Characterization of Key Aroma-Active Compounds in Two Types of Peach Spirits Produced by Distillation and Pervaporation by Means of the Sensomics Approach

As a deep-processed product of peach, the aroma characteristics of peach spirit have not been systematically studied, and there has been no research on improving the aroma quality through process improvement. Pervaporation technology was used for the first time in the production of peach spirit instead of distillation, and its critical aroma compounds were analyzed compared with distilled peach spirit. Compared to the distilled peach spirit, pervaporation produced peach spirit presented stronger fruity, honey, and acidic aromas, and lighter cooked-apple aroma. Sixty-two and 65 aroma-active regions were identified in the distilled and pervaporation produced peach spirits, and 40 and 43 of them were quantified. The concentrations of esters, lactones, and acids were significantly higher in the pervaporation produced peach spirit than those in the distilled peach spirit, while terpenoids showed opposite tendency. Both of the overall aromas of distilled and pervaporation produced peach spirits were reconstituted successfully by the compounds with OAV ≥ 1. The omission tests identified 10 and 18 compounds as important aroma compounds for distilled and pervaporation-produced peach spirits, respectively. The differences in the key aroma compounds between the two types of peach spirits explained the differences in the aroma profiles.


Introduction
Peach (Prunus persica L.), a typical stone fruit, originated in China and has been cultivated for more than two thousand years [1]. The varieties of peaches are divided into six categories, including sweet, crisp, honey, yellow-fleshed, flat, and nectarine, according to their texture, shape, and skin characteristics [2]. The market time of peaches is mainly from May to September in a vintage, and peaches are often wasted because they are not sold and transported in time [3]. Peach fruit is rich in nutrients, containing essential amino acids, vitamins, organic acids, dietary fiber, folic acid, minerals, and dietary antioxidants. The protein content of peaches is twice that of apples and grapes, and seven times that of pears [3][4][5].
According to the statistics of the 2020 China Fruit Industry Development Report (compiled by the China Fruit Circulation Society), the area and production of peaches in China ranked 1st in the world in 2019, with 89.0 million hm 2 and 15.993 million tons, respectively [6]. The annual production of peach fruit worldwide is gradually increasing, from 13.2 million tons in 2000 to 24.6 million tons in 2020 [7]. However, the fruits are not storage resistant, dehydration, shriveling, softening, decrease in ascorbic acid content, and decay after harvest [8]. Based on the huge scale of peach fruit cultivation in the world, it is necessary to develop the deep processing industry of peach. Currently, processed products of peach include peach juice [9], jam [10], dried fruit [11], fermented peach wine [1,3], and

Description of Samples
The peaches used for winemaking were sampled from an orchard in Pinggu District, Beijing, and immediately transferred to the laboratory in Haidian District in October 2021. The peach variety was Prunus persica (L.) Batsch cv. Yanhong. The winemaking process is shown in Figure 1. Peaches were cleaned, de-haired, dried, and crushed by mixer (Joyoung, Beijing, China) with potassium metabisulphite to a total SO 2 level of 30 mg/kg to prevent enzymatic browning and miscellaneous bacteria contamination. A 0.1 g/mL of pectinase ZYM AROM MP (Enartis, Novara, Italy) was added to the peaches and digested at 20 ± 1 • C for 12 h. Twenty liters of clarified juice (total sugar 83 g/L, pH 4.2) were obtained by filtration and transferred into glass fermenters for fermentation. Sugar and tartaric acid (food grade) was added to adjust the total sugar to 240 g/L, and the pH value to 3.5. Then the peach juice was inoculated with 4 g/L Saccharomyces cerevisiae Aroma White yeast (Enartis, Novara, Italy), and controlled the fermentation temperature at 18 ± 2 • C. During the fermentation, the specific gravity and temperature of the fermentation broth were monitored every day. After fermentation, peach wine was ranked three times to clarify. Subsequently, the fermented peach wine was used as raw material to produce peach spirits. Part of the peach wine was distilled twice using a mini-Charente pot still and obtained a spirit with 70% (v/v) ethanol. Then, the alcohol content was adjusted to 40% with pure water. The final spirit was distillation spirit (DS). The other part of the peach wine was separated using the pervaporation method twice, generating a peach spirit with 40% (v/v) ethanol, which was named pervaporation spirit (PS). The process condition parameters of pervaporation are described in Supplementary Table S1. The spirits were stored in a cellar with a stable environment (14 • C, 75% relative humidity) for further study.
compounds. The results will help to understand the aroma characterization of peach spirit, and compare the effects of the different separation methods on the aroma of spirits, providing the theoretical foundation to produce high-quality peach spirit.

