Dynamic changes in the aroma profile of Qingzhuan tea during its manufacture

Discrimination and prediction of Qingzhuan tea storage year using quantitative chemical profile combined with multivariate analysis: Advantages of MRMHR based targeted quantification metabolomics

The duration of storage significantly influences the quality and market value of Qingzhuan tea (QZT). Herein, a high-resolution multiple reaction monitoring (MRMHR) quantitative method for markers of QZT storage year was developed. Quantitative data alongside multivariate analysis were employed to discriminate and predict the storage year of QZT. Furthermore, the content of the main biochemical ingredients, catechins and alkaloids, and free amino acids (FAA) were assessed for this purpose. The results show that targeted marker-based models exhibited superior discrimination and prediction performance among four datasets. The R2Xcum, R2Ycum and Q2cum of orthogonal projection to latent structure-discriminant analysis discrimination model were close to 1. The correlation coefficient (R2) and the root mean square error of prediction of the QZT storage year prediction model were 0.9906 and 0.63, respectively. This study provides valuable insights into tea storage quality and highlights the potential application of targeted markers in food quality evaluation.

Combined multi-omics approach to analyze the flavor characteristics and formation mechanism of gabaron green tea

Gabaron green tea (GAGT) has unique flavor and health benefits through the special anaerobic treatment. However, how this composite processing affects the aroma formation of GAGT and the regulatory mechanism was rarely reported. This study used nontargeted metabolomics and molecular sensory science to overlay screen differential metabolites and key aroma contributors. The potential regulatory mechanism of anaerobic treatment on the aroma formation of GAGT was investigated by transcriptomics and correlation analyses. Five volatiles: benzeneacetaldehyde, nonanal, geraniol, linalool, and linalool oxide III, were screened as target metabolites. Through the transcriptional-level differential genes screening and analysis, some CsERF transcription factors in the ethylene signaling pathway were proposed might participate the response to the anaerobic treatment. They might regulate the expression of related genes in the metabolic pathway of the target metabolites thus affecting the GAGT flavor. The findings of this study provide novel information on the flavor and its formation of GAGT.

Characterization of volatile compounds and identification of key aroma compounds in different aroma types of Rougui Wuyi rock tea

In this study, we characterized the aroma profiles of different Rougui Wuyi rock tea (RGWRT) aroma types and identified the key aroma-active compounds producing these differences. The roasting process was found to have a considerable effect on the aroma profiles. Eleven aroma compounds, including linalool, β-ionone, geraniol, indole, and (E)-nerolidol, strongly affected the aroma profiles. An RGWRT aroma wheel was constructed. The rich RGWRT aroma was found to be dominated by floral, cinnamon-like, and roasty aromas. Human olfaction was correlated with volatile compounds to determine the aromatic characteristics of these compounds. Most key aroma-active compounds were found to have floral, sweet, and herbal aromas (as well as some other aroma descriptors). The differences in key compounds of different aroma types were found to result from the methylerythritol phosphate, mevalonic acid and shikimate metabolic pathways and the Maillard reaction. Linalool, geraniol, and (E,E)-2,4-heptadienal were found to spontaneously bind to olfactory receptors.

Co-occurrence network and functional profiling of the bacterial community in the industrial pile fermentation of Qingzhuan tea: Understanding core functional bacteria

The distinct quality of Qingzhuan tea is greatly influenced by the bacterial community but was poorly characterized. Therefore, this study investigated the Co-occurrence network and functional profiling of the bacterial community, with special attention paid to core functional bacteria in the industrial pile fermentation. Microbiomics analysis indicated that Klebsiella and Pantoea dominated raw tea leaves, and were rapidly replaced by Pseudomonas in pile fermentation, but substituted mainly by Burkholderia and Saccharopolyspora in final fermented tea. Bacterial taxa were grouped into 7 modules with the dominant in module I, III, and IV, which were involved in flavor formation and biocontrol production. Functional profiling revealed that "penicillin and cephalosporin biosynthesis" increased in pile fermentation. Twelve bacterial genera were identified as core functional bacteria, in which Klebsiella, Pantoea, and Pseudomonas also dominated the pile fermentation. This work would provide theoretical basis for its chemical biofortification and quality improvement by controlling bacterial communities.

