https://orcid.org/0000-0002-3697-2982
?Mathematical formulae have been encoded as MathML and are displayed in this HTML version using MathJax in order to improve their display. Uncheck the box to turn MathJax off. This feature requires Javascript. Click on a formula to zoom.ABSTRACT
Jiangxi Congou black tea is one of the earliest Congou black tea in China. In order to explore its sensory characteristics and molecular sensory basis, quantitative description analysis (QDA) and gas chromatography-mass spectrometry (GC-MS) were used to analyze the sensory characteristics and volatile components of 11 Congou black tea samples from Jiangxi Province in China. The results showed that the aroma characteristics of Jiangxi Congou black were composed of 11 aroma descriptors, namely, sweet-smell, floral, fruity, roasted, spicy, fresh, green, oil, off-odor, woody and smoky. There was also interaction effect between sensory attributes of aroma, which were significant negative correlation between floral, and fruity, and green. The presence of roasted incense could significantly enhance the olfactory perception of off-odor and smoky, and off-odor was an important attribute that affects the aroma quality of Jiangxi Congou black tea. Volatile components (i.e. 74 kinds) were identified, among which the highest content and species were alcohols, followed by esters, and the lowest were heterocycles and terpenes. The results of partial least square variable import project (PLS-VIP) showed that there were 29 volatile compounds that had an important contribution to the aroma characteristics of Congou black tea, and the volatile compounds that had an important influence on six aroma attributes were obtained.
Introduction
Tea plant [Camellia Sinensis] is a perennial woody economic crop, of which tender shoots or leaves are the most effective economic part.[Citation1] It can be made into green tea, black tea, white tea, oolong tea, yellow tea and dark tea by different processing techniques.[Citation2] Black tea can be divided into Congou black tea, Souchong black tea and Broken black tea. Congou black tea has been evolved from Souchong black tea. The representative Congou black teas in China are Keemum black tea, Yunnan black tea, Sichuan black tea, Jiangxi black tea and so on.[Citation3] Jiangxi Congou black tea is one of the earliest Congou black teas in China, including Ning black tea and Fuliang black tea. It is famous for its special “apple flavor” and sweet mellow taste, which is favored by the market and consumers.[Citation4] At present, there are few reports on the aroma characteristics of Jiangxi Congou black tea, and the important contribution of volatile components to the aroma characteristics is not clear. Therefore, its quality cannot be controlled stably.
Untargeted metabonomics study targets all endogenous metabolites in biological samples. As many metabolites as possible could be extracted by the ideal collection and preparation method during untargeted metabonomics.[Citation5,Citation6] Therefore, in recent years, more and more scientists have applied it to the field of tea studies to obtain effective secondary metabolism rules of tea plants and different metabolites among different tea species, so as to deepen the understanding of endogenous information of tea.[Citation7,Citation8] QDA is not only a flavor inspection method with the function of analysis, description and quantification, but also a practical quality evaluation tool.[Citation9–11] This method has also been gradually applied to the field of tea sensory evaluation. The descriptors of green tea infusion taste,[Citation12] green tea aroma,[Citation13] black tea aroma,[Citation14] and Dianhong flavor[Citation15] have been achieved, respectively, and their intensity to obtain the flavor characteristics of related tea categories and improve people’s sensory awareness of the quality of tea leaves were evaluated by QDA. Multivariate statistical analysis has been developed from classical statistics, and is a comprehensive analysis method.[Citation16] It can screen out the statistical relationships between multiple variables and multiple indicators, reduce complexity to simplicity, and clearly let us understand the characteristics of research objects, as a result, it is more suitable for agricultural research.[Citation17]
In this study, GC-MS and QDA were used to analyze the volatile compounds and sensory characteristics of the representative Jiangxi black tea. This study also revealed the correlation between metabolites of Jiangxi Congou black tea and sensory attributes of flavor, and analyzed the volatile compound variables’ importance to the sensory attributes of aroma.
Materials and methods
Materials
Eleven samples of Congou black tea produced in Jiangxi Province were taken as the research objects (Table S1). The traditional processing method of Congou black tea was conducted in the processing of samples, including fresh leaf picking, withering, rolling, fermentation, drying and enhancing aroma. Each sample was packed in a special aluminum foil bag of the same specification and kept in a refrigerator at – 4 °C until analysis.
QDA of Congou black tea samples
Selection of evaluators
According to the theoretical system of QDA, the personnel involved in sensory evaluation must be consistent, sensitive, objective and controllable.[Citation18] On the basis of the above theories, a three-point experiment and sequence experiment of general taste screening were conducted, and the personnel whose accuracy rate was not less than 80% shall be regarded as qualified. Six tea evaluators (three females, three males, 20–30 years old) were selected as the evaluation group.
Construction of aroma descriptors and reference system
Three gram of each sample was placed into the evaluation cup, and then 150 mL of boiling water was added. After brewing for 5 min, the tea infusion was filtered and evaluated by the evaluation team. The flavor profile of Jiangxi Congou black tea samples was described by evaluators, then combined with the flavor descriptors of Congou black tea reported in the existing literature and the traditional classic flavor characteristics of Jiangxi Congou black tea to build the flavor system of Jiangxi Congou black tea. The final aroma descriptors of Jiangxi black tea were confirmed and the reference system was established according to the method reported by Mao[Citation18] and Li[Citation19] ().
Table 1. Descriptors and reference compounds of aroma of Jiangxi Congou black tea
Sensory evaluation
Three g of each sample was placed into the evaluation cup, and then 150 mL of boiling water was added. After brewing for 5 min, the tea infusion was filtered and evaluated by evaluation team. The tea evaluation team used the scale strength of 1–15 combined with the established Congou black tea aroma descriptors to evaluate the sensory strength, where 1–3 = weakest, 3–5 = weaker, 5–7 = weak, 7–9 = medium, 9–11 = strong, 11–13 = stronger, 13–15 = strongest, and marked the position that best reflects the sensory strength.
