Characterization of the Key Aroma Compounds in Different Aroma Types of Chinese Yellow Tea

Yellow tea is one of the six major tea categories in China. The floral fragrance type, high-fired fragrance type, fresh fragrance type, and corn-like fragrance type were the main aroma types of yellow tea screened by QDA. A total of 103 volatiles were identified in yellow teas by HS-SPME/GC-MS analysis. Using multivariate analysis and OAVs, forty-seven aroma compounds were identified as key aroma compounds for the formation of different aroma types of yellow teas. Among them, 8, 14, 7, and 18 key aroma compounds played an important role in the formation of aroma characteristics of floral fragrance, high-fired fragrance, fresh fragrance, and corn-like fragrance types of yellow teas, respectively. Furthermore, PLS analysis revealed that 12 aroma compounds were the key contributors to the ‘floral and fruity’ and ‘sweet’ attributes, five aroma compounds contributed to the ‘roasted’ attribute, and four aroma compounds related to the ‘fresh’ and ‘grassy’ attributes. This study provides new insights into the aroma characteristics formation of different aroma types of yellow teas and will provide a valuable theoretical basis for improving the flavor quality of yellow tea during the manufacturing process.


Introduction
Tea is a kind of widely consumed beverage ranking second only to water around the world [1]. Yellow tea is one of the six major tea categories in China, which is famous for its unique appearance and flavors, such as yellow dry tea, yellow infusion, and yellow brewed leaves that have an elegant fragrance and smooth taste. Meanwhile, yellow tea also possesses several healthy benefits for humans, such as anti-cancer, anti-bacterial, antioxidant, and gastrointestinal protecting effects [2][3][4]. The main manufacturing process of yellow tea are fixing, rolling, sealed yellowing, and drying. Among them, sealed yellowing is the key processing stage that contributes to the typical flavor characteristic formation of yellow tea [2]. Although yellow tea is increasingly deeply loved by consumers, its key aroma compounds that responsible for the special aroma formation are still unknown. It is difficult to establish flavor directed processing technology for yellow tea. Therefore, the study of yellow tea, particularly, research on its key aroma compounds for this special aroma, has important value for improving the flavor quality of yellow tea during the manufacturing process.
Tea aroma is a critical criterion to evaluate the overall quality of yellow tea [4]. Many aroma components were identified in yellow tea by previous studies, but different yellow tea possesses a wide range of key aroma compounds. This might result from the different raw materials, processes, and parameters of different yellow teas. The detailed information is summarized in Table S1. Our previous study indicated that different levels of tenderness of tea leaves will form the different characteristic aromas and key aroma compounds in yellow tea. Phenylacetaldehyde was identified as the key aroma compound in the budtype yellow tea, due to its special fresh aroma; Ethyl hexanoate, benzyl alcohol, geraniol, phenethyl alcohol, citral, neral, and myrcene were identified as major contributors in budleaf yellow tea due to their flowery, fruity and sweet aroma notes; 2-pyrrole formaldehyde, 3-ethyl-2,5-dimethylpyrazine, 2-ethyl-5-methylpyrazine, and 2,3-diethyl-5-methylpyrazine are the key aroma compounds in multi-leaf yellow tea due to their roasted aroma notes [5]. Another study also revealed that key aroma compounds, including (E,E)-3,5-octadien-2one, (Z)-linalool oxide (furanoid), (E)-2-heptenal, naphthalene, geraniol, (E)-linalool oxide (furanoid), styrene, linalool, α-ionone, 1-octen-3-ol, (E)-β-ionone, and (E,Z)-3,5-octadien-2-one, presented great differences in the different tenderness levels of yellow tea [6]. In addition, the different manufacturing processes can significantly affect the aroma profile of yellow tea and lead to the different characteristics of aroma in yellow tea. A previous study revealed that the high temperature roasting process was essential for special flavor formation of yellow teas with strong 'roasted', 'nutty', and 'woody' odors, as well as weak 'fatty' and 'fruity' odors [7]. This was also the key process for unique 'crispy-rice-like' odor formation in yellow tea [8]. In addition, different temperatures of roasting processes could result in different aroma characteristics of yellow tea, causing odors such as 'fresh', 'tender corn', 'ripe corn', and so on [9]. The degree of yellowing process significantly affects the composition of the aroma profile in yellow tea [10].
In summary, it is reasonable to hypothesize that different raw materials and processing technologies will lead to different compositions of the aroma profile, as well as cause different types of characteristic aromas in yellow tea. However, up to now, there are scarcely any systematic comparative study focused on the composition of key aroma compounds in different aroma types of yellow tea. Therefore, in this study, our goals are to (a) screen out the main aroma types of yellow tea by sensory-directed flavor analysis; (b) identify the key aroma compounds responsible for characteristic aroma formation in the different aroma types of yellow tea; (c) illuminate the correlation the aroma compounds and sensory attributes. Our findings will provide important information for the key aroma compounds that contribute to the different characteristic aroma formation of yellow tea, and also provide valuable information for improving the aroma quality of yellow tea during the manufacturing process.