Effects of Monascus purpureus on ripe Pu-erh tea in different fermentation methods and identification of characteristic volatile compounds

The present study aimed to explore the key volatile compounds (VCs) that lead to the formation of characteristic flavors in ripe Pu-erh tea (RIPT) fermented by Monascus purpureus (M. purpureus). Headspace solid-phase microextraction coupled with gas chromatography/mass spectrometry (HS-SPME-GC-MS), orthogonal partial least square-discriminant analysis (OPLS-DA) were employed for a comprehensive analysis of the VCs present in RIPT fermented via different methods and were further identified by odor activity value (OAV). The VCs 1,2-dimethoxybenzene, 1,2,3-trimethoxybenzene, (E)-linalool oxide (pyranoid), methyl salicylate, linalool, β-ionone, β-damascenone were the key characteristic VCs of RIPT fermented by M. purpureus. OAV and Gas chromatography-olfactometry (GC-O) further indicated that β-damascenone was the highest contribution VCs to the characteristic flavor of RIPT fermented by M. purpureus. This study reveals the specificities and contributions of VCs present in RIPT under different fermentation methods, thus providing new insights into the influence of microorganisms on RIPT flavor.

Monitoring the dynamic changes in aroma during the whole processing of Qingzhuan tea at an industrial scale: From fresh leaves to finished tea

Aroma is one of the most outstanding quality characteristics of Qingzhuan tea (QZT), but its formation is still unclear. Thus, the volatile organic compounds (VOCs) during the whole processing of QZT were investigated by headspace solid-phase microextraction/gas chromatography-mass spectrometry. Based on 144 identified VOCs, the results showed that de-enzyming, sun-drying, and piling fermentation were the key processes of QZT aroma formation. Furtherly, 42 differential VOCs (VIP > 1.0 and p < 0.05) and 16 key VOCs (rOAV > 1.0 and/or ROAV > 1.0) were screened. Especially, sulcatone and β-ionone (rOAV > 100 and ROAV > 10) were considered the most important contributors to the aroma of QZT. The metabolisms of key VOCs were mainly involved in oxidative degradation of fatty acids, degradation of carotenoids, and methylation of gallic acid. This study could help to more comprehensively understand the aroma formation in QZT processing at an industrial scale.

Analysis of the key aroma components of Pu'er tea by synergistic fermentation with three beneficial microorganisms

Aroma is a key indicator of the quality and value of Pu'er tea. A total of 36 aroma components were detected,which Saccharomyces, Rhizopus, and Aspergillus niger, were in the ratios of 2:1:2, 2:2:2, and 2:3:2 inoculated to ferment Pu'er tea, comparing with natural fermentation. In addition, 12 key aroma compounds were identified by analysing ROAVs. Methoxyphenyl compounds and β-ionone were the primary contributors to the formation of aged and woody aroma when fermenting Pu'er tea naturally or using Rhizopus, while linalool and its oxides, benzyl alcohol, hexanal, and limonene were the primary contributors to the formation of floral and fruity aroma when fermenting Pu'er tea using synergistic fermentation with Saccharomyces, Rhizopus, and Aspergillus niger. This study identified the key aroma components of the Pu'er tea fermented using five methods, which revealed and demonstrated the potential application of synergistic effects of different microorganisms in the changes of aroma of Pu'er tea.

Exploring core functional fungi driving the metabolic conversion in the industrial pile fermentation of Qingzhuan tea

The distinct sensory quality of Qingzhuan tea is mainly formed in pile fermentation by a group of functional microorganisms but the core functional ones was poorly characterized. Therefore, this study investigated the dynamic changes in the fungal community and metabolic profile by integrating microbiomics and metabolomics, and explored the core functional fungi driving the metabolic conversion in the industrial pile fermentation of Qingzhuan tea. Indicated by microbiomics analysis, Aspergillus dominated the entire pile-fermentation process, while Thermoascus, Rasamsonia, and Cylindrium successively abounded in the different stages of the pile fermentation. A total of 50 differentially changed metabolites were identified, with the hydrolysis of galloyl/polymeric catechins, biosynthesis of theabrownins, oxidation of catechins, N-ethyl-2-pyrrolidinone substitution of catechins, and deglycosylation of flavonoid glucosides. Nine fungal genera were identified as core functional fungi, in which Aspergillus linked to the hydrolysis of polymeric catechins and insoluble polysaccharides as well as biosynthesis of theabrownins, while Thermoascus participated in the biosynthesis of theabrownins, deglycosylation of flavonoid glucosides, and N-ethyl-2-pyrrolidinone substitution of catechins. These findings would advance our understanding of the quality formation of Qingzhuan tea and provide a benchmark for precise inoculation for its quality improvement.

