Revelation of the discrepancy of volatile compounds in fig (Ficus carica) via gas chromatography ion‐mobility spectrometry

Abstract The volatile compounds of fig (Ficus carica) are influenced by various factors. To explore the composition and difference of volatile compounds among figs, gas chromatography ion mobility spectrometry (GC‐IMS) was used to study the volatiles of figs from various regions, diverse cultivars, and after treatment with different drying methods. Aldehydes were the main volatile compounds in Bojihong from Shandong, while esters, ketones, and alcohols were the main volatile compounds in Bojihong from Sichuan and Guangdong. The volatiles of Branswick and Banane were similar, but differed significantly from those of Bojihong. Drying had the most significant effect on fig volatiles, which greatly reduced the content of benzaldehyde, (E)‐2‐hexenal, 2‐methylbutanal aldehydes, lost the content of esters such as isoamyl acetate, butyl acetate, ethyl butyrate, and generated some ketones and ethers. The results showed that Bojihong from Shandong was more suitable for the processing of subsequent fig drying products.

Volatile compounds are one of the most important indicators of fruit quality (Cagliero et al., 2012).The unique volatile compounds in figs are influenced by various aspects (Villalobos et al., 2018), such as cultivars, geographical location, and processing methods.In research conducted by Fella et al., the composition of VOCS in ripe fruit changed, and Zidi et al. stated that the VOCS of fig fruit had different characteristics in distinct ripening stages (Fella et al., 2022;Zidi et al., 2021).As figs are perishable and difficult to store (Paolucci et al., 2020), drying has become an important way to preserve figs in China (Slatnar et al., 2011).The most common drying treatments are natural drying, vacuum drying, hot air drying, microwave drying, freeze drying, and explosion puffing drying.Hot air drying and freeze drying are the most common drying methods in fruit and vegetable processing.Vallejo et al. (2012)) found that dried figs had higher total amounts of phenolics than fresh fruits.Explosion puffing drying can make the product taste crispy and retain the nutrients of the dried product, and is mostly used in combination with other drying techniques (Chen et al., 2017).As one of the main processing methods for figs, drying has different effects on the volatile substances of figs during processing.
Gas chromatography-mass spectrometry (GC-MS) has been used to identify the volatile compounds in figs (Mujić et al., 2012).
Although it can be easily operated using pretreatment methods such as HiSorb, it still has the disadvantages of being time-consuming, high cost, the complexity of food matrices, and the isobars and isomers that cannot be separated by MS (Fella et al., 2023;Hearn et al., 2022;Hernández-Mesa et al., 2019).By contrast, ion mobility chromatography (IMS) is an ultra-sensitive, efficient, and easy-touse technique for the detection of trace gases.The combination of GC and IMS allows not only analysis at normal pressure but also differential analysis and the generation of visual fingerprints, thereby making data processing easier (Castell et al., 2022;Leng et al., 2021;Liu et al., 2021).However, there are only a few studies on the application of gas phase-ion mobility chromatography (GC-IMS) to analyze the combined volatile compound composition and differentiation of fresh and dried figs.
To our knowledge, there is no reporting comparing the VOCS of the above-mentioned regions, cultivars, and after drying treatment of figs.In this study, the volatile compounds of figs from different regions and cultivars and those produced using different drying treatments were comparatively analyzed using GC-IMS, fingerprinting, and principal component analysis (PCA).Our findings lay the theoretical foundation for the selection and breeding of new figs in China, the consumption of fresh fruits, and the processing of products.

| Hot air drying (HD)
Fresh figs were sliced to a thickness of 5 mm and dried for 24 h at a regulated air speed of 0.5 m/s and a temperature of 60°C.The moisture content of the dried figs was less than 10%.

| Freeze-drying (FD)
Fresh fruits were sliced to a thickness of 5 mm, prefrozen at −80°C for 12 h, and then freeze-dried in a vacuum at 1 kPa and a tray temperature of 27°C for 48 h.The moisture content of the dried figs was less than 10%.

| Explosion puffing drying (EPD)
Fresh figs were sliced to a thickness of 1 cm and pre-dried at 80°C for 4 h to maintain a moisture content of approximately 30%, after which they were closed and softened for 24 h.After softening, puffing was carried out at 85°C with a pressure of 0.1 MPa and a puffing time of 5 min.After 5 min, the vacuum valve was opened to allow the figs to puff rapidly, after which the temperature was cooled to 60°C, and the samples were kept in a vacuum for 2.5 h.After completion, cooling water was passed over the fruits to cool the samples to 20°C, and this temperature was maintained for 10 min, after which a normal pressure was applied.After drying, the moisture content of the figs was less than 10%.

