Characterization of Key Aroma-Active Compounds in Rough and Moderate Fire Rougui Wuyi Rock Tea (Camellia sinensis) by Sensory-Directed Flavor Analysis and Elucidation of the Influences of Roasting on Aroma

Contribution of tea stems to large-leaf yellow tea aroma

Large-leaf yellow tea (LYT) is processed from both leaves and stems, resulting in a distinctive rice crust-like aroma. Tea stems may contribute differently to the aroma of LYT than leaves. This study aimed to clarify the specific contribution of stems to LYT. The volatile compounds in different components of LYT were extracted and analyzed using a combination of headspace solid-phase microextraction and stir bar sorptive extraction coupled with gas chromatography-olfactory-mass spectrometry. The results revealed high concentrations of compounds with roasty attributes in stems such as 2-ethyl-3,5-dimethylpyrazine (OAV 153-208) and 2-ethyl-3,6-dimethylpyrazine (OAV 111-140). Aroma recombination and addition experiments confirmed that the roasty aroma provided by stems plays a pivotal role in the formation of the distinctive flavor of LYT. This study offers novel insights into the contribution of stems to the aroma of LYT, which can be used for processing and quality enhancement of roasted tea.

Comparison and elucidation of the changes in the key odorants of precooked stewed beef during cooking-refrigeration-reheating

The key odorants contributing to the warmed-over flavor (WOF) of reheated precooked stewed beef were characterized using a sensomics approach. Overall, 36 odorants were identified, and based on flavor dilution factors, odor activity values, aroma recombination, and omission test, 11 compounds mainly derived from lipid oxidation were characterized as the key odorants contributing to the formation of WOF. In particular, 3-(methylthio)propanal, which was positively correlated with meaty aroma, was implicated in an overall increase in WOF. Thus, these odorants were elected as potential markers of WOF in the reheated precooked stewed beef. In summary, the WOF of the precooked stewed beef could be attributed to an overall increase in lipid oxidation products and a decrease in the odorants with desirable aromas. The characterization of WOF in precooked stewed beef will aid in the flavor quality control of precooked stewed beef dishes.

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.

Characterization and correlations of dominant microorganisms and volatile compounds in fermentation process of Yangjiang douchi

BACKGROUND

Yangjiang douchi (YD) is a traditional fermented soybean product, which is popular in Chinese cuisine for its unique flavor. However, due to its high salt content and unstable flavor, its competitiveness in the international market is gradually weakening. Microorganisms have a key role in the production process of YD because it is a fermented food but the effect of microorganisms on the volatile compounds of YD is also not currently clear.

RESULTS

In this paper, aroma compounds and microbial diversity in different fermentation stages of YD were analyzed using gas chromatography-mass spectrometry/olfactometry (GC-MS/O) and IlluminaMiseq system sequencing. A total of 78 aroma-active compounds were detected throughout the fermentation process and they influenced the formation of flavor in YD. Fungi flora were relatively single in YD, and bacteria were rich and varied. A total of 418 species of bacteria were present during fermentation, with unclassified_Staphylococcus, Staphylococcus_kloosii, and Bacillus_velezensis_Bacillus predominating. There were 25 species of fungi at the species level, and Aspergillus minisclerotigenes (OTU 4) played a dominant role in the whole fermentation process.

CONCLUSION

Staphylococcus and Bacillus in the bacterial genus were strongly correlated with most flavor compounds detected, and A. minisclerotigenes in the fungi were more relevant to flavor compounds. This research provides a theoretical basis for the enhancement of the flavor of traditional fermented douchi in China. © 2024 Society of Chemical Industry.

Volatilomics and Macro-Composition Analyses of Primary Wuyi Rock Teas of Rougui and Shuixian Cultivars from Different Production Areas

Wuyi Rock Tea (WRT) is cherished for its exceptional "rock flavor" and its quality shows obvious regional differences. However, the flavor characteristics of Primary Wuyi Rock Teas (PWRTs) from different production areas remain unclear. Here, the Camellia sinensis var. sinensis cv. 'Rougui' and 'Shuixian', two quintessential cultivars for making WRT, planted in Zhengyan, Banyan, at high elevations, and Waishan production areas were used to make PWRTs. We conducted a comprehensive comparison of the sensory attributes, volatile organic compounds (VOCs), and macro-compositions of PWRTs of 'Rougui' and 'Shuixian' cultivars from different producing areas. Sensory evaluation indicated that both 'Rougui' and 'Shuixian' PWRTs from Zhengyan exhibited the best flavor qualities, followed by those from Banyan, at high altitudes, and Waishan production areas. The results of the determination and analysis of VOCs showed 680 VOCs in 'Rougui' and 'Shuixian' PWRTs, and that the different production areas mainly influenced the quantitative pattern of VOCs and rarely the qualitative composition. Integrated multivariate statistical analysis methods revealed that benzyl alcohol, hotrienol, butanoic acid, 2-methyl-, hexyl ester, benzene, (2-nitroethyl)-, and geranyl isobutyrate may be the key VOCs affecting the aroma differences in PWRTs from different production areas. In addition, water-extractable substances, tea polyphenols, caffeine, and free amino acids may be the important macro-compositions that distinguish PWRTs from different production areas. The metabolite basis for differences in the flavor qualities of PWRTs across production areas was elucidated, which may be helpful for the production of high-quality WRT.