Characteristic volatiles of Fu brick tea formed primarily by extracellular enzymes during Aspergillus cristatus fermentation

Fu brick tea (FBT) has unique "fungal flower" aroma traits, but its source of crucial aroma compounds is still controversial. Aspergillus cristatus is the dominant fungus that participated in the fermentation of FBT. In this study, volatiles of Aspergillus cristatus and corresponding fermented FBT were examined using GC × GC-Q-TOFMS. A total of 59 volatiles were shared by three strains of Aspergillus cristatus isolated from representative FBT. Among them, 1-octen-3-ol and 3-octanone were the most abundant. A total of 133 volatiles were screened as typical FBT volatiles from three FBTs fermented by the corresponding fungi. Aspergillus cristatus and FBT had only 29 coexisting volatiles, indicating that the volatiles of Aspergillus cristatus could not directly contribute to the aroma of FBT. The results of no significant correlation between volatile content in FBT and volatile content in Aspergillus cristatus suggested that intracellular metabolism of Aspergillus cristatus was not a direct driver of FBT aroma formation. Metabolic pathway analysis and proteomic analysis showed that the aroma in FBT was mainly formed by the enzymatic reaction of extracellular enzymes from Aspergillus cristatus. This study enriched our understanding of Aspergillus cristatus in the aroma formation process of FBT.

Geographical differentiation of garlic based on HS-GC-IMS combined with multivariate statistical analysis

Garlic is famous for its unique flavor and health benefits. An effective means of authenticating garlic's origin is through the implementation of the Protected Geographical Indication (PGI) scheme. However, the prevalence of fraudulent behavior raises concerns regarding the reliability of this system. In this study, garlic samples from six distinct production areas (G1: Cangshan garlic, G2: Qixian garlic, G3: Dali single clove garlic, G4: Jinxiang garlic, G5: Yongnian garlic, and G6: Badong garlic) underwent analysis using HS-GC-IMS. A total of 26 VOCs were detected in the samples. The differences in VOCs among the different garlic samples were visually presented in a two-dimensional topographic map and fingerprint map. Principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) were employed to demonstrate the capacity of the HS-GC-IMS method for effectively distinguishing garlic samples from different geographical sources. Further screening based on the p-value and VIP score threshold identified 12 different aroma substances, which can be utilized for the identification of garlic from different producing areas. The fusion of HS-GC-IMS with multivariate statistical analysis proved to be a rapid, intuitive, and efficient approach for identifying and categorizing garlic VOCs, offering a novel strategy for ascertaining garlic origin and ensuring quality control.

Effect of cooking methods on metabolites of deep purple-fleshed sweetpotato

The effects of different cooking methods on purple-fleshed sweetpotato (PFSP) metabolites were systematically explored, containing the changes of starch, soluble sugar, volatile organic compounds and non-target metabolites after steaming, boiling and baking. Compared to raw samples, the steamed samples showed the greatest changes in starch (degraded from 53.01% to 39.5%) and soluble sugar content (increased from 11.82% to 29.08%), while the baked samples showed insignificant changes in starch (51.06%). In total, 64 volatile organic compounds were identified in PFSP, with aldehydes decreasing and terpenes increasing after cooking. However, most of them were low in content and contributed weak aroma for PFSP. More importantly, 871 non-volatile metabolites were detected in PFSP, and 83.5% of which were well-preserved after cooking, while most of the changes were concentrated in phenylpropanoids, amino acids and carbohydrates. This study enriches the understanding of quality changes after PFSP cooking and helps consumers choose the right cooking method.