Sample Collection and Chemicals
Ninety-seven yellow tea samples were collected from the tea market all over China in 2021 (Table S2). The C7-C40 n-alkanes and ethyl decanoate (99.99%) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Fifty authentic standards were purchased from J&K Chemical Ltd. (Beijing, China) (Table S3). All the solvents were of chromatography grade, and all the chemicals were of analytical reagent grade, unless otherwise stated.

Sensory Analysis
Sensory analysis was approved by the Hunan Agricultural University Institutional Review Board Committee (TSF-780-2021). Twelve panelists (six males and six females, aged from 25 to 55 years) were selected from the Tea Science Department in Hunan Agricultural University. All participants received written information about the study and signed informed consent forms to participate. The yellow tea samples with representative aroma characteristics were selected by sensory evaluation according to the Chinese standards 'Methodology of Sensory Evaluation of Tea' (GB/T 23776-2018) and submitted to the quantitative descriptive analysis (QDA) [11]. Each sample was evaluated three times. The detailed information about sensory evaluation is described in Supplement Material S1.

Qualitative and Quantitative Analysis
The volatile compounds were analyzed by the HS-SPME/GC-MS method according to our previous study [12]. Briefly, each sample was initially ground and homogenized. The tea powder (1 g), NaCl (0.5 g), and ethyl decanoate (10 µL, 8.64 mg/L) were introduced into a 20 mL headspace bottle and infused with 5 mL of boiling water. The vial was immediately sealed and kept at 80 • C for 10 min. A 65-µm polydimethylsiloxane/divinylbenzene (PDMS/DVB) fiber (Supelco, Bellefonte, PA, USA) was exposed to the sample for 40 min. Next, the SPME fiber was inserted into GC injection port (230 • C for 5.0 min) and subsequently analyzed. Each sample was analyzed three times.
GC conditions: An Agilent 7890B GC system coupled with an Agilent 5977A MSD mass spectrometer (Agilent, Santa Clara, CA, USA) was used. An Agilent HP-5MS capillary column (30 m × 0.25 mm i.d. × 0.25 µm film thickness) was used for separation; the carrier gas was helium (purity > 99.999%) with a constant flow rate of 1 mL/min. The injection mode was set to splitless. The oven temperature was initially held at 50 • C for 3 min, raised to 170 • C at a rate of 3 • C/min, raised to 190 • C at a rate of 5 • C/min, then raised to 250 • C at a rate of 15 • C/min, and finally held at temperature for 2 min. MS conditions: EI ionization energy, ion source temperature, quadrupole temperature, and mass scanning range were set to 70 eV, 230 • C, 150 • C, and 35-400 atomic mass units (amu), respectively. Retention indices (RIs), authentic standards, and mass spectra matching in the standard NIST17 library were used for identification. RIs were calculated after analyzing C7-C40 n-alkane series under the same chromatographic conditions. Moreover, some commercially available standards were applied to verify the analysis results under the same conditions. The volatile compounds with available standards were quantitated according to our previous study. For the volatile compounds without the available standards, the quantitation was carried out using the standard that had the same carbon atom or a similar functional structure [12].

Multivariate Analysis
The data were preprocessed by mean centering and scaling prior to analysis. Principal component analysis (PCA), hierarchical cluster analysis (HCA), supervised orthonormal partial least-squares discriminant analysis (OPLS-DA), and partial least-squares analysis (PLS) were performed with SIMCA-P+ (Version 14.0, Umetrics, Umea, Sweden). The aroma compounds were used as the X variables and the aroma sensory attributes as the Y variables for PLS analysis. All data were presented as the mean values ± SD. Significant differences between groups were declared significant at p < 0.05.

