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Journal of Food Science and Technology logoLink to Journal of Food Science and Technology
. 2018 Jan 12;55(3):1185–1195. doi: 10.1007/s13197-018-3034-0

Flavor characteristics and chemical compositions of oolong tea processed using different semi-fermentation times

Pan-Pan Liu 1,2, Jun-Feng Yin 1, Gen-Sheng Chen 1, Fang Wang 1, Yong-Quan Xu 1,✉
PMCID: PMC5821678  PMID: 29487461

Abstract

Flavor characteristics and chemical compositions of Tieguanyin oolong tea processed using different semi-fermentation times were investigated. Six flavor attributes of the teas, namely, astringency, bitterness, umami, sweet aftertaste, floral flavor, and green fruity flavor, were analysed. With extended semi-fermentation time, the taste intensity of sweet aftertaste increased, and the aroma intensity of floral and green fruity flavors increased, while the intensities of astringency, bitterness, and umami showed no clear trend. With increasing semi-fermentation time, the concentrations of gallated catechins, myricetin-rhamnose, quercetin-rutinoside, myricetin, and theanine greatly decreased, while those of total theaflavins, vitexin-rhamnose, kaempferol-galactose, kaempferol-rutinoside, vitexin, quercetin, and kaempferol increased significantly. The intensity of bitter taste was positively correlated with the concentrations of total catechins and gallated catechins. The intensity of astringent taste strongly correlated with the flavonoid concentrations, and that of sweet aftertaste positively correlated with the concentrations of (−)-epigallocatechin and (−)-epicatechin. However, dose-over-threshold analysis revealed that catechins, theaflavin, flavonol glycosides, and caffeine are the main taste-active compounds contributing to the taste of Tieguanyin oolong tea. The concentrations of total volatiles and most of the esters increased markedly with the semi-fermentation time, while the concentrations of low aldehydes showed a significant decrease. The flavor index was consistent with the intensity of floral aroma, increasing from 0.59 (12 h) to 0.84 (24 h) and then decreasing to 0.63 (32 h). Results of this work suggest that the flavor change is mainly due to the variation of taste-active and aroma-active compounds in oolong tea.

Keywords: Tieguanyin oolong tea, Flavor attributes, Semi-fermentation, Chemical compounds

Introduction

Tea is one of the most popular and widely consumed beverages in the world. Tieguanyin, a premium variety of oolong tea prepared by light semi-fermentation (20–50%), is very popular in China (Chen et al. 2013). Three kinds of Tieguanyin with different flavors (Zhengzuo, Xiaoqing, and Tuosuan) that are produced by varying the semi-fermentation time are consumed in China (Xin et al. 2012).

The flavor of oolong tea has been reported to be related to a combination of various compounds (Chen et al. 2010b), such as catechins (bitterness), amino acids (freshness), soluble sugar (sweetness), theaflavins (briskness), and thearubigin (mellowness). A series of flavonol glycosides have been found to be a major contributor to the astringent taste of black tea infusion (Scharbert et al. 2004). Many of the main taste compounds in oolong tea, such as 2 alkaloids, 11 flavan-3-ols, 8 organic acids and esters, and 3 theaflavins, have been identified by high-performance liquid chromatography (HPLC) combined with UV spectrophotometry and mass spectrometry (Dou et al. 2007).

Semi-fermentation has been found to significantly influence the flavor of oolong tea (Wang et al. 2001). Semi-fermentation during the processing of oolong tea involves natural browning reactions induced by oxidative enzymes in the cells of tea leaves (Haslam 2003). However, the flavor characteristics of Tieguanyin oolong tea are not yet understood. This study aimed to investigate the flavor characteristic of Tieguanyin oolong tea from fresh tea leaves processed for different semi-fermentation times. We analyzed the relationship between flavor and chemical components. Our results may provide useful information for modifying fermentation process for varying flavors of Tieguanyin oolong teas by changing its flavor characteristics and concentrations of its chemical components.

Materials and methods

Chemicals

Standard catechins [gallocatechin (GC), epigallocatechin (EGC), epigallocatechin gallate (EGCG), epicatechin (EC), gallocatechin gallate (GCG), epicatechin gallate (ECG)), theaflavins (theaflavin (TF), theaflavin-3-gallate (TF3G), theaflavin-3′-gallate (TF3′G), and theaflavin-3,3′-digallate (TFDG)], flavonol glycosides [vitexin (Vit), quercetin (Que), myricetin (Myr), kaempferol (Kae), myricetin-rhamnose (Myr-rha), myricetin-galactose (Myr-gala), quercetin-galactose (Que-gala), quercetin-glucose (Que-glu), vitexin-rhamnose (Vit-rha), kaempferol-galactose (Kae-gala), kaempferol-rhamnose (Kae-rha), and kaempferol-glucose (Kae-glu)], amino acids (phosphoserine, aspartic acid, threonine, serine, glutamic acid, theanine, alanine, cysteine, tyrosine, phenylalanine, β-alanine, γ-aminobutyric acid, histidine, tryptophan, lysine, and arginine), caffeine, sodium glutamate, glucose, and volatiles (linalool, cis-3-hexenyl hexanoate, ethyl caprate and n-paraffins C6–C20) were purchased from Sigma (Shanghai, China). Quercetin-rutinoside (Que-rut) and quercetin-rhamnose (Que-rha) were purchased from Mansite Bio-Technology Co., Ltd. (Chengdu, China).

Tea leaves

Tea plant shoots [Camellia sinensis (L.) O. Kuntze cv. Tieguanyin] with four leaves and a bud were harvested in April 2013 from Anxi County in Fujian Province. After the tea leaves were wilted at 35 °C for 40 min, they were spread in an air-conditioned room (20 °C) to dry for 1 h. We then tossed and rolled the leaves three times (5 min the first time, 15 min the second time, and 40 min the third time), leaving them for 2 h in between to allow them to partially ferment. Subsequently, they were panned for different times (12, 16, 20, 24, 28, and 32 h) in an air-conditioned room (20 °C, 70% relative humidity). Afterward, the fermented leaves were panned at 170 °C for 12 min, kneaded and loosened five times, and then roasted at 110 °C for 30 min to a final moisture content of 4%. The dry leaves were finally stored in a freezer at − 20 °C for further use.

Preparation of tea infusion

Three grams of Tieguanyin oolong tea sample was infused with 150 mL of freshly boiled pure water for 6 min, in accordance with the Chinese national standards of tea sensory evaluation (Gong et al. 2009). The infusions were immediately used for sensory evaluation and chemical composition analysis.

Sensory evaluation

Reference materials containing monomers of epigallocatechin gallate, caffeine, sodium glutamate, glucose, linalool, and cis-3-hexenyl hexanoate were used in descriptive analysis. They were used at different concentrations (Table 1). Prior to the evaluation of the oolong tea infusion, panelists were screened and trained to mark the flavor of reference materials at different concentrations. A 10-point scale based on the method described by Alasalvar et al. (2012) was used for scoring. The evaluation, which was based on a 10 cm linear scale of 0 (weakest) to 10 (strongest), was done by ten well-trained panelists. Each evaluation was randomly replicated three times on different days.

Table 1.

Effect of semi-fermentation time on the taste intensity of Tieguanyin oolong tea infusions

Attributes Definition Reference materials (1–10 points) 12 h 16 h 20 h 24 h 28 h 32 h
Astringent Feeling of tightening in mouth 0.2–10 g/L EGCG 3.1b 2.5c 2.1c 4.0a 3.0b 3.5ab
Bitter The degree of bitterness in mouth 0.2–10 g/L Caffeine 1.7ab 1.9a 1.7ab 1.4b 1.6b 1.5b
Umami Flavor of umami 0.05–1 g/L Sodium glutamate 2.8a 2.6ab 3.0a 2.3b 2.2b 2.5ab
Sweet aftertaste Sweet remained in mouth after spitting out 5–100 g/L Glucose 3.2c 4.1b 4.4b 3.8bc 5.4a 5.5a
Floral Flavor of flower 10−6–10−3 g/L Linalool 6.4c 7.5b 8.2a 8.6a 5.6d 4.1e
Green fruity Flavor of green fruit 10−5–10−3 g/L cis-3-Hexenyl hexanoate 2.6d 3.3c 3.7c 3.7c 6.3b 7.6a
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Data are means (± standard deviation) of three replicates

a–cData marked with different letters in the same row are not significantly different (p > 0.05)

Analysis of chemical compositions

Catechins and theaflavins

Catechins and theaflavins were analyzed by HPLC (Liang et al. 2003). The tea infusion was filtered through a 0.2 μm Millipore filter before injection [Model Waters 600E, Waters (Shanghai) Corporation, Shanghai, China)] The HPLC conditions were as follows: 10 μL injection volume, a 5 μm Diamonsil™ C18 (4.6 mm × 250 mm) column, 35 °C temperature, acetonitrile/acetic acid/water (6:1:193) solution as mobile phase A, acetonitrile/acetic acid/water (60:1:139) solution as mobile phase B, linear gradient of 100% mobile phase A to 100% mobile phase during the first 45 min and then 100% mobile phase B up to 60 min, 1 mL/min flow rate, and a Waters 2487 ultraviolet detector [Waters (Shanghai) Corporation] set at 280 nm.

Flavonol glycosides and flavonols

Flavonol glycosides and flavonols were analyzed by HPLC (Wu et al. 2012). The tea infusion was filtered through a 0.2 μm Millipore filter before injection [Model Waters 600E, Waters (Shanghai) Corporation]. The HPLC conditions were as follows: 10 μL injection volume, a 5 μm Diamonsil™ C18 (4.6 mm × 250 mm) column, 25 °C temperature, acetonitrile/formic acid/water (20:1:646) solution as mobile phase A, and acetonitrile/formic acid/water (200:1:466) solution as mobile phase B. The gradient elution was started at 100% mobile phase A and 0% mobile phase B in which the mobile phase A concentration is maintained for 23 min. The mobile phase B concentration was thereafter increased linearly to 50% within 7 min, to 62.5% within 10 min, to 80% within 5 min, and to 100% within 3 min. Finally, the mobile phase B concentration was maintained at 100% for 35 min. The flow rate was 1 mL/min, and the detector was a Waters 2487 ultraviolet detector [Waters (Shanghai) Corporation,] set at 360 nm.

Free amino acids

Five milliliters of tea infusion was evaporated, and the dried sample was dissolved in 0.02 N HCl solution. Free amino acid components were determined with an amino acid analyzer (Hitachi 835-50, Tokyo, Japan). The relevant conditions were a single microbore stainless-steel column (2.6 mm inside diameter, 15 cm length), a maximum of five programmable eluting buffers, and one regenerant. The column material was a Hitachi custom cation-exchange #2619F resin. Buffers and ninhydrin flow rates were 0.25 and 0.30 mL/min, respectively. In brief, amino acids separated by cation-exchange chromatography were detected spectrophotometrically after postcolumn reaction with ninhydrin reagent. The free amino acid content of each sample was calculated through the area normalization method (Zhao et al. 2008).