Effects of pile-fermentation on the aroma quality of dark tea from a single large-leaf tea variety by GC × GC-QTOFMS and electronic nose

Aroma is one of the significant quality factors of dark tea (DT). However, for a single large-leaf tea variety, there are few studies analyzing the effect of pile-fermentation on the aroma quality of DT. The GC × GC-QTOFMS, electronic nose (E-nose) and GC-olfactometry (GC-O) techniques were employed to analysis the difference of tea products before and after pile-fermentation. A total of 149 volatile metabolites (VMs) were identified, with 92 VMs exhibiting differential characteristics. Among these, 31 VMs with OAV > 1.0 were found to be correlated with E-nose results (|r| > 0.8). Additionally, GC-O analysis validated seven major differential metabolites. Notably, naphthalene, 2-methylnaphthalene, and dibenzofuran were found to enhance the woody aroma, while (Z)-4-heptenal, 2-nonenal and 1-hexanol were associated with an increase in mushroom, fatty and sweet odors, respectively. Moreover, 1-octen-3-ol was linked to reducing pungent fishy smell. These findings could provide a certain theoretical basis for understanding the influence of pile-fermentation on the aroma quality of dark tea.

Comprehensive applications of metabolomics on tea science and technology: Opportunities, hurdles, and perspectives

With the development of metabolomics analytical techniques, relevant studies have increased in recent decades. The procedures of metabolomics analysis mainly include sample preparation, data acquisition and pre-processing, multivariate statistical analysis, as well as maker compounds' identification. In the present review, we summarized the published articles of tea metabolomics regarding different analytical tools, such as mass spectrometry, nuclear magnetic resonance, ultraviolet-visible spectrometry, and Fourier transform infrared spectrometry. The metabolite variation of fresh tea leaves with different treatments, such as biotic/abiotic stress, horticultural measures, and nutritional supplies was reviewed. Furthermore, the changes of chemical composition of processed tea samples under different processing technologies were also profiled. Since the identification of critical or marker metabolites is a complicated task, we also discussed the procedure of metabolite identification to clarify the importance of omics data analysis. The present review provides a workflow diagram for tea metabolomics research and also the perspectives of related studies in the future.

Revealing the chemical differences and their application in the storage year prediction of Qingzhuan tea by SWATH-MS based metabolomics analysis

It's generally believed that the longer the storage, the better the quality of dark tea, but the chemical differences of Qingzhuan tea (QZT) with different storage years is still unclear. Herein, in this work, an untargeted metabolomic approach based on SWATH-MS was established to investigate the differential compounds of QZT with 0-9 years' storage time. These QZT samples were roughly divided into two categories by principal component analysis (PCA). After orthogonal projections to latent structures discriminant analysis (OPLS-DA), 18 differential compounds were putatively identified as chemical markers for the storage year variation of QZT. Heatmap visualization showed that the contents of catechins, fatty acids, and some phenolic acids significantly reduced, flavonoid glycosides, triterpenoids, and 8-C N-ethyl-2-pyrrolidinone-substituted flavan-3-ols (EPSFs) increased with the increase of storage time. Furthermore, these chemical markers were verified by the peak areas corresponding to MS2 ions from SWATH-MS. Based on the extraction chromatographic peak areas of MS and MS2 ions, a duration time prediction model was built for QZT with correlation coefficient R2 of 0.9080 and 0.9701, and RMSEP value of 0.85 and 1.24, respectively. This study reveals the chemical differences of QZT with different storage years and provides a theoretical basis for the quality evaluation of stored dark tea.

Variations of main quality components of matcha from different regions in the Chinese market