Extraction and analysis of volatile compounds
Extraction of volatile components
Headspace solid-phase microextraction (HS-SPME) method was used to extract volatile aroma compounds. 1.5 g of ground tea sample was put in 20 mL headspace injection vial, and 20 μL of ethyl decanoate (0.02 mg/mL, in ethanol) was added. The vial was incubated at 60°C, for 15 min with shaking at 450 rpm (5S on, 2S off). Then 50/30 μm DVB/CAR/PDMS, fiber (Sigma) was inserted into the headspace part of the vial for extraction for 60 min. Then the fiber was injected into GC port, at 250°C for 5 min, and then GC-MS separation and identification were carried out.
GC-MS analysis conditions
Agilent 7890b-5977b (GC-MS)-PAL RSI 120 was used for the analysis of volatile substances. DB-WAX capillary column (30 m × 0.25 mm × 0.25 μm, Agilent J&W Scientific, Folsom, CA, USA) was used for separation and high-purity helium (≥99.999%) was used as carrier gas. Constant flow rate was 1.0 mL/min, and inlet temperature was 260°C. Splitless injection mode was used and solvent delay time was set as 1.5 min. Oven temperature was kept, at 40°C for 3 min, and rising to 220°C at 4°C/min for 10 min. Mass spectrometry conditions: electron bombardment ion source (EI), ion source temperature was 230°C, quadrupole temperature was 150°C, mass interface temperature was 280°C, electron energy 70 ev. Detector voltage: 901 v, scanning mode: scan, mass scanning range: 20–650 m/z.
GC-MS quantitative analysis
Ethyl decanoate was used as internal standard for extraction and detection, and the content of volatile components in the sample was calculated according to the following formula:
Data statistics and analysis
Correlation analysis and variance analysis were carried out by SPSS 20.0, and PLS-VIP was carried out by SIMCA 14.1.
Results and discussion
QDA of aroma characteristics of Jiangxi Congou black tea
Analysis of variance
The evaluation results of 11 aroma attributes of the samples are shown in . The results of variance analysis-LSD multiple comparisons showed that the similar aroma characteristics of Jiangxi Congou black tea were strong sweet flavor, and other sensory attributes were statistically different. Woody fragrance mainly refers to the smell of rosin, hay, camphor wood incense and resin. It is common in flavor characteristics for dark tea, and rarely found in black tea.[Citation20,Citation21] In the sample of this study, No. 3, No. 4 and No. 11 samples showed strong woody, among them, the raw material of No.11 sample was produced by ancient tea plant, while two other samples were products made by different manufacturers with leaves of Ningzhou variety. At present, Jiangxi Congou black tea with woody flavor and its origin have not been reported. In this paper, we speculated that the woody of No.11 black tea could be derived from fresh leaf materials, and the woody of No. 3 and No. 4 may be produced in the processing, especially in the process of drying, firewood was used as fuel to provide heat source for tea drying, and because of the adsorption of palmitic acid and terpene in tea, giving them a woody. In general, floral, fruity and sweet-smell are key sensory factors to attract consumers.[Citation18,Citation19] The intensity difference of floral among samples was larger than others, and the difference of fruity was relatively low. These samples of No. 1-No. 8 were Ning black tea, which is famous for its special “apple flavor”. In the process of QDA evaluation, no significant “apple flavor” was found, and the fruity intensity of the samples was low, only No. 1 and No. 7 samples could reach the “strong” level, black tea aroma was characterized by the comprehensive effect of complex volatile compounds, but also affected by the activity value of flavor substances, meteorological factors and cultivation management.[Citation22] It is gratifying that except that the green of No. 1 and No. 2 and the oil of No. 7 were at the “weaker” and “weak” levels, the intensity of the unpleasant smell of other samples, such as green, oil, off-odor and smoky were basically at the “weakest” level.
Table 2. QDA results of Jiangxi Congou black tea
Correlation analysis between the intensity of aroma attributes and the overall aroma of samples
The correlation analysis was used to explore the correlation between the aroma attributes of tea infusion and the overall aroma. The results were shown in Table S2. Sweet-smell, floral, fruity, fresh and woody were positively correlated with the overall aroma, but did not reach a significant level. Roasted, spicy, green, oily, off-odor and smoky were negatively correlated with the overall aroma, among which the off-odor reached a significant correlation level (p ≤ 0.05). Fruity is a pleasant odor, and high-intensity fruity could improve the overall aroma of Congou black tea,[,Citation23,Citation24] but this study found that the correlation between fruity and the overall aroma quality attained only 0.205, about 60% lower than that of sweet-smell and floral. Synthesizing the results of QDA and correlation analysis, it was suggested that the fruity might not be the characteristic flavor of Jiangxi Congou black tea. The floral was extremely significantly negatively correlated with the fruity and green respectively. This study speculates that the floral and fruity have certain similarities in human olfaction and brain recognition. In the discrimination process, there was a trade-off expression of “synergy and competition”, that was, the combination of low intensity and high intensity expressed the side with high intensity, but the low-strength attribute aroma was suppressed or weakened.