Odor and taste characteristics, transduction mechanism, and perceptual interaction in fermented foods: a review

Fermentation is a critical technological process for flavor development in fermented foods. The combination of odor and taste, known as flavor, is crucial in enhancing people's perception and psychology toward fermented foods, thereby increasing their acceptance among consumers. This review summarized the determination and key flavor compound screening methods in fermented foods and analyzed the flavor perception, perceptual interactions, and evaluation methods. The flavor compounds in fermented foods could be separated, purified, and identified by instrument techniques, and a molecular sensory science approach could identify the key flavor compounds. How flavor compounds bind to their respective receptors determines flavor perception, which is influenced by their perceptual interactions, including odor-odor, taste-taste, and odor-taste. Evaluation methods of flavor perception mainly include human sensory evaluation, electronic sensors and biosensors, and neuroimaging techniques. Among them, the biosensor-based evaluation methods could facilitate the investigation of the flavor transduction mechanism and the neuroimaging technique could explain the brain's signals that relate to the perception of flavor and how they compare to signals from other senses. This review aims to elucidate the flavor profile of fermented foods and highlight the significance of comprehending the interactions between various flavor compounds, thus improving the healthiness and sensory attributes.

Dynamic changes and the effects of key procedures on the characteristic aroma compounds of Lu'an Guapian green tea during the manufacturing process

As a kind of green tea with unique multiple baking processes, the flavor code of Lu'an Guapian (LAGP) has recently been revealed. To improve and stabilize the quality of LAGP, further insight into the dynamic changes in odorants during the whole processing is required. In this study, 50 odorants were identified in processing tea leaves, 14 of which were selected for absolute quantification to profile the effect of processes. The results showed that spreading is crucial for key aroma generation and accumulation, while these odorants undergo significant changes at the deep baking stage. By adjusting the conditions of the spreading and deep baking, it was found that low-temperature (4 °C) spreading for 6 h and low-temperature with long-time baking (final leaf temperature: 102 °C, 45 min) could improve the overall aroma quality. These results provide a new direction for enhancing the quality of LAGP green tea.

Metabolomics analysis of flavor differences in Shuixian (Camellia sinensis) tea from different production regions and their microbial associations

Shuixian is renowned for its "rock flavor". However, the variations in Shuixian flavor are unclear, as the discussion mainly considers regional factors and overlooks the role of microorganisms. Sensory evaluation of Shuixian from three different regions (Zhengyan, Banyan, and Waishan) revealed that each had unique flavor characteristics: a woody aroma with slight acidity, a strong floral and fruity aroma with good freshness, and a distinct sweet aroma and sourness. Metabolomic analyses have revealed that 2-methylpyrazine was a crucial component of the woody aroma, whereas other metabolites contributed to sweet aroma, freshness, and acidity. Moreover, examinations of the relationship between flavor metabolites and microorganisms revealed that fungi had a more pronounced influence on the metabolite content of Shuixian. The study evaluated the role of fermentation microorganisms in shaping the flavor based on Shuixian flavor analyses, contributing to further research into the "rock flavor", as well as potential microbial interventions.

GC-MS Combined with Proteomic Analysis of Volatile Compounds and Formation Mechanisms in Green Teas with Different Aroma Types

Aroma is one of the key factors for evaluating the quality of green tea. A tender aroma (NX) and floral-like aroma (HX) are two types of high-quality aroma of green tea. In this work, the different aroma types of baked green tea were classified by sensory evaluation. Then, seven tea samples with a typical tender or floral-like aroma were selected for further volatile component analysis by GC-MS. A total of 43 aroma compounds were identified in two different aroma types of baked green tea samples. The PCA showed that linalool, geraniol, 3-hexenyl butyrate, and 3-hexenyl hexanoate were the major volatiles contributing to the HX. On the other hand, most of the alcohol volatiles, such as 1-octanol, 1-octen-3-ol, 1-dodecanol, 1-hexadecanol, phenylethyl alcohol, benzyl alcohol, aldehydes and some hydrocarbons contributed more to the NX. In addition, the chemical composition analysis showed that the content of free amino acids was higher in NX green tea samples, while the content of catechins was relatively higher in HX tea samples. A proteomic analysis revealed that most of the enzymes involved in VPBs pathways, such as phenylalanine ammonialyase, peroxidase, and shikimate-O-hydroxycinnamoyl transferase, were more abundant in NX than in HX tea samples. These results laid a foundation for the aroma formation mechanism of different aroma types of baked green tea and provided some theoretical guidance for the breeding of specific aroma varieties.

Dynamic Changes in Qidan Aroma during Roasting: Characterization of Aroma Compounds and Their Kinetic Fitting

Qidan is one of the most famous varieties of Wuyi Rock tea and has a strong aroma. The aroma-active compounds in Qidan subject to different roasting times were analyzed using solid-phase microextraction two-dimensional gas chromatography-olfactometry-mass spectrometry (SPME-GC×GC-O-MS), and a total of 92 aroma-active compounds were detected. Multivariate statistical analysis showed that the roasting time had a significant effect on the aroma characteristics of Qidan, and that the key products in the Maillard reaction accumulated with the extension of the roasting time; these key products were screened out according to the calculation of the odor activity values (OAVs), from which kinetic equations were established. It was found that the levels of 2-methylbutