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. 2025 Aug 6;29:102867. doi: 10.1016/j.fochx.2025.102867

Volatile metabolomics reveals mechanisms of aroma enhancement in green tea via combination drying

Zhibin Ye a, Wenjing Huang a, Zhenbin Chen a, Mengying Zhang a, Shengmei Xie a, Jixin Zhang a, Huan Zhou a, Qian Xu b, Jingming Ning a,⁎,1
PMCID: PMC12356019  PMID: 40823138

Abstract

Drying is key to green tea aroma. Solvent-assisted flavor evaporation–gas chromatography–mass spectrometry and quantitative descriptive analysis were used to characterize the aroma profiles of sun-dried (SD), hot-air-dried (HD), and combination-dried (CD) green tea samples. Sun drying leads to the formation of sweet and floral aromas in green tea, whereas hot-air drying leads to the formation of malty, fatty, and roasted aromas. Gas chromatography–olfactometry and quantitative analysis were used to screen 24 key odorants, with an odor activity value of ≥1, primarily consisting of unsaturated fatty acid degradation products. These products included (Z)-4-heptenal, (E)-2-nonenal, (E,E)-2,4-heptadienal, (E,E)-2,4-decadienal, 1-octen-3-ol, nonanal, decanal, (E)-2-octenal, and octanal. Quantitative and in vitro simulation experiments revealed that sun drying mitigated the thermal degradation of unsaturated fatty acids, thereby reducing the production of fatty odorants. The results confirmed that the SD and CD samples had lower fatty odor intensity than the HD sample.

Keywords: Green tea, Combination drying, Metabolomics, Odor activity value, Fatty acid

Graphical abstract

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Highlights

  • •

    The combined-dried green tea with sun drying followed by hot-air drying was studied.

  • •

    Sun drying enhances the formation of sweet and floral aromas in green tea.

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    Hot-air drying enhances the formation of malty, roasted, and fatty aromas.

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    Thermal degradation of unsaturated fatty acids promotes the formation of fatty aromas.

1. Introduction

Tea, popular for its unique flavor, is also renowned for its antioxidant, anti-aging, antiseptic, and anti-inflammatory effects (Zhao et al., 2019). Green tea, widely produced in China, is made through the processes of spreading, fixing, rolling, and drying (Xia et al., 2014). The aroma of tea is a crucial criterion for consumers when assessing its quality. Green tea can have clean, floral, chestnut-like, bean-like, and roasted aromas (Tu et al., 2023), depending on its processing technology (Han et al., 2016).

Drying is crucial for the development of green tea aroma, with hot-air drying, sun drying, and pan frying being widely used methods (Wan et al., 2003). Each drying method is associated with unique aroma characteristics in green tea. Pan frying and hot-air drying are typically performed at high temperatures, which cause tea shoots to rapidly lose their water content. High-temperature drying produces volatile heterocyclic compounds with caramel-like and roasted aromas. These volatiles are produced by the Maillard reaction of reducing sugars and amino acids. For example, high-temperature drying produces 3-ethyl-2,5-dimethylpyrazine, which contributes to roasted and bean-like aromas in green tea. In contrast to hot-air drying, pan frying is conducted at higher temperatures, leading to the production of more heterocyclic compounds. Specifically, pan frying generates caramel-like odorants, while cauldron frying produces chestnut-like odorants (Tu et al., 2023). This chestnut-like aroma is attributed to a combination of eight odorants, including heptanal, benzaldehyde, and others (Zhu et al., 2018). Sun drying is a low-temperature drying method in which tea shoots are exposed to sunlight to enable them to slowly lose their water content. Pu-erh raw tea, as a kind of sun-dried (SD) green tea, tends to have a sweet and floral aroma (Pang et al., 2019). This aroma is attributable to the presence of floral odorants such as terpenes, which are produced at low-temperature (Yu et al., 2023). Research has identified terpenes and phenols with floral, sweet, and woody aromas as the primary odorants in pu-erh raw tea (Pang et al., 2019). Given these findings, temperature is a determining factor that influences the aroma profile of green tea. Selecting an appropriate drying method to enhance the quality of green tea and meet the needs of consumers remains a topic of interest.

Over the past decades, volatile metabolomics has emerged as a crucial approach for the analysis and identification of aromatic compounds in foods. By integrating qualitative and quantitative analyses with human sensory evaluations, volatile metabolomics has revealed the intricate relationships between processing methods and aroma profiles (Flaig et al., 2020). Wang, Ma, et al. (2020) extracted and identified the volatiles in Longjing tea. Using gas chromatography olfactometry (GC-O), they identified 14 odorants as key contributors to the aroma of Longjing tea, each with an odor activity value (OAV) of ≥1. They validated their results using aroma recombination. Similarly, Yin et al. (2023) compared the aroma profiles of yellow tea roasted at different temperatures through volatile metabolomics. Using in vitro simulations, they confirmed that theanine substantially contributed to the formation of pyrazines which were responsible for the roasted aroma of yellow tea.

