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. 2022 Feb 7;13:100248. doi: 10.1016/j.fochx.2022.100248

Characterization of key aroma compounds and relationship between aroma compounds and sensory attributes in different aroma types of Fu brick tea

Zheng Xuexue a,b, Hong Xin a,b, Jin Youlan a,c, Wang Chao a, Liu Zhonghua a,b,c, Huang Jianan a,b,⁎, Li Qin a,b,c,⁎
PMCID: PMC9040021  PMID: 35499020

Graphical abstract

graphic file with name ga1.jpg

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Chemical compounds studied in this article: Benzyl alcohol (PubChem CID: 244); linalool (PubChem CID: 6549); nerolidol (PubChem CID: 5284507); β-ionone (PubChem CID: 5352481); α-terpineol (PubChem CID: 17100); acetophenone (PubChem CID: 7410); (E,Z)-2,6-nonadienal (PubChem CID: 643731); hexanal (PubChem CID: 6184); dihydro-β-ionone (PubChem CID: 519382); (E,E)-2,4-nonadienal (PubChem CID: 5283339).

Keywords: Fu brick tea, Different aroma types, Key aroma compounds, Sensory attributes, Multivariate statistical analysis

Highlights

  • •

    Aroma characteristics of Fu brick tea were classified into three types.

  • •

    Key aroma compounds in three aroma types of Fu brick tea were identified.

  • •

    Relationship between aroma compounds and aroma attributes was illuminated.

Abstract

Aroma is one of the most important sensory properties of tea. Floral-fungal aroma type, ripe-fungal aroma type and fresh-fungal aroma type were the main aroma types of Fu brick tea by QDA. A total of 112 volatile compounds were identified and quantified in tea samples by HS-SPME/GC–MS analysis. Ten voaltiles in floral-fungal aroma type, eleven voaltiles in ripe-fungal aroma type, and eighteen voaltiles in fresh-fungal aroma type were identified as key aroma compounds for the aroma characteristics formation in three aroma types of Fu brick tea. In addition, PLS analysis revealed that 3,4-dehydro-β-ionone, dihydro-β-ionone, (+)-carotol and linalool oxide Ⅱ were the key contributors to the ‘floral and fruity’ attribute, α-terpineol contributed to ‘woody’ and ‘stale’ attributes, and thirteen aroma compounds related to ‘green’ attribute. Taken together, these findings will provide new insights into the formation mechanism of different aroma characteristics in Fu brick tea.

1. Introduction

Fu brick tea is a typical post-fermented dark tea, which is mainly produced in Hunan, Shaanxi and Zhejiang provinces of China. The manufacturing process of Fu brick tea includes steaming, piling, pressing, fermentation, and drying (Li et al., 2019, Ling et al., 2010, Xu et al., 2007). Microbial fermentation was considered as the key step in the process of Fu brick tea, which helped to form its unique ‘fungal flower’ aroma and mellow taste (Ling et al., 2010). In recent years, Fu brick tea has attracted global interest owing to its special flavor and health benefits, such as anti-obesity, anti-hyperlipidemia, anti-oxidant, anti-tumor, anti-microbial, and others (Fu et al., 2011, Li et al., 2013, Mo et al., 2008, Xu et al., 2011).

Tea aroma is one of the most important sensory properties reflecting the quality of tea (Zhu et al., 2018). ‘Fungal flower’ aroma is the basic aroma characteristics of Fu brick tea, which is formed by a variety of volatile compounds with different aroma attributes mixed together (Li et al., 2020). However, the aroma characteristics of Fu brick tea vary greatly with cultivation conditions, processing, regions and storage time. Previous study indicated that the ‘fungal flower’ aroma of aged Fu brick tea was dominated with ‘stale’ attribute and was enriched by methyl hexadecanoate, 1-octanol, methyl laurate, methyl tetradecanoate, 1-heptadecanol (Huang, Wang, Zeng, & Lai, 2011). Otherwise, the ‘fungal flower’ aroma of Fu brick tea processing in Hunan province was dominated with ‘floral’, ‘woody’, ‘green’ attributes and was enriched in methyl salicylate, acetophenone, cedrol, benzyl alcohol (Li et al., 2020). The ‘fungal flower’ aroma of Fu brick tea processing in Shaanxi province was dominated with ‘roasted’ attribute and enriched in heteroxic compounds, such as 2-pentylfuran and tetramethylpyrazine. The ‘fungal flower’ aroma of Fu brick tea producing from Zhejiang province was usually dominated by the ‘green’ attribute and was riched in low grade fatty aldehydes such as hexanal (Cao et al., 2018, Zhao et al., 2017). The ‘fungal flower’ aroma of Fu brick tea processed from Sichuan province was dominated by ‘floral and fruity’ attribute and its main volatile components are methyl salicylate, geranylacetone and β-ionone (Nie et al., 2019). To sum up, it is reasonable to speculate that there are different ‘fungal flower’ aroma characteristics, which might result in different aroma types in Fu brick tea. But up to now, there was no research focus on the aroma characteristics in different aroma types of Fu brick tea.

Therefore, the aims of the present study were to (a) classify the aroma characteristics of Fu brick tea in different aroma types according to the sensory evaluation; (b) identify the key aroma compounds contributing to the formation of different aroma characteristics in Fu brick tea; (c) illuminate the relationship between the aroma compounds and sensory attributes in Fu brick tea. This study is of significant importance for providing information to enhance our understanding of the mechanisms on different aroma characteristics formation in Fu brick tea.

2. Method and material

2.1. Chemicals

The C7-C40 n-alkanes and ethyl decanoate (99.99%) were obtained from Sigma-Aldrich (St. Louis, MO). Forty two authentic standards were purchased from J&K chemical Ltd. (Beijing, China). All the chromatographic solvents were of chromatography grade and all of the chemicals were of analytical reagent grade unless otherwise stated.

2.2. Samples and sensory analysis

Sixty Fu brick tea samples were collected from the tea market all over China. Sensory analysis was approved by Hunan Agricultural University Institutional Review Board Committee (#TSF-780-2020). Sensory evaluation was performed by eight well-trained panelists from the Tea Science Department in Hunan Agricultural University (four males and four females, aged from 25 to 55 years) according to the Chinese standards “Methodology of Sensory Evaluation of Tea” (GB/T 23776-2018). Before the experiment, each panelist had to complete a 60-hours of sensory evaluation training with different aroma types of Fu brick tea in 20 days. All participants received written information about the study, and they signed informed consent to participate. Sensory quantitative description analysis (QDA) was performed according previous study (Li, Luo, Wang, Fu, & Zeng, 2019). Briefly, 3.0 g tea sample was infused with 150 mL of boiled water for 7 min in a special tea cup. Each sample was coded with a three-digit number and randomly offered to panelists after brewing. The aroma descriptors were evaluated and discussed by panelist panel. The intensity of aroma attribute was scored by panelists using a scale from 0 to 10. Each score was expressed as mean value. Each sample was evaluated three times by each panelist in different days.

2.3. Qualitative and quantitative analysis of the volatiles in Fu brick tea by HS-SPME/GC–MS analysis

The volatile profile of Fu brick tea was extracted by headspace solid-phase micro-extraction (HS-SPME) using a 65 µm polydimethylsiloxane/divinylbenzene (PDMS/DVB) fiber (Supelco, Bellefonte, PA, USA) and was analysed using a GC system (Agilent 5977B, Agilent Technologies Inc., CA, USA) equipped with a mass spectrometer (Agilent 5977A, Agilent Technologies Inc., CA, USA), according to our previous study (Li et al., 2020). Ethyl decanoate (8.64 mg/L) was used as internal standard. An Agilent HP-5MS capillary column (30 m × 0.25 mm i.d. × 0.25 μm film thickness) was employed for separating the volatile compounds, and helium (purity > 99.999 %) was used as carrier gas with a constant flow rate of 1 mL/min. The injector temperature was set at 250 °C with splitless model. The GC oven temperature was set at 50 °C for 2 min, then increased at a rate of 3 °C/min to 160 °C and held for 1 min; lastly, it increased to 280 °C at a rate of 15 °C/min and kept for 1 min. The mass spectrometer conditions were set as follows: ionization mode, EI; ion source temperature, 230 °C; quadrupole temperature, 150 °C; electron energy, 70 eV; full scan mode, mass scan range 35–400 atomic mass units (amu). Each sample was analyzed three times.