Volatiles

A Solid Phase Micro-extraction (SPME) device (Supelco, Bellefonte, PA, USA) consisting of a fused silica fiber coated with 50/30 µm divinylbenzene/carboxen/polydimethylsiloxane was used. The fiber was preconditioned for 10 min in the injection port of the gas chromatograph at 260 °C to remove any volatiles remaining on the fiber before each extraction. Three grams of dry tea sample was transferred to a 500 mL vial and infused with 150 mL of boiling water, and then 20 μL Eethyl caprate solution was added. The vial was kept in a water bath at 50 °C to equilibrate for 5 min, and then the SPME fiber was exposed for 30 min to the headspace while the sample was maintained at 50 °C. After sampling of the SPME liquid, the SPME fiber was introduced into the gas chromatograph injector in splitless mode, and held there for 3 min to allow analyte thermal desorption.

An Agilent (Las Vegas) 6890 gas chromatograph interfaced with an Agilent HP 5973 MSD ion-trap mass spectrometer was used to perform the analysis. A DB-5MS capillary column (60 m × 0.25 mm × 0.32 µm) was used separately. The gas chromatograph oven temperature was programmed as follows: holding at 50 °C for 5 min, an increase to 180 °C at a rate of 3 °C/min, holding at 180 °C for 2 min, an increase to 250 °C at a rate of 10 °C/min, and finally holding at 250 °C for 3 min. The carrier gas, helium (> 99.999% percentage purity), was used at a constant flow rate of 1 mL/min. The mass spectrometer was operated in the EI mode at an electronic energy of 70 eV. The injector and ion source temperatures were 250 and 230 °C, respectively, and the scanning range for MS was 35–400 AMU. Identification of the volatile compounds was based on the National Institute of Standards and Technology library (98L). The linear retention indices (RI) calculated by using n-paraffins C6–C20 as external references (Alasalvar et al. 2012) were referenced against previously published Kovat indices. The concentrations of the constituents calculated on the basis of the IS solution were in µg/L, and experiments for each tea sample were in triplicate.

Statistical analysis

All recorded results, which were from three replicates, were reported as means. Significant differences between means were analyzed by one-way ANOVA using SPSS (version 16, SPSS Inc., Chicago, IL, USA). Correlation between data (in terms of r) was evaluated using Pearson’s correlation analysis.

Results and discussion

Effect of semi-fermentation time on the taste of Tieguanyin oolong teas

The taste and aroma attributes of Tieguanyin oolong tea infusion were influenced by the semi-fermentation time (Table 1). With increasing semi-fermentation time, the semi-fermentation degree increased, and the intensities of sweet aftertaste and green fruit flavor increased, while the intensity of flower flavor increased and then decreased; the intensities of astringent, bitter, and umami taste showed no significant trend. The above changes may be due to the alteration of taste-related compounds in the infusion (Table 1). Semi-fermentation degree has also been reported to affect the aroma quality of Tieguanyin oolong tea via changes in chemical composition. It is therefore interesting how semi-fermentation time affects the flavor of oolong teas.

Effect of semi-fermentation time on the concentration of taste-active compounds in Tieguanyin oolong tea infusion

Catechins and theaflavins

A mixture of six catechins (GC, EGC, EGCG, EC, GCG, and ECG) and four theaflavins (TF, TF3′G, TF3″G, and TFDG) were identified by HPLC (Table 2). The total catechin concentration varied between 380.85 and 389.81 mg/L. We determined EGCG (126.65–146.60 mg/L), EGC (74.44–89.66 mg/L), EC (66.43–74.19 mg/L), and ECG (37.73–43.32 mg/L) to be the main catechins, with all four constituting 87.2% of the total catechins on average (Table 2). With increasing semi-fermentation time, the total catechin concentration decreased. In particular, the concentration of gallated catechins greatly decreased because they hydrolyze into simple catechins and gallic acid. Furthermore, the gallic acid concentration increased markedly during processing of the oolong tea because of EGCG decomposition (Chen et al. 2013). The total theaflavin concentrations of the oolong teas ranged from 14.19 to 16.97 mg/L. TF was the most abundant compound, representing 77.8–85.8% of the total theaflavins. These results indicate that the semi-fermentation of oolong tea affects theaflavin synthesis. The total catechin concentrations decreased during the fermentation, while the total theaflavin concentrations tended to increase. Catechins can be oxidized into theaflavins and thearubigins by oxidase (Vuong et al. 2010).

Table 2.