Description of Samples
The peaches used for winemaking were sampled from an orchard in Pinggu District, Beijing, and immediately transferred to the laboratory in Haidian District in October 2021. The peach variety was Prunus persica (L.) Batsch cv. Yanhong. The winemaking process is shown in Figure 1. Peaches were cleaned, de-haired, dried, and crushed by mixer (Joyoung, Beijing, China) with potassium metabisulphite to a total SO2 level of 30 mg/kg to prevent enzymatic browning and miscellaneous bacteria contamination. A 0.1 g/mL of pectinase ZYM AROM MP (Enartis, Novara, Italy) was added to the peaches and digested at 20 ± 1 °C for 12 h. Twenty liters of clarified juice (total sugar 83 g/L, pH 4.2) were obtained by filtration and transferred into glass fermenters for fermentation. Sugar and tartaric acid (food grade) was added to adjust the total sugar to 240 g/L, and the pH value to 3.5. Then the peach juice was inoculated with 4 g/L Saccharomyces cerevisiae Aroma White yeast (Enartis, Novara, Italy), and controlled the fermentation temperature at 18 ± 2 °C. During the fermentation, the specific gravity and temperature of the fermentation broth were monitored every day. After fermentation, peach wine was ranked three times to clarify. Subsequently, the fermented peach wine was used as raw material to produce peach spirits. Part of the peach wine was distilled twice using a mini-Charente pot still and obtained a spirit with 70% (v/v) ethanol. Then, the alcohol content was adjusted to 40% with pure water. The final spirit was distillation spirit (DS). The other part of the peach wine was separated using the pervaporation method twice, generating a peach spirit with 40% (v/v) ethanol, which was named pervaporation spirit (PS). The process condition parameters of pervaporation are described in Supplementary Table S1. The spirits were stored in a cellar with a stable environment (14 °C, 75% relative humidity) for further study.

Aroma Extract Dilution Analysis with GC
This method referred to Li et al. [23] with modification. Fifty milliliters of spirit were diluted to 10% ethanol by volume with ultrapure water, then saturated with NaCl, and extracted by 50 mL redistilled dichloromethane three times. The organic phase was washed with 50 mL Na 2 CO 3 solution (0.5 mol/L, pH = 10.0) three times to obtain neutral/basic fraction (NBF). The aqueous phase was acidified by 4.0 mol/L HCl to adjust the pH to 2.0, and extracted by redistilled dichloromethane again to obtain the acidic fraction (AF). Anhydrous Na2SO4 was added to both NBF and AF fractions to remove water, then filtered and concentrated samples to 500 µL using a rotary evaporator. Both NBF and AF samples were stored at −20 • C. One microliter sample was injected into the injector of GC for the analysis of GC-MS and GC-O. For each sample, three parallel experiments were conducted.
2.3.2. Headspace Solid-Phase Microextraction (HS-SPME) SPME was used to extract volatile compounds to compensate for the lack of highly volatile compounds in the LLE extracted samples. According to the previous method with slight modifications [24]. A SPME fiber (50/30 µm DVB/CAR/PDMS) was used to extract the aroma compounds via HS-SPME. The spirit samples were diluted to 10% ethanol by volume with ultrapure water. Six milliliters of the diluted sample with 3.0 g NaCl were placed into a 20 mL vial capped by a silicon septum tightly. The prepared samples were incubated in a thermostatic bath at 40 • C for 30 min, then inserted a SPME fiber into the vital and extracted at 40 • C for 30 min with stirring at 500 rpm/min. After extraction, the fiber was inserted into the injector of GC to desorb for 8 min followed by GC-O or GC-MS analysis. For each sample, three parallel experiments were conducted.

Aroma Extract Dilution Analysis (AEDA)
AEDA of the concentrates (NBF and AF) obtained by LLE was performed according to the method of Zhao et al. [25]. The original extract by LLE was used as the sample. Either AF or NBF, each concentrated fraction was stepwise diluted 3-fold with dichloromethane as the solvent in a series of 1:3, 1:9, 1:27, . . . , 1:6561 times. All dilutions were analyzed by GC-O. Referred to Al-Dalali et al. [26] with slight modification, for HS-SPME, AEDA was performed by setting a series of the split ratio at the end of the column, to dilute the sample with helium in a series of 1:3, 1:9, 1:27, . . . , 1:6561 and analyzed by GC-O. Three trained panelists carried out this research, and each sample was repeated in triplicate by each panelist. The maximum dilution factor that at least two panelists could perceive the odorants is the FD factor.