Currently, research into green tea drying primarily focuses on single drying techniques such as hot-air drying, sun drying, and pan frying (Tu et al., 2023; Yu et al., 2023). During hot-air drying and pan frying, high temperatures cause the loss of floral odorants. Sun drying, typically performed at normal temperatures, requires an extended period, which leads to high moisture levels in the tea samples. These high moisture contents are not conducive to storage or preservation (Gulati et al., 2003). Therefore, a hybrid drying method that ensures high drying efficiency while retaining floral odorants is urgently needed. In this study, the same batch of rolled tea shoots was employed for sun drying, hot-air drying, and combination drying. Volatile metabolomics was employed to elucidate the mechanisms underlying the formation of aroma in green tea. In addition, the impact of the hybrid drying method on the aroma characteristics of green tea was examined. Overall, this study broadens consumers' choices of green tea aroma profiles and provides a basis for aroma-driven processing.

2. Materials and methods

2.1. Reagents and materials

(Z)-4-Heptenal, (E,E)-2,4-heptadienal, (E,E)-2,4-decadienal, β-ionone, β-damascenone, dimethyl sulfide, 3-methylbutanal, 1-octen-3-ol, 2-methylbutanal, linalool, (E)-2-nonenal, benzeneacetaldehyde, 2-ethyl-3,5-dimethylpyrazine, (Z)-jasmone, geraniol, (Z)-linalool oxide (pyranoid), nonanal, decanal, methional, (E)-2-octenal, nonanoic acid, phenylethyl alcohol, octanal, eugenol, ethyl decanoate, methyl oleate, methyl linoleate, and methyl linolenate were purchased from Shanghai Macklin Biochemical (Shanghai, China). β-Damascenone was purchased from Shanghai Haohong Biomedical Technology (Shanghai, China). Purity of all the above compounds was ≥99 %. The 50/30 μm divinylbenzene/carboxen/polydimethylsiloxane solid-phase microextraction (SPME) fiber was purchased from Supelco (Bellefonte, PA, USA).

2.2. Tea samples

Tea shoots (Camellia sinensis var. sinensis cv. Zhuye, one bud and three leaves) were harvested in April 2024 from Qimen County, Anhui Province, China. All processed shoots came from the same batch. The shoots were spread (24 °C ± 1 °C, 12 h, indoor), fixed (leaf temperature 100 °C ± 5 °C, 160 s, to achieve enzyme inactivation), and rolled (12 min) to obtain rolled tea shoots. They were then divided into three groups for drying. The first group of shoots were directly dried in a hot-air oven (100 °C, 40 min) to obtain a hot-air-dried (HD) tea sample. The second group of shoots were exposed to sunlight (ambient temperature 17 °C–31 °C, light intensity 30.2–121.9 klx) for 6 h until their moisture content decreasing to 30.30 %, and half of the sun-dried (SD) tea shoots were then placed in a hot-air oven (100 °C, 15 min) to obtain a combination-dried (CD) sample. The third group of tea shoots were subjected to an additional 5 h sun-drying treatment (ambient temperature 17 °C–31 °C, light intensity 30.2–121.9 klx) using the remaining partially SD shoots to obtain a fully SD sample. One kilogram of rolled tea shoots was used for each drying method. Tea shoots were fixed using a pan dryer (6CCT-110B, Huangshan Baiyun Machinery Co., Ltd.), rolled using a rolling machine (6CR-25, Nanning Chuangyu Tea Machinery Co., Ltd.), and dried using a hot-air oven (6CHZ-8B, Huangshan Qimen County Qita Tea Machinery Co., Ltd.).

2.3. Sensory evaluation based on Chinese national standards

An evaluation panel comprising 10 panelists (5 men and 5 women) with 5–48 years of experience in tea evaluation was formed. The evaluation procedure was based on the GB/T 23776–2018 standard (National Technical Committee 339 on Tea of Standardization Administration of China, 2018). Each sample (3.0 g) was brewed in 150 mL of boiling water for 5 min, then filtered, and subjected to aroma evaluation.

2.4. Extraction of volatiles and fatty acids

2.4.1. Extraction of volatiles by using headspace SPME (HS-SPME)

The protocol reported in the previous study (Feng et al., 2019) was adopted in this study. After the samples were brewed as mentioned earlier, the infusion was filtered and rapidly cooled. Next, 10 mL of the infusion was mixed with ethyl decanoate (5 μL, 5 μg/mL), stabilized for 15 min (40 °C), and adsorbed in headspace with an SPME fiber for 30 min, followed by rapid desorption at 250 °C at the inlet.

2.4.2. Extraction of volatiles by using solvent assisted flavor evaporation (SAFE)

After the tea samples were brewed as mentioned earlier, ethyl decanoate (1000 μg/mL, 2 μL) was added, and then the mixture was transferred to a SAFE apparatus for distillation under reduced pressure (40 °C, < 10−3 Pa). Next, volatiles were cooled to a solid state with liquid nitrogen in a collection device and allowed to stand at room temperature to obtain a liquid distillate. This distillate was extracted with dichloromethane (30 mL), and the lower layer was retained. The extraction process was repeated three times, with the lower layers from each extraction being combined. Finally, the extract was dewatered and then concentrated to 100 μL by nitrogen purging for gas chromatography–mass spectrometry (GC–MS) detection.

2.4.3. Extraction of fatty acids

The protocol reported in the previous study was used in this study (Huang et al., 2024). To 0.1 g of tea powder, 0.5 mL of reagent A (n-hexane/isopropanol, 3:2, v/v) and 0.25 mL of aqueous sodium sulfate (1:15, m/v) were added, mixed thoroughly, centrifuged, and the supernatant was retained. This procedure was repeated for the lower residue, and the two supernatants were combined. After the extract was dried using a nitrogen stream at 4 °C, 3 mL of reagent B (methanol/toluene/sulfuric acid, 44:5:1, v/v/v) was added, heated in a water bath at 80 °C for 1 h under sealed conditions, and then cooled. Then, 1 mL of heptane was added, mixed thoroughly, and allowed to separate into layers, with the upper layer retained. The procedure was repeated three times, and the volume was adjusted to 2 mL. Finally, the water was removed, and the solution was obtained.