Effect of semi-fermentation time on the concentrations of catechins and theaflavins in Tieguanyin oolong tea infusions (μg/mL)

Semi-fermentation time Catechins Theaflavins
GC EGC EGCG EC GCG ECG Total TF TF3G TF3’G TFDG Total
12 h 21.47c 74.44c 146.40a 66.43b 28.79a 43.32a 380.85b 11.28d 0.37c 2.39a 0.45c 14.49c
16 h 22.97c 84.33ab 144.98a 68.48b 27.57a 41.48ab 389.81a 11.05d 0.46c 2.27a 0.41c 14.19c
20 h 25.34b 81.38b 140.11b 69.28b 24.20b 42.40ab 382.71b 12.42c 0.41c 2.50a 0.47c 15.80b
24 h 25.00b 79.80b 139.86b 74.19a 23.86b 40.13b 382.84b 12.70c 1.45a 0.41b 0.77b 15.33b
28 h 25.41b 89.66a 133.13c 73.73a 20.27c 41.29ab 383.49b 13.75b 1.12b 0.66b 0.95a 16.48a
32 h 26.03a 83.65ab 126.65d 72.63a 21.59bc 37.73c 368.28c 14.55a 1.18b 0.47b 0.77b 16.97a
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Data are means (± standard deviation) of three replicates

a–dData marked with different letters in the same row are not significantly different (p > 0.05)

Catechins and their oxidation products mainly account for the flavor and astringent character of oolong tea (Chen et al. 2010a). Catechins have been found to greatly contribute to the astringent and bitter taste of tea (Lin et al. 1998). Gallated catechins have bitterness and astringency stronger than those of simple tea catechins (Obanda et al. 2001). In our study, the intensity of astringent taste and the concentrations of total catechins and gallated catechins showed no significant correlations. However, the intensity of bitter taste and the concentrations of total catechins (r = 0.79, p < 0.05) and gallated catechins (r = 0.63, p < 0.05) had significant correlations. Theaflavins have been reported to have an astringent taste and affect the color of tea liquor (Scharbert et al. 2004). In our study, the intensity of astringent taste and the concentrations of theaflavins had no significant correlation. Flavone glycosides are an important source of astringent taste (Scharbert et al. 2004), with their hydrolysis influencing the intensity of astringent taste. Therefore, the variation of astringent and bitter tastes may be due to the overall effect of catechins, theaflavins, and flavonoid glycosides.

The non-gallated catechins EGC and EC are the main contributors to the sweet aftertaste of green tea infusion (Zhang et al. 2016), and the intensities of the sweet aftertaste increases with the concentrations of EGC and EC. In the present study, both the intensity of sweet aftertaste and the concentrations of EGC and EC increased with the semi-fermentation time. The intensity of sweet aftertaste and the concentrations of EGC (r = 0.83, p < 0.05) and EC (r = 0.62, p < 0.05) of the infusions were significantly correlated, indicating that non-gallated catechins EGC and EC contributed to the sweet aftertaste.

Flavonol glycosides and flavonols

Ten flavonol glycosides were detected in the teas that had been semi-fermented for different times. The parent flavonols were Vit, Que, Myr, and Kae, which were conjugated to the monosaccharides galactose (gala), glucose (glu), rhamnose (rha), and rutinoside (rut). Myr-gala, Vit-rha, Que-gala, and Kae-gala were found to be the main flavonol glycosides, consistent with the results of Wu et al. (2012). With extended semi-fermentation time, the total concentrations of flavonol glycosides and flavonols showed a slight increase, which may be due to the hydrolysis of the flavonol diglycosides, flavonol triglycosides, and flavonol tetraglycosides (Wang et al. 2001). Flavonol monoglycosides can also be hydrolyzed into flavones and monosaccharides. Meanwhile, the concentrations of Vit-rha, Kae-gala, Kae-rut, Vit, Que, and Kae increased significantly, while the concentrations of Myr-rha, Que-rut, and Myr decreased significantly (Table 3). We did not detect by HPLC, however, flavonol diglycosides, flavonol triglycosides, and flavonol tetraglycosides because of the lack of standards for these substances.

Table 3.

Effect of semi-fermentation time on the concentrations of flavonol glycosides and flavones in Tieguanyin oolong tea infusions (μg/mL)

Semi-fermentation time (h) Myr-gala Vit-rha Myr-rha Que-rut Que-gala Que-glu Kae-gala Kae-rut Kae-glu Que-rha Vit Myr Que Kae Total
12 0.904a 0.389bc 0.018ab 0.036a 0.346ab 0.079a 0.232b 0.027b 0.019a 0.004ab 0.376b 0.045a 0.008c 0.007b 2.490b
16 0.850a 0.371c 0.020a 0.028ab 0.358ab 0.079a 0.233b 0.028b 0.019a 0.003b 0.384b 0.032b 0.008c 0.010ab 2.423b
20 0.930a 0.393bc 0.018ab 0.022b 0.327b 0.084a 0.241b 0.026b 0.020a 0.005a 0.371b 0.024c 0.013bc 0.011ab 2.485b
24 0.951a 0.426b 0.017b 0.019bc 0.366ab 0.080a 0.263ab 0.027b 0.018a 0.004ab 0.411ab 0.026c 0.016b 0.014a 2.638ab
28 0.820a 0.414b 0.016b 0.017bc 0.334b 0.078a 0.279a 0.030ab 0.021a 0.004ab 0.446a 0.025c 0.017b 0.013a 2.514b
32 0.883a 0.507a 0.016b 0.015c 0.384a 0.087a 0.281a 0.032a 0.023a 0.005a 0.447a 0.024c 0.022a 0.013a 2.739a
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Data are means (± standard deviation) of three replicates

a–dData marked with different letters in the same column are not significantly different (p > 0.05)

Flavonol glycosides, except catechins, have been reported to be the main astringent compounds in black tea (Scharbert and Hofmann 2005). Que-gala, Que-glu, Kae-glu, Myr-glu, and Myr-gala positively contributed to the astringent taste of black tea infusion, as shown by their dose-over-threshold factors (the specific value of the concentration and its threshold) of > 1.0. In the present study, the total concentrations of flavonol glycosides and flavonols were found to significantly positively correlate with the intensity of astringency (r = 0.72, p < 0.05). However, only the concentrations of Vit-rha (r = 0.57, p < 0.05), Que-gala (r = 0.62, p < 0.05), Kae-gala (r = 0.56, p < 0.05), and Vit (r = 0.57, p < 0.05) significantly correlated with the intensity of astringency, indicating that flavonol glycosides also contributed to the astringency of oolong tea infusions.