Conditions of GC-MS/GC-O
GC-MS and GC-O were performed employing an Agilent 7890B GC equipped with an Agilent 5977A MS or a sniffing port (ODP3 C200; Gerstel, Mülheim an der Ruhr, Germany). All the samples were analyzed on a DB-WAX capillary column (60 m × 250 µm i.d., 0.25 µm film thickness; Agilent Technologies Inc, Santa Clara, CA, USA). Helium (99.999%) was used as carrier gas at a constant flow rate of 1.5 mL/min. The injector temperature was 250 • C in splitless mode. The initial temperature of the oven was 40 • C, then raised to 50 • C at a rate of 10 • C/min, and held for 5 min; then raised to 80 • C at 5 • C/min, and held for 5 min; finally raised to 230 • C at 5 • C/min and held for 10 min. The flow split ratio at the end of the column was 1:1 to the MS held at (250 • C) and sniffing-port (230 • C). MS (the electron ionization mode) was in full scan mode with an acquisition range of m/z 30-350 fragments and ionization energy of 70 eV. The ion source temperature was 230 • C.

Identification of Aroma Compounds
The identification of aroma compounds was achieved by comparing the mass spectrum, linear retention index (RI), and aroma characteristics of the standards or those reported in references. RIs of the compounds detected were calculated by the retention times of a series of n-alkanes (C7-C25). The identified aroma compounds were listed in Table 1.

Direct Injection (DI) Combined with GC-MS
The aromatic compounds, 3-methyl-1-butanol and 1-hexanol were quantified by DI-GC-MS using the internal standard calibration method, according to Wang et al. [27] The sample (990 µL) was spiked with 10.0 µL mixed internal standards (IS1, pentyl acetate 10,000 mg/L; IS2, 4-heptanol 10,000 mg/L; IS3, 2-ethylbutanoic acid, 10,000 mg/L) in a 2.0 mL injection vial, and 1 µL sample was directly injected into the injection port of GC-MS with a 10:1 split ratio. The mass spectrum was chosen as selective ion detection (SIM) mode, and other conditions were the same in Section 2.3.4. Each sample was repeated in triplicate. The mixed standard solutions were prepared as follows. Each standard compound was dissolved in absolute ethanol at high concentrations to prepare the base solution. Then, the standard stock solutions were added to the model solution (40% ethanol by volume, pH 4.0), then diluted into 10 concentrations. The same amount of internal standards was added to the standard solutions. The detection and analysis method was conducted as samples. Detailed information on the calibration curves is listed in Table S2.

LLE Combined with GC-MS
Two hundred and fifty microliters of the internal standard mixture, containing 200 mg/L pentyl acetate (IS1), 200 mg/L 4-heptanol (IS2), and 200 mg/L 2-ethylbutanoic acid (IS3), were added to 30 mL spirit sample. Then, the recombined sample was diluted to 10% ethanol by volume with ultrapure water, saturated with NaCl, and extracted by 50 mL of dichloromethane three times. Anhydrous Na 2 SO 4 was added to the mixed organic phases overnight to remove water. After filtration, the organic phase was concentrated to 500 µL by spin and nitrogen (99.9999%) blowing as the final sample for detection. One microliter extracted concentrate was submitted to GC-MS with a split ratio of 20:1, and the compounds were quantified using SIM mode. Other conditions were the same with Section 2.3.4. Each sample was conducted in triplicate. The quantitative method was carried out according to Li et al. [23]. The base solutions of standard compounds were diluted in dichloromethane, and other operations were described in Section 2.4.1. Detailed information on the calibration curves is listed in Table S2.

HS-SPME Combined with GC-MS
The aroma compounds, not quantified by DI-GC-MS and LLE-GC-MS, were detected employing HS-SPME combined with GC-MS. Before detection, spirit samples were diluted to 10% ethanol by volume, then 100 µL of the mixed internal standards (IS1, pentyl acetate, 200 mg/L; IS2, 4-heptanol, 200 mg/L; IS3, 2-ethylbutanoic acid, 200 mg/L) was also added. Other operating steps and parameters of HS-SPME and GC-MS were the same as that described in Sections 2.3.2 and 2.3.4. A synthetic matrix was prepared by mixing the standard stock solution in a 10% (v/v) ethanol/water solution at pH 4.0, and diluted to ten different concentrations. The internal standards, the same with those in the samples, were also added to the standard solutions. Detailed information on the calibration curves is listed in Table S2.

Sensory Analysis
The sensory panel consisted of 16 trained panelists in the olfactory experiments and quantitative descriptions of aromas, aged 23-26 years, with a male to female ratio of 1:1. All panelists were from the Beijing Key Laboratory of Flavor Chemistry, Beijing Technology and Business University. Before the sensory evaluation experiment, the panelists were trained for 15 min every day for 3 months to improve the accuracy and reproducibility of the test. Sensory tests were performed in a sensory evaluation laboratory at 20 ± 1 • C. First, the panelists described the aroma profile of peach spirits by evaluating the samples, and a list of aroma attributes was obtained based on the frequency and intensity described by the panelists. According to the determined method of Zhu et al. [28], eight sensory attributes were selected, including malty, cooked-apple, fruity, floral, honey, grass, acidic, and alcoholic. Standard solutions of 3-methylbutan-1-ol (malty), (E)-β-damascenone (cookedapple), ethyl 3-methylbutanoate (fruity), phenylethyl alcohol (floral, honey-like), phenethyl acetate (honey), hexanal (grass), acidic (acetic acid), and alcohol (alcoholic) were used as references in the experiments, respectively [28,29]. Panelists evaluated the similarity of the samples using a 7-point scale. The intensity started at 0 (imperceptible) and increased at 0.5 step each time to 3 (strongly perceptible) [29]. The results gained from 16 evaluators were averaged and plotted as a radar diagram.