2.5. GC–MS analysis

Regarding the determination of volatiles, modifications to the oven temperature and injection volume were made based on the method reported in the previous study (Huang et al., 2022). HP-5 ms and DB-FFAP columns were used. For HS-SPME, the column oven temperature increased from 40 °C (held for 3 min) to 160 °C at 4 °C/min and then to 250 °C at 12 °C/min (held for 2 min). For SAFE, the HP-5 ms column oven temperature increased from 40 °C (held for 5 min) to 120 °C at 5 °C/min, to 220 °C at 4 °C/min, and to 280 °C at 12 °C/min (held for 5 min); the DB-FFAP column oven temperature increased from 40 °C (held for 5 min) to 150 °C at 6 °C/min and then to 230 °C at 4 °C/min (held for 15 min). The injection volume of the SAFE samples was 3 μL.

Regarding the determination of fatty acids, the method was consistent with the previous study (Huang et al., 2024). The retention indices (RIs) were determined through the analysis of retention times of all compounds and a homologous series of n-alkanes (C6–C40) using HP-5 ms and DB-FFAP capillary columns. Compounds were identified by matching their RIs with those listed in the NIST Chemistry WebBook (https://webbook.nist.gov) and in the report (Huang et al., 2022), and their mass spectra were compared against the NIST 2023 database in MSD ChemStation (match score ≥ 70).

2.6. Odorant identification through GC-O-MS

Volatiles were extracted using the two methods described in 2.4.1, 2.4.2. The effluent from the column was split 1:1 between the MS detector (5977B) and the olfactory detection port (ODP4; Gerstel, Mülheim, Germany), operating under the same conditions as those used in the GC–MS analysis.

Odorants were identified using the method reported in the previous study (Wu et al., 2025). A panel consisting of seven panelists with experience in aroma identification conducted a GC-O-MS-based experiment. Each evaluator was given time to prepare for a sniffing evaluation before the GC-O-MS program was run. Panelists were required to record data on retention time, odor quality, and sniffing intensity. Sniffing intensity was quantitatively scored from 0 to 5, with 0.5 points for each interval. An odor was considered to be present if it was identified by at least four out of the seven panelists. The aroma intensity (AI, %) was calculated by dividing the average sniffing intensity of all panelists by the maximum scale (“5”). The frequency of detection (F, %) represents the percentage of panelists who detected the odorant. The modified detection frequency (MF, %) was calculated as follows (Gu et al., 2022).

MF%=AI%×F% (1)

2.7. Quantitative analysis of volatiles and fatty acids

The protocol reported in the previous study was adopted in this study (Wu et al., 2025). A calibration curve was constructed with the ratio of the target compound's peak area to the internal standard's peak area on the vertical axis and the ratio of their concentrations on the horizontal axis. Briefly, the volatiles identified by GC-O-MS were screened as potential key volatiles. The relative contents of these volatiles were used as a reference to prepare ethanol solutions with different concentrations. Assuming the relative content of the target compound is C, the calibration curve includes a series of concentrations: 1/4C, 1/2C, C, 2C, and 4C. A specific amount of ethyl decanoate was added to each solution as an internal standard. For SPME calibration, ethyl decanoate (5 μg/mL, 5 μL) was mixed with 10 mL of a mixed standard solution, and the mixture was subjected to HS-SPME extraction followed by GC–MS analysis to obtain the calibration curve. For SAFE calibration, the concentration of ethyl decanoate in the mixed standards was increased to 20 μg/mL and directly injected. Detection conditions were the same as in Section 2.5.

Fatty acids were quantitatively analyzed using the external standard method. Briefly, methylated fatty acids were dissolved in ethanol to obtain a series of solutions with different concentration gradients. Detection conditions were the same as in Section 2.5.

2.8. Aroma quantitative descriptive analysis (QDA) and addition experiments

SD, CD, and HD samples were subjected to an aroma QDA. The same evaluation panel sniffed each infusion and described its best matching aroma profile. The panel reached a consensus regarding six profiles: sweet, floral, fatty, roasted, citrus-like, and malty. To help the evaluation panel better understand the properties of these profiles, phenylethyl alcohol (1400 μg/L), β-ionone (0.21 μg/L), (E)-2-nonenal (4 μg/L), 2-ethyl-3,5-dimethylpyrazine (0.4 μg/L), geraniol (32 μg/L), and 2-methylbutanal (15 μg/L) were identified as the compounds responsible for sweet, floral, fatty, roasted, citrus-like, and malty aromas, respectively. The intensity of each aroma profile was expressed on a scale of 0 to 5, with 0.5 points for each interval.

An addition experiment was conducted to validate the accuracy of the methodology and findings. The key volatile compounds added were those with an OAV of ≥1. Of the 24 odor-active compounds (OAV ≥ 1), 11 were more abundant in SD sample and 13 in HD sample. Each compound was spiked into the three tea samples at differing levels so that all final concentrations were standardized to the highest value observed across the tea samples. Finally, a QDA was conducted.