Volatile compounds were identified based on retention indices (RIs), authentic standards, and mass spectra matching in the standard NIST17 library. C7-C40 n-alkane mixture was employed for determination of RIs. Each standard was mix with ‘volatile-free’ tea to obtain the standard curves. The ‘volatile-free’ tea was prepared by vacuum concentration to remove volatile compounds according to a previous study (Du, Wang, Li, Xiao, Li, & Xu, 2013). For the volatile compounds without available standards, the quantitation was carried out using the standard that had the same carbon atom or a similar functional structure (Li et al., 2020).

2.4. Odor activity values (OAVs) calculation

OAV was calculated by dividing the concentration of volatile compound with its odor threshold (OT) reported by the references. Volatile compounds with OAV > 1 were considered as an aroma-active compound, which played an important role for the aroma characteristics formation of tea samples (Mao, Lu, Li, Ye, Wei, & Tong, 2018).

2.5. Statistical analysis

The data were preprocessed by mean centering and scaling prior to analysis. Principal component analysis (PCA), hierarchical cluster analysis (HCA), orthogonal partial least squares discriminant analysis (OPLS-DA) and partial least squares analysis (PLS) were performed by SIMCA-P+ (Version 14.0, Umetrics, Umea, Sweden). Analysis of variance (ANOVA) was performed by SPSS (Version 22.0, IBM, Armonk, NY, USA). All data were presented as the mean value ± SD. Significant differences between groups were declared significant at p < 0.05.

3. Results and discussion

3.1. Aroma characteristics of Fu brick tea

After sensory evaluation, 25 samples with typical flavor characteristics of Fu brick tea were selected from 60 samples and used for subsequent sensory QDA analysis. In total, 40 attributes that described the aroma characteristics were obtained (Fig. 1A). The flavor wheel was consist of 3 tiers, 1 first-tier descriptor, 10 s-tier descriptors and 40 third-tier descriptors. The third-tier descriptors were precise descriptions to Fu brick tea, such as cherry fragrance, orchid sweet, old house smell and mushroom scent. The second-tier descriptors were the summary of the third-tier descriptors, that is, the concrete aroma attributes of Fu brick tea, which were split into 10 categories, including fermented, fruity, floral, nutty, stale, woody, green, hay flavor, fishy smell and herbal. All of the second-tier descriptors have been often used to describe the characteristics of Fu brick tea in other reports (Li et al., 2019, Li et al., 2020, Lv et al., 2014, Nie et al., 2019). It is worth mentioning that the ‘fungal flower’ aroma attribute could be a complex aroma, which might be formed by a variety of volatiles with woody, floral and mint attributes mixed together (Li et al., 2020). So, this descriptor was not presented in the flavor wheel independently. After discussion, all panelists agreed to use the five attributes with higher odor intensity as useful terms for subsequent evaluation of the aroma characteristics of Fu brick tea, including ‘floral and fruity’, ‘stale’, ‘woody’, ‘green’ and ‘herbal’.

Fig. 1.

Fig. 1

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Quantitative descriptive analysis of different aroma types of Fu brick tea. (A) Flavor wheel. (B) JHX: floral-fungal aroma type. (C) SJX: ripe-fungal aroma type. (D) QJX: fresh-fungal aroma type.

Using the QDA method, 25 samples were clustered into three groups based on sensory intensity (Fig. 1B–D). The first group had strong ‘floral and fruity’ attribute, while weaked in the other attributes. The second group, with a typical ‘stale’ attribute, also was rich in ‘woody’ attribute. The third group had strong ‘green’ attribute, whereas the other attributes were moderate. These noticeable differences suggested that the samples had significantly different flavor and intensities. Generally, eight Fu brick tea samples with strong ‘floral and fruity’ attribute were classified as ‘floral-fungal’ aroma (JHX) type, eight samples with strong ‘stale’ and ‘woody’ attributes were classified as ‘ripe-fungal’ aroma (SJX) type, and nine samples with strong ‘green’ attribute were classified as ‘fresh-fungal’ aroma (QJX) type based on the sensory evaluation.

3.2. Comparison of volatile profiles in three aroma types of Fu brick tea

To illuminate the volatile profiles of three aroma types of Fu brick tea, a comprehensive analysis was performed by HS-SPME/GC–MS. A total of 112 volatile compounds were identified and quantified in Fu brick tea samples by absolute quantitative method (Table 1 and Table S1). For clarification, the identified compounds were further classified into 10 different sub-classes, including 26 hydrocarbons, 21 ketones, 20 alcohols, 20 aldehydes, 9 esters, 6 heteroxy compounds, 3 phenols, 2 lactones, 1 pyrrole and 4 others. Generally, the total content of volatile compounds in QJX type was much higher than that in JHX and SJX types of samples (Fig. 2A). Alcohols, ketones, aldehydes, hydrocarbons were the predominant volatile categories in Fu brick tea, which totally accounted for 85.47% in the identified volatile compounds (Fig. 2B). Previous studies also reported that alcohols, ketones, aldehydes, hydrocarbons might be the main contributing substance to the characteristic aroma formation of Fu brick tea, which was consistent with our present results (Li et al., 2019, Xu et al., 2007). Notably, alcohols were the most volatiles category in Fu brick teas (29.11%, 26.91%, and 24.32%), followed by ketones, aldehydes and hydrocarbons in the three aroma types of Fu brick tea, but it was not consistent with previous studies that reported ketones was the most abundant volatile category (Lv et al., 2014, Shi et al., 2019). This difference might be due to the absolute quantitative method used in our present study, otherwise, the relative quantitative method, such as peak area normalization method used in previous studies, might result in the lower relative content of alcohol compounds (Lv et al., 2014, Shi et al., 2019). Furthermore, alcohols, ketones and hydrocarbons were more abundant in JHX type, whereas ketones and aldehydes were more abundant in SJX and QJX types of Fu brick tea.

Table 1.

Qualitative and quantitative results of volatile components among three aroma types of Fu brick tea.

Compounds CAS Quantitative ions (m/z)a RIb/RIc Identificationd Concentration (μg/L)
 Proportion (%)
JHX SJX QJX JHX% SJX% QJX%
Esters
Sebacic acid, di(2-hexyl) ester 1000355–65-8 185,98,143,166 1851/- MS 1.49 ± 0.93 1.96 ± 1.33 1.46 ± 0.62 0.03 ± 0.01 0.04 ± 0.03 0.02 ± 0.01
Octyl 4-methoxycinnamate 5466–77-3 161,178,133,207 2329/- MS 9.58 ± 0.16 9.71 ± 0.44 9.69 ± 0.37 0.17 ± 0.03 0.18 ± 0.05 0.11 ± 0.02
Diethyl phthalate 84–66-2 149,177,105,121 1592/1602 MS RI 13.17 ± 4.57 10.81 ± 1.18 29.42 ± 47.91 0.23 ± 0.1 0.21 ± 0.07 0.35 ± 0.52
Diisobutyl phthalate 84–69-5 149,57,223,104 1870/1869 MS RI 26.67 ± 7.3 18.37 ± 7.02 20.76 ± 7.48 0.46 ± 0.16 0.35 ± 0.06 0.24 ± 0.06
Methyl hexadecanoate 112–39-0 74,143,87,227 1923/1926 MS RI 2.81 ± 0.23 2.6 ± 0.17 2.87 ± 0.26 0.05 ± 0.01 0.05 ± 0.01 0.03 ± 0
Ethyl palmitate 628–97-7 88,101,157,241 1990/1992 MS RI 0.66 ± 0.49 0.42 ± 0.07 0.54 ± 0.13 0.01 ± 0.01 0.01 ± 0 0.01 ± 0
Dibutyl phthalate 84–74-2 149,150,223,205 1965/1965 MS RI 14.09 ± 0.54 11.51 ± 0.59 12.5 ± 1.08 0.24 ± 0.05 0.22 ± 0.07 0.15 ± 0.03
Methyl salicylate 119–36-8 120,152,92,121 1191/1191 MS RI STD 294.09 ± 169.47 225.83 ± 219.33 167.88 ± 68.53 5.11 ± 3.06 4.28 ± 2.73 1.97 ± 0.91
2,2,4-trimethyl-1,3-pentanediol diisobutyrate 6846–50-0 71,111,43,83 1595/- MS 31.62 ± 18.18 12.12 ± 4.83 26.03 ± 13.98 0.55 ± 0.51 0.23 ± 0.13 0.31 ± 0.14