Aroma Reconstitution Experiments
In order to reconstitute the original aroma of DS and PS spirit, the aroma active compounds with OAV ≥ 1 were blended in hydroalcoholic solution (40% ethanol by volume, pH 4.0) according to their concentrations quantified in the samples ( Table 2). The recombinants were mixed and then balanced for 30 min. The panelists evaluated these reconstituted samples as described in Section 2.5.

Omission Experiments
The omission models were constructed by omitting one or one class of selected aroma compounds from those used in aroma reconstitution experiments. Then each omission model was distinguished by comparison with the corresponding complete reconstituted solution using triangle tests, and panelists need to select the most different one in each group. At last, analyze the frequency that panelists distinguished correctly. Each sample was labeled with a three-digit code randomly, and each test was repeated three times on different dates. The significance of each test was calculated according to the previous reference [30].

Statistical Analysis
The concentrations of aroma compounds were expressed as mean ± standard deviation (SD). OAV was calculated by dividing the concentration of a compound by its odor . Radar diagrams were carried out using origin 2021 statistical software (OriginLab Corporation, Northampton, MA, USA). The one-way analysis of variance (ANOVA) test was used to analyze the significant difference between two samples using the SPSS 26 software package (SPSS Inc., Chicago, IL, USA). Principal component analysis (PCA) was performed using origin 2021 statistical software. The RIs of aroma compounds were calculated using Qualitative Analysis 10.0 (Agilent Technologies Inc., Santa Clara, CA, USA).

Aroma Profile Analysis of Peach Spirits
To clarify the differences in the aroma profiles between distilled peach spirit and pervaporation produced peach spirit, sensory analysis was performed. Malty, cookedapple, fruity, floral, honey, grass, acidic, and alcoholic were chosen as odor descriptors, and the aroma profiles of both peach spirits were extremely different ( Figure 2). The scores of cooked-apple property was significantly higher in the distilled peach spirit, while fruity, honey and acidic aroma properties were stronger in the pervaporation produced peach spirit. There was no obvious difference in the intensities of grass, floral, malty, and alcoholic notes. The differences in aroma profiles of distilled peach spirit and pervaporation produced peach spirit were caused by different separation processes. different dates. The significance of each test was calculated according to the previou erence [30].

Statistical Analysis
The concentrations of aroma compounds were expressed as mean ± standard d tion (SD). OAV was calculated by dividing the concentration of a compound by its threshold (Sha et al., 2017). Radar diagrams were carried out using origin 2021 stat software (OriginLab Corporation, Northampton, MA, USA). The one-way analysis o iance (ANOVA) test was used to analyze the significant difference between two sa using the SPSS 26 software package (SPSS Inc., Chicago, IL, USA). Principal comp analysis (PCA) was performed using origin 2021 statistical software. The RIs of a compounds were calculated using Qualitative Analysis 10.0 (Agilent Technologie Santa Clara, CA, USA).

Aroma Profile Analysis of Peach Spirits
To clarify the differences in the aroma profiles between distilled peach spirit an vaporation produced peach spirit, sensory analysis was performed. Malty, cookedfruity, floral, honey, grass, acidic, and alcoholic were chosen as odor descriptors, an aroma profiles of both peach spirits were extremely different ( Figure 2). The sco cooked-apple property was significantly higher in the distilled peach spirit, while f honey and acidic aroma properties were stronger in the pervaporation produced spirit. There was no obvious difference in the intensities of grass, floral, malty, and holic notes. The differences in aroma profiles of distilled peach spirit and pervapo produced peach spirit were caused by different separation processes.