| GC-IMS analysis
Volatile compounds were analyzed by GC-IMS (FlavourSpec®, the G.A.S.Department of Shandong Haineng Science Instrument Co., Ltd., Shandong, China).The samples were weighed at 2 g into a 20-mL headspace vial and then incubated at 500 rpm at 40°C for 15 min.200 μL of the sample was injected into a head injector at 45°C.GC-IMS was carried out for 20 min using an Agilent gas chromatogram (FS-SE-54-CB-1, 15 m ID: 0.53 mm GC column) with a column temperature of 60°C and N 2 flow rate of 150 mL/min in a capillary split column.The flow rate program was 2 mL/min for 2 min and 100 mL/min for 18 min.The NIST and IMS databases built into the software were used to identify volatile compounds.

| Statistical analysis
Data were collected and processed using Laboratory Analytical Viewer Gallery Plot 2.2.1 software of the analytical instrument, and fingerprints were drawn by Reporter and Gallery Plot plug-ins, while PCA was performed using SIMCA 14.1 (UMetrics AB, Umea, Sweden).
The R language was used for the cluster analysis and heat map.All data were made in triplicate and presented as mean ± standard deviation.GC-IMS spectroscopy was used to identify all volatile compounds, and the Gallery Plot plug-in was used to generate fingerprints (Figure 2d).A total of 27 volatile compounds were detected, including 14 esters, 9 aldehydes, 2 ketones, 1 alcohol, and 1 furan.Hexanal (M and D), furfural, (E)-2-hexenal (M and D), benzaldehyde (M and D), and isopropyl acetate were detected in Bojihong-sd-peel, which were significantly higher than those in other samples (labeled with a red rectangle).While ethyl butyrate, ethyl propanoate, 2-pentanone, isopropyl acetate, hexanal (M and D), and furfural were unique in Bojihong-sdpulp (labeled with purple rectangle).Guangdong were close to each other.However, the peel and pulp of figs from the same regions were separately distributed.The load chart showed that Bojihong-sc and Bojihong-gd were mainly esters and ketones, while Bojihong-sd was mainly aldehydes (Figure 2f).

| Effect of cultivars on volatile compounds in figs
Similarly, the volatile compounds of three fig samples (Bojihong, Branswick, and Banane) were analyzed by fingerprint analysis (Figure 3).Twenty-eight volatile compounds were detected, including 18 esters, 9 aldehydes, and 1 ketone (Figure 3a).The results of PCA in Figure 3b showed that, except for Bojihong, Branswick and Banane had similar volatile compounds.It was observed that a number of esters and ketones are in Branswick and Banane, and some aldehydes and a few esters are in Bojihong based on the load chart (Figure 3c).The results of PCA in Figure 4b showed that the volatile compounds were significantly different after three drying methods, but the volatile compounds of different cultivars of figs after the same drying method were similar.Further difference analysis of the fig samples

| Effect of drying treatments on volatile compounds in figs
showed that the main compounds after HD and FD treatment were aldehydes, and the main compounds after EPD treatment were ketones, ethers, esters, and a small number of alcohols (Figure 4b,c).
The volatile compounds obtained by GC-IMS were analyzed and clustered, and the concentration of each volatile compound was indicated by different colors in the heat map (Figure 4d).The results were the same as the PCA results.Drying treatment caused a significant decrease in esters, with a contemporary increase in ketones and ethers (Figure 5a-f).Aldehydes also showed significant changes after drying treatment; the most remarkable difference was a significant decrease in benzaldehyde and (E)-2-hexenal in Bojihong and 2-methylbutanal in Branswick and Banane.However, 3-methylbutanal, butanal, and 2-methylpropanal were generated after drying treatment.It could also be seen from the table that most of the esters of the three varieties of figs were lost after drying, and isopropyl acetate was generated (Table 1).In addition, methyl-5-hepten-2-one, dihydro-2(3 h)-furanone (M and D), 3-hydroxybutan-2-one (M and D), 2,3-butanedione, 2-Butanone (M and D), dimethyl sulfide, and 2-methyl-1-propanol were generated.