Different Aroma Type of Yellow Tea
In this study, 26 yellow tea samples with typical aroma characteristics were selected from 97 yellow tea samples for QDA (Table S2). As shown in Figure 1A, a flavor wheel was established by sensory evaluation. The flavor wheel consisted of three tiers, one firsttier descriptor, five s-tier descriptors, and forty-six third-tier descriptors. The five s-tier descriptors have been widely used to describe the aroma characteristics of yellow tea in previous studies, including 'floral and fruity', 'roasted', 'sweet', 'fresh', and 'green' [7,8].

Different Aroma Type of Yellow Tea
In this study, 26 yellow tea samples with typical aroma characteristics were selected from 97 yellow tea samples for QDA (Table S2). As shown in Figure 1A, a flavor wheel was established by sensory evaluation. The flavor wheel consisted of three tiers, one firsttier descriptor, five s-tier descriptors, and forty-six third-tier descriptors. The five s-tier descriptors have been widely used to describe the aroma characteristics of yellow tea in previous studies, including 'floral and fruity', 'roasted', 'sweet', 'fresh', and 'green' [7,8]. According to the second-tier descriptors and their sensory intensities, the aroma characteristics of 26 yellow tea samples could be divided into four aroma types, namely floral fragrance (H1-H7), high-fired fragrance (HG1-HG4), fresh fragrance (Q1-Q6), and corn-like fragrance (Y1-Y9). The floral fragrance type of yellow tea had a predominantly 'floral and fruity' attribute, whereas the other attributes were only weakly present ( Figure  1B). The high-fired fragrance type of yellow tea mainly featured high intensity in the 'roasted' attribute, while the 'floral and fruity' attribute was moderate ( Figure 1C). The fresh fragrance type of yellow tea had a strong 'fresh' attribute, while the 'green' and 'sweet' attributes were moderate ( Figure 1D). The corn-like fragrance type of yellow tea was marked by strong 'sweet' and 'floral and fruity' attributes, while the 'roasted' attribute was moderate ( Figure 1E).

Volatile Profile in Different Aroma Types of Yellow Tea
The volatile profiles of different aroma types of yellow teas were analyzed by HS-SPME/GC-MS with external standard method (Table S3). A total of 103 volatile compounds were identified and quantified in the different aroma types of yellow teas (Table  1), and 101, 91, 90, and 97 volatile compounds were identified in the floral fragrance, highfired fragrance, fresh fragrance, and corn-like fragrance types of yellow teas, respectively. These identified volatile compounds belong to seven categories, including 16 alcohols, 2 phenols, 13 nitrogenous compounds, 10 aldehydes, 33 hydrocarbons, 13 ketones, and 16 esters. As shown in Figure S1A, alcohols (33.48%) and aldehydes (21.90%) were the predominant categories in the floral fragrance type of yellow tea, and (Z)-linalool oxide (furanoid), (E)-linalool oxide (furanoid), linalool oxide (pyranoid), hotrienol, (Z)-citral, and citral were the most abundant volatiles compared with the other aroma types of yellow tea ( Figure S1B). Indeed, the alcohols and aldehydes have been reported to provide 'floral' and 'fruity' attributes, which have a good coordinating effect on the aroma profiles of tea products [6,22]. Nitrogenous compounds (60.10%) and hydrocarbons (17.27%) were According to the second-tier descriptors and their sensory intensities, the aroma characteristics of 26 yellow tea samples could be divided into four aroma types, namely floral fragrance (H1-H7), high-fired fragrance (HG1-HG4), fresh fragrance (Q1-Q6), and corn-like fragrance (Y1-Y9). The floral fragrance type of yellow tea had a predominantly 'floral and fruity' attribute, whereas the other attributes were only weakly present ( Figure 1B). The high-fired fragrance type of yellow tea mainly featured high intensity in the 'roasted' attribute, while the 'floral and fruity' attribute was moderate ( Figure 1C). The fresh fragrance type of yellow tea had a strong 'fresh' attribute, while the 'green' and 'sweet' attributes were moderate ( Figure 1D). The corn-like fragrance type of yellow tea was marked by strong 'sweet' and 'floral and fruity' attributes, while the 'roasted' attribute was moderate ( Figure 1E).