Identification of Aroma Compounds in Peach Spirits
To comprehensively research the key aroma compounds of peach spirits, and also investigate the odorant compounds responsible for the differences in aroma profiles influenced by two separation processes, samples were extracted by both LLE and SPME, analyzed using AEDA-GC-O and GC-MS, and identified according to RI, standards, aroma and mass spectrum. In distilled peach spirit, a total of 62 odorous regions were detected with FD factors ranging from 1 to 6561, including 22 esters, 9 alcohols, 6 aromatic compounds, 7 terpenes, 1 acetal, 2 carbonyl compounds, 1 lactone, 8 acids, 3 sulfides, and 3 unknown odorants ( Table 1). As for the pervaporation produced peach spirit, 65 odoractive regions were detected, including 22 esters, 11 alcohols, 6 aromatic compounds, 5 terpenes, 1 acetal, 4 carbonyl compounds, 5 lactones, 7 acids, 1 sulfide and 3 unidentified odorants. In both peach spirits, the members of esters and alcohols were the most abundant, which was not related to the separation methods, the same as the previous report [31]. There were more terpenoids, while fewer esters, alcohols and lactones in distilled peach spirit than in pervaporation produced peach spirit.
The numbers of esters in both peach spirits were the same, but the components were different. Ethyl dodecanoate, isoamyl decanoate, ethyl tetradecanoate, ethyl pentadecanoate and ethyl 9-decenoate were only identified as aroma-active compounds in distilled peach spirit, while ethyl isobutyrate, ethyl 2-methylbutyrate, ethyl 3-methyl-butanoate, ethyl 2-hydroxy-3-methyl butyrate and ethyl 3-hydroxybutyrate were identified as aromaactive compounds only in pervaporation produced peach spirit. In peach brandy, ethyl dodecanoate, ethyl tetradecanoate and ethyl 9-decenoate were also identified [22,31]. In addition, the short-chain esters, including ethyl isobutyrate, ethyl isovalerate, and ethyl butanoate, were only detected or with higher FD factors in the pervaporation produced peach spirit compared to distilled peach spirit. While esters with over 14 carbon atoms were mainly detected in the distillation spirit. However, the contribution of esters to the peach spirits produced by different separation methods should be researched deeply.
Terpenoids have been identified as important contributors to the aroma profile of spirit [34,35]. Contrary to the case of lactones, terpenoids were identified more in distilled peach spirit than in pervaporation produced peach spirit. And citronellol, dihydro-β-ionol, and (E)-β-damascenone exhibit higher FD values in distilled peach spirit compared to pervaporation produced peach spirit. In addition, trans-nerolidol, trans-β-ionone, and (E,E)-farnesol were identified as aroma-active compounds only in the distilled peach spirit.