Heteroxy compounds
1,2-dimethoxybenzene 91–16-7 138,123,95,77 1144/1148 MS RI STD 37.29 ± 13.06 58.66 ± 32.57 107.37 ± 51.96 0.65 ± 0.28 1.11 ± 0.46 1.26 ± 0.67
1,3-dimethoxybenzene 151–10-0 138,107,122,95 1164/- MS 26.7 ± 6.28 29.62 ± 23.61 30.47 ± 14.86 0.46 ± 0.28 0.56 ± 0.25 0.36 ± 0.17
5-methoxy-6,7-dimethylbenzofuran 35355–35-2 161,131,145,176 1465/1468 MS RI 24.85 ± 5.68 62 ± 16.59 46.55 ± 14.11 0.43 ± 0.22 1.18 ± 0.43 0.55 ± 0.17
4-methylanisole 104–93-8 122,77,91,107 929/- MS STD 6.77 ± 1.87 5.65 ± 1.53 22.61 ± 9.98 0.12 ± 0.02 0.11 ± 0.05 0.27 ± 0.13
2-pentylfuran 3777–69-3 82,53,81,138 987/988 MS RI STD 8.73 ± 2.53 4.41 ± 2.87 20.86 ± 4.11 0.15 ± 0.03 0.08 ± 0.03 0.25 ± 0.05
cis-anethol 104–46-1 148,133,121,77 1281/1283 MS RI 5.32 ± 2.03 5.16 ± 1.84 8.28 ± 6.86 0.09 ± 0.03 0.1 ± 0.03 0.1 ± 0.12



Ketones
Geranylacetone 3796–70-1 43,69,151,107 1451/1449 MS RI STD 186.72 ± 67.15 145.01 ± 60.04 241.66 ± 54.02 3.24 ± 0.88 2.75 ± 1.74 2.84 ± 0.8
Dihydro-β-ionone 17283–81-7 126,161,176,121 1436/1438 MS RI 35.05 ± 0.44 35.8 ± 1.19 35.47 ± 0.56 0.61 ± 0.12 0.68 ± 0.22 0.42 ± 0.07
α-ionone 127–41-3 121,93,136,192 1425/1425 MS RI STD 96.18 ± 22.32 78.04 ± 25.69 120.34 ± 30.4 1.67 ± 0.27 1.48 ± 0.71 1.41 ± 0.39
β-ionone 79–77-6 177,123,135,159 1484/1483 MS RI STD 37.6 ± 12.36 15.59 ± 6.08 45.33 ± 12.72 0.65 ± 0.18 0.3 ± 0.1 0.53 ± 0.14
3,4-dehydro-β-ionone 1203–08-3 175,91,147,190 1481/1482 MS RI 26.29 ± 7.1 13.89 ± 2.27 26.13 ± 7.49 0.46 ± 0.07 0.26 ± 0.06 0.31 ± 0.1
Benzophenone 119–61-9 105,182,77,51 1624/1623 MS RI 6.99 ± 0.94 7.07 ± 1.35 8.2 ± 4.35 0.12 ± 0.03 0.13 ± 0.05 0.1 ± 0.05
(E)-3-nonen-2-one 18402–83-0 125,55,97,140 1137/1134 MS RI 17.49 ± 5.45 14.67 ± 3.21 26.89 ± 4.53 0.3 ± 0.09 0.28 ± 0.12 0.32 ± 0.1
4-methyleneisophorone 20548–00-9 150,135,107,91 1215/- MS 13.74 ± 0.96 15.37 ± 1.97 16.17 ± 1.17 0.24 ± 0.04 0.29 ± 0.09 0.19 ± 0.03
2,5-pyrrolidinedione,1-ethyl- 2314–78-5 127,56,84,112 1132/1134 MS RI 14.06 ± 3.43 20.98 ± 17.04 24.37 ± 12.93 0.24 ± 0.04 0.4 ± 0.42 0.29 ± 0.14
Dihydro-a-ionone 31499–72-6 121,136,95,176 1413/- MS 79.8 ± 23.15 56.71 ± 33.74 121.26 ± 30.26 1.39 ± 0.37 1.08 ± 0.88 1.43 ± 0.38
Hexahydrofarnesylacetone 502–69-2 58,109,194,137 1845/1846 MS RI 62.23 ± 4.09 56.38 ± 2.32 64.92 ± 3.13 1.08 ± 0.16 1.07 ± 0.36 0.76 ± 0.16
3,5-dihydroxyacetophenone 51863–60-6 152,137,109,81 1152/- MS 464.33 ± 146.01 133.35 ± 64.38 393.76 ± 131.95 8.07 ± 1.73 2.53 ± 0.74 4.63 ± 1.82
3-formyl-5,5-dimethyl-2-cycolohexen-1-one 56621–35-3 96,152,109,68 1140/- MS 11.25 ± 4.58 7.9 ± 4.5 11.6 ± 2.73 0.2 ± 0.07 0.15 ± 0.14 0.14 ± 0.05
Ionone 8013–90-9 136,192,177,121 1274/- MS 24.78 ± 2.24 21.03 ± 1.82 26.75 ± 2.72 0.43 ± 0.1 0.4 ± 0.14 0.31 ± 0.07
4-t-butylpropiophone 81561–77-5 147,175,190,105 1379/- MS 12.79 ± 2.06 10.73 ± 0.54 12.8 ± 1.81 0.22 ± 0.05 0.2 ± 0.06 0.15 ± 0.02
Pulegone 89–82-7 110,152,81,95 1244/1237 MS RI STD 81.33 ± 35.64 126.8 ± 40.55 149.09 ± 45.32 1.41 ± 0.61 2.41 ± 1.29 1.75 ± 0.72
2,2,6-trimethyl-cyclohexanone 2408–37-9 82,69,140,56 1030/1030 MS RI 27.27 ± 6.13 7.47 ± 4.21 25.48 ± 7.67 0.47 ± 0.12 0.14 ± 0.05 0.3 ± 0.06
(E,E)-3,5-octadien-2-one 30086–02-3 95,81,109,43 1068/1074 MS RI 44.83 ± 19.41 65.01 ± 35.37 201.01 ± 67.52 0.78 ± 0.33 1.23 ± 0.71 2.36 ± 0.73
Methylheptenone 110–93-0 108,93,69,83 982/981 MS RI STD 24.5 ± 7.36 30.11 ± 10.62 51.9 ± 10.24 0.43 ± 0.13 0.57 ± 0.32 0.61 ± 0.21
Acetophenone 98–86-2 105,120,77,51 1061/1061 MS RI STD 37.65 ± 6.71 83.13 ± 79.03 58.11 ± 10.66 0.65 ± 0.16 1.58 ± 2.44 0.68 ± 0.11
3-octen-2-one 1669–44-9 55,111,126,97 1035/1040 MS RI STD 0 ± 0 0 ± 0 3.02 ± 2.09 0 ± 0 0 ± 0 0.04 ± 0.03