GC-MS results and analysis of Jiangxi Congou black tea
Main volatile components of Congou black tea
The qualitative and quantitative results of the volatile compounds of Jiangxi Congou black tea were shown in Table S3. 74 volatile compounds were obtained from 11 samples, which were divided into 8 types, including 20 alcohols, 10 aldehydes, 8 ketones, 6 acids, 12 esters, 3 terpenes, 9 hydrocarbons and 7 heterocycles. The total amount of volatile components in 11 samples could reach 5557.2 ~ 9129.9 μg/kg. In terms of quantity and content, the main volatile components of 11 samples were alcohols, accounting for 55.7% ~ 71.3% of the total volatile compounds in the samples, followed by esters, acids, aldehydes, ketones, hydrocarbons, heterocycles and terpenes, with the content of respectively 9.6% ~ 18.0%, 4.6% ~ 14.5%, 3.4% ~ 6.4%, 1.8% ~ 4.8%, 2.8 ~ 3.7%, 0.6% ~ 3.8%, 0.5% ~ 1.2%. Wang et al.[Citation25] analyzed the aroma components of Chinese famous black tea, the results showed that 60 kinds of volatile components were identified, mainly including alcohols, esters, aldehydes, ketones, alkanes, etc. Alcohols were the most important components in the aroma of Chinese famous black tea, followed by esters and aldehydes; Luo et al.[Citation26] identified the aroma components of Sichuan black tea, and found that the volatile components mainly included alcohols, aldehydes, esters, ketones, hydrocarbons, acids, etc., and the main aroma contribution components were alcohols, with the relative content of 45.97% ~ 63.78%; Lei et al.[Citation27] analyzed the volatile components of Keemun black tea, and the results showed that 50 kinds of volatile components were identified, mainly alcohols, acids and aldehydes, with the contents of 30.52% ~ 49.13%, 22.84% ~ 38.52% and 10.37% ~ 16.15%. Compared with the previous research results, it was found that the alcohol content of Jiangxi black tea was the highest, and the proportion of aroma components was larger than other Congou black tea, which was the main reason for the difference between Jiangxi black tea aroma characteristics and other black tea. Among the volatile components of Jiangxi black tea, geraniol was the largest, followed by phenylethyl alcohol, benzyl alcohol, linalool, trans-linalool oxide (furanoid) and methyl salicylate. Similar to this result, Cloughley thought that the aroma of Chinese black tea was mainly geraniol, Assam black tea was mainly linalool, while Dajiling and Yunnan black tea belongs to intermediate type.[Citation28] After studying the aroma characteristics of Chinese black tea, Tadakazu divided Chinese black tea into three types. The first type was dominated by linalool and its oxides, the second was linalool and geraniol, and the third was geraniol.[Citation26] According to the classification method of Zhuweizhongyi, Jiangxi black tea belonged to the third type of Congou black tea which was dominated by geraniol.
Correlation analysis of volatile components and sensory properties of Congou black tea
The correlation analysis results of volatile components and aroma sensory attributes of the samples were shown in Table S4. Sweet-smell was the dominant aroma feature of Congou black tea,[Citation29] which was also confirmed by the sensory evaluation results of QDA (). The volatile components with positive correlation and correlation coefficient greater than 0.4 had 3,7-dimethyl-1,5,7-octatrien-3-ol, 2-nonanol, trans-linalool oxide (furanoid), benzene acetaldehyde, butanoic acid, 1,1-dimethyl-2-phenylethyl ester, methyl salicylate, among them, the correlation of trans-linalool oxide (furanoid) reached an extremely significant level (P ≤ 0.01). Floral is a common flavor characteristic in black tea and oolong tea.[Citation30] In this study, there were 25 kinds of volatile compounds with positive correlation with floral and correlation coefficient greater than 0.4, although there were many kinds of volatile components with positive correlation with floral, none of them have reached a significant level. In addition to the types of volatile components and their flavor characteristics, the floral of the sample was also affected by its aroma activity value and inter-compound interaction effects, including additive, antagonistic, or weakened phenomena.[Citation24] According to the correlation analysis, no significant positive correlation was found between the volatile components and the fruity, while there were 18 volatile components with negative correlation and correlation coefficient greater than 0.4. Through the comparison, we found that there were 10 kinds of volatile components positively correlated with floral and negatively correlated with fruity, and the correlation coefficients were greater than 0.4. This conclusion confirmed the sensory evaluation results of QDA, that is, there is a trade-off between floral and fruity. Fresh is a common type of odor type in green tea and oolong tea, but relatively low in black tea.[Citation18] Twelve volatile components were negatively correlated with fresh and correlation coefficient greater than 0.4, 2(5 H)-furanone, dihydro-4-hydroxy-2(3 H)-furanone and 1-(1 H-pyrrol-2-yl)-ethanone reached significant level. Woody was common in dark tea,[Citation11] but in this sample, it was found that some of the Jiangxi Congou black tea had high strength woody, and the 28 volatile components were positively correlated with woody and the correlation coefficient was greater than 0.4. Through correlation analysis, only methyl salicylate was found to have a significant negative correlation with woody, and the structure types of volatile components affecting woody were relatively rich, including capranols, aldehydes, ketones, acids, esters and heterocyclic compounds. The roasted tea is generally formed, released and accumulated in the process of drying or enhancing aroma, which can improve the aroma quality of tea to a certain extent.[Citation21] According to the analysis, in addition to the heterocyclic compounds that were furaneol and methyl-pyrazine, which were roasted, were positively correlated to roasted, the compounds with green or fresh, such as 1-hexanol, linalool, orcinol, ((Z)-2-penten-1-ol and (Z)-3-hexen-1-ol, were positively correlated with it. In the correlation analysis of sensory attributes of QDA (Table S2), it was also found that green and roasted, which were opposite to each other, appeared up in the same tea, and had a certain positive correlation. Spicy refers to spicy and pungent scents such as mint, holly, ginger and pepper. According to the previous analysis, spicy was negatively correlated with the overall aroma quality of tea. Seventeen volatile components were positively correlated with spicy and had a correlation coefficient greater than 0.4, 3,7-dimethyl-1,5,7-octatrien-3-ol, geraniol and phenylethyl alcohol reached an extremely significant level; (Z)-2-penten-1-ol, (Z)-3-hexen-1-ol, orcinol, pentanoic acid and 2-heptanone were negatively correlated with spicy and the correlation coefficient greater than 0.4. Most of the volatile components positively correlated with spicy were floral. Our previous results also manifested that floral and spicy were positively correlated (Table S2). Green was a sensory characteristic of the fresh leaves of tea plant, which would gradually lose or transform with the tea processing, but some of it still was retained in the finished tea, thus affecting the aroma of tea.[Citation19] Volatile components were positively correlated with green include (Z)-2-penten-1-ol, linalool, benzene acetaldehyde, etc. These components are aromatic compounds with green; Volatile components were negatively correlated with green include 6,10,14-trimethyl-2-pentadecanone, hexadecanoic acid, methyl ester, etc. These components are aromatic compounds with fat smell.[Citation18] Oily basically was to point to butter taste, adipose taste, grease, which had a negative correlation with tea aroma quality.[Citation19] The main compounds negatively related to oily were alpha-terpineo, 1-hexanol, cedrol, orcinol, thymol, benzaldehyde, 2-epi-alpha-funebrene, diisobutyl phthalate and azulene. In addition to some volatile components with unpleasant odors, irregular processing conditions could also cause off-odor in tea.[Citation18] The volatile components with a correlation coefficient greater than 0.4 and positively correlation with off-odor had (Z)-3-hexen-1-ol, (+)-beta-cedrene, furaneol and methyl-pyrazine. A significant positive correlation was found in linalool. (Z)-2-penten-1-ol, (Z)-3-hexen-1-ol and p-cresol were significantly positively correlated with smoky, and linalool was significantly negatively correlated with smoky. Most of the volatile components which promoted the aroma quality of tea had positive effects on the smoky, but in the correlation analysis of sensory sub-attribute strength, only the correlation between the roasted and smoky was statistically positive (Table S2). In the current research, there is no reasonable explanation for this phenomenon.
Screening of key contribution compounds of aroma characteristics of Jiangxi Congou black tea
PLS-VIP analysis was carried out with volatile components as independent variable x and all aroma sensory properties as dependent variable y. The results are shown in , three principal components were extracted through automatic fitting. The total explanatory rate of the model was 74.8%, and the explanatory rate of the three principal components was 39.2%, 19.8% and 15.7%. Eleven samples could be effectively distinguished from each other on the principal component, which indicated that the model had high reliability and could be used for the next PLS analysis. PLS-VIP scatter diagram of the sensory attributes and volatile components of samples was shown in , the distance between the aroma sensory attributes and the volatile components indicated the degree of correlation, and the smaller the distance, the greater the correlation. Twenty-nine volatile compounds with a VIP value greater than 1 mapped to the principal component were shown in , including 12 alcohols, 5 esters, 3 terpenes, 3 heterocycles, 2 aldehydes, 2 hydrocarbons, 1 ketone and 1 acid, namely, linalool, geraniol, benzyl alcohol, 2-furanmethanol, benzene acetaldehyde, methyl hexadecanoate, diisobutyl phthalate, furaneol, dimethylbenzylcarbinyl butyrate, nerolidol, 3,7-dimethyl-1,5,7-octatrien-3-ol, orcinol, phenol, methyl-pyrazine, (Z)-3,7-dimethyl-1,3,6-octatriene, p-cresol, trans-linalool oxide (furanoid), 2-epi-alpha-funebrene, methyl salicylate, phenylethyl alcohol, beta.-phellandrene, 2,3,6-trimethyl-naphthalene, 6,10,14-trimethyl-2-pentadecanone, 2-nonanol, (Z)-3-hexenyl ester hexanoic acid, 3-furaldehyde, 2-ethoxytetrahydrofuran, 3-methyl-butanoic acid, 1-hexanol. The results showed that the 29 volatile components were the key contributors to the aroma of Jiangxi Congou black tea. Previous studies have shown that nerolidol, benzene acetaldehyde, linalool, phenylethyl alcohol, methyl salicylate, benzyl alcohol, geraniol, methyl hexadecanoate, trans-linalool oxide (furanoid) and (Z)-3,7-dimethyl-1,3,6-octatriene were the key aroma components of Chinese Congou black tea.[Citation25–27] These components also contribute to the aroma of Jiangxi black tea. Different from previous reports, this study found that 2-furanmethanol, diisobutyl phthalate, furaneol, dimethylbenzylcarbinyl butyrate, 3,7-dimethyl-1,5,7-octatrien-3-olorcinol, phenol, methyl-pyrazine, p-cresol, 2-epi-alpha-funebrene, beta.-phellandrene, 2,3,6-trimethyl-naphthalene, 6,10,14-trimethyl-2-pentadecanone, 2-nonanol, (Z)-3-hexenyl ester hexanoic acid, 3-furaldehyde, 2-ethoxytetrahydrofuran, 3-methyl-butanoic acid and 1-hexanol were also the main components contributing to the aroma quality of Jiangxi Congou black tea, which may be the main reason for the difference of aroma quality between Jiangxi Congou black tea and other black tea.
Figure 1. Distribution of samples on the first two principal components
Figure 2. PLS scatter diagram of volatile compounds and sensory properties of samples
Figure 3. VIP values of volatile compounds to the overall aroma sensory attributes
Screening of key contribution compounds for each aroma attribute
In order to further explore the key contribution components of each aroma sub-attribute, PLS-VIP analysis was carried out with each aroma sub-attribute strength as the dependent variable y and the volatile components as the independent variable x, and the key components with important contribution to six aroma sub-attributes were obtained. The components with VIP value greater than 1 were shown in Table S5. According to the requirements of PLS-VIP, the prediction models of roasted, green, oily, off-odor, smoky and volatile components were poor, which mainly showed that RMSEE and RMSECV were too large, while the regression coefficient R2 of fitting equation of dependent variable prediction model was smaller, which didn’t meet the requirements of the model of screening important variable projection, so the VIP screening of the five aroma sub-attributes could not be carried out, which needs to be further discussed in combination with other methods.
Sweet-smell -VIP
There were 15 volatile compounds which have important influence on sweet-smell, including 5 alcohols, 3 acids, 3 esters, 2 aldehydes, 1 ketones and 1 heterocycles (Table S5). The main characteristics of these five alcohols were floral and fruity.[Citation31] The research results of Lei et al.[Citation27] also showed that these five alcohols were the main contribution components of sweet-smell. The three acids were hexanoic acid, (E)-3-hexenoic acid and pentanoic acid, which belonged to unpleasant aromatic components.[Citation32] The previous correlation analysis results also showed that it had a large negative correlation with sweet-smell (Table S4). Another PLS-VIP analysis was carried out with dihydroactinidiolide, phenylacetic acid methyl ester, benzaldehyde, 2-acetyl-5-methylfuran, 2(3 H)-furanon, 2,3,6-trimethylnaphthalene, (z)-2-pentene-1-ol and geraniol as the extensions items. The results showed that the influence of the quadratic of geraniol and benzyl alcohol and the interaction term between them on sweet-smell over 1, the VIP of the linear and quadratic were 1.66 and 2.93 respectively. The influence of the latter linear was under 1, and the quadratic and its interaction with geraniol were both greater than 1, indicating a positive additive effect between the two (Table S5).
Floral-VIP
There were 14 volatile compounds that had a great influence on floral, including 6 alcohols, 3 acids, 3 esters, 1 ketones, and 1 heterocyclics (Table S5). According to the correlation analysis, except linalool, the others were positively correlated with floral (Table S4). This contradicts the conclusion had been reported that linalool had a positive contribution on the floral of Congou black tea.[Citation18] Linalool is an important aromatic substance in tea, of which the aroma is characterized by the floral with green, and the content of linalool has a close influence on the terpene index of tea aroma substances.[Citation33] Taking methyl salicylate, (+)-beta-cedrene, alpha-ionone, (Z)-3-methyl-2-(2-pentenyl)-2-cyclopenten-1-one, 1-hydroxy-2-propanone, (E)-6,10-dimethyl-5,9-undecadien-2-one, benzene acetaldehyde, alpha.-ethylidene-benzene acetaldehyde, 1-methyl-naphthalene, 2,3,6-trimethyl-naphthalene, geraniol, linalool, phenethyl alcohol, trans-linalool oxide (furanoid) that put up floral components as extended variables, PLS-VIP analysis was conducted to observe VIP between them and floral attributes (Table S5). The results revealed that VIP of 4 quadratic and 20 interaction terms was above 1, which had a great influence on floral, which indicated that there was a serious interaction effect between volatile compounds of floral. The existence of the extension had great influence on the linear, and the overall trend of change was different. The main manifestations included decreases, unchanged, and increases of the value of the original compounds of linear with VIP above 1, of which the degree changed involved addition and disappearance. The newly added compounds with VIP greater than 1 incorporated pentanoic acid, ethyl hexanoate, (Z)-3-hexenyl ester hexanoic acid and benzaldehyde, and the disappearance was trans-linalool oxide (furanoid). In the extension term, the VIP of the interaction term of trans-linalool oxide (furanoid) with geraniol, phenylethyl alcohol and linalool were more than 1, and phenylethyl alcohol and methyl salicylate are the most frequent. It could be considered that these two components were the key volatile components of the floral of Jiangxi Congou black tea. The aroma type of phenylethanol was rose aroma, while that of methyl salicylate was fresh and floral.[Citation34] These two components not only contributed to the floral aroma of black tea, but also contributed to the aroma of floral green tea and floral oolong tea.[Citation35,Citation36]
Fruity-VIP
There were 30 volatile components that had key influence on fruity, which were the most components with VIP greater than 1 in the identified aroma attributes, including 10 alcohols, 7 aldehydes, 4 esters, 4 carbohydrates, 3 acids, 1 ketone and 1 terpene (Table S5). Combined with correlation analysis, these components had a high negative correlation with fruity (Table S4), which were the mainly floral, and only benzaldehyde, ethyl 3-hexenoater and (Z)-3-hexenyl ester hexanoic acid are multiple-flavor compounds with fruity.[Citation37] In our research, it failed to show a promoting effect on fruity. In the correlation analysis of aroma sensory attributes, it also indicated that there was a significant negative correlation between floral and fruity (Table S2). Mao[Citation18] found that there was little correlation between floral and fruity aroma of black tea, while Li[Citation19] found that there was a negative correlation between floral and fruity aroma in principal component analysis. Therefore, based on the previous research results, the 30 volatile components might had an important negative impact on fruity. Among the 74 volatile components identified by GC-MS, 17 kinds had the characteristics of fruity (Table S3). However, in PLS scatter diagram of sensory attribute and volatile components, only (Z)-2-penten-1-ol and (Z)-3-hexen-1-ol were close to fruity (). The correlation analysis of sensory attributes and volatile components, PLS and PLS-VIP all suggested that there were less components promoting the fruity of Congou black tea. PLS-VIP analysis was again performed with ethyl 3-hexenoate, ethyl palmitate, methyl hexadecanoate, (Z)-3-hexenyl ester hexanoic acid, ethyl hexanoate, d-limonene, 2-heptanone, 6,10,14-trimethyl-2-pentadecanone, benzaldehyde, hexanal, 2-acetyl-5-methylfuran, 1-(2-furanyl)-ethanone, furaneol, nerolidol, nerol, benzyl alcohol, methyl-pyrazine as an extension item (Table S5). It was found that there were 14 new linear components with a VIP above 1, and the original linear VIP value basically showed an increase; the extended items with a VIP greater than 1 were mainly interactive items, and the quadratic square only had a pleasant lemon scent for d-limonene, the frequency of this compound in the interaction item was also the highest, indicating that there are many components with an interactive effect with it, which could be used as the key research object of processing fruity black tea.
Spicy-VIP
Through analysis, 10 volatile compounds with a VIP of greater than 1 for spicy were captured, all of them were positively correlated with spicy (Table S5). Among them, 3,7-dimethyl-1,5,7-octatrien-3-ol, geraniol, phenethyl alcohol, (E)-6,10-dimethyl-5,9-undecadien-2-one, alpha-ionone, and (Z)-3,7-dimethyl-1,3,6-octatriene had a significant positive correlation with spicy (Table S4), the VIP values of the first two were the largest, which were 5.66 and 2.14, respectively, indicating that both were key influencers of spicy. In the PLS scatter diagram (), the components closest to the spicy were respectively (Z)-3,7-dimethyl-1,3,6-octatriene, 3,7-dimethyl-1,5,7-octatrien-3-ol, phenylethyl alcohol and (E)-6,10-dimethyl-5,9-undecadien-2-one, and the analysis results were basically consistent front and back. According to the flavor characteristics of 10 compounds, all of them presented floral, and the previous correlation analysis results also manifested that there was a positive correlation between floral and spicy (Table S2). After adding the extension analysis, no quadratic square term and interaction term that had great impacts on spicy were found, and the VIP value of the original linear term didn’t fluctuate significantly (Table S5).