Quantification and OAV Analysis of Aroma Compounds
To further analyze the differences in aroma characteristics between peach spirits produced by distillation or pervaporation, a total of 53 compounds out of 85 compounds identified by GC-O were quantified using either DI-GC-MS, LLE-GC-MS or HS-SPME-GC-MS (Table 2). Other qualitative compounds were not quantified due to a lack of standards or trace concentration. For quantitative analysis, the standard curves of all compounds were constructed using the internal standard method, exhibited good linear correlation coefficients (R 2 > 0.990), and the relative standard deviation (RSD) of the triplicate samples was ≤15%. Details were shown in Table S2. OAVs were calculated to evaluate the contribution of aroma compounds in the alcoholic beverage matrix (Table 2). A compound is considered to have a contribution to the aroma profile when its OAV is greater than 1. The different OAV and FD values for the same compound are due to its different thresholds in the different matrixes.
Fourteen aroma compounds with OAVs ≥ 1 were found in distilled peach spirit, and 21 aroma compounds with OAVs ≥ 1 were identified in the pervaporation produced peach spirit. In both of the peach spirits, the OAVs of isoamyl acetate, ethyl hexanoate, 3-methyl-1-butanol,1,1-diethoxyethane, and (E)-β-damascenone were relatively higher than other compounds, and their FD factors were also markedly high, which certified the importance of these compounds to the overall aroma of the peach spirit. While phenethyl acetate, phenethyl alcohol and 3-methylbutanoic acid, with obviously high FD factors, had significantly low OAVs, even below 1.
Most esters contribute fruity and floral sensory properties to wines and spirits. The total concentration of all the esters in the pervaporation produced peach spirit was more than in the distilled peach spirit. In the distilled peach spirit, there were 5 esters having greater concentrations than their thresholds, while 8 esters were quantified in the pervaporation produced peach spirit with OAVs ≥ 1. The OAVs of isobutyl acetate, ethyl isovalerate, and ethyl hexanoate in the pervaporation produced peach spirit were significantly higher than those in the distilled peach spirit, especially ethyl hexanoate, which contributed sweet and fruity aroma. Sun et al. [18] reported that compared to the raw wine sample, the contents of ethyl acetate and ethyl hexanoate were concentrated 5.94 times and 1.74 times, respectively, after being separated by the pervaporation process. It indicated pervaporation process was probably positive to concentrate these esters. While ethyl butanoate and ethyl octanoate had higher OAVs in the distilled peach spirit compared to pervaporation produced peach spirit. A similar result was also reported in Cognac produced by Charente pot distillation, that ethyl octanoate and ethyl butanoate had high concentrations and important contributions to brandy [36]. Higher concentrations and OAVs of esters in the pervaporation produced peach spirit explained why the intensity of fruity property was stronger in the pervaporation produced peach spirit than in the distilled peach spirit. As mentioned above, esters preferred to pass through organophilic membranes and be received into the permeate to make spirits, especially short-chain esters with high hydrophobicity, which contribute to fruity scents [37].
Alcohols are mainly generated during the fermentation process. After separation, the total concentration of alcohols in the pervaporation produced peach spirit was significantly higher than that in the distilled peach spirit. Although about 10 alcohols were identified, and 5 and 7 alcohols were quantified in the distilled peach spirit and pervaporation produced peach spirit, respectively, only 3 alcohols had OAVs greater than 1. 3-Methyl-1-butanol (malty and nail polish-like) and 1-hexanol (green) presented OAVs as 7 and 3 in the distilled peach spirit, and as 7 and 6 in the pervaporation produced peach spirit. While isobutanol was only quantified in the pervaporation spirit with the OAV as 22, contributing malty aroma property. Combined with FD factors, the aroma contribution of these three compounds needs attention and further research. Although there were significant differences in the concentrations and OAVs of alcohols in the distilled peach spirit and pervaporation produced peach spirit, the intensities of malty and grass in the two peach spirits had no difference. The reason might be the perceptual interaction of alcohols with other aroma ingredients in a complex matrix environment [38].
Lactones were characteristic aroma compounds of peach and peach wine, contributing to peach and apricot odor. Lactones were mainly concentrated in the pervaporation produced peach spirit. A total of 4 lactones were quantified. Among them, although the FD factor of γ-butyrolactone was low, it was the most abundant lactone in both peach spirits, presenting a fruity aroma, and its content in pervaporation produced peach spirit was 5 times higher than in distilled peach spirit. γ-Decalactone with high FD factor had an OAV as 5 in pervaporation produced peach spirit, presenting apricot and peach aroma. The OAVs of γ-hexalactone and γ-nonanolactone were below 1. Therefore, γ-butyrolactone and γ-decalactone were the main peach-aroma contributors in peach spirit. The differences in lactone concentrations and OAVs between the two peach spirits were in agreement with the typical peach aroma property being stronger in the pervaporation produced peach spirit. Due to high carbon content and the presence of oxygen-containing heterocycles, lactones has high hydrophobicity, which account for the abundance of lactones in pervaporation produced peach spirit [37,39,40]. On the other hand, all these lactones had high boiling points (>200 • C), which resulted in being concentrated hardly by distillation. It implies that producing peach spirit by pervaporation membrane technology was beneficial to retaining the typical aroma of peach.
Terpenoids are important aroma compounds contributing to floral and sweet odor, with relatively low thresholds. Except dihydro-β-ionol, all the terpenoids identified by GC-O were quantified in the two peach spirits. A total of 6 terpenoids were quantified in distilled peach spirit. Among them, citronellol, (E)-β-damascenone, and linalool had OAVs greater than or equal to 1, presenting rose, sweet and floral notes, respectively. Based on the OAV and FD factors, (E)-β-damascenone was potentially the most critical aroma compound in the distilled peach spirit, and it was also suggested as an important aroma compound in Cognac by Uselmann and Schieberle [36]. A total of 3 terpenoids were quantified in pervaporation produced peach spirit, (E)-β-damascenone and eugenol having OAVs greater than 1. A comparative analysis of the terpenoids showed that citronellol and (E)-β-damascenone were more concentrated in the distilled peach spirit, with OAVs 20 times higher than those in the pervaporation produced peach spirit. The above results indicated that the contribution of terpenoids is greater in distilled peach spirit than in pervaporation produced peach spirit. The glycoside-bound form terpenoid is usually 2-8 times that of free form [41]. Acid hydrolysis under mild conditions and continuous heating could liberate volatile compounds from their glycosyl portion during the Charente pot distillation process [42,43]. For example, the heat treatment of diol precursors, as well as the thermal degradation of β-carotene in aqueous media, could form and additionally convert many norisoprenoids [43,44]. Moreover, in the previous research, (E)-β-damascenone has also been proved to be formed during the distillation process, and the concentration of (E)β-damascenone increased along with the distillation time [45]. Therefore, distillation process is benefit for terpenoids accumulation, while the terpenoid concentration in the pervaporation produced peach spirit was lower due to lack of heating treatment during pervaporation. However, eugenol showed a different accumulation pattern and was only detected in pervaporation produced peach spirit. This might be due to the fact that the chemical structure of eugenol contains a phenyl group, which is more hydrophobic than other terpenoids and facilitates concentrating by pervaporation [37,40].
A total of 7 volatile acids were quantified in the peach spirits, which mainly contributed to the acidic aroma. The concentration of each acid in the pervaporation produced peach spirit was significantly higher than those in the distilled peach spirit, except decanoic acid, which was only detected in the distilled peach spirit. In the distilled peach spirit, the OAVs of all the acids were less than 1. In the pervaporation produced peach spirit, the concentrations of 2-methylpropanoic acid and 3-methylbutanoic acid were the highest among the acids, with OAV of 8 and 4, respectively. The presence of high concentrations of 3-methylbutanol has also been reported in the production of the Brazilian spirit Cachaça using pervaporation [17]. Other acids had extremely low contributions and even made no effect on the overall aroma based on the OAVs. The above results indicated that, compared to other compound classes, acids presented little sensory attribute in the peach spirits. While the comparison between the two peach wines, because the pervaporation process was more conducive to the enrichment of acids, the acidic aroma property was significantly stronger in the pervaporation produced peach spirit. Franitza et al. [46] explained that as acids had higher boiling points than ethanol, losses would occur during the distillation process, while the acids were not affected by the boiling points during the pervaporation.
In the peach spirits, 1,1-diethoxyethane, the only detected acetal compound, had markedly high OAVs with concentrations of more than 4000 µg/L, presenting a sweet sensory attribute. As for furans, furfural was quantified with OAVs of 2 and 3 in the distilled peach spirit and pervaporation produced peach spirit, respectively. Although ethyl 2-furoate was also detected in the distilled peach spirit, the OAV was below 1. Six aromatic compounds with relatively high FD factors were quantified, but all of them had little aroma contribution according to the OAVs, except phenethyl acetate in the distilled peach spirit.