Free amino acids

We determined 16 free amino acids in teas semi-fermented for different times (Table 4). The total amino acid concentration varied between 224.85 and 257.71 mg/L. Theanine was the most abundant amino acid, accounting for 63.9–69.7% of the total amino acids. Glutamic acid was the next, representing 5.4–10.0% of the total amino acids. As the duration of semi-fermentation increased, the concentrations of alanine, cysteine, histidine, tryptophan, and lysine gradually increased (Table 4). In contrast, the concentrations of theanine, aspartic acid, glutamic acid, and arginine declined with the increase in semi-fermentation time. The concentrations of glutamic acid and theanine decreased during the semi-fermentation probably because of conversion of glutamic acid to γ-aminobutyric acid (Wang et al. 2006) or their reaction with sugar to form aroma compounds (Dev Choudhury and Bajaj 1979).

Table 4.

Effect of semi-fermentation time on the concentrations of free amino acids in Tieguanyin oolong tea infusions (μg/mL)

Semi-fermentation time (h) Phosphoserine Aspartic acid Threonine Serine Glutamic acid Theanine Alanine Cysteine Tyrosine
12 1.08a 7.88a 1.98b 4.61a 22.20a 162a 8.03c 2.95c 0.63b
16 1.08a 8.59a 2.15b 5.01a 23.25a 157a 9.90b 3.04c 0.81b
20 1.06a 8.26a 2.09b 4.85a 22.92a 154b 9.02bc 3.39c 0.74b
24 1.09a 6.08b 1.98b 4.48ab 20.25b 153b 8.68bc 7.93b 0.76b
28 1.05a 6.35b 2.03b 4.58ab 21.11b 149c 9.93b 10.1b 0.75b
32 1.11a 3.41c 2.39a 3.97b 13.93c 144d 24.4a 16a 1.39a
Semi-fermentation time (h) Phenylalanine β-Alanine γ-Aminobutyric acid Histidine Tryptophane Lysine Arginine Total
12 3.44a 0.13 0.62b 0.56c 10.1c 0.43c 8.53a 234b
16 3.23a 0.15 0.62b 0.60c 11.3c 0.49c 4.99c 233b
20 3.66a 0.15 0.74b 0.61c 10.8c 0.49c 6.79b 229c
24 3.30a 0.11 0.63b 0.74b 12.5bc 0.42c 2.75d 225d
28 3.47a 0.15 0.69b 0.73b 13.7b 1.05b 2.97d 228 cd
32 3.41a 0.34 0.87a 1.05a 16.3a 1.39a 3.20d 237a
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Data are means (± standard deviation) of three replicates

a–dData marked with different letters in the same row are not significantly different (p > 0.05)

Previous works have shown that free amino acids greatly contribute to the delicate taste and characteristic flavor of tea (Wang et al. 2010). Amino acids have usually been found to have a sweet taste or umami taste. Nishimura and Kato (1988) reported that glycine and alanine have considerably strong sweetness and that monosodium glutamate has the typical umami taste. Theanine is a characteristic amino acid in tea infusion and is a key compound for its umami taste (Hayashi et al. 2008). Leaf wounding and fermentation during tea processing could lead to protein degradation, which increases the levels of some free amino acids. Notably, theanine not derived from tea protein hydrolysis is derived from glutamic acid and ethylamine (Suzuki et al. 2002). Theanine has been found to counteract the bitterness and astringency of oolong tea by increasing its sweetness (Chen et al. 2010b). γ-Aminobutyric acid has been found to be another important flavor compound of oolong tea (Chen et al. 2010b). However, we observed no significant correlation between the concentrations of total free amino acids and the intensity of umami taste (r = − 0.25, p > 0.05) in the oolong teas.

Effect of semi-fermentation time on the dose-over-threshold factor of taste-active compounds in Tieguanyin oolong tea infusion

The dose-over-threshold (Dot) factors of taste-active compounds in the tea were analyzed according to the method described by Scharbert and Hofmann (2005). The Dot factor, which is the ratio of concentration to the taste threshold, was used to evaluate the taste contribution of the individual taste compounds. The taste-active compounds were classified according to the taste qualities into six groups, namely, compounds imparting puckering astringency and rough oral sensation, compounds imparting mouth-drying and velvety-like astringency, bitter-tasting compounds, sweet-tasting compounds, and umami-like taste compounds (Table 5).

Table 5.