2.9. Simulation experiments

Three groups of simulation experiments were conducted depending on the contents of unsaturated fatty acids (USFAs) detected in the rolled tea shoots. A certain concentration of oleic acid (0.50 mg), linoleic acid (LA, 1.00 mg), and α-linolenic acid (ALA, 2.82 mg) was prepared and dissolved in 0.5 mL of ethanol. Subsequently, the mixture was volume-determined to 10 mL and transferred to a headspace flask to ensure that its concentration was identical to that of the USFAs detected in the rolled tea shoots. Simulation experiments were conducted using oven heating. Three drying conditions were simulated: SD (24 °C, 11 h), CD (24 °C for 6 h followed by 100 °C for 15 min), and HD (100 °C, 40 min). The same HS-SPME adsorption method and detection conditions were used as described in Section 2.4.1 and Section 2.5.

2.10. Data analysis

The results were expressed as the mean ± standard deviation for three replicates. Qualitative and quantitative analyses of compounds were performed using MSD ChemStation (version E.02.02; Agilent, Santa Clara, CA, USA). Multivariate statistical analysis was conducted using SIMCA (version 14.1; Sartorius, Göttingen, Niedersachsen, Germany). All statistical analyses were conducted using one-way analysis of variance in IBM SPSS Statistics (version 26.0; IBM, Armonk, NY, USA). Visualization charts were created using Chiplot (https://www.chiplot.online) and Origin (version 2024; OriginLab, Northampton, MA, USA).

3. Results and discussion

3.1. Sensory evaluation based on Chinese national standards

After the evaluation panel evaluated the aromas of the three green tea samples, they indicated that each sample had a unique aroma: a sweet and floral aroma in the SD sample, a chestnut-like aroma in the HD sample, and a sweet and chestnut-like aroma in the CD sample. The aroma evaluation scores for the SD, CD, and HD samples were 86.10 ± 1.10, 87.80 ± 1.03, and 85.50 ± 1.51, respectively. The SD and HD samples present distinct aroma profiles, mainly resulting from their different drying methods (Pang et al., 2019; Tu et al., 2023). The CD sample achieved the highest aroma evaluation score, as it integrates the superior characteristics of both SD and HD samples. To investigate the causes of aroma differences among the three tea samples, the volatiles were first analyzed by GC–MS, and key odorants were then identified by GC-O and quantification.

3.2. Effect of drying method on volatiles in tea samples

SAFE can mitigate the loss of heat-sensitive volatiles under vacuum, which is suitable for extracting trace compounds. HS-SPME is suitable for extracting volatiles with high volatility through headspace adsorption. Combining SAFE with HS-SPME allows for adequate extraction of tea volatiles (Flaig et al., 2020).

In this study, 86 volatiles were identified in the samples. Table S1 presents the characteristics of these substances. The total relative concentrations of volatiles were 645.56 ± 21.73 μg/L in the SD sample, 592.01 ± 31.76 μg/L in the CD sample, and 461.40 ± 28.85 μg/L in the HD sample, with no significant difference (p ≥ 0.05) observed between the SD and CD samples. The total relative concentrations of volatiles in the SD and CD samples were significantly higher (p < 0.05) than those in the HD sample, indicating that the loss of volatiles in the CD sample was lower than that in the HD sample due to its shorter period of high-temperature drying. Depending on their chemical characteristics, 86 volatiles were classified into seven categories (Fig. 1): aldehydes (25), heterocyclic compounds (16), alcohols (16), ketones (11), esters (8), acids (5), and other substances (5). Among these seven categories, alcohols were the most prevalent (38.19 %–53.56 %), followed by heterocyclic compounds (15.01 %–29.56 %), aldehydes (11.60 %–12.57 %), acids (7.38 %–9.41 %), esters (3.67 %–6.22 %), ketones (2.78 %–4.41 %), and other substances (0.60 %–4.16 %). In terms of the relative total concentration of aldehydes, alcohols, esters, and acids, the SD sample exhibited the highest level, the CD sample had an intermediate level, while the HD sample showed the lowest level (Table S1). In the HD sample, a large proportion of heterocyclic substances was observed, primarily attributable to the Maillard reaction induced by the prolonged and high-temperature drying method of the HD sample (Guo et al., 2018).

Fig. 1.

Fig. 1

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Relative contents of volatile compounds in different categories and their percentages in the SD, CD, and HD samples. The number of volatile compounds in each category is shown.

To further understand the effect of the drying method on the volatiles of the samples, a multivariate statistical analysis was conducted on the volatile metabolite contents of different samples. Principal component analysis (Fig. 2A) revealed that each drying method resulted in a distinct aroma profile. Hierarchical cluster analysis (Fig. 2B) indicated that all samples belonged to three groups, with the SD and CD samples clustering into a single group, suggesting that the volatile metabolite profile of the CD sample was similar to that of the SD sample. Of the 86 volatiles identified, 82 were detected in all three tea samples, while only 4 were unique to the CD and HD samples, namely 4-methyl-3-penten-2-one, 5-methyl-2-furancarboxaldehyde, 2-ethyl-3,5-dimethylpyrazine, and 3,4-dimethyl-3-pyrrolin-2-one (Fig. 2C), whose formation may be attributable to prolonged high-temperature drying. Pairwise comparisons of volatile content indicated that low-temperature drying was more effective in preserving aromatic compounds (Fig. 2D). In summary, the total relative contents of volatiles in the HD sample were lower than those in the SD and CD samples, which may be attributable to the high-temperature drying process of the HD sample.