Aroma Reconstitution Test
To verify the aroma contribution of odorants with high OAVs to peach spirits, aroma reconstitution tests were carried out. Fourteen odorants with OAV ≥ 1 quantified in distilled peach spirit were dissolved at natural concentrations (Table 2) in 40% ethanol aqueous solution to simulate the original spirit. Similarly, 20 odorants with OAV ≥ 1 quantified in pervaporation produced peach spirit were also dissolved at natural concentrations (Table 2) in another 40% ethanol aqueous solution. Then the reconstituted solutions were evaluated by 16 trained panelists. As shown in Figure 3a, the results of ANOVA indicated that no significant difference existed between the reconstituted solution and distilled peach spirit. The aroma profile of reconstitution was similar to the distilled peach spirit, especially in the honey, malty, fruity, and cooked-apple aroma properties, while the intensities of the alcoholic, floral, and acidic aroma were a little lower than those in the original spirit. As shown in Figure 3b, the aroma profiles of pervaporation produced peach spirit and corresponding reconstitution were nearly overlapped with high similarity of alcoholic, malty, floral, and honey sensory. The intensities of grass and acidic aroma were slightly lower, and the intensities of cooked-apple and fruity were slightly stronger in reconstituted solution compared to the original spirit. There were 5 unknown compounds and several unquantified compounds being unconsidered in the aroma reconstitution test, especially some of them with high FD factors. In addition, the interactions among aroma compounds also influence the aroma profile. These might be the reasons for differentiations presented between the reconstitutions and original spirits.

Omission Test
Omission experiments were performed to investigate the importance of odorants with OAV ≥ 1 on the aroma profiles of distilled peach spirit and pervaporation produced peach spirit. Seventeen omission models and 25 omission models were prepared based on omission of a compound or a class of compounds to compare to the reconstituted models of distilled peach spirit and pervaporation produced peach spirit, respectively (Table 3).

Omission Test
Omission experiments were performed to investigate the importance of odorants with OAV ≥ 1 on the aroma profiles of distilled peach spirit and pervaporation produced peach spirit. Seventeen omission models and 25 omission models were prepared based on omission of a compound or a class of compounds to compare to the reconstituted models of distilled peach spirit and pervaporation produced peach spirit, respectively (Table 3).