Taste qualities, taste thresholds, concentrations, and dose-over-threshold factors of Tieguanyin oolong tea

Taste-active components Concentration in tea infusion (μmol/L) Thresholda (μmol/L) Dose-over-thresholdb factor
12 h 16 h 20 h 24 h 28 h 32 h
Group 1: compounds imparting puckering astringency and rough oral sensation
GC 344 540 0.56 0.59 0.65 0.64 0.64 0.67
EGC 1098 520 2.01 2.25 2.16 2.11 2.36 2.24
EGCG 1284 190 7.21 7.08 6.80 6.76 6.40 6.20
EC 1077 930 1.06 1.08 1.09 1.16 1.15 1.15
GCG 219 390 0.69 0.66 0.57 0.56 0.47 0.52
ECG 382 260 1.61 1.53 1.56 1.47 1.50 1.40
TF 94.6 16 5.35 5.20 5.81 5.91 6.38 6.87
TF3G 8.49 15 0.15 0.18 0.16 0.57 0.44 0.47
TF3’G 2.38 15 0.95 0.90 0.98 0.16 0.26 0.19
TFDG 3.71 13 0.17 0.16 0.18 0.29 0.35 0.29
Total 19.8 19.6 20.0 19.6 20.0 20.0
Group 2: compounds imparting mouth-drying and velvety-like astringency
Que-rut 0.12 0.00115 201 157 119 107 94 85
Vit-rha 3.09 2.80 1.03 0.97 1.02 1.10 1.07 1.33
Myr-gala 8.31 2.7 2.98 2.78 3.03 3.08 2.64 2.90
Que-gala 3.31 0.43 7.41 7.60 6.92 7.69 7.00 8.19
Que-glu 0.72 0.65 1.11 1.11 1.17 1.11 1.07 1.23
Kae-rut 0.19 0.25 0.78 0.79 0.74 0.78 0.84 0.91
Kae-gala 2.46 6.7 0.33 0.33 0.34 0.37 0.39 0.40
Kae-glu 0.17 0.67 0.27 0.27 0.28 0.26 0.30 0.32
Theanine 1192 6000 0.19 0.19 0.20 0.20 0.20 0.20
γ-Aminobutyric acid 7.69 20 0.38 0.38 0.45 0.38 0.42 0.54
Total 215 171 133 122 108 101
Group 3: bitter-tasting compounds
Caffeine 1477 500 2.73 2.66 2.78 2.95 2.76 2.73
EGCG 1284 380 3.60 3.54 3.40 3.38 3.20 3.10
EC 1077 930 1.06 1.08 1.09 1.16 1.15 1.15
GCG 219 390 0.69 0.66 0.57 0.56 0.47 0.52
Phenylalanine 25.2 58,000 0.0005 0.0004 0.0005 0.0004 0.0005 0.0005
Tyrosine 5.30 5000 0.001 0.001 0.001 0.001 0.001 0.002
Tryptophane 77.2 4400 0.014 0.016 0.015 0.017 0.019 0.023
Lysine 3.62 3400 0.001 0.001 0.001 0.001 0.003 0.004
Arginine 19.9 2800 0.022 0.013 0.017 0.007 0.007 0.008
Total 8.12 7.97 7.87 8.08 7.61 7.54
Group 4: umami-like taste compounds
Aspartic acid 57.6 4000 0.019 0.021 0.020 0.014 0.015 0.008
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aTaste threshold concentration was taken from the literature (Scharbert and Hofmann 2005)

bThe dose-over-threshold factor is the ratio of the concentration to the taste threshold

Table 5 shows that the puckering astringency and rough oral sensation, the Dot factors of which are all > 1.0, are mainly due to EGC, EGCG, EC, ECG, and TF. This result is different from that for black tea, the puckering astringency and rough oral sensations of which are mainly due to EGCG (Scharbert and Hofmann 2005) because of oxidization of most of the catechins. Catechins primarily have an astringent and bitter taste, with the taste intensity increasing with the concentration (Narukawa et al. 2010). The mouth-drying and velvety-like astringency was mainly due to Que-rut, Vit-rha, Myr-gala, Que-gala, and Que-glu, while that of Dajiling black tea was mainly due to flavonol glycosides, namely, five flavonol-3-monoglycosides, two flavonol-3-3diglycosides, and one flavonol-3-triglycoside. Besides the type of flavon-3-ol aglycon, the type and the sequence of the individual monosaccharides in the glycosidic chain were the key factors affecting astringency perception of flavon-3-ol glycosides (Scharbert et al. 2004). Caffeine, EGCG, and EC are the main contributors of bitter taste of Tieguanyin oolong tea, whereas caffeine is the only quantitatively predominant bitter tastant of Dajiling black tea (Scharbert and Hofmann 2005). Compounds with umami-like taste mainly included amino acids (Table 5). According to the concentrations and the taste threshold values, most of the umami-like taste compounds have a Dot of < 1, which means that these compounds do not contribute significantly to the taste of Tieguanyin oolong tea.

With extended fermentation time, the total Dot factor of the compounds imparting puckering astringency and rough oral sensation was almost unchanged, but the total Dot factor of the compounds imparting mouth-drying and velvety-like astringency decreased (Table 5). However, both Dot factors were not significantly correlated with the intensity of astringency probably because of the interaction between the two groups of compounds. With extended fermentation time, the total Dot factor of the bitter-tasting compounds decreased, similar to the intensity of the bitterness (Table 5).