Fresh-VIP
There were 18 volatile compounds that affected the fresh of Congou black tea, including 6 alcohols, 5 esters, 3 acids, 2 aldehydes and 2 ketones (Table S5), among them, the 1-(1 H-pyrrol-2-yl)-ethanone had the highest VIP (3.62), and the correlation analysis results also revealed that it had a significant negative correlation with the fresh (Table S4). The VIP of 3,7-dimethyl-1,5,7-Octatrien-3-ol, (E)-6,10-dimethyl-5,9-undecadien-2-one, methyl salicylate and (Z)-3-hexenyl ester hexanoic acid with fresh characteristics to Congou black tea were also greater than 1, and they had a certain degree of positive correlation with fresh. In the PLS scatter diagram of the overall sensory properties and volatile components of the sample, the nearest components to fresh were methyl salicylate and d-limonene (). The correlation analysis results also displayed that d-limonene was positively correlated with fresh (Table S4). D-limonene presents pleasant lemon flavor, which was fresh feeling.[Citation38] At present, there is no report on the relationship between d-limonene and fresh. In the future, the relationship between d-limonene and the characteristics of fresh can be studied by recombination experiments of flavor compounds to simulate tea infusion. After adding the extension analysis (Table S5), no quadratic square term and interaction term which had great influence on fresh were found, and the VIP value of the original linear did not fluctuate significantly.
Woody-VIP
There were 14 components influencing the woody of Congou black tea, including 4 alcohols, 4 acids, 3 esters, 1 aldehyde, 1 terpene, and 1 heterocycle, among them, linalool and methyl salicylate were negatively correlated with woody, while other components were positively correlated. After PLS-VIP analysis with (+)-beta-cedrene, 2-epi-alpha-funebrene, 4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-buten-2-one, hexanal, cedrol, alpha-terpineol and trans-linalool oxide (furanoid) as extension terms, no VIP value more than 1 quadratic square term and interaction term were obtained, while the VIP value of the original linear was increasing, and a new key compound ethyl hexanoate appeared, and the VIP value of it increased from 0.97 to 1.09. The aroma characteristic of ethyl hexanoate is fruit odor, which is the main aroma contributing component of many fruit wines. In this study, ethyl hexanoate was found to be one of the main contribution components of black tea woody aroma after the interaction of other aroma components. It has not been reported in other studies, and its specific interaction needs to be further studied.[Citation39,Citation40]
Conclusion
This work was the first systematic study on the aroma characteristics and volatile components of Jiangxi Congou black tea. The aroma characteristics of Jiangxi Congou black were composed of 11 aroma descriptors, namely, sweet-smell, floral, fruity, roasted, spicy, fresh, green, oily, off-odor, smoky and woody. Seventy-four kinds of volatile components were identified by HS-SPME-GC-MS, among which the highest content and species were alcohols, followed by esters, and the lowest content and species were heterocycles and terpenes. 29 key compounds affecting aroma characteristics of Jiangxi Congou black tea were determined by PLS-VIP, namely, linalool, geraniol, benzyl alcohol, 2-furanmethanol, benzene acetaldehyde, methyl hexadecanoate, diisobutyl phthalate, furaneol, dimethylbenzylcarbinyl butyrate, nerolidol, 3,7-dimethyl-1,5,7-octatrien-3-ol, orcinol, phenol, methyl-pyrazine, (Z)-3,7-dimethyl-1,3,6-octatriene, p-cresol, trans-linalool oxide (furanoid), 2-epi-alpha-funebrene, methyl salicylate, phenylethyl alcohol, Beta.-phellandrene, 2,3,6-trimethyl-naphthalene, 6,10,14-trimethyl-2-pentadecanone, 2-nonanol, (Z)-3-hexenyl ester hexanoic acid, 3-furaldehyde, 2-ethoxytetrahydrofuran, 3-methyl-butanoic acid, 1-hexanol. Furthermore, the key volatile components affecting six sensory attributes of aroma were also screened. This study provided a data reference for the comprehensive understanding of the aroma characteristics of Jiangxi Congou black tea. The key volatile components affecting the aroma properties were confirmed by multivariate statistical means, which provided some important theoretical basis for further understanding the aroma characteristics of volatile components monomers in Jiangxi Congou black tea.
Supplemental Material
Download MS Word (460 KB)Acknowledgments
The research was funded by Modern Agricultural Industrial Technology System of Jiangxi Province (JXARS-02), National Modern Industrial Technology System (CARS-19), Jiangxi Key Laboratory of Tea Quality and Safety Control (20192BCD40007). The authors declare that they have no conflict of interest.
Supplementary material
Supplemental data for this article can be accessed on the publisher’s website.
Additional information
Funding
References
- Zeng, L.; Watanabe, N.; Yang, Z. Understanding the Biosyntheses and Stress Response Mechanisms of Aroma Compounds in Tea (Camellia Sinensis) to Safely and Effectively Improve Tea Aroma. Crit. Rev. Food Sci. Nutr. 2018, 59, 1–14. DOI: 10.1080/10408398.2018.1506907.
- Xue, Z.; Wang, J.; Chen, Z.; Ma, Q.; Guo, Q.; Gao, X.; Chen, H. Antioxidant, Antihypertensive, and Anticancer Activities of the Flavonoid Fractions from Green, Oolong, and Black Tea Infusion Waste. J. Food Biochem. 2018, 426, e12690. DOI: 10.1111/jfbc.12690.
- Dong, C.; Liang, G.; Hu, B.; Yuan, H.; Jiang, Y.; Zhu, H.; Qi, J. Prediction of Congou Black Tea Fermentation Quality Indices from Color Features Using Non-Linear Regression Methods. Entific Rep. 2018, 8, 10535. DOI: 10.1038/s41598-018-28767-2.
- Li, W.; Yang, P.; Li, D. Developmental Situation and Countermeasures of Jiangxi Provincial Tea Industry in 13th Five-year. Jiangxi Agricult. J. 2016, 28, 119–122. DOI: 10.19386/j.cnki.jxnyxb.2016.12.24.