Fig. 2.

Fig. 2

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(A) Principal component analysis score plot. (B) Hierarchical cluster analysis plot. (C) Venn diagram of the total number of volatile compounds in the SD, CD, and HD samples. (D) Comparison of volatile compound concentrations in the SD, CD, and HD samples.

3.3. Effects of different drying methods on volatiles in tea samples

Of the 25 aldehydes identified, 14 compounds with 6–10 carbon atoms were detected in the tea samples, including nonanal (Fig. 3). The higher drying temperature and longer drying time of the HD sample (100 °C and 40 min, respectively) compared with the SD sample (17 °C–31 °C and 15 min, respectively) may have led to the formation of aldehydes in the HD sample through the thermal degradation of USFAs. Moreover, the highest contents of (Z)-3-hexenal and hexanal were detected in the HD sample, which may be attributable to the thermal degradation of ALA and LA to aldehydes under hot-air drying conditions (Ho et al., 2015). Xu et al. (2020) reported that hexanal, 3-octen-2-one, and (E,E)-3,5-octadien-2-one, as typical degradation products of USFAs, contributed to the bean-like aroma of pulse protein isolates. High concentrations of these compounds were detected in the HD samples, which may indicate their role in the transition to a chestnut-like aroma. The low concentration of (E)-2-pentenal and high concentration of 2-methylfuran detected in HD samples may be due to the oxidation-catalyzed formation of 2-methylfuran from (E)-2-pentenal (Adams et al., 2011). Therefore, various oxidative degradation products, such as aldehydes and furans, may be produced during hot-air drying. The biosynthetic precursors of methyl jasmonate, (Z)-jasmone, and jasmine lactone are also USFAs (Zeng et al., 2018). These substances were detected in high concentrations in the SD sample and low concentrations in the CD and HD samples. This discrepancy may be attributable both to the blockage of the biosynthesis pathway with the weakening of enzyme activity during the fixation and to the loss of these substances during the drying process of the CD and HD samples.

Fig. 3.

Fig. 3

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Heatmap of the relative content of each volatile compound in the SD, CD, and HD samples. The size of each circle corresponds to the relative content of the respective compound. Colors differentiate categories of volatile compounds.

Volatiles in glycosidically bound form are less likely to be released compared with those in the free form. During tea processing, enzymes in damaged tea tissues hydrolyze glycosidic bonds upon contact with their substrate, releasing volatiles. 2-Phenylethanol, phenylacetaldehyde, benzaldehyde, benzyl alcohol, geraniol, linalool, (Z)-3-hexenol, methyl salicylate, linalool oxides, β-damascenone, furaneol, and other compounds are biosynthesized through enzymatic reactions either during the spreading process or at the derivatization stage (Ho et al., 2015; Yang et al., 2013). In this study, endogenous enzymes were inactivated, and the enzymatic hydrolysis pathways were blocked after the fresh tea shoots were fixed. Among all three tea samples, the SD sample contained the highest concentrations of these compounds, likely because of their accumulation during drying at normal temperatures and the lower velocity of airflow at the surface of the leaf.

Carotenoid-derived aromatic compounds are produced by enzymatic oxidation, thermal degradation, or photo-oxidation of carotenoids during tea processing. β-Damascenone and β-ionone are primarily produced by the degradation of β-carotene. In addition, β-cyclocitral, safranal, α-ionone, ketoisophorone, geranylacetone, and dihydroactinidiolide are produced by the thermal degradation or photo-oxidation of carotenoids (Kawakami & Kobayashi, 2002). The contents of these compounds were higher in the SD and CD samples, which is likely attributable to the fact that sun drying and combination drying play a role in the formation of carotenoid-derived aromatics through photo-oxidation. Although drying at 100 °C promotes the thermal degradation of carotenoids, the high-temperature drying process of the CD and HD samples may have led to the volatilization of aromatic substances, and the loss of aroma in the HD samples was increased by prolonging the drying time at 100 °C.

The Maillard reaction refers to chemical reactions between amino acids and carboxylic compounds at ambient or high temperatures. During the reaction, the generated α-dicarbonyl compounds react with α-amino acids and undergo Strecker degradation. In green tea, the majority of heterocyclic substances, such as furans, pyrroles, pyrans, and pyrazines, are generated by the Maillard reaction (Li et al., 2022). In this study, the high temperatures used in the hot-air drying of the CD (3.28 ± 0.53 μg/L) and HD (8.83 ± 0.12 μg/L) samples may have facilitated a thermal degradation reaction leading to the accumulation of dimethyl sulfide (Table S1). Strecker degradation of the precursors S-methylmethionine and methionine produces dimethyl sulfide (Yu et al., 2022). Similarly, Strecker degradation produces 2-methylbutanal, 3-methylbutanal, 2-methylpropanal, phenylacetaldehyde, indole, and methional (Zhai et al., 2022). Yang, Chen, et al. (2022) reported that the concentrations of 1-ethyl-1H-pyrrole, 2-methylfuran, and 1-ethyl-1H-pyrrole-2-carboxaldehyde, substantially increased during drying because of the Maillard reaction, consistent with the results of the present study. In addition, significant (p < 0.05) differences in the concentrations of the heterocyclic compounds 3-methylfuran and 3,4-dimethyl-3-pyrrolin-2-one were observed in the SD (0.06 ± 0.01 μg/L, not found), CD (0.10 ± 0.01 μg/L, 34.98 ± 2.00 μg/L), and HD (0.13 ± 0.00 μg/L, 48.39 ± 3.35 μg/L) samples (Table S1). The high concentrations of these two compounds in the tea samples with high-temperature drying may be attributed to the Maillard reaction. Guo et al. (2018) reported that 4-methyl-3-penten-2-one was produced after the full fire processing of oolong tea at 150 °C.