Volatile Profile in Different Aroma Types of Yellow Tea
The volatile profiles of different aroma types of yellow teas were analyzed by HS-SPME/GC-MS with external standard method (Table S3). A total of 103 volatile compounds were identified and quantified in the different aroma types of yellow teas (Table 1), and 101, 91, 90, and 97 volatile compounds were identified in the floral fragrance, high-fired fragrance, fresh fragrance, and corn-like fragrance types of yellow teas, respectively. These identified volatile compounds belong to seven categories, including 16 alcohols, 2 phenols, 13 nitrogenous compounds, 10 aldehydes, 33 hydrocarbons, 13 ketones, and 16 esters. As shown in Figure S1A, alcohols (33.48%) and aldehydes (21.90%) were the predominant categories in the floral fragrance type of yellow tea, and (Z)-linalool oxide (furanoid), (E)-linalool oxide (furanoid), linalool oxide (pyranoid), hotrienol, (Z)-citral, and citral were the most abundant volatiles compared with the other aroma types of yellow tea ( Figure S1B). Indeed, the alcohols and aldehydes have been reported to provide 'floral' and 'fruity' attributes, which have a good coordinating effect on the aroma profiles of tea products [6,22]. Nitrogenous compounds (60.10%) and hydrocarbons (17.27%) were the main categories in the high-fired fragrance type of yellow teas ( Figure S1A). Among them, 1-ethyl-1H-pyrrole, 2,4-dimethyl-3-ethylpyrrole, 2,5-dimethyl-3-ethylpyrazine, 2methyl-5-ethylpyrazine, methyl anthranilate, and p-xylene were the most abundant volatile compounds compared with the other aroma types of yellow teas ( Figure S1B). These pyrroles, pyrazines, and their derivatives are usually generated from the Maillard reaction during roasting and baking treatments, which are thought to contribute to the formation of the 'roasted' aroma [1,7]. Esters (45.06%) and alcohols (28.03%) were the dominant categories in the fresh fragrance type of yellow tea ( Figure S1A). Among them, ethyl hexanoate, ethyl nonanoate, and (E)-3-hexen-1-ol were the most abundant chemical structures, compared with the other aroma types of yellow tea ( Figure S1B). Esters have been reported to play an important role in the fresh aroma characteristics formation of yellow tea [6]. Hydrocarbons (21.68%) and ketones (11.62%) were the dominant categories in the corn-like fragrance type of yellow tea ( Figure S1A). Compared with the other aroma types, styrene, (E)-β-ocimene, β-cedrene, toluene, mesityl oxide, 2,3-octanedione, and β-ionone were the most abundant in the corn-like fragrance type ( Figure S1B). Hydrocarbons and ketones with 'sweet', 'floral', and 'fruity' aroma notes, which are found to dominate the volatile compounds of black tea and are considered to provide a unique aroma for black tea [23,24]. The results showed that there were significant differences in the composition of volatile compounds of yellow teas with different aroma types.
To investigate the similarities and differences of volatile compounds among the four aroma types of yellow teas, PCA and HCA were performed based on the quantitative results of the identified volatile compounds. The results of PCA (the total variance of model was 89%) ( Figure 2A) and HCA ( Figure 2B) showed that the yellow teas could be divided into four groups according to their aroma types, which indicated that there were significant differences in the volatile profile among the four aroma types of yellow teas. Based on these results, six OPLS-DA models were established to investigate the discriminatory volatile compounds ( Figure S2). Among the four types of yellow tea, 75 discriminative volatile compounds, which might play an important role in the formation of different aroma characteristics and contribute to the distinction of these four aroma types of yellow teas ( Figure S3), were identified. To investigate the similarities and differences of volatile compounds among the four aroma types of yellow teas, PCA and HCA were performed based on the quantitative results of the identified volatile compounds. The results of PCA (the total variance of model was 89%) ( Figure 2A) and HCA ( Figure 2B) showed that the yellow teas could be divided into four groups according to their aroma types, which indicated that there were significant differences in the volatile profile among the four aroma types of yellow teas. Based on these results, six OPLS-DA models were established to investigate the discriminatory volatile compounds ( Figure S2). Among the four types of yellow tea, 75 discriminative volatile compounds, which might play an important role in the formation of different aroma characteristics and contribute to the distinction of these four aroma types of yellow teas ( Figure S3), were identified.