- Herrero, M.; Simó, C.; García-Cañas, V.; Ibáñez, E.; Cifuentes, A. Foodomics: MS-based Strategies in Modern Food Science and Nutrition. Mass Specromet. Rev. 2012, 31, 49–69. DOI: 10.1002/mas.20335.
- Bakir, S.; Capanoglu, E.; Hall, R. D.; Devos, R. C, H. Variation in Secondary Metabolites in a UniqueSet of Tomato Accessions Collected in Turkey. Food Chem. 2020, 317, 126406. DOI: 10.1016/j.foodchem.2020.126406.
- Chen, S.; Liu, H.; Zhao, X.; Li, X.; Shan, W.; Wang, X.; Wang, S.; Yu, W.; Yang, Z.; Yu, X. Non-targeted Metabolomics Analysis Reveals Dynamic Changes of Volatile and Non-volatile Metabolites during Oolong Tea Manufacture. Food Res. Int. 2020, 128, 108778. DOI: 10.1016/j.foodres.2019.108778.
- Arkorful, E.; Yu, Y.; Chen, C.; Lu, L.; Hu, S.; Yu, H.; Ma, Q.; Thangaraj, K.; Periakaruppan, R.; Jeyaraj, A.; et al. Untargeted Metabolomic Analysis Using UPLC-MS/MS Identifies Metabolites Involved in Shoot Growth and Development in Pruned Tea Plants (Camellia Sinensis (L.) O. Kuntz). Sci. Hortic. 2020, 264, 109164. DOI: 10.1016/j.scienta.2019.109164.
- Oliver, P.; Cicerale, S.; Pang, E.; Keast, R. A. Comparison of Temporal Dominance of Sensation (TDS) and Quantitative Descriptive Analysis (QDA) to Identify Flavors in Strawberries. J. Food Sci. 2018, 83, 1094–1102. DOI: 10.1111/1750-3841.14096.
- Braghieri, A.; Piazzolla, N.; Galgano, F.; Condelli, N.; De Rosa, G.; Napolitano, F. Effect of Preservative Addition on Sensory and Dynamic Profile of Lucanian Dry-sausages as Assessed by Quantitative Descriptive Analysis and Temporal Dominance of Sensations. Meat Sci. 2016, 122, 68–75. DOI: 10.1016/j.meatsci.2016.07.020.
- Li, H.; Luo, L.; Wang, J.; Fu, D.; Zeng, L. Lexicon Development and Quantitative Descriptive Analysis of Hunan Fuzhuan Brick Tea Infusion. Food Res. Int. 2019, 120, 275–284. DOI: 10.1016/j.foodres.2019.02.047.
- Jin, X. The Research on Taste Compounds of Green Tea; Southwest University, Ed: Chongqing, 2007. DOI:10.7666/d.y1074229.
- Lee, J.; Chambers, D. H. A Lexicon for Flavor Descriptive Analysis of Green Tea. J. Sens. Stud. 2007, 22, 256–272. DOI: 10.1111/j.1745-459X.2007.00105.x.
- Alasalvar, C.; Topal, B.; Serpen, A.; Bahar, B.; Pelvan, E.; Gökmen, V. Flavor Characteristics of Seven Grades of Black Tea Produced in Turkey. J. Agricult. Food Chem. 2012, 60, 6323–6332. DOI: 10.1021/jf301498p.
- Pang, X.; Qin, Z.; Zhao, L.; Cheng, H.; Hu, X.; Song, H.; Wu, J. Development of Regression Model to Differentiate Quality of Black Tea (Dianhong): Correlate Aroma Properties with Instrumental Data Using Multiple Linear Regression Analysis. Int. J. Food Sci. Technol. 2012, 47, 2372–2379. DOI: 10.1111/j.1365-2621.2012.03112.x.
- Belmonte-Sánchez, J. R.; Romero-González, R.; Martínez Vidal, J. L.; Arrebola, F. J.; Garrido Frenich, A. 1H NMR and Multi-technique data Fusion as Metabolomic Tool for the Classification of Golden Rums by Multivariate Statistical Analysis. Food Chem. 2020, 317, 126363. DOI: 10.1016/j.foodchem.2020.126363.
- Innamorato, V.; Longobardi, F.; Cervellieri, S.; Cefola, M.; Pace, B.; Capotorto, I.; Gallo, V.; Rizzuti, A.; Logrieco, A. F.; Lippolis, V. Quality Evaluation of Table Grapes during Storage by Using 1H NMR, LC-HRMS, MS-eNose and Multivariate Statistical Analysis. Food Chem. 2020, 315, 126247. DOI: 10.1016/j.foodchem.2020.126247.
- Mao, S. Quality Analysis and Control of Congou Black Tea Based on Sensomics; Southwest University, Ed: Chongqing, 2018.
- Li, X. Relationship Between Sensory Character and Chemical Components of Congou's Flavour; Southwest University, Ed: Chongqing, 2015.
- Li, D.; Weng, Y.; Du, L.; Wang, C.; Ma, L.; Xiao, D. Detection of Volatile Compounds in Different Brands of Pu-erh Tea Using Electronic Nose and GC-MS. Food Fermenat. Indust. 2019, 45, 237–245. DOI: 10.13995/j.cnki.11－1802/ts.017410.
- Pang, X.; Yu, W.; Cao, C.; Yuan, X.; Qiu, J.; Kong, F.; Wu, J. Comparison of Potent Odorants in Raw and Ripened Pu-Erh Tea Infusions Based on Odor Activity Value Calculation and Multivariate Analysis: Understanding the Role of Pile Fermentation. J. Agric. Food Chem. 2019, 67, 13139–13149. DOI: 10.1021/acs.jafc.9b05321.
- Xiao, Z.; Wang, H.; Niu, Y.; Zhu, J.; Ma, N. Analysis of Aroma Components in Four Chinese Congou Black Teas by Odor Active Values and Aroma Extract Dilution Analysis Coupled with Partial Least Squares Regression. Food Sci. 2018, 39, 242–249.