In the present study, 4-methyl-3-penten-2-one was detected only in the CD and HD samples, but not in the SD sample, suggesting that this compound is typically generated at high temperatures. Notably, the concentrations of the five organic acids were lowest in the HD sample and highest in the SD sample, which may be attributable to degradation for extended periods at high temperatures.

In conclusion, the prolonged low-temperature drying process reduces the loss of volatiles derived from two distinct pathways: those biosynthesized during the growth and development of fresh tea shoots, and those released through glycosidic bond cleavage by glycosidases. This process retains most of the aroma, including the sweet and floral volatiles that positively contribute to tea's aroma profile (Pang et al., 2019). However, it also retains low-boiling-point substances like 1-octen-3-ol, which may negatively impact the aroma but contribute to the formation of sun-dried flavor. Sun drying, due to its lower temperature, cannot generate the characteristic volatile compounds produced at high temperatures. Instead, hot-air drying promotes the formation of Maillard reaction products, such as nitrogen-containing heterocyclic compounds (Guo et al., 2018). Four volatiles specific to CD and HD samples can serve as potential differentiators between SD and HD samples. Additionally, it was observed that most of the fatty unsaturated aldehydes accumulate in the HD sample, which may be a characteristic feature of high-temperature drying.

3.4. Screening of key compounds by GC-O and quantitative analysis

To identify potential odor-active compounds, 34 odorants were further characterized using GC-O, following their initial identification via characteristic ions and RIs on two different capillary columns (Tables S2 and S3). Depending on their aroma characteristics, the 34 compounds were classified into seven categories, namely sweet, malty, fatty, unpleasant, roasted, citrus-like, and floral. These compounds were subsequently quantified, and their OAVs were calculated. Odorants with OAVs of ≥1 are considered to be odor-active (Wang, Ma, et al., 2020). In this study, the same criterion for odor activity was adopted. Twenty-four odor-active compounds (Table S4), including seven fatty odorants ((E,E)-2,4-heptadienal, (E,E)-2,4-decadienal, 1-octen-3-ol, (E)-2-nonenal, decanal, (E)-2-octenal, and octanal), five sweet odorants (dimethyl sulfide, benzeneacetaldehyde, phenylethyl alcohol, eugenol, and β-damascenone), three roasted odorants ((Z)-linalool oxide, 2-ethyl-3,5-dimethylpyrazine and methional), three citrus-like odorants (linalool, nonanal, and geraniol), two malty odorants (3-methylbutanal and 2-methylbutanal), two floral odorants ((Z)-jasmone and β-ionone), and two unpleasant odorants ((Z)-4-heptenal and nonanoic acid), were screened. The following OAVs were obtained: dimethyl sulfide (SD, < 1; CD, < 1; HD, 2), 3-methylbutanal (SD, < 1; CD, < 1; HD, 1), 2-methylbutanal (SD, < 1; CD, < 1; HD, 1), (Z)-4-heptenal (SD, 227; CD, 233; HD, 248), methional (SD, 2; CD, 3; HD, 3), 1-octen-3-ol (SD, 25; CD, 12; HD, 2), octanal (SD, < 1; CD, 1; HD, 1), (E,E)-2,4-heptadienal (SD, 158; CD, 204; HD, 262), benzeneacetaldehyde (SD, 15; CD, 12; HD, 5), (E)-2-octenal (SD, 2; CD, 2; HD, 4), 2-ethyl-3,5-dimethylpyrazine (SD, not detected; CD, 10; HD, 11), linalool (SD, 19; CD, 17; HD, 11), nonanal (SD, 2; CD, 3; HD, 4), phenylethyl alcohol (SD, 1; CD, 1; HD, 1), (E)-2-nonenal (SD, 13; CD, 14; HD, 16), (Z)-linalool oxide (SD, 6; CD, 5; HD, 5), decanal (SD, 2; CD, 3; HD, 4), geraniol (SD, 8; CD, 6; HD, 5), nonanoic acid (SD, 2; CD, 1; HD, < 1), (E,E)-2,4-decadienal (SD, 58; CD, 60; HD, 62), eugenol (SD, 1; CD, 1; HD, 1), β-damascenone (SD, 60; CD, 38; HD, 31), (Z)-jasmone (SD, 10; CD, 10; HD, 9), and β-ionone (SD, 53; CD, 44; HD, 38). Odorants with an MF greater than 70 % were considered to significantly contribute to the overall aroma (Gu et al., 2022). As shown in Fig. 4 and Table S2, seven of the 24 previously identified odor-active compounds made significant contributions, namely (Z)-4-heptenal, methional, 1-octen-3-ol, (E,E)-2,4-heptadienal, benzeneacetaldehyde, maltol, and phenylethyl alcohol. These compounds exhibited an MF greater than 70 % across all samples.

Fig. 4.