PCA
To further investigate the key odorants to distinguish the spirits produced by the distillation process of pervaporation process, PCA analysis was performed using the concentrations of all of the key aroma compounds in the peach spirits ( Figure 4). The first principal component (PC1) and the second principal component (PC2) explained 93.4% and 5.1%, respectively, indicating these two principal components could represent the original data. Figure 4a showed that PC1 could separate the distilled peach spirit and pervaporation produced peach spirit well, distributed by the negative and positive half-axes, respectively. Ethyl hexanoate, ethyl isobutyrate, isobutyl acetate, 3-methylbutyl acetate, ethyl isovalerate, γ-decalactone, ethyl decanoate, isobutanol, 1-hexanol, γ-butyrolactone, 2-methylpropanoic acid, 3-methylbutanoic acid, eugenol, and furfural were positively corresponding to the aroma characterization of pervaporation produced peach spirit, while 1,1-diethoxyethane, phenethyl acetate, citronellal, (E)-β-damascenone, and ethyl octanoate were highly related to the overall aroma of distilled peach spirit. The distinctive aroma compounds and aroma profiles of peach spirits were formed by different separation methods.
To further investigate the key odorants to distinguish the spirits produced b distillation process of pervaporation process, PCA analysis was performed using th centrations of all of the key aroma compounds in the peach spirits ( Figure 4). Th principal component (PC1) and the second principal component (PC2) explained and 5.1%, respectively, indicating these two principal components could represe original data. Figure 4a showed that PC1 could separate the distilled peach spirit an vaporation produced peach spirit well, distributed by the negative and positive half respectively. Ethyl hexanoate, ethyl isobutyrate, isobutyl acetate, 3-methylbutyl a ethyl isovalerate, γ-decalactone, ethyl decanoate, isobutanol, 1-hexanol, γ-butyrola 2-methylpropanoic acid, 3-methylbutanoic acid, eugenol, and furfural were pos corresponding to the aroma characterization of pervaporation produced peach while 1,1-diethoxyethane, phenethyl acetate, citronellal, (E)-β-damascenone, and eth tanoate were highly related to the overall aroma of distilled peach spirit. The disti aroma compounds and aroma profiles of peach spirits were formed by different s tion methods.

Conclusions
The aroma profiles and aroma-active compounds of peach spirits produced b tillation and pervaporation were analyzed by sensory analysis, identification, quan tion and OAVs, and aroma recombinant and omission tests. The peach spirit produc distillation showed stronger property of cooked-apple, and the intensities of fruity, h and acidic were significantly higher in the peach spirit produced by pervaporation. AEDA with GC-O and GC-MS, 62 and 65 aroma-active compounds were identif distilled peach spirit and pervaporation produced peach spirit, respectively. A them, 14 and 20 compounds were considered as important contributors to aroma p of distilled peach spirit and pervaporation produced peach spirit, respectively, com with their concentrations and OAVs. Furtherly, both of the aroma profiles of di peach spirit and pervaporation produced peach spirit were reconstituted by corres ing important odorants successfully. At last, using omission tests, isoamyl acetate hexanoate, 1,1-diethoxyethane, 1-hexanol, (E)-β-damascenone, γ-butyrolactone phenethyl acetate were proved to be key aroma compounds in the peach spirit pro by distillation, while ethyl isobutyrate, ethyl isovalerate, isoamyl acetate, ethyl hexa ethyl octanoate, 1,1-diethoxyethane, 1-hexanol, (E)-β-damascenone, eugeno methylpropanoic acid, 3-methylbutanoic acid, γ-butyrolactone and γ-decalactone the key aroma compounds in the peach spirit produced by pervaporation. Moreov

Conclusions
The aroma profiles and aroma-active compounds of peach spirits produced by distillation and pervaporation were analyzed by sensory analysis, identification, quantification and OAVs, and aroma recombinant and omission tests. The peach spirit produced by distillation showed stronger property of cooked-apple, and the intensities of fruity, honey, and acidic were significantly higher in the peach spirit produced by pervaporation. Using AEDA with GC-O and GC-MS, 62 and 65 aroma-active compounds were identified in distilled peach spirit and pervaporation produced peach spirit, respectively. Among them, 14 and 20 compounds were considered as important contributors to aroma profiles of distilled peach spirit and pervaporation produced peach spirit, respectively, combined with their concentrations and OAVs. Furtherly, both of the aroma profiles of distilled peach spirit and pervaporation produced peach spirit were reconstituted by corresponding important odorants successfully. At last, using omission tests, isoamyl acetate, ethyl hexanoate, 1,1-diethoxyethane, 1-hexanol, (E)-β-damascenone, γ-butyrolactone and phenethyl acetate were proved to be key aroma compounds in the peach spirit produced by distillation, while ethyl isobutyrate, ethyl isovalerate, isoamyl acetate, ethyl hexanoate, ethyl octanoate, 1,1-diethoxyethane, 1-hexanol, (E)-β-damascenone, eugenol, 2-methylpropanoic acid, 3methylbutanoic acid, γ-butyrolactone and γ-decalactone were the key aroma compounds in the peach spirit produced by pervaporation. Moreover, the peach spirits produced by different separation processes also could be distinguished by the key aroma compounds. This study was the first work to identify the key odor-active volatile compounds in peach spirit through sensory approaches, and to compare the aroma profiles of peach spirits produced by distillation and pervaporation technology systematically. The results are beneficial to understanding the aroma characteristics of peach spirits comprehensively and provide a theoretical foundation for the production of peach spirits and the use of a new separation technique.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/foods11172598/s1, Table S1: The process condition parameters of pervaporation membrane in this study; Table S2: Calibration curves for volatile compounds analyzed in this study. Reference [18] is cited in the supplementary materials.