| DISCUSS ION
Volatile compounds are one of the important factors affecting the aroma of figs.However, different regions, cultivars, and drying treatments can affect the content of volatile compounds.Previous studies have reported that fig substances from the Mediterranean coast were mostly dominated by ketones, alcohols, and aldehydes (Sertkaya et al., 2021).However, our study found that esters were the main volatile compounds in the peel and pulp of fig samples in China by GC-IMS.Esters, important volatiles in most fruits, are fatty acids synthesized through enzymatic metabolism and contribute to the formation of fig flavor (Sertkaya et al., 2021).In this study, isopropyl acetate, ethyl butyrate, and ethyl propanoate were unique to Bojihong in Shandong, while ethyl 2-methylbutyrate, propyl butanoate, methyl 2-methylbutanoate, and ethyl 2-methylpropanoate were unique to Bojihong peels of Guangzhou.Sadiye et al. (Gozlekci et al., 2011)  hydes (pentanal and butanal) may be derived from the oxidation of linoleic acid (Yang et al., 2021;Zhang et al., 2019); the newly formed 3-methylbutanal and 2-methylpropanal are generated by Strecker degradation of leucine and valine, respectively, during the Maillard reaction (Pu et al., 2019).High drying temperatures and long drying times can lead to the loss of esters, resulting in the formation of more alcohols (Guo et al., 2018).
Ketones were confirmed to be the main volatiles of figs after drying (Russo et al., 2017).Higher concentrations were detected in dried treated samples, such as methyl-5-hepten-2-one, dihydro-2(3 h)-furanone (M and D), and 2,3-butanedione, which increased in content and were probably produced due to oxidation, enzymes, and microbial fermentation.
When the same species of figs were dried in various ways, there were differences in volatile compounds.After vacuum expansion drying treatment, aroma compounds such as 2-methylpropanal, 2-butanone, butanal, dimethyl sulfide, 3-methylbutanal, and 3-pentanone had great changes.As an effective and promising drying method, explosion puffing drying is applied to many kinds of fruits and vegetables.For example, apples that were pretreated by freezing explosion puffing drying had strong DPPH (1,1-diphenyl-2-picrylhy drazyl free radical -for in vitro antioxidant evaluation), hydroxyl radical, and FRAP (ferric ion reducing antioxidant power -for in vitro antioxidant evaluation) scavenging ability, and the aroma compounds also changed significantly (Feng et al., 2021).Furthermore, vacuum-expanded and dried yam slices have higher hardness and brittleness, and the volatile components of the product were changed (Gao et al., 2021).and 2021 Taishan Industry Leading Talent Project.
Figs are widely grown in China, and the main cultivars are Bojihong, Branswick, and Banane.Figs from different regions and cultivars have distinct aromas and contain various volatile components.Therefore, it is important to study the volatile components of figs from various regions and cultivars for the subsequent selection and breeding of seeds, consumption of fresh fruits, and processing of products.
To study the difference in volatile compounds of figs caused by different regions, the GC-IMS spectra of six fig samples (Bojihongsd-peel, Bojihong-sd-pulp, Bojihong-gd-peel, Bojihong-gd-pulp, Bojihong-sc-peel, and Bojihong-sc-pulp) were analyzed.Figure 2a shows the 2D topographic spectra of volatile substances in fig samples from different regions that could be used for qualitative analysis.The background of GC-IMS was blue, and the red vertical line at abscissa 1.0 was the normalized active ion peak (RIP).Each point on either side of the RIP peak represented a volatile compound, and the color reflected the concentration of the compound, with white color representing a lower concentration and red meaning a higher concentration.Most of the peak signals had a retention time of 100-600 s and a drift time of 1.0-1.75s.In order to distinguish the differences among the fig samples from different origins more clearly, the topographical plots of other samples were derived in Figure 2b,c with Bojihong-sd-peel and Bojihong-sd-pulp as references, respectively.The red color indicates a higher concentration than the reference substance, and the blue color indicates a lower concentration than the reference substance.There were a large number of red and blue dots in Figure 2b,c, which demonstrated a significant difference among the fig samples.