Matcha has a unique aroma of seaweed-like, which is popular with Chinese consumers. In order to effectively understand and use matcha for drinks and tea products, we roundly analyzed the variation of main quality components of 11 matcha samples from different regions in the Chinese market. Most of matcha samples had lower ratio of tea polyphenols to amino acids (RTA), and the RTA of 9 samples of matcha was less than 10, which is beneficial to the formation of fresh and mellow taste of matcha. The total volatile compounds concentrations by HS-SPME were 1563.59 ~ 2754.09 mg/L, among which terpenoids, esters and alcohols were the top three volatile components. The total volatile compounds concentrations by SAFE was 1009.21 ~ 1661.98 mg/L, among which terpenoids, heterocyclic compounds and esters ranked the top three. The 147 volatile components with high concentration (&gt;1 mg/L) and no difference between samples are the common odorants to the 11 samples of matcha. The 108 distinct odorants had differences among the matcha samples, which were important substances leading to the different aroma characteristics. Hierarchical cluster analysis (HCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) showed that 11 samples of matcha were well clustered according to different components. Japanese matcha (MT, MY, ML, MR, MJ) could be clustered into two categories. The aroma composition of Guizhou matcha (GM1, GM2) was similar to that of Japanese matcha, 45 volatile components (decanal, pyrazine, 3,5-diethyl-2-methyl-, 1-hexadecanol, etc. were its characteristic aroma components. The aroma characteristics of Shandong matcha and Japanese matcha (ML, MR, MJ) were similar, 15 volatile components (γ-terpinene, myrtenol, cis-3-hexenyl valerate, etc.) were its characteristic aroma components. While Jiangsu matcha and Zhejiang matcha have similar aroma characteristics due to 225 characteristic aroma components (coumarin, furan, 2-pentyl-, etc). In short, the difference of volatile components formed the regional flavor characteristics of matcha. This study clarified the compound basis of the flavor difference of matcha from different regions in the Chinese market, and provided a theoretical basis for the selection and application of matcha in drinks and tea products.

Comparative Analysis of the Quality in Ripe Fruits of Cuiguan Pear from Different Regions

The Cuiguan pear is called "June snow" and the skin is thin; the meat is crisp and juicy; the taste is thick and fresh; and the juice is rich and sweet. In this study, the volatile organic compounds and the sensory and physicochemical parameters of the Cuiguan pear from four different regions of China (Sichuan (SC), Shangdong (SD), Chongming (CM), Zhuanghang (ZH)) were assessed. The highest differences in the physicochemical parameters were observed between four regions. The volatile fingerprints of GC-IMS showed great differences in the volatile of the Cuiguan pear, which suggested that the aroma of pears could be largely impacted by origin areas. (E)-ethyl-2-hexenoate can be used to distinguish between the 'CM' and pears from other regions. High contents of 2-heptanone, 1-pentanol, 1-butanol, 3-methylbutanol, butyl 2-methylbutanoate, heptyl acetate and butyl acetate were observed in the 'SD'. Dimethyl trisulfide, 6-methyl-5-hepten-2-one, 3-hydroxy-2-butanone, 1-penten-3-one, beta-pinene, γ-terpinene, propanal, (e)-2-pentenal, (e)-2-heptenal, 1-pentanol and 3-methyl-1-pentanol were primarily contained in the 'ZH'. Principal component analysis showed that there was very good discrimination based on the information obtained from GC-IMS for four samples. These findings were in agreement with the sensory analysis. In the opinion of the respondents to the consumer test, 'ZH' resulted in the most appreciated sample based on the average scores of the acceptability. This study provides some reference for the development and utilization of the Cuiguan pear.

Impact of different cooking methods on the flavor and chemical profile of yellow-fleshed table-stock sweetpotatoes (Ipomoea batatas L.)

This study investigated the impact of baking, boiling, and steaming on the taste, flavor, and chemical profile of yellow-fleshed sweetpotatoes (YFSP). Baked YFSP were sweeter, more palatable, and more flavorful than both steamed and boiled YFSP. Baking increased the YFSP soluble sugar content from 9.12% to 36.65%. Specifically, maltose increased by 200-fold and this possibly accounted for the sweetness of baked YFSP. From the Gas Chromatography-Mass Spectrometry analysis, the contents of furans and terpenes increased with baking, endowing baked YFSP with an aroma. On the contrary, boiling retained more carotenoids than the other cooking methods. Although cooking clearly altered YFSP, bioactive substances were predominantly preserved as only 72 out of 706 metabolites were identified as differentially accumulated metabolites between cooked and raw samples. Taken together, baked YFSP had high levels of sugars and volatile compounds, and the three cooking methods had little effect on chemical compounds. This comprehensive evaluation of cooked YFSP is a basis for sweetpotato processing and consumer choice.