Hydrocarbons
Dehydro-ar-ionene 30364–38-6 157,172,142,109 1350/1349 MS RI 77.44 ± 15.89 32.02 ± 13.1 59.79 ± 19.76 1.35 ± 0.26 0.61 ± 0.17 0.7 ± 0.2
α-ionene 475–03-6 159,174,160,131 1353/1352 MS RI 39.59 ± 7.15 21.85 ± 9.08 33.77 ± 9.44 0.69 ± 0.13 0.41 ± 0.06 0.4 ± 0.11
1,2,3-trimethoxybenzene 634–36-6 168,153,177,105 1311/1316 MS RI STD 36.37 ± 10.22 136.91 ± 90.13 187.02 ± 68.07 0.63 ± 0.27 2.6 ± 1.6 2.2 ± 0.75
1-methylnaphthalene 90–12-0 142,166,141,115 1304/1306 MS RI STD 3.28 ± 0.95 1.06 ± 0.98 5.71 ± 2.01 0.06 ± 0.02 0.02 ± 0.02 0.07 ± 0.03
Naphthalene 91–20-3 128,64,77,102 1178/1178 MS RI STD 4.68 ± 2.8 1.67 ± 1.99 7.72 ± 2.52 0.08 ± 0.07 0.03 ± 0.03 0.09 ± 0.03
Phenylethylene 100–42-5 104,78,103,58 883/888 MS RI 29.96 ± 9.59 32.67 ± 11.56 50.62 ± 22.12 0.52 ± 0.2 0.62 ± 0.28 0.6 ± 0.18
1-ethylcyclohexene 1453–24-3 67,81,95,110 1008/- MS 62.25 ± 37.55 123.2 ± 57.79 319.75 ± 128.28 1.08 ± 0.59 2.34 ± 1.44 3.76 ± 1.16
2,6,10,14-tetramethylpentadecane 1921–70-6 71,57,113,85 1703/1703 MS RI 8.28 ± 2.6 4.6 ± 1 6.97 ± 2.94 0.14 ± 0.08 0.09 ± 0.04 0.08 ± 0.03
2,2′,5,5′-tetramethylbiphenyl 3075–84-1 195,210,165,180 1680/1675 MS RI 5.95 ± 2.18 4.56 ± 0.56 6.04 ± 1.44 0.1 ± 0.05 0.09 ± 0.03 0.07 ± 0.03
α-cedrene 469–61-4 119,161,204,93 1409/1409 MS RI STD 139.09 ± 128.7 244.08 ± 231.94 125.48 ± 55.02 2.42 ± 2.15 4.63 ± 4.44 1.48 ± 0.52
Calamenene 483–77-2 159,204,137,123 1521/1522 MS RI 33.15 ± 14.82 31.97 ± 18.2 33.85 ± 8.1 0.58 ± 0.26 0.61 ± 0.16 0.4 ± 0.07
3,4-dimethoxytoluene 494–99-5 152,137,167,109 1236/1241 MS RI STD 11.9 ± 2.51 10.15 ± 8.01 20.11 ± 14.7 0.21 ± 0.21 0.19 ± 0.1 0.24 ± 0.17
3,5-dihydroxyamylbenzene 500–66-3 124,180,137,81 1523/1528 MS RI 111.34 ± 30.54 89.39 ± 36.43 162.6 ± 37.48 1.93 ± 0.44 1.7 ± 0.67 1.91 ± 0.42
(+)-β-cedrene 546–28-1 161,204,120,93 1418/1420 MS RI 25.43 ± 13.95 37.82 ± 26.96 25.57 ± 6.71 0.44 ± 0.26 0.72 ± 0.51 0.3 ± 0.06
1,6-dimethylnaphthalene 575–43-9 156,141,177,121 1412/1410 MS RI 23.73 ± 4.56 17.24 ± 4.2 29.21 ± 4.88 0.41 ± 0.09 0.33 ± 0.11 0.34 ± 0.07
3,4-diethylbiphenyl 61141–66-0 195,165,210,180 1708/- MS 5.13 ± 1.25 4.11 ± 0.35 5.11 ± 1.04 0.09 ± 0.03 0.08 ± 0.03 0.06 ± 0.02
Hexadecane 638–36-8 71,57,43,85 1806/1809 MS RI 5.12 ± 1.43 3.27 ± 0.39 4.29 ± 1.14 0.09 ± 0.04 0.06 ± 0.02 0.05 ± 0.01
2,6-di-tert-butyl-p-benzoquinone 719–22-2 135,220,177,121 1464/- MS 30.61 ± 12.98 24.49 ± 7.52 34.02 ± 19.35 0.53 ± 0.2 0.46 ± 0.18 0.4 ± 0.25
2-vinylnaphthalene 827–54-3 154,153,128,76 1374/1381 MS RI 28.88 ± 7.07 20.29 ± 5.3 34.13 ± 7.65 0.5 ± 0.09 0.38 ± 0.08 0.4 ± 0.1
Fluorene 86–73-7 166,165,83,109 1573/1572 MS RI 71.62 ± 40.97 35.29 ± 37.67 94.06 ± 64.19 1.24 ± 0.67 0.67 ± 0.44 1.11 ± 1.09
Heptadecane 629–78-7 57,71,85,43 1697/1700 MS RI 12.4 ± 0.36 11.8 ± 0.09 12.06 ± 0.19 0.22 ± 0.05 0.22 ± 0.07 0.14 ± 0.02
m-xylene 108–38-3 91,105,77,56 860/864 MS RI 150.04 ± 41.27 116.47 ± 68.89 267.14 ± 106.14 2.61 ± 0.63 2.21 ± 0.97 3.14 ± 0.91
Phenanthrene 85–01-8 178,176,111,151 1775/1778 MS RI 4.96 ± 0.47 4.64 ± 0.7 5.53 ± 0.76 0.09 ± 0.01 0.09 ± 0.02 0.07 ± 0.01
Acenaphthene 83–32-9 153,77,154,123 1476/- MS 26.92 ± 14.2 13.16 ± 3.39 14.97 ± 2.27 0.47 ± 0.22 0.25 ± 0.05 0.18 ± 0.04
Octadecane 593–45-3 57,71,85,99 1796/1800 MS RI 11.9 ± 0.14 11.63 ± 0.04 11.72 ± 0.06 0.21 ± 0.04 0.22 ± 0.07 0.14 ± 0.02
2-methylnaphthalene 91–57-6 142,141,115,139 1286/1285 MS RI STD 4.35 ± 1.45 1.22 ± 1.36 8.81 ± 3.37 0.08 ± 0.03 0.02 ± 0.02 0.1 ± 0.05