F
I G U R E 2 GC-IMS analysis of three different regions of figs.(a) 2D-topographic plots; (b and c) the difference comparison topographic plots; (d) fingerprints of volatile compounds; (e) PCA plot; and (f) loading scatter plot.F I G U R E 3 GC-IMS analysis of three different cultivars of figs.(a) Fingerprints of volatile compounds; (b) PCA plot; and (c) loading scatter plot.F I G U R E 4 GC-IMS analysis of three different drying treatments of figs.(a) Fingerprints of volatile compounds; (b) PCA plot; (c) loading scatter plot; and (d) heat map.| 1295 LIU et al.
pointed out that no esters were detected in the peels of two cultivars of Karabakunya and Sultan Selim in Turkey.The two cultivars of Branswick and Banane in Shandong were mostly esters, including methyl 2-methylbutanoate-M, butanoic acid methyl ester (M and D), ethyl propanoate, ethyl butyrate, ethyl 2-methylbutyrate (M and D), and ethyl acetate.In addition, methyl 2-methylpropanoate, ethyl 2-methylpropanoate, methyl hexanoate, and methyl 2-methylbutanoate-D were detected in the pulps of the two cultivars, while Bojihong was different from the two.Mujić et al. (2014)) found ethyl acetate to be the ester with high content in Croatian dried figs and also found esters such as ethyl ester, methyl salicylate, and 1-butanol-3-methyl acetate, which were not found in figs from China.Aldehydes are considered to be an important class of volatiles affecting fruit volatiles, which are more likely to be formed through the oxidation of fatty acids or the degradation of amino acids.Benzaldehyde is formed by the catalytic oxidation of benzyl alcohol by dehydrogenase.It was identified as the main compound of figs in Portugal and was also detected in fig samples from Shandong Province.Moreover, furfural, hexanal, and (E)-2-hexenal were detected in fig samples from Shandong Province.While in the Bojihong and two other cultivar samples from Guangdong and Sichuan, the aldehyde content was lower with 2-methylbutanal and 3-methylbutanal, and these compounds were also reported before (Oliveira et al., 2010).Drying is an important step in the postprocessing of figs, and the aroma compounds change according to the drying method and temperature changes.Aldehydes are the main aroma compounds in dried figs, and most aldehydes are produced by the cleavage of unsaturated fatty acids through the Maillard reaction.Linear alde- The above results revealed the differences in volatile compounds of figs from three different regions of Shandong, Sichuan, and Guangdong; the differences between Bojihong, Branswick, and Banane; and the differences and trends of Bojihong by hot air drying, freeze-drying, and explosion puffing drying.It provides a theoretical reference for the development of fig by-products.F I G U R E 5 Changes in the quantity of volatile compounds of figs.(a) Changes in the quantity of volatile compounds in Bojihong after drying; (b) changes in the quantity of volatile compounds in Branswick after drying; (c) changes in the quantity of volatile compounds in Banane after drying; (d) changes in the quantity of aldehydes in Bojihong after drying; (e) changes in the quantity of aldehydes in Branswick after drying; and (f) changes in the quantity of aldehydes in Banane after drying.TA B L E 1 Volatile compounds of figs after different drying treatments.
Molecular weight (MW); Retention index (RI); Retention time (RT); Drift time (DT); The letters represent the significant difference (p < 0.05) between VOCS of figs after different drying treatments.TA B L E 1 (Continued) In this paper, the GC-IMS technique was used to analyze the volatile compounds of the three regions and cultivars, as well as the drying treatments of the figs.Twenty-seven volatile compounds were detected in the three regions, including 14 esters, 9 aldehydes, 2 ketones, 1 alcohol, and 1 furan.PCA analysis could clearly distinguish the three, and Bojihong from Shandong was mainly hexanal, (E)-2-hexenal, benzaldehyde, and furfural.The main volatile compounds of Bojihong from Sichuan and Guangdong were esters such as ethyl acetate, butyl acetate, isoamyl acetate, ethyl butyrate, ethyl propanoate, and acetic acid, 2-methylpropyl ester, as well as 3-hydroxybutan-2-one and other ketones, and 2-methylbutan-1-ol and other alcohols.Twenty-eight volatiles were detected in the three cultivars, and PCA results showed that methyl 2-methylpropanoate, ethyl 2-methylpropanoate, methyl hexanoate, and methyl 2-methylbutanoate-D were specific to Banane and Branswick, compared to Bojihong.Twenty-nine volatile compounds were detected in figs after treatment with different drying methods.The volatile compounds of the identical cultivars of figs were significantly different after different drying treatments.And there were substantial changes in volatile compounds after drying treatment.This initial progress provides a theoretical basis for further studies on the factors influencing the different discrepancies among figs.Data curation (equal); formal analysis (equal); writingoriginal draft (equal).Rui Sun: Conceptualization (equal); project administration (equal); supervision (equal); writing -review and editing (equal).Qiu Wu: Methodology (equal); resources (equal); visualization (equal); writing -original draft (equal).Ming Jia: Validation (equal); visualization (equal); writing -original draft (equal).Tingjuan Yu: Validation (equal); visualization (equal); writing -original draft (equal).Yanling Han: Formal analysis (equal); visualization (equal); writing -original draft (equal).Mingguan Yang: Resources (equal); writing -review and editing (equal).Lei Sun: Supervision (equal); writing -review and editing (equal).ACK N OWLED G M ENTS This work was supported by the Jinan 20 Rules of High School [grant number 2020GXRC024]; Qilu University of Technology 2022 major innovation project of the pilot of integration of science, education and industry [grant number 2022JBZ01-08]; Shandong Province Natural Science Foundation, China [grant number ZR2022MC130];