including temperature, pH, O2, the LAB fermentation was the prominent pathway whatever NF or IF (Sanlier et al., 2017). Many other microorganisms were inhibited by LAB. Hence, two fermentation methods produced the similar volatile compounds, but IF might shorten fermentation period and promote FMP quality. It was proved that the use of inoculated microorganism starters guaranteed the agreeable sensory properties, including volatile compounds (Woutets et al., 2013; Zhao et al., 2015).

In summary, main volatile compounds, including esters, aldehydes, alcohols, and acids, notably changed during fermentation. Almost all esters, aldehydes, and some alcohols posed an obvious decrease, and some alcohols and all acids posed an obvious increase during fermentation process. These results were related to microbial fermentation. However, there were no obvious differences in volatile compounds between NF and IF samples. Hence, further analysis was needed to explore volatile compounds during NF and IF process.

3.5. Analysis based on PCA results

Principal component analysis, a multivariate statistical analysis, was used to turn original correlated variables into linearly uncorrelated variables by several related transformation. These uncorrelated variables obtained by PCA could reflect the relationships among the observed variables (Cirlini et al., 2019; Yao et al., 2015). The correlated variables could be distinguished by the positive or negative score values in PC1 and PC2. Yilmaztekin and Sislioglu (2015) found that most important volatile compounds in fermented and raw European cranberrybush were divided into four uncorrelated parts by PCA, and PCA discriminated the fermentation stage as three groups. In our research, PCA obtained by original date was used to compare the difference in principal compounds (Figure 6). An obvious separation of two principal compounds was found during fermentation process and two fermentation methods (NF and IF). Two principal compounds (PC1 and PC2) were up to 87% in variation of originate date. PC1 was up to 74%, but PC2 was only up to 13% in variation of originate date. As shown in Figure 6, the samples showed a well separation degree each other, and three parts were presented in PCA based on previous analysis and heat map. The higher the separation degree, the lower the correlation in different samples. Samples in different parts posed low correlation. According to PCA, an obvious difference was found comparing samples on 0 days (NF and IF) with other samples (NF and IF), as shown in red frame. PC1 and PC2 of samples on 0 days were the positive score values. The result indicated that fermentation time played a notable influence on volatile compounds of FMP. The other samples could be well distinguished based on the score values of PC1 and PC2. The samples on 6 days were clustered in the yellow frame, but few differences were found between NF and IF samples. Principal compounds of samples on 12, 18, and 24 days of NF and IF were clustered in green frame. The samples on 12 and 18 days during IF have negative score values of PC1 and PC2, but the negative score values of PC1 and the positive score values of PC2 in other samples that were found. According to above results, it was found that there were few differences in principal compounds in the late fermentation time. It is indicated that the late fermentation ability was weakened because of less nutrition substance. In summary, the PCA well discriminated the stage of NF and IF as three groups (0 days; 6 day; and 12, 18, and 24 days). Based on above results, fermentation time played a key role in change of volatile compounds, but the two fermentation methods posed little effect on volatile compounds.

FIGURE 6.

FIGURE 6

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PCA of volatile compounds during NF and IF process. IF: inoculated fermentation; IF‐0, IF‐6, IF‐12, IF‐18, IF‐24: IF samples were obtained on 0, 6, 12, 18, and 24 days; NF: natural fermentation; NF‐0, NF‐6, NF‐12, NF‐18, NF‐24: NF samples were obtained on 0, 6, 12, 18, and 24 days