Aldehydes
trans-2-hexenal 6728–26-3 69,83,98,55 843/846 MS RI STD 321.75 ± 87.14 308.27 ± 86.85 565.18 ± 237.91 5.59 ± 1.2 5.85 ± 2.11 6.64 ± 2.07
Hexanal 66–25-1 56,82,95,100 788/800 MS RI STD 129.62 ± 96.94 233.01 ± 88.35 572.45 ± 256.08 2.25 ± 1.8 4.42 ± 2.38 6.73 ± 2.35
Benzaldehyde 100–52-7 106,51,78,106 954/957 MS RI STD 62.64 ± 8.63 57.39 ± 17.62 123.65 ± 31.86 1.09 ± 0.11 1.09 ± 0.22 1.45 ± 0.3
Citral 5392–40-5 69,84,97,137 1266/1268 MS RI 8.74 ± 1.83 19.73 ± 11.09 17.72 ± 6.94 0.15 ± 0.02 0.37 ± 0.14 0.21 ± 0.09
(E)-2-octenal 2548–87-0 70,55,41,83 1054/1056 MS RI 31.09 ± 9.04 39.92 ± 10.16 77.26 ± 28.24 0.54 ± 0.18 0.76 ± 0.36 0.91 ± 0.28
Octanal 124–13-0 43,84,100,110 1000/1001 MS RI 20.24 ± 14.75 17.4 ± 17.68 12.83 ± 18.77 0.35 ± 0.28 0.33 ± 0.42 0.15 ± 0.24
Nonanal 124–19-6 57,98,82,114 1101/1101 MS RI 56.71 ± 36.96 39.64 ± 24.15 177.34 ± 85.64 0.99 ± 0.67 0.75 ± 0.57 2.08 ± 0.78
trans-2-decenal 3913–81-3 70,83,121,55 1257/1258 MS RI 15.23 ± 0.19 16.74 ± 2.9 16.83 ± 0.95 0.26 ± 0.05 0.32 ± 0.09 0.2 ± 0.04
(E,E)-2,4-heptadienal 4313–03-5 81,67,79,110 993/997 MS RI STD 23.04 ± 17.14 37.17 ± 22.92 89.97 ± 62.12 0.4 ± 0.25 0.71 ± 0.56 1.06 ± 0.65
(E,E)-2,4-decadienal 25152–84-5 81,95,152,67 1288/- MS 2.95 ± 0.04 2.97 ± 0.08 3.37 ± 0.34 0.05 ± 0.01 0.06 ± 0.02 0.04 ± 0.01
(E,Z)-2,6-nonadienal 557–48-2 41,70,94,69 1150/1155 MS RI 66.22 ± 3.83 64.37 ± 5.24 86.92 ± 14.74 1.15 ± 0.21 1.22 ± 0.31 1.02 ± 0.07
(E,E)-2,4-nonadienal 5910–87-2 81,138,95,67 1210/1213 MS RI STD 8.92 ± 10.36 26.47 ± 22.54 123.15 ± 68.78 0.15 ± 0.17 0.5 ± 0.59 1.45 ± 0.79
Decanal 112–31-2 57,82,95,112 1203/1200 MS RI STD 1.46 ± 2.42 0 ± 0 11.12 ± 9.05 0.03 ± 0.04 0 ± 0 0.13 ± 0.09
Heptaldehyde 111–71-7 70,81,55,96 896/899 MS RI STD 22.93 ± 8.56 21.24 ± 6.64 59.13 ± 22.31 0.4 ± 0.14 0.4 ± 0.13 0.7 ± 0.2
β-cyclocitral 432–25-7 152,137,123,109 1217/1218 MS RI STD 29.31 ± 4.65 21.27 ± 6.36 40.08 ± 8.4 0.51 ± 0.07 0.4 ± 0.11 0.47 ± 0.1
2,3-dihydro-2,2,6-trimethylbenzalhyde 116–26-7 107,121,150,91 1196/1196 MS RI 19.78 ± 3.66 10.16 ± 1.26 18.56 ± 2.74 0.34 ± 0.05 0.19 ± 0.05 0.22 ± 0.05
2-undecenal 2463–77-6 70,83,97,121 1361/1362 MS RI 15.12 ± 0.08 15.16 ± 0.11 15.54 ± 0.22 0.26 ± 0.05 0.29 ± 0.09 0.18 ± 0.03
1-cyclohexene-1-acetaldehyde 472–67-2 104,107,151,123 1253/1254 MS RI 7.04 ± 0.32 7.18 ± 0.92 9.71 ± 3.05 0.12 ± 0.02 0.14 ± 0.05 0.11 ± 0.05
trans-2-heptenal 18829–55-5 83,55,83,70 950/956 MS RI 5.47 ± 2.81 11.51 ± 5.54 25.73 ± 10.37 0.1 ± 0.05 0.22 ± 0.17 0.3 ± 0.11
Benzylcarboxaldehyde 122–78-1 91,120,92,65 1039/1040 MS RI STD 9.87 ± 18.83 3.34 ± 5.96 20.5 ± 10.23 0.17 ± 0.26 0.06 ± 0.08 0.24 ± 0.12



Phenols
4-amino-3-methylphenol 2835–99-6 123,122,106,94 1045/- MS 164.56 ± 50.11 66.69 ± 14.1 132.95 ± 47.7 2.86 ± 0.93 1.26 ± 0.42 1.56 ± 0.71
4-(2-butyl)phenol 99–71-8 121,150,93,109 1279/- MS 38.22 ± 2.53 37 ± 0.95 42.69 ± 3.75 0.66 ± 0.13 0.7 ± 0.22 0.5 ± 0.13
2,4-di-tert-butylphenol 96–76-4 191,57,162,206 1509/1514 MS RI STD 0.19 ± 0.44 0.11 ± 0.28 0.26 ± 0.63 0 ± 0.01 0 ± 0.01 0 ± 0.01



Alcohols
α-terpineol 98–55-5 121,136,93,81 1188/1188 MS RI STD 68.35 ± 19.5 254.28 ± 276.39 48.79 ± 60.26 1.19 ± 0.34 4.82 ± 3.59 0.57 ± 0.81
Geraniol 106–24-1 69,123,93,111 1251/1251 MS RI STD 40.92 ± 30.75 128.08 ± 251.68 39.99 ± 14.37 0.71 ± 0.43 2.43 ± 3.28 0.47 ± 0.13
1-octen-3-ol 3391–86-4 57,72,85,99 974/974 MS RI STD 27.41 ± 4.44 23.56 ± 3.97 44.71 ± 5.07 0.48 ± 0.05 0.45 ± 0.15 0.53 ± 0.11
Cedrol 77–53-2 95,150,135,119 1598/1598 MS RI STD 64.44 ± 55.53 106.33 ± 113.46 84.06 ± 43.35 1.12 ± 1.07 2.02 ± 2.27 0.99 ± 0.5
Linalool 78–70-6 93,121,71,136 1098/1098 MS RI STD 140.67 ± 74.52 109.3 ± 134.63 97.9 ± 55.73 2.44 ± 1.46 2.07 ± 1.89 1.15 ± 0.67
Phenethyl alcohol 60–12-8 91,122,92,65 1109/1108 MS RI STD 232.35 ± 66.59 230.19 ± 156.58 446.62 ± 208.42 4.04 ± 1.03 4.37 ± 1.97 5.25 ± 2.72
cis-1-p-menthanol 3901–95-9 71,98,123,141 1128/- MS 5.04 ± 1.85 4.09 ± 0.86 7.26 ± 2.76 0.09 ± 0.05 0.08 ± 0.04 0.09 ± 0.05
(-)-α-cadinol 481–34-5 95,161,105,204 1656/1652 MS RI 0.73 ± 1.03 0.16 ± 0.26 0.54 ± 0.79 0.01 ± 0.02 0 ± 0 0.01 ± 0.01
(R,R)-(+)-hydrobenzoin 52340–78-0 107,108,79,77 1020/- MS 20.57 ± 1.57 19.39 ± 6.11 18.4 ± 2.3 0.36 ± 0.06 0.37 ± 0.16 0.22 ± 0.04
2,6-dimethylcyclohexanol 5337–72-4 71,95,110,128 1105/- MS 57.69 ± 13.78 38 ± 11.06 77.47 ± 12.79 1 ± 0.18 0.72 ± 0.31 0.91 ± 0.16
Ledol 577–27-5 58,107,182,95 1402/- MS 12.39 ± 0.57 11.97 ± 0.27 12.79 ± 0.46 0.22 ± 0.04 0.23 ± 0.07 0.15 ± 0.03
t-cadinol 5937–11-1 161,95,204,121 1642/1647 MS RI 11.37 ± 1.9 9.95 ± 0.54 10.3 ± 0.39 0.2 ± 0.06 0.19 ± 0.05 0.12 ± 0.02
Linalool oxide (trans-pyranoid) 39028–58-5 68,94,151,59 1166/- MS 62.29 ± 35.63 50.02 ± 22.94 47.01 ± 18.87 1.08 ± 0.45 0.95 ± 0.21 0.55 ± 0.16
Terpinen-4-ol 562–74-3 71,93,111,154 1174/1179 MS RI STD 17.16 ± 4.83 9.47 ± 3.13 10.7 ± 4.72 0.3 ± 0.08 0.18 ± 0.04 0.13 ± 0.06
2-ethylhexanol 104–76-7 57,98,112,83 1025/1026 MS RI 184.45 ± 148.43 196.17 ± 131.88 456.38 ± 387.04 3.2 ± 2.85 3.72 ± 3.23 5.37 ± 4.08
Benzyl alcohol 100–51-6 108,79,91,107 1029/1031 MS RI STD 522.72 ± 307.55 215.1 ± 180.57 517.89 ± 219.95 9.08 ± 4.09 4.08 ± 2.81 6.09 ± 2.14
Linalool oxide Ⅰ 5989–33-3 59,94,111,68 1070/1068 MS RI 49.73 ± 12.63 46.54 ± 17.17 37.56 ± 7.71 0.86 ± 0.2 0.88 ± 0.09 0.44 ± 0.13
Linalool oxide Ⅱ 34995–77-2 59,111,137,94 1086/1085 MS RI 79.4 ± 35.27 67.39 ± 41.48 55.33 ± 15.11 1.38 ± 0.58 1.28 ± 0.41 0.65 ± 0.17
Nerolidol 7212–44-4 69,81,93,107 1561/1569 MS RI STD 72.32 ± 98.35 7.9 ± 15.08 28.79 ± 21.15 1.26 ± 1.39 0.15 ± 0.32 0.34 ± 0.25
Heptanol 111–70-6 70,56,69,98 964/969 MS RI STD 20.01 ± 2.36 21.3 ± 2.03 27.08 ± 4.43 0.35 ± 0.05 0.4 ± 0.13 0.32 ± 0.11
Pyrrole
Tetramethylpyrazine 1124–11-4 136,54,42,95 1083/- MS STD 31.66 ± 2.51 131.83 ± 166.84 186.31 ± 148.7 0.55 ± 0.08 2.5 ± 2.06 2.19 ± 1.34