- Nicole, K.; Eugene, B.; Yuriy, G.; Albert, R. Differentiation of Key Biomarkers in Tea Infusions Using a Target/nontarget Gas Chromatography/mass Spectrometry Work Flow. Food Res. Int. 2018, 113, 414–423. DOI: 10.1016/j.foodres.
- Wang, C.; Zhang, C.; Kong, Y.; Peng, X.; Li, C.; Liu, S.; Du, L.; Xiao, D.; Xu, Y. A Comparative Study of Volatile Components in Dianhong Teas from Fresh Leaves of Four Tea Cultivars by Using Chromatography-Mass Spectrometry, Multivariate Data Analysis, and Descriptive Sensory Analysis. Food Res. Int. 2017, 100, 267–275. DOI: 10.1016/j.foodres.2017.07.013.
- Wang, Q.; Chen, D.; Xu, Y.; Yin, J. Study on the Aroma Components in Chinese Famous Black Tea. J. Chin. Inst. Food Sci. Technol. 2013, 13, 195–200. DOI: 10.16429/j.1009-7848.2013.01.030.
- Luo, X.; Li, L.; Ma, C.; Zhao, X. SPME-GC-MS Analysis of Aroma Components in Black Tea Prepared with Main Tea Cultivars in Sichuan. Food Sci. 2016, 37, 173–178. DOI: 10.7506/spkx1002-6630-201616028.
- Lei, P.; Huang, J.;.; Ding, Y.; Xu, Y.; Fang, W. Quality Characteristics of Keemun Black Tea from Various Regions. Food Sci. 2015, 36, 144–149. DOI: 10.7506/spkx1002-6630-201510029.
- Wan, X.;. Biochemistry of Tea, 3th ed.; China Agricultural Press: Beijing, 2003.
- Wu, D.; Zhu, X. Ning Black Tea on the Ten Thousand Kilometer Tea Ceremony. Chinese Tea. 2016, 38, 35–36. DOI: 10.3969/j..1000-3150.2016.09.016.
- Guo, X.; Song, C.; Ho, C.; Wan, X. Contribution of L-theanine to the Formation of 2,5-dimethylpyrazine, a Key Roasted Peanutty Flavor in Oolong Tea during Manufacturing Processes. Food Chem. 2018, 263, 18–28. DOI: 10.3389/fphar.2018.00195.
- Mao, S.; Lu, C.; Li, M.; Ye, Y.; Wei, X.; Tong, H. Identification of Key Aromatic Compounds in Congou Black Tea by Partial Least-Square Regression with Variable Importance of Projection Scores and Gas Chromatography-Mass Spectrometry/gas Chromatography-olfactometry. J. Sci. Food Agric. 2018, 98, 5278–5286. DOI: 10.1002/jsfa.9066.
- Chen, X.; Chen, D.; Jiang, H.; Jiang, H.; Sun, H.; Zhang, C.; Zhao, H.; Li, X.; Yan, F.; Chen, C.; et al. Aroma Characterization of Hanzhong Black Tea (Camellia Sinensis) Using Solid Phase Extraction Coupled with Gas Chromatography-Mass Spectrometry and Olfactometry and Sensory Analysis. Food Chem. 2018, 274, 130–136. DOI: 10.1016/j.foodchem.2018.08.124.
- Mei, X.; Liu, X.; Zhou, Y.; Wang, X.; Zeng, L.; Fu, X.; Li, J.; Tang, C.; Dong, F.; Yang, Z. Formation and Emission of Linalool in Tea (Camellia Sinensis) Leaves Infested by Tea Green Leaf Hopper (Empoasca (Matsumurasca) Onukii Matsuda). Food Chem. 2017, 237, 356–363. DOI: 10.1016/j.foodchem.2017.05.124.
- Li, H.; Luo, L.; Ma, M.; Zeng, L. Characterization of Volatile Compounds and Sensory Analysis of Jasmine Scented Black Tea Produced by Different Scenting Processes. J. Food Sci. 2018, 83, 2718–2732. DOI: 10.1111/1750-3841.14340.
- Ma, L.; Liu, Y.; Cao, D.; Gong, Z.; Liu, P.; Jin, X. Analysis of Aroma Components of Green Teas Made from Leaves of High-Quality Tea Strains in Hubei Province. Food Sci. 2019, 40, 251–256. DOI: 10.7506/spkx1002-6630-20180206-088.
- Chen, L.; Yu, W.; Zhang, Y.; Xiang, L.; Wang, L.; You, Z. Aroma Profiling of Oolong Tea by SDE and HS-SPME in Combination with GC-MS. J. Tea Sci. 2019, 39, 692–704. DOI: 10.13305/j.cnki.jts.2019.06.008.
- Mu, B.; Zhu, Y.; Lv, H.; Yan, H.; Peng, Q.; Lin, Z. The Enantiomeric Distributions of Volatile Constituents in Different Tea Cultivars. Food Chem. 2018, 265, 329–336. DOI: 10.1016/j.foodchem.2018.05.094.
- Xiang, Z.; Chen, X.; Qian, C.; He, K.; Xiao, X. Determination of Volatile Flavors in Fresh Navel Orange by Multidimensional Gas Chromatography Quadrupole Time-of-flight Mass Spectrometry. Anal. Lett. 2020, 53, 614–626. DOI: 10.1080/00032719.2019.1662429.
- Zhao, N.; Wei, X.; Fan, M.; Zhang, J.; Qi, Y.; Xu, J.; Zhang, L. Analysis of Volatile Components of Kiwifruit Wines Made from Different Cultivars Using Solid Phase Microextraction-Gas Chromatography-Mass Spectrometry Combined with Electronic Nose. Food Sci. 2019, 40, 249–255. DOI: 10.7506/spkx1002-6630-20181227-322.
- Mao, J.; Li, Y. Comparative Analysis of Aroma Components of Black Chokeberry Wine and Cabernet Sauvignon Wine. Food Sci. 2019, 40, 270–276. DOI: 10.7506/spkx1002-6630-20180813-128.