Effect of semi-fermentation time on the concentrations of volatile compounds

Gas chromatography–mass spectrometry combined with headspace solid-phase microextraction was performed to analyze volatile compounds in oolong tea samples semi-fermented for different times. Fifty-five compounds were tentatively identified from their retention index and mass spectra (Table 6). The volatile components consisted of 15 aldehydes, 13 esters, 8 alcohols, 6 ketones, 5 aromatic hydrocarbons, 4 alkenes, 1 acid, and 3 miscellaneous compounds. Quantitatively, esters (26.7–39.4%), alcohols (16.0–26.7%) and aldehydes (13.4–18.8%) were dominant in the volatiles of the teas. (Z)-3-Hexenyl hexanoate (64.34–162.81 μg/L), nerolidol (63.85–109.03 μg/L), indole (40.93–60.44 μg/L), limonene (26.10–75.92 μg/L), and decanal (14.00–45.23 μg/L) were the major compounds. The tea aroma depended on a mix of abundant volatile compounds and their interaction. Notably, the odor thresholds were vital to the overall aroma of the tea. These volatile flavor compounds (VFCs) have been previously identified in oolong tea (Lin et al. 2013), green tea (Shimoda et al. 1995), and black tea (Kawakami et al. 1995). The VFCs were divided into two groups, Groups I and II, which impart inferior aroma and sweet flowery aroma, respectively (Owuor et al. 1990). The flavor index (FI), which was calculated from the ratios of Group II and Group I, increased from 0.59 (12 h) to 0.84 (24 h) and then decreased to 0.63 (32 h). The variations of FI are consistent with the intensity of floral aroma.

Table 6.

Main volatile compounds in Tieguanyin oolong tea identified by HS-SPME/GC–MS using a DB-5 capillary column (μg/L)

No. Compounds RI MI 4 h 8 h 12 h 16 h 20 h 24 h
1 Pentanal 680 MS,RIL 0.16b 0.14b 0.14b 0.23ab 0.31a 0.22ab
2 Toluene 766 MS,RIL 0.86ab 0.90ab 0.99a 1.09a 0.81ab 0.68b
3 Hexanal 802 MS,RIL 2.84b 2.65b 3.19ab 3.85a 2.73b 2.23b
4 2-Hexenal 852 MS,RIL 0.81ab 0.64b 1.06a 0.90a 0.96a 1.04a
5 Ethylbenzene 859 MS,RIL 0.79ab 0.71ab 0.92a 0.92a 0.70ab 0.49b
6 o-Xylene 868 MS,RIL 1.93ab 1.58b 2.46a 2.07a 1.23b 1.49b
7 2-Heptanone 883 MS,RIL 0.26a 0.24a 0.23a 0.21a 0.29a 0.26a
8 Heptanal 901 MS,RIL 0.99c 1.05c 0.94c 1.32b 1.96a 0.64d
9 (E)-2-Heptenal 957 MS,RIL 1.08ab 0.89b 0.62b 1.27a 1.01ab 0.79b
10 Benzaldehyde 961 MS,RIL 3.64b 3.19b 4.05b 4.17b 3.75b 6.57a
11 1-Octen-3-ol 982 MS,RIL 0.73b 1.08ab 0.82b 0.78b 1.34a 0.70b
12 6-Methyl-5-heptene-2-one 986 MS,RIL 2.41b 2.88ab 3.43a 3.71a 2.57b 2.29b
13 2-Pentylfuran 990 MS,RIL 1.39a 1.22a 1.45a 1.54a 1.14a 1.41a
14 trans,trans-2,4-Heptadienal 993 MS,RIL 1.67b 2.05ab 1.88ab 2.84a 1.72ab 1.39b
15 Octanal 1004 MS,RIL 4.52a 4.89a 5.39a 3.29ab 2.19b 2.80b
16 Limonene 1030 MS,RIL 57.03b 63.39ab 75.92a 27.92c 26.10c 27.56c
17 (Z)-β-ocimene 1037 MS,RIL 2.45b 2.33b 2.76ab 2.15b 3.07ab 3.95a
18 Benzeneacetaldehyde 1045 MS,RIL 5.84c 6.23c 7.18bc 8.93bc 10.91b 17.07a
19 2-Octen-1-al 1058 MS,RIL 2.23a 1.75a 1.88a 2.14a 2.82a 2.04a
20 3,5-Octadien-2-one 1070 MS,RIL 0.72a 0.56ab 0.61ab 0.65ab 0.41b 0.46b
21 (Z)-linalool oxide 1074 MS,RIL 0.42b 0.41b 0.49b 0.89a 0.59b 0.45b
22 (E)-linalool oxide 1089 MS,RIL 0.44ab 0.42ab 0.49a 0.56a 0.40b 0.38b
23 Linalool 1101 MS,RIL 7.91a 6.35ab 6.98ab 8.20a 6.18ab 4.73b
24 Nonanal 1106 MS,RIL 14.31a 13.52a 15.21a 9.07b 7.23b 8.94b
25 Phenylacetonitrile 1035 MS,RIL 2.52c 2.83c 3.31bc 4.13bc 4.89b 11.07a
26 (E)-3-Hexenyl Butyrate 1143 MS,RIL 9.03b 11.50ab 10.13ab 9.03b 8.01b 13.29a
27 (Z)-2-Nonenal 1162 MS,RIL 0.53b 0.59b 0.68ab 0.85a 0.67ab 0.64ab
28 Naphthalene 1187 MS,RIL 3.04b 3.41b 3.96b 5.94a 3.66b 1.77c
29 Hexyl butyrate 1191 MS,RIL 0.88b 0.72b 0.77b 0.70b 0.94b 2.16a
30 Methyl salicylate 1194 MS,RIL 4.31b 4.46b 4.80b 7.18a 4.68b 4.08b
31 Safranal 1202 MS,RIL 0.27a 0.33a 0.41a 0.34a 0.44a 0.38a
32 Decanal 1207 MS,RIL 37.24a 45.23a 39.82a 16.28b 17.94b 14.00b
33 β-Cyclocitral 1222 MS,RIL 3.82ab 3.45ab 4.31a 3.70ab 2.66b 3.54ab
34 (Z)-3-Hexenyl isovalerate 1232 MS,RIL 2.59b 2.72b 3.12b 2.11b 2.95b 6.69a
35 Geraniol 1256 MS,RIL 1.37b 1.06b 0.93b 1.31b 1.77b 4.51a
36 Phenethyl acetate 1261 MS,RIL 0.55b 0.45b 0.47b 0.55b 0.65b 1.35a
37 (E)-2-Decenal 1269 MS,RIL 1.22a 1.08a 1.42a 1.01a 1.65a 1.16a
38 (E)-2-Hexenyl Caproate 1288 MS,RIL 0.66a 0.58a 0.76a 0.53a 0.63a 0.95a
39 Indole 1295 MS,RIL 40.93b 42.18b 45.77b 41.93b 54.81ab 60.44a
40 Methyl geranate 1305 MS,RIL 3.89b 3.52b 4.53b 5.15b 6.55b 14.37a
41 1-Methylnaphthalene 1320 MS,RIL 0.71a 0.66a 0.82a 0.65a 0.63a 0.60a
42 (Z)-3- Hexenyl hexanoate 1381 MS,RIL 148.85a 128.96a 162.81a 73.42b 64.34b 106.42b
43 Hexyl hexanoate 1386 MS,RIL 0.71c 0.73c 0.72c 0.83c 1.43b 2.87a
44 α-Ionone 1424 MS,RIL 1.19b 1.29b 1.20b 1.07b 1.51a 1.50a
45 Phenylethyl isobutyrate 1440 MS,RIL 1.49c 1.64c 1.92c 2.57bc 3.75b 17.45a
46 (E)-Geranyl acetone 1449 MS,RIL 0.93a 1.25a 1.12a 0.86a 1.18a 1.07a
47 (E)-β-Farnesene 1453 MS,RIL 0.91b 0.82b 1.08ab 1.03ab 1.17ab 1.35a
48 β-Ionone 1482 MS,RIL 3.81b 4.82a 4.38a 3.37b 3.09b 3.91ab
49 α-Farnesene 1496 MS,RIL 7.62b 8.13b 8.86b 7.22b 10.40ab 13.29a
50 Geranyl butyrate 1540 MS,RIL 1.11c 0.83c 0.92c 1.69c 2.81b 4.01a
51 Nerolidol 1565 MS,RIL 67.08c 63.85c 71.24c 68.24c 86.46b 109.03a
52 α-Cedrol 1616 MS,RIL 0.45b 0.53b 0.63ab 0.71a 0.45b 0.38b
53 Phenethyl butyrate MS 0.65d 0.53d 0.88 cd 1.12c 1.80b 6.47a
54 Palmitic acid MS 0.11b 0.12b 0.11b 0.52a 0.66a 0.60a
55 Dibutyl phthalate MS 0.59b 0.75ab 0.66b 0.96ab 1.34a 0.83ab
Total 464.49b 458.09b 521.63a 353.70c 374.34c 498.76b
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a–dData marked with different letters in the same row are not significantly different (p > 0.05)