Lactones
Dihydroactinolide 17092–92-1 111,137,180,109 1525/1526 MS RI STD 81.5 ± 82.12 7.03 ± 13.12 128.46 ± 56.33 1.42 ± 1.08 0.13 ± 0.17 1.51 ± 0.59
γ-nonanolactone 104–61-0 85,119,162,91 1359/1360 MS RI 90.57 ± 43.11 60.18 ± 25.26 111.24 ± 64.52 1.57 ± 0.57 1.14 ± 0.79 1.31 ± 1.17



Others
Caffeine 58–08-2 194,109,85,71 1846/1846 MS RI 1.12 ± 0.41 1.16 ± 0.47 1.38 ± 0.31 0.02 ± 0 0.02 ± 0.01 0.02 ± 0
Oleic acid 112–80-1 57,69,95,71 2172/2140 MS RI 11.54 ± 0 11.53 ± 0 11.54 ± 0 0.2 ± 0.04 0.22 ± 0.07 0.14 ± 0.02
Palmitic acid 57–10-3 73,129,57,83 1958/1963 MS RI 6.98 ± 2.34 5.33 ± 1.68 7.57 ± 2.03 0.12 ± 0.02 0.1 ± 0.03 0.09 ± 0.02
(+)-carotol 465–28-1 161,119,71,85 1768/- MS 9.32 ± 0.22 9.46 ± 0.41 9.45 ± 0.23 0.16 ± 0.04 0.18 ± 0.05 0.11 ± 0.02
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a

Ions monitored for quantitation. The underlined ions were the quantified ones while the others were the identified ones.

b

Retention index of compounds on HP-5MS.

c

Retention index of compounds in reference.

d

“MS” mass spetrum comparison using NIST17 library. “RI” retention index in agreement with literature value. “STD” confirmed by authenic standards.

Fig. 2.

Fig. 2

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Differences of the volatiles content (A) and proportion (B) in three aroma types of Fu brick tea. JHX: floral-fungal aroma type. SJX: ripe-fungal aroma type. QJX: fresh-fungal aroma type.

To obtain a preliminary overview of similarities and differences among the different aroma types of Fu brick tea, unsupervised PCA were carried out based on the absolute content of identified volatile compounds, and most of the chemical phenotypes of the samples were well discriminated according to the aroma types of Fu brick tea. As shown in Fig. 3A (PC1, 26.4%; PC2, 13.1%; R2X = 0.852), there was a clear differentiation among the three aroma types of Fu brick tea samples, suggesting that the volatile profile of tea samples differed dramatically in their content levels. Furthermore, HCA analysis was also performed to distinguish the similarities and differences of volatile composition and content in different tea samples, which was consistent with the PCA results (Fig. 3B). Overall, Fu brick teas could be divided into three groups based on their aroma types: JHX type (JHX-1 ∼ JHX-8), SJX type (SJX-1 ∼ SJX-8) and QJX type (QJX-1 ∼ QJX-9). However, the absolute content of volatile compounds could not reflect the contribution of each compounds for the aroma characteristics formation in tea samples due to the absolute content of volatiles in QJX type samples was much higher than that in the other two types. Therefore, the percentage content of each volatile was used to explore the different volatile components in three types of Fu brick tea. Based on these results, three OPLS-DA models were established to investigate the differential volatiles contributing to distinguish different aroma types, including OPLS-DA model Ⅰ (JHX type and SJX type), OPLS-DA models Ⅱ (JHX type and QJX type) and OPLS-DA models III (SJX type and QJX type), respectively (Fig. S1). Two conditions need to be fulfilled for identifying the discriminatory volatiles in three aroma types Fu brick tea: the values of predictive component variable importance in the projection (VIP) ≥ 1 and p-value ≤ 0.05. Based on these criteria, 75 volatiles were identified as discriminatory volatile compounds (VIP > 1.0, p < 0.05) among the three groups. Obviously, the level of these discriminatory volatiles showed a dramatically different among the three aroma types of Fu brick tea (Fig. S2). In total, there were 29 discriminatory volatile compounds displayed the highest amount in JHX type samples, while 27 and 19 discriminatory volatile compounds had the highest content in SJX type and QJX type samples, respectively. Moreover, the higher level of discriminatory ketone and alcohol compounds in JHX type of Fu brick tea, including β-ionone, 3,5-dihydroxyacetophenone, ionone, benzyl alcohol, linalool, nerolidol, linalool oxide (trans-pyranoid), might cause the aroma characteristics of JHX type of Fu brick tea differentiate from that in the other two aroma types (Huang et al., 2011, Lv et al., 2014, Shi et al., 2019). Also, the higher level of discriminatory ketone and aldehyde compounds in SJX type of Fu brick tea, such as acetophenone, dihydro-β-ionone, pulegone, (E,Z)-2,6-nonadienal, might contributed to the difference formation in the aroma characteristics of SJX type of Fu brick tea, as well as the higher level of discriminatory aldehyde compounds in QJX type of Fu brick tea, including hexanal, (E,E)-2,4-heptadienal, benzaldehyde, nonanal, (E,E)-2,4-nonadienal (Li et al., 2019, Li et al., 2020, Nie et al., 2019, Shi et al., 2019, Xu et al., 2007).

Fig. 3.

Fig. 3

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PCA (A) and HCA (B) analysis based on the identified volatile compounds in different aroma types of Fu brick tea. JHX: floral-fungal aroma type. SJX: ripe-fungal aroma type. QJX: fresh-fungal aroma type.