Key Aroma Compounds in Different Aroma Types of Yellow Tea
It is well known that the characteristic aroma of yellow tea is formed by a series of volatile compounds with a certain composition and proportion. The characteristic aroma mainly depends on the concentrations of aroma compounds and their OAVs. OAV is usually used to evaluate the contribution of the aroma compound to the odor of tea [13]. Therefore, the aroma attributes and OAVs of all the identified volatile compounds were calculated and listed in Table S4. Sixty-one volatile compounds were identified with OAV > 1 in at least one aroma type of yellow tea. Twenty-four volatile compounds with OAVs > 1 were common in all yellow tea samples, and 12 (linalool, phenethyl alcohol, 1-octen-3-ol, α-ionone, β-ionone, dehydro-β-ionone, (E,E)-2,4-heptadienal, phenyl acetaldehyde, methyl anthranilate, ethyl hexanoate, myrcene, and α-muurolene) had OAVs > 10.
Most of aroma compounds with high OAVs (OAV > 10) possess 'floral', 'fruity', 'citrus', 'green', and 'fatty' odors, which endow yellow tea with clean and pure scents and play a major role in the formation of a variety of aroma types of yellow teas [6].  Combined with the results of OPLS-DA, a total of 47 discriminative aroma compounds were identified in the four aroma types of yellow tea based on the three conditions: the values of predictive component variable importance in the projection (VIP) ≥ 1.0, p ≤ 0.05 and OAV > 1 (Figure 3). Among these discriminative aroma compounds, eight, fourteen, seven, and eighteen aroma compounds with the highest levels in the floral fragrance, high-fired fragrance, fresh fragrance, and corn-like fragrance types of yellow teas, respectively. High contents of these discriminative aroma compounds might play an important role in the formation of different aroma characteristics of yellow tea. Therefore, these discriminative aroma compounds were recognized as key aroma compounds in different aroma types of yellow tea. calculated and listed in Table S4. Sixty-one volatile compounds were identified with OAV > 1 in at least one aroma type of yellow tea. Twenty-four volatile compounds with OAVs > 1 were common in all yellow tea samples, and 12 (linalool, phenethyl alcohol, 1-octen-3-ol, α-ionone, β-ionone, dehydro-β-ionone, (E,E)-2,4-heptadienal, phenyl acetaldehyde, methyl anthranilate, ethyl hexanoate, myrcene, and α-muurolene) had OAVs > 10. Most of aroma compounds with high OAVs (OAV > 10) possess 'floral', 'fruity', 'citrus', 'green', and 'fatty' odors, which endow yellow tea with clean and pure scents and play a major role in the formation of a variety of aroma types of yellow teas [6].
Combined with the results of OPLS-DA, a total of 47 discriminative aroma compounds were identified in the four aroma types of yellow tea based on the three conditions: the values of predictive component variable importance in the projection (VIP) ≥ 1.0, p ≤ 0.05 and OAV > 1 (Figure 3). Among these discriminative aroma compounds, eight, fourteen, seven, and eighteen aroma compounds with the highest levels in the floral fragrance, high-fired fragrance, fresh fragrance, and corn-like fragrance types of yellow teas, respectively. High contents of these discriminative aroma compounds might play an important role in the formation of different aroma characteristics of yellow tea. Therefore, these discriminative aroma compounds were recognized as key aroma compounds in different aroma types of yellow tea. In the floral fragrance types of yellow tea, (Z)-citral, citral, jasmone, (E)-linalool oxide (furanoid), decanal, geraniol, (E)-2-octen-1-ol, and safranal were identified as the key In the floral fragrance types of yellow tea, (Z)-citral, citral, jasmone, (E)-linalool oxide (furanoid), decanal, geraniol, (E)-2-octen-1-ol, and safranal were identified as the key aroma compounds. Among them, (Z)-citral and citral with 'citrus' odor have been reported to be responsible for the formation of strongly fruity fragrance in lemon basil and hops [25,26]. €-Linalool oxide (furanoid) possesses a 'floral' odor and has been widely detected in yellow tea, green tea, black tea, and oolong tea. It has been considered as the important compound that contributes to 'floral' aroma formation in tea products [1,6,8,27,28]. In addition, as a vital odoriferous compound of honey, safranal has been regarded as an important aromaactive compound because of its 'ripe fruit' and 'honey' odor [6,14,16,29]. (E)-2-Octen-1-ol emits a 'grassy' odor and has been considered as an aroma-active compound responsible for the 'green' aroma characteristics formation of asam sunti [30].
In the fresh fragrance type of yellow tea, indole, phenethyl alcohol, (E)-3-hexen-1-ol, αterpineol, ethyl hexanoate, ethyl nonanoate, and ethyl octanoate were identified as the key aroma compounds. Esters are well-known aroma compounds that play an important role in the properties of beverages. Ethyl nonanoate, ethyl hexanoate, and ethyl octanoate are the representative ester compounds that can provide 'floral', 'fruity', and 'waxy' odors [6,34]. For example, ethyl hexanoate has been identified as an important aroma compound in oolong tea with a 'pineapple' odor, which contributes to the 'sweet fruit' aroma formation of special aroma characteristics of black tea [23,35]. Ethyl nonanoate has been considered as a potential aroma-active compound in fruits, and it also contributes to the formation of 'rose-like' aroma in black tea [35,36]. In addition, (E)-3-Hexen-1-ol has a 'green' note and has been reported to contribute to the formation of 'green', 'grass', and 'fresh' odors of black tea and oolong tea [16,23].
In addition, characteristic aromas were also impacted by the interactions among volatile compounds or volatile and non-volatile compounds [39]. Zhu et al. revealed that mixed aroma compounds with similar structures mainly present a synergistic effect and additive action, and a masking effect was found among aroma compounds with different structures in oolong tea [39]. Vicente Ferreira et al. also reported that the non-volatile higher alcohol content had an important impact on the characteristic aroma formation of wine [40][41][42].