RI retention indices

Method of identification: MS identification by comparison with mass spectra; RIL identification using retention indices

Extending the semi-fermentation time significantly increased the concentrations of benzaldehyde, (Z)-β-ocimene, benzeneacetaldehyde, phenylacetonitrile, hexyl butyrate, (Z)-3-hexenyl isovalerate, geraniol, phenethyl acetate, indole, methyl geranate, hexyl hexanoate, coumarin, (E)-β-farnesene, phenylethyl isovalerate, α-farnesene, nerolidol, phenethyl butyrate, and myristic acid, most of which are esters. Meanwhile, concentrations of octanal, limonene, linalool, nonanal and decanal, which were mainly from low aldehydes, showed a significant decrease over the same period. In contrast to the fermentation time of 12 h, a fermentation time of 32 h led to an increase in the amounts of phenylethyl isobutyrate, phenethyl butyrate, and palmitic acid by 1171.1, 995.4 and 545.5%, respectively. As mentioned previously, the amounts of phenylacetaldehyde, benzeneacetonitrile, methyl salicylate, geraniol, and indole increase after semi-fermentation (Wang et al. 2008). The concentrations of these volatile compounds tended to increase with the semi-fermentation time and greatly contribute to the flowery aroma of oolong tea, which form by glycoside hydrolysis during tea processing (Wang et al. 2011).

Conclusion

The intensities of astringency, bitterness, umami, and sweet aftertaste of Tieguanyin oolong tea changed with the semi-fermentation time, with the change being mainly due to the concentration variation of catechins, flavonol glycosides, and free amino acids in the tea. The flavor index was consistent with the intensity of floral aroma. Results of this work suggest that the flavor change is correlated with the taste-active and aroma-active compounds in oolong tea.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (31101248, 31671861), Natural Science Foundation of Zhejiang (LR17C160001), and the Innovation Project for Chinese Academy of Agricultural Sciences (CAAS-ASTIP-2014-TRICAAS) and the Young Elite Scientist Sponsorship Program by CAST.

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