Fig. 4

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Odorants, their odor properties, and their MFs for the SD, CD, and HD samples. Odorants are classified into seven categories depending on their aromatic properties. The size of each circle corresponds to the magnitude of the MF. nd, compound not detected by GC-O.

Fatty odorants account for the largest fraction of odorants in green tea. Similarly, fatty odorants constituted 43 % of the odorants with an MF greater than 70 %, suggesting that these odorants are integral to the aroma composition of green tea. The fatty odorants include 1-octen-3-ol, octanal, (E,E)-2,4-heptadienal, (E)-2-octenal, (E)-2-nonenal, decanal, and (E,E)-2,4-decadienal. In this study, with the exception of 1-octen-3-ol, these compounds were detected at high concentrations in the HD sample. The high concentration of 1-octen-3-ol in the SD sample may indicate that it is a potential odorant with sun-dried odor characteristics. Drying at high temperatures led to the thermal degradation of USFAs, producing a series of unsaturated aldehydes. All six unsaturated aldehydes had high OAVs and fatty aromas. (Z)-4-Heptenal (OAV = 227–248, fish-oil-like) is also an unsaturated aldehyde, but it has an unpleasant odor and had the largest OAV among all odor-active compounds. This also suggests that tea aroma is a coordinated state achieved by a variety of odorants.

Many odorants contribute to the sweet aroma of green tea, such as dimethyl sulfide (corn-like), benzeneacetaldehyde (honey-like and rose-like), phenylethyl alcohol (honey-like and rose-like), and β-damascenone (apple-like). Among these compounds, β-damascenone (OAV = 31–60) and benzeneacetaldehyde (OAV = 5–15) are the strongest contributors to this sweet aroma. β-Damascenone, primarily produced from carotenoids or glycosides (Ho et al., 2015; Kawakami & Kobayashi, 2002), is one of the major contributors to the key characteristic aroma of most teas (Tao et al., 2022). These two compounds were detected in high concentrations in the SD sample, indicating that sun drying may have facilitated the formation of β-damascenone through photo-oxidation, while its lower drying temperature reduced the volatilization of benzeneacetaldehyde. Notably, benzeneacetaldehyde and phenylethyl alcohol, with MF exceeding 70 %, significantly influence the aroma profile of green tea, presenting rose-like and honey-like olfactory characteristics. This indicates that these compounds, given their high concentrations and unique sensory traits, are likely to be more easily perceived and distinguished through olfactory means, thus contributing prominently to the sensory experience of green tea.

A roasted aroma is a typical characteristic of HD or pan-fried green tea (Tu et al., 2023). (Z)-Linalool oxide (pyranoid) (earthy), methional (cooked potato-like), and 2-ethyl-3,5-dimethylpyrazine (roasted) were identified as the main contributors to the roasted aroma of the tea samples. Flaig et al. (2020) analyzed the aromatic extracts of Longjing tea infusions and discovered that methoxypyrazines and pyrazines were the volatiles contributing to the earthy aroma of these infusions. 2-Ethyl-3,5-dimethylpyrazine was detected, which is formed via the Maillard reaction between alanine or threonine and glucose at high temperatures (Li et al., 2022). This compound was detected in the CD and HD samples, but not in the SD sample. Notably, the OAV of this compound was ≥1 in both the CD and HD samples, indicating the key role that high-temperature drying plays in the production of pyrazine as the main roasted odorant.

Linalool (citrus-like), nonanal (citrus-like and soapy), and geraniol (citrus-like) are compounds with a citrus-like aroma. Linalool and geraniol are monoterpene alcohols. In tea shoots, they are released by their synthase enzymes from their geranyl pyrophosphate precursors. Linalool and geraniol are both abundant in tea (Feng et al., 2019). Wang, Ma, et al. (2020) reported that linalool and geraniol accounted for the majority of alcohols in Longjing green tea, with both having odor-active properties in tea infusions. Schuh and Schieberle (2006) indicated that, among all alcohols, geraniol and linalool had the highest OAVs in Darjeeling black tea. These compounds have made aroma contributions in both green and black tea studies, and their different concentrations and ratios enable them to have different effects on different types of tea. In this study, geraniol was present at higher concentrations in tea samples than linalool. However, due to linalool's lower threshold, it exhibited a higher OAV than geraniol.

Both 2-methylbutanal and 3-methylbutanal have a malty aroma. These aldehydes are produced from the Strecker degradation. According to the literature, 3-methylbutanal is a volatile compound that is crucial for the chestnut-like aroma of green tea during the stages of spreading, fixing, and drying (He et al., 2023; Wang, Hua, et al., 2020; Yang, Qian, et al., 2022). Hence, Strecker degradation may shift the aroma of tea to a chestnut-like aroma, causing the aroma of HD tea to differ from that of SD tea.

3.5. QDA and aroma addition experiments

To better understand the aroma characteristics of the tea samples, a QDA with the same brewing method outlined in Section 2.3 was performed. As shown in Fig. 5A, highly significant differences (p < 0.01) in the sweet, fatty, roasted, and floral aromas; significant differences (p < 0.05) in the malty aroma were observed among the three tea samples. Sun drying promoted the formation of a sweet and floral aroma, primarily because of the accumulation of floral odorants such as β-ionone and (Z)-jasmone and sweet odorants such as benzeneacetaldehyde and β-damascenone. In addition, hot-air drying promoted the formation of malty, roasted, and fatty aromas, which were attributable to the generation of methylbutanals, 2-ethyl-3,5-dimethylpyrazine, and fatty aldehydic odorants during the drying process.