3.3. Differential aroma compounds in three aroma types of Fu brick tea

It is well known that the contribution of a volatile compound to the overall aroma of tea was not only depended on the concentration but also relied on its odor threshold. In our study, the odor descriptions and OAV values of volatile compounds were presented in Table S2, based on the reported references. Generally, there were 54 volatiles with OAV > 1 and 17 volatiles with OAV < 1. It has shown that the OAV value was directly proportional to the contribution degree of aroma (Liu, Zhou, & Xu, 2008). In this study, there are 28 aroma compounds with OAV > 10 were common to the three aroma types of Fu brick tea, including eight aroma compounds with floral attribute, nine with green attribute, four with woody attribute, two with fatty attribute, one with pungent attribute, two with stale attribute, one with earthy attribute and one with fruity attribute. All these aroma compounds provided important contribution to the basic aroma characteristics formation of Fu brick tea. Xu et al. (2007) reported that the compounds with floral and stale attributes in combination with some components of the raw material, which were together contributed to the ‘fungal flower’ aroma characteristics formation in Fu brick tea. However, some volatile compounds lacking OAV values might also contribute to the aroma characteristics formation of Fu brick tea. Fox example, (+)-β-cedrene and α-cedrene were found to be important compounds contributing to the woody flavour in Pu-erh tea (Xu et al., 2016, Lv et al., 2012). t-Cadinol was considered to have an important contribution to the formation of on the chestnut-like aroma in green tea (Wang, Hua, Jiang, Yang, Wang, & Yuan, 2020).

Combined with OPLS-DA results, 39 aroma compounds were considered as the discriminatory aroma compounds (VIP > 1, p < 0.05, OAV > 1) among the three aroma types of Fu brick tea (Fig. 4). For JHX type of Fu brick tea, there are 10 aroma compounds considered as discriminatory aroma compounds, including benzyl alcohol, linalool, linalool oxide Ⅱ dehydro-ar-ionene, nerolidol, α-ionene, β-ionone, 2,2,4-trimethyl-1,3-pentanediol diisobutyrate, β-cyclocitral, 3,4-dehydro-β-ionone. The above discriminatory aroma compounds were presented at the highest level in the JHX type of Fu brick tea when compared with the other two aroma types. The aroma characteristics of JHX type of Fu brick tea was dominated by floral and fruity attributes, mixing with stale and woody attributes to form the floral-fungal aroma characteristics. Benzyl alcohol has the highest content in all volatile components of Fu brick tea, accounting for 6.4%. Also, previous studies reported that benzyl alcohol provided an important contribution to the floral attribute in Fu brick tea (Li et al., 2020, Wang et al., 2019). Linalool and linalool oxide Ⅱ possessing typical floral attribute, could enhance the intense of floral attribute and decreased the intense of roasted attribute in instant white tea (Demyttenaere & Willemen, 1998). In addition, those two compounds were also identified as the important contributors to the ‘fungal flower’ aroma formation of Fu brick tea (Xu et al., 2007). Nerolidol is a sesquiterpene present in many teas, which was one of the most important factors to form the floral and fruity aroma in oolong tea and was also identified as the key contributor to the roses and apples aroma of black tea (Xu et al., 2016, Zhu et al., 2015). β-Ionone with floral attribute was derived from carotenoids, which played an important role in the aroma characteristic formation of many teas, such as Fu brick tea, due to its extremely low odor threshold (Li et al., 2020). Furthermore, β-cyclocitral, impart fruity attribute, which could be produced by the degradation of β-carotene, was also considered as one of the important compounds to form the ‘fungal flower’ aroma of Fu brick tea (Wang, Ma, Shi, Zhu, Lin, & Lv, 2020). Dehydro-ar-ionene presented with woody attribute, which was generated by the hydrolysis of aroma glycosides and contributed to the aroma characteristics formation in oolong tea (Guo, Ho, Wan, Zhu, Liu, & Wen, 2021).

Fig. 4.

Fig. 4

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Heatmap analysis for the discriminatory aroma compounds among the three aroma types of Fu brick tea. JHX: floral-fungal aroma type. SJX: ripe-fungal aroma type. QJX: fresh-fungal aroma type.

For the SJX type of Fu brick tea, 11 aroma compounds presented the highest level compared with the other two aroma types were identified as discriminatory aroma compounds, including tetramethylpyrazine, α-terpineol, 1,2,3-trimethoxybenzene, pulegone, acetophenone, (E,Z)-2,6-nonadienal, dihydro-β-ionone, citral, trans-2-decenal, (+)-carotol, (E,E)-2,4-decadienal. Those compounds impart woody, stale and fatty attributes were found to dominate in SJX type of Fu brick tea, combining with those of which contributed to roasted, floral, fruity, green and mint scents to form the ripe-fungal aroma characteristics. α-Terpineol with woody note has been considered as an important aroma compound for the smoky or stale aroma characteristics in Pu-erh tea, which was hydrolyzed from glycoside precursors by microbial enzyme during the post-fermentation process (Lv, Zhong, Lin, Wang, Tan, & Guo, 2012). Dehydro-β-ionone with woody note was widely presence in tea, as the fermented product of β-ionone by fungal enzyme. It was also reported to be the key aroma compound for the ‘aging fragrance’ characteristics formation in the aging-storage Qingzhuan tea (Zhang et al., 2021). 1,2,3-trimethoxybenzene is well known as a typical stale compound that was considered to be the most important contributor to the stale characteristics of Pu-erh tea (Lv et al., 2014). It was transformed by fungal methylate action from gallic acid and the methylation of phenolic hydroxyl groups by Aspergillus niger during the pile-fermentation process (Lv et al., 2014). (E,E)-2,4-decadienal with fatty note was also considered as an important aroma compound in Chinese dark tea (Cao et al., 2018). Tetramethylpyrazine is a pyrazine compound with roasted attribute, which contributed to the characteristic aging fragrance formation of Chinese liquor and aged vinegar (Zhang et al., 2020, Zhu et al., 2016). Furthermore, acetophenone was mainly contributed to a floral note, which was identified as the basic aroma compounds of Fu brick tea (Li et al., 2020, Shi et al., 2019). Citral is a monoterpene aldehyde with fruity note that could be converted from carotenoids during the post-fermentation of tea, and it was considered to be a major constituent of essential oils in many lemon-scented aromatic plants (Dudai, Weinstein, Krup, Rabinski, & Ofir, 2005). (E,Z)-2,6-nonadienal was the key odorant in Chinese black teas and Darjeeling black tea, which could enhance green aroma in tea infusion (Chen et al., 2019, Greger and Schieberle, 2007). Pulegone was identified as the main aroma compound of peppermint that was usually used as herbal teas, spices and essential oil, due to its minty note and high relative odor intensity (Diaz Maroto, Castillo, Castro Vazquez, Gonzalez Vinas, & Perez Coello, 2007).