The Impact of Different Withering Approaches on the Metabolism of Flavor Compounds in Oolong Tea Leaves

In this study, complementary metabolomic and proteomic analyses were conducted on the solar- and indoor-withered oolong tea leaves, and freshly plucked leaves as the control, for the purpose to reveal the mechanisms underlying the initial formation of some flavor determinants during the early stage of oolong tea processing. As a result, a total of 978 non-volatile compounds and 152 volatile compounds were identified, the flavonoids and several esters were differently accumulated in various tea samples. In total, 7048 proteins were qualitatively and quantitatively determined, the analysis on pathway enrichment showed that phenylpropanoid, flavonoid metabolisms, and protein processing in endoplasmic reticulum were the major pathways discriminating the different tea samples. The joint protein-metabolite analysis showed that the multiple stresses such as dehydration, heat, and ultra-violet irradiation occurred during the withering step induced the dynamic and distinct changes in the biochemical network in the treated leaves compared to fresh leaves. The significant decreases in flavonoids, xanthine alkaloids, and several amino acids contributed to the alleviation of bitter or astringent taste of withered leaves, although the decomposition of L-theanine resulted in the loss of umami flavor over the solar-withering step. Moreover, the fruity or floral aromas, especially volatile terpenoids and phenylpropanoids/benzenoids, were retained or accumulated in the solar withered leaves, potentially aiding the formation of a better characteristic flavor of oolong tea made by indoor withered tea leaves. Distinct effects of solar- and indoor-withering methods on the flavor determinant formation provide a novel insight into the relationship between the metabolite accumulation and flavor formation during the withering step of oolong tea production.

Variation in Volatile Compounds of Raw Pu-Erh Tea upon Steeping Process by Gas Chromatography-Ion Mobility Spectrometry and Characterization of the Aroma-Active Compounds in Tea Infusion Using Gas Chromatography-Olfactometry-Mass Spectrometry

Steeping process is an important factor for aroma release of tea, which has rarely been investigated for the aroma changes of raw Pu-erh tea (RAPT). In addition, the comprehensive aroma characteristics identification of RAPT infusion is necessary. In this study, GC-IMS coupled with principal component analysis (PCA) was used to clarify the difference of volatile profiles during the steeping process of RAPT. Furthermore, the volatiles contained in the RAPT infusion were extracted by three pretreatment methods (HS-SPME, SBSE, and SAFE) and identified using GC-O-MS. According to the odor activity value, 28 of 66 compounds were categorized as aroma-active compounds. Aroma recombination and omission experiments showed that "fatty", "green", "fruity", and "floral" are considered to be the main aroma attributes of RAPT infusion with a strong relationship with 1-octen-3-one, 1-octen-3-ol, (E)-2-octenal, β-ionone, linalool, etc. This study will contribute a better understanding of the mechanism of the RAPT steeping process and volatile generation.

Geographical differentiation of Molixiang table grapes grown in China based on volatile compounds analysis by HS-GC-IMS coupled with PCA and sensory evaluation of the grapes

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Volatile fingerprints of three Molixiang grapes were established using HS-GC-IMS.

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GC-IMS coupled with PCA could distinguished the three Molixiang grapes well.

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Sensory evaluation revealed significant (P ≤ 0.05) difference among the three samples.

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E-2-octenal, styrene and benzaldehyde were potential geographical marker compounds.

In this study, the volatile fingerprints of GC-IMS showed great differences on the volatile profiles of Molixiang grapes collected from three different regions of China, which suggested that aroma of table grapes could be largely impacted by origin areas. Butyl lactate, E-2-octenal and Z-2-pentanol were mainly contained in MLX-A, the grapes sampled from Ningbo, China. High contents of p-cymene, styrene and γ-terpinene were observed in MLX-B grapes sampled from Beizhen, China. In addition, benzaldehyde and methyl benzoate were major contained in MLX-C grapes sampled from Zhangzhou, China. The PCA results revealed effective differentiation of samples from different geographical origin based on the information obtained from GC-IMS. Furthermore, sensory evaluation showed that the aroma characters of grapes from different geographical origin were significantly different (P ≤ 0.05). E-2-octenal, styrene and benzaldehyde might serve as the geographical marker compounds of origin area based on the results of GC-IMS analysis and sensory evaluation.