Relationship between Aroma-Active Compounds and Sensory Attributes
To explore the relationships between the aroma-active compounds (OAV > 1) and sensory attributes, a PLS model was established. Two latent variables were included in the PLS model, which represented 86.90% of X-matrix variance (aroma-active compounds) and explained 99.30% of Y-matrix variance (sensory attributes). As shown in Figure 4, the positions of the sensory attributes and aroma-active compounds of yellow tea between the two ellipses indicated that they were well explained by the PLS model. Among these samples, the floral fragrance (H1-H7) and corn-like fragrance types of yellow teas (Y1-Y9) are mainly located on the upper part of PC2. The fresh fragrance type of yellow tea samples (Q1-Q6) are mainly located in the negative region of PC1 and PC2. The high-fired fragrance type yellow tea samples (HG1-HG4) are mainly located in the positive region of PC1 and negative region of PC2. This clearly indicates that tea samples can be divided into four groups according to their aroma types.
between the two ellipses indicated that they were well explained by the PLS model. Among these samples, the floral fragrance (H1-H7) and corn-like fragrance types of yellow teas (Y1-Y9) are mainly located on the upper part of PC2. The fresh fragrance type of yellow tea samples (Q1-Q6) are mainly located in the negative region of PC1 and PC2. The high-fired fragrance type yellow tea samples (HG1-HG4) are mainly located in the positive region of PC1 and negative region of PC2. This clearly indicates that tea samples can be divided into four groups according to their aroma types. The five attributes ('floral and fruity', 'roasted', 'fresh', 'grassy', and 'sweet') that significantly correlated with some aroma-active compounds were located between the inner and outer ellipses. The 'floral and fruity', 'sweet', and 'roasted' attributes were located on the positive dimension, and the 'fresh' and 'green' attributes were located on the negative dimension of the first PC1. The floral fragrance and corn-like fragrance types of yellow tea samples were strongly associated with 'floral and fruity' and 'sweet' attributes, the high-fired fragrance type of yellow tea was strongly associated with 'roasted' attribute, The five attributes ('floral and fruity', 'roasted', 'fresh', 'grassy', and 'sweet') that significantly correlated with some aroma-active compounds were located between the inner and outer ellipses. The 'floral and fruity', 'sweet', and 'roasted' attributes were located on the positive dimension, and the 'fresh' and 'green' attributes were located on the negative dimension of the first PC1. The floral fragrance and corn-like fragrance types of yellow tea samples were strongly associated with 'floral and fruity' and 'sweet' attributes, the high-fired fragrance type of yellow tea was strongly associated with 'roasted' attribute, and the fresh fragrance type of yellow tea was strongly associated with 'fresh' and 'green' attributes. Furthermore, the 'floral and fruity' and 'sweet' attributes were positively correlated to l-menthone, heptanal, naphthalene, 2-methylnaphthalene, 1-methylnaphthalene, methyl heptenone, decanal, styrene, safranal, toluene, para-xylene, and acetophenone. A previous study suggested that the 'floral and fruity' attribute was related to the 'sweet' attribute in tea [43]. The 'roasted' attribute was positively correlated to methyl anthranilate, 2,3-diethyl-5-methylpyrazine, 2-methyl-5-ethylpyrazine, 2,5-dimethyl-3-ethylpyrazine, and 3,5-diethyl-2-methyl pyrazine. It is well known that the intensity of the 'roasted' attribute mainly depends on the content of pyrazine compounds, which has been confirmed by a previous study [1]. The 'fresh' and 'green' attributes were positively correlated with (E)-3-hexen-1-ol, ethyl hexanoate, ethyl nonanoate, and ethyl octanoate. Previous studies have shown that ethyl esters with low detection thresholds usually exist in large amounts in wine and play a vital role in the formation of fruity aroma characteristics [44]. Meanwhile, it has also been reported that these esters make an important contribution to the fresh aroma characteristics of yellow tea, which differs from green and black teas [6].