For the QJX type of Fu brick tea, there were 18 compounds considered as discriminatory aroma compounds, including hexanal, (E,E)-3,5-octadien-2-one, nonanal, dihydroactinolide, benzaldehyde, (E,E)-2,4-nonadienal, 1,2-dimethoxybenzene, (E,E)-2,4-heptadienal, (E)-2-octenal, heptaldehyde, (E)-2-heptenal, 4-methylanisole, 2-pentylfuran, benzylcarboxaldehyde, decanal, 2-methylnaphthalene, naphthalene, 1-methylnaphthalene. The highest content of these 18 compounds were found in the fresh-fungal aroma type of Fu brick tea compared with the other types. The aroma characteristics of QJX type of Fu brick tea was mainly formed by green attribute, combining with fruity, stale and pungent notes to form the fresh-fungal aroma characteristics. (E,E)-3,5-Octadien-2-one, (E,E)-2,4-heptadienal, (E,E)-2,4-nonadienal, benzylcarboxaldehyde and decanal exhibit a freshly cut grass smell and were all identified as the key aroma compounds to form green attribute of the post-fermentation dark tea in previous studies, such as Fu brick tea and Pu-erh tea (Cao et al., 2018, Li et al., 2020, Lv et al., 2012, Shi et al., 2019). Hexanal, (E)-2-octenal, 2-pentylfuran and benzylcarboxaldehyde also with typical green attribute was often reported to contribute to form the characteristic green aroma of non-fermentation or full-fermentation tea, such as green tea and black tea (Kang et al., 2019, Wang et al., 2020, Zhu et al., 2018). Benzaldehyde is an important odor compound with fruity note, which often present in the chestnut-like aroma of green tea (Zhu et al., 2018). Naphthalene, 1-methylnaphthalene and 2-methylnaphthalene contributed with pungent and herbal-like notes, which could be degraded by microorganisms (Tanguler, Selli, Sen, Cabaroglu, & Erten, 2017). Naphthalene with pungent note, converted from long-chain hydrocarbons, was identified as the key odorant in yellow tea and Wuyi rock tea (Liu et al., 2021, Shi et al., 2021). 1-Methylnaphthalene also possessing pungent note was reported to contribute to the aroma of Liupao tea, which was usually used as a potential indicator to evaluate the degree of fermentation (Ma et al., 2020). 1,2-Dimethoxybenzene mainly contributed to a stale note, which was regard as important compound in the formation of the special flavor in Pu-erh tea (Lv et al., 2012).

3.4. Relationship between aroma compounds and sensory attributes in three aroma types of Fu brick tea

To explore the relationships among the aroma compounds (OAV > 1), sensory attributes and tea samples, PLS analysis was performed. Two latent variables were included in the PLS model, which represents 82.8% of X-matrix variance, explained 79.6% of Y-matrix variance. As shown in Fig. 5, samples appeared to be divided into three groups according to the different aroma types. Among them, floral-fungal aroma type of samples were mainly distributed at the positive value of PC1 and PC2, ripe-fungal aroma type of samples were mainly located at the positive values of PC1 and negative values of PC2, fresh-fungal aroma type of samples were mainly distributed at the negative value of PC1 and PC2. The first PC was defined by the aroma descriptors showing ‘green’ attribute on the negative dimension and ‘herbal’, ‘floral and fruity’, ‘stale’, ‘woody’ attributes on the positive dimension.

Fig. 5.

Fig. 5

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PLS plots for the tea samples, the sensory attributes and aroma compounds (OAV＞1). M38: 3,4-dehydro-β-ionone. M36: dihydro-β-ionone. M42: (+)-carotal. M14: linalool oxide. M21: α-terpineol. M43: 2-pentylfuran. M45: decanal. M16: nonanal. M4: heptaldehyde. M40: dihydroactinolide. M53: 4-methylanisole. M23: (E,E)-2,4-nonadienal. M44: benzylcarboxaldehyde. M51: (E,E)-3,5-octadien-2-one. M46: 2-methylnaphthalene. M30: 1-methylnaphthalene. M20: naphthalene. M5: benzaldehyde.

The ‘green’ attribute was positively correlated to 13 compounds including decanal (M45), nonanal (M16), heptaldehyde (M4), (E,E)-2,4-nonadienal (M23), benzylcarboxaldehyde (M44), benzaldehyde (M5), naphthalene (M20), 1-methylnaphthalene (M30), 2-methylnaphthalene (M46), (E,E)-3,5-octadien-2-one (M51), 2-pentylfuran (M43), 4-methylanisole (M53) and dihydroactinolide (M40). Previous study has shown that most of aldehydes were generally associated with ‘green’ attributes (Xu et al., 2007). The ‘floral and fruity’ attribute was significantly positive related to 3,4-dehydro-β-ionone (M38), dihydro-β-ionone (M36), (+)-carotol (M42) and linalool oxide Ⅱ (M14). It is well known that ketone compounds are very important volatile compounds in various teas, providing a special floral attribute, and appear to be the unique volatile compounds in Fu brick tea (Lv et al., 2014). The ‘woody’ and ‘stale’ attributes were only significantly related to α-terpineol (M21). α-Terpineol is a tertiary monoterpenoid alcohol, which could be produced by many microorganisms and was also considered to be an important component of the basic aroma of dark tea (Nie et al., 2019, Sales et al., 2020). In addition, it should be noted that ‘herbal’ attribute is located in the inner ellipse, which indicated that this attribute was not well described by the PLS model, perhaps due to the masking interactions with other attributes.

4. Conclusion

In summary, Fu brick tea could be divided into three aroma types by QDA analysis according to the differences of aroma characteristics, including floral-fungal aroma type, ripe-fungal aroma type and fresh-fungal aroma type. Among them, floral-fungal aroma type of Fu brick tea had strong ‘floral and fruity’ attribute, ripe-fungal aroma type with strong ‘stale’ and ‘woody’ aroma attributes, and green-fungal aroma type with strong significant ‘green’ attribute. A total of 112 volatile compounds were identified and quantified in Fu brick tea samples by HS-SPME/GC–MS analysis. According to OAV analysis, 54 voaltile compounds were identified as aroma compounds. Combined with OPLS-DA analysis, ten aroma compounds dominated with floral and fruity attributes in JHX type samples, eleven aroma compounds dominated with woody, stale and fatty attributes in SJX type samples, and eighteen aroma compounds dominated with green attribute in QJX type samples were identified as key aroma compounds that were responsible for the different aroma characteristics formation in the three aroma types of Fu brick tea. Furthermore, 3,4-dehydro-β-ionone, dihydro-β-ionone, (+)-carotol and linalool oxide Ⅱ were identified as key contributors to ‘floral and fruity’ attribute, ‘α-terpineol was contributed to ‘woody’ and ‘stale’ attributes, and thirteen aroma compounds that mainly composed of aldehydes were related to ‘green’ attribute by PLS analysis. This study provided the useful information for understanding the formation mechanisms of different aroma characteristic in Fu brick tea. Further research can focus on whether the volatile compounds with low OAVs have contribution to the aroma characteristics formation and the interaction of these aroma compounds in the different aroma types of Fu brick tea.

CRediT authorship contribution statement

Zheng Xuexue: Investigation, Visualization, Software, Writing – original draft. Hong Xin: Investigation, Visualization, Software, Writing – original draft. Jin Youlan: Investigation, Methodology, Software, Validation, Resources. Wang Chao: Validation, Writing – review & editing. Liu Zhonghua: Resources, Writing – review & editing. Huang Jianan: Conceptualization, Investigation, Writing – review & editing, Project administration, Funding acquisition. Li Qin: Conceptualization, Investigation, Writing – original draft, Writing – review & editing, Project administration, Funding acquisition.

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 by the National Natural Science Foundation of China (31871764); and the China Tea Research System Project (CARS-23); and the Open Project Fund of College of Horticulture of Hunan Agricultural University (2021YYXK003); and the “1515 Talent Project” of Hunan Agricultural University.

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.fochx.2022.100248.

Contributor Information

Huang Jianan, Email: jianan-huang@hotmail.com.

Li Qin, Email: liqinvip@hotmail.com.

Appendix A. Supplementary data

The following are the Supplementary data to this article:

Supplementary data 1
mmc1.docx (2.1MB, docx)
Supplementary data 2
mmc2.docx (5MB, docx)
Supplementary data 3
mmc3.docx (24KB, docx)
Supplementary data 4
mmc4.xlsx (156.8KB, xlsx)

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Associated Data

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Supplementary Materials

Supplementary data 1
mmc1.docx (2.1MB, docx)
Supplementary data 2
mmc2.docx (5MB, docx)
Supplementary data 3
mmc3.docx (24KB, docx)
Supplementary data 4
mmc4.xlsx (156.8KB, xlsx)

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