3.6. Clustering analysis based on heat map

To further analyze the differences in volatile compounds of FMP during fermentation process, the cluster analysis was obtained based on the heat map, as shown in Figure 7. In the heat map, the samples were clustered in the horizon, and volatile compounds were clustered in the vertical. There was the high correlation degree in the same category. The samples showed a higher correlation degree each other with the shortening of Euclidean distance. Based on the vertical mode, all samples were clustered into three main categories. Samples on 0 days were clustered together under NF and IF, as the first category. Hence, NF samples were highly similar with IF ones on 0 days. According to the color depth, samples on 0 days showed high content of volatile compounds, such as hexyl hexanoate, methyl octanoate, gamma‐butyrolactone, phenylacetaldehyde, dimethyl disulfide, and acetone. The second category was the NF and IF samples on 6 day. The other samples presented a high correlation between NF (12–24 days) and IF (12–24 days), as the third category. In the late fermentation time, several volatile compound content was up to a high level, including 2‐methylbutanoic acid, 2‐methylpropionic acid, 2‐acetylfuran, and ethyl acetate. On the basis of vertical analysis in heat map, it could be concluded that volatile compounds changed with the prolonging of fermentation time, especially in the early fermentation time. Above results were similar with PCA and previous analysis. Based on the horizontal mode, all volatile compounds in FMP could be classified into two main categories according to the variation tendency (increase and decrease). In heat map, the content of hexyl hexanoate, methyl octanoate, gamma‐butyrolactone, E‐2‐hexenol, 2‐methylbutanol, dimethyl disulfide, acetone, and 1‐butanol obviously decreased to a low level. Apparently, these volatile compounds were clustered into a category with the shortening of Euclidean distance, as the first category. The result was related to change in microorganisms during fermentation process. However, methyl benzoate, ethyl acetate, linalool, 3‐methylbutanol, acetoin, 2‐ethylfuran, 2‐acetylfuran, 2‐methylpropionic acid, and 2‐methylbutanoic acid were clustered into another category, showing an increase tendency in content. LAB fermentation could turn the carbohydrate to other end products, such as acids and alcohols, so promoted the increase in above volatile compounds. These results from heat map further confirmed that HS‐GC‐IMS was a reliable way to detect the volatile compounds in FMP during NF and IF periods.

FIGURE 7.

FIGURE 7

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Heat map clustering of volatile compounds during NF and IF process. D: dimer; IF: inoculated fermentation; IF‐0, IF‐6, IF‐12, IF‐18, IF‐24: IF samples were obtained on 0, 6, 12, 18, and 24 days; M: monomer; NF: natural fermentation; NF‐0, NF‐6, NF‐12, NF‐18, NF‐24: NF samples were obtained on 0, 6, 12, 18, and 24 days

4. CONCLUSION

The study investigated the change in volatile compounds of FMP during NF and IF process using the HS‐GC‐IMS instrument and related supplementary analysis software. A total of 53 volatile compounds were identified in all samples, including 12 esters, 17 aldehydes, 13 alcohols, four ketones, three furans, two acids, one pyrazine, and one ether. With the prolonging of fermentation time, most esters, aldehydes, and alcohols obviously decreased, especially for isoamyl hexanoate, methyl octanoate, gamma‐butyrolactone, phenylacetaldehyde, methional, and E‐2‐hexenol. However, 2‐methylbutanoic acid, 2‐methylpropionic acid, linalool, and ethyl acetate increased to a high level during fermentation process. Above volatile compounds were the indicators of flavor at the end of fermentation time, playing a key role in unique flavor of FMP. Hence, the fermentation time possessed an obvious effect on change in volatile compounds. However, volatile compounds in NF and IF samples showed slight differences at the same fermentation time. Based on PCA and heat map, the fermentation process in all samples was well discriminated as three stages (0 days; 6 day; and 12, 18, and 24 days), and all volatile compounds were divided into two categories (increase and decrease). Above results proved that HS‐GC‐IMS was an effective method to analyze the change in volatile compounds in FMP during NF and IF process.

CONFLICT OF INTEREST

The authors declare that they do not have any conflict of interest.

HUMAN AND ANIMAL RIGHTS

No animals or humans were used for studies that are the basis of this manuscript.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No. 31601525), Natural Science Foundation of Hunan Province (2019JJ50256), the Introduces Talent Projects of Hunan Agricultural University (20654/540490316002), and Funds for Distinguished Young Scientists of Changsha (KQ1905025).

Chen Y, Xu H, Ding S, et al. Changes in volatile compounds of fermented minced pepper during natural and inoculated fermentation process based on headspace–gas chromatography–ion mobility spectrometry. Food Sci Nutr. 2020;8:3362–3379. 10.1002/fsn3.1616

Yuyu Chen and Haishan Xu contributed equally to this work.

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