Conclusions
According to the difference of aroma characteristics, yellow teas were divided into four main aroma types by QDA, including the floral fragrance, high-fired fragrance, fresh fragrance, and corn-like fragrance. Among them, floral fragrance yellow tea had a predominant 'floral and fruity' attribute, high-fired fragrance yellow tea was mainly featured with high intensity in the 'roasted' attribute, fresh fragrance yellow tea had a strong 'fresh' attribute, and corn-like fragrance yellow tea was marked by strong 'sweet' and 'floral and fruity' attributes. In addition, a total of 103 volatile compounds were identified and quantified in yellow teas by HS-SPME/GC-MS. According to multivariate statistical analysis and OAVs, 47 volatile compounds were identified as the key aroma compounds for the formation of different aroma types of yellow tea. Among them, eight aroma compounds dominated with 'floral and fruity' attribute in the floral fragrance type, fourteen aroma compounds dominated with 'roasted' attribute in the high-fired fragrance type, seven aroma compounds dominated with 'floral and fruity' and 'grassy' attributes in the fresh fragrance type, and eighteen aroma compounds dominated with 'floral and fruity' and 'sweet' attributes in the corn-like fragrance type. Furthermore, PLS analysis revealed that the 'floral and fruity' and sweet attributes were positively correlated to l-menthone, heptanal, naphthalene, 2-methylnaphthalene, 1-methylnaphthalene, methyl heptenone, decanal, styrene, safranal, toluene, para-xylene, and acetophenone. The 'roasted' attribute was positively correlated to methyl anthranilate, 2,3-diethyl-5-methylpyrazine, 2-methyl-5-ethylpyrazine, 2,5-dimethyl-3-ethylpyrazine, and 3,5-diethyl-2-methyl pyrazine. The 'fresh' and 'grassy' attributes were positively correlated to (E)-3-hexen-1-ol, ethyl hexanoate, ethyl nonanoate, and ethyl octanoate. Overall, this study provides new insights into the key aroma compounds that are responsible for the characteristic aroma in different aroma types of yellow tea, and will provide an important theoretical basis for establishing flavor directed processing of yellow tea.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/foods12010027/s1, Supplement Material S1: The detail information of sensory evaluation; Figure S1: Differences of the volatile categories (A) and discriminatory volatiles (B) among the four aroma types of yellow tea; Figure Table S1: The main volatile compounds in different raw material or processing parameters of yellow tea; Table S2: Yellow tea samples information; Table S3: The standard curve of identified volatile compounds in yellow tea;   Institutional Review Board Statement: The study was conducted in accordance with the Declaration of Helsinki, and approved by the Hunan Agricultural University Institutional Review Board Committee (TSF-780-2021).