Fig. 5.

Fig. 5

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(A) QDA radar map of the SD, CD, and HD samples. ⁎Significant (p < 0.05). ⁎⁎Highly significant (p < 0.01). (B) QDA radar map of the SD, CD, and HD samples after the addition of 24 odor-active compounds.

To validate the accuracy of the GC-O and quantitative analysis results, an aroma addition experiment was conducted. Depending on the odor-active compound with the highest content in the tea samples, a certain amount of this compound was added to each tea infusion to reach a balance. Subsequently, QDA was performed again. As shown in Fig. 5B, no significant differences (p ≥ 0.05) were observed between the aroma profiles of the three samples after the added compounds were included.

3.6. Effect of the thermal degradation of fatty acids on aroma

Thermal degradation of USFAs produces aldehydes, alcohols, ketones, and other substances. In green tea, the majority of fatty acids are unsaturated (Bhuyan et al., 1991). In order to confirm the effects of different drying methods on fatty acid degradation and to explore the origins of fatty odorants as the primary odor-active compounds, the degradation of USFAs in tea samples was simulated under different drying conditions. The USFA with the highest concentration in green tea was identified as ALA (4.55–5.33 mg/g), followed by LA (1.62–1.85 mg/g) and oleic acid (0.67–0.95 mg/g) (Table S5 and Fig. 6A–C). A significant difference was observed among the USFA concentrations of the SD, CD, and HD samples (p < 0.05). Each sample exhibited a different degree of degradation relative to the rolled tea shoots. After drying, it was found that the CD and HD samples had more USFA degradation compared with the SD sample. These results indicated that high-temperature hot-air drying promoted the degradation of the three USFAs in the CD and HD samples. The results showed that the three USFAs in the SD sample were stable and that sun drying at ambient temperatures (17 °C–31 °C) was insufficient to induce major degradation. Next, an ethanol solution containing these fatty acids was dissolved in an aqueous substrate, and the simulation procedure was completed in an oven. A total of 15 aldehydes, 2 ketones, and 1 alcohol were identified (Fig. 6D). All 18 compounds were detected in the samples, except for (E,Z)-2,6-nonadienal. These simulation results were consistent with those from prior content analyses. Of these compounds, seven were identified as odor-active compounds with a fatty aroma, namely (E,E)-2,4-heptadienal, (E,E)-2,4-decadienal, (E)-2-nonenal, nonanal, decanal, (E)-2-octenal, and octanal. Collectively, these findings indicate that the high temperatures used in hot-air drying promote the degradation of USFAs and the formation of key fatty odorants in green tea.

Fig. 6.

Fig. 6

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(A–C) Concentrations of three unsaturated fatty acids in the rolled tea shoots (RS) and in the SD, CD, and HD samples. (D) Heatmap of the relative contents of 18 volatile compounds produced from the thermal degradation of oleic acid, linoleic acid, and α-linolenic acid.

4. Conclusion

In this study, the aroma profiles of SD, CD, and HD samples were explored. The results revealed that each drying method had a unique effect on the aroma of green tea. QDA revealed that the SD sample had a strong sweet and floral aroma, whereas the HD sample had a distinctive malty, roasted, and fatty aroma. The total relative concentrations of volatiles were found to be similar in the CD and SD samples, indicating that brief hot-air drying may increase the efficiency of drying while maintaining the aroma quality of SD green tea. GC-O and quantitative analyses identified 24 odorants with OAVs ≥1. These odorants primarily contributed to a fatty aroma, and they were confirmed via an aroma addition experiment as the main difference between the three tea samples. In vitro thermal simulation experiments confirmed that these fatty odorants were mainly produced from the thermal degradation of USFAs at a high temperature and that they were the main odorants contributing to the strong fatty aroma of HD green tea.

This study showed that the combination drying method can significantly enhance productivity while preserving the distinctive sweet and floral aromas characteristic of sun-dried green tea. However, the optimal extent of sun drying to maximize economic benefits remains a topic that warrants further investigation. Additionally, the role of fatty odorants in shaping and enhancing the aroma profiles of various green teas requires deeper exploration. In summary, this study offers a practical guide for selecting appropriate green tea drying methods.

CRediT authorship contribution statement

Zhibin Ye: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Wenjing Huang: Validation, Formal analysis. Zhenbin Chen: Data curation. Mengying Zhang: Supervision, Formal analysis. Shengmei Xie: Visualization. Jixin Zhang: Visualization. Huan Zhou: Visualization. Qian Xu: Resources. Jingming Ning: Writing – review & editing, Resources, Project administration, Funding acquisition, Conceptualization.

Ethical statements

The study protocol was reviewed and approved by the Human Sensory Ethics Review Committee of Anhui Agricultural University. All participants in the sensory experiments provided their informed consent. Additionally, they were given the option to withdraw from the experiment at any time. Personal information was protected, and no sensory data were released without authorization from the participants.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported from the National Key Research & Development Program (2021YFD1601102) and the earmarked fund for Agriculture Research System of China (CARS-19).

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.fochx.2025.102867.

Appendix A. Supplementary data

Supplementary material

mmc1.docx (90.8KB, docx)

Data availability

Data will be made available on request.

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Associated Data

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Supplementary Materials

Supplementary material

mmc1.docx (90.8KB, docx)

Data Availability Statement

Data will be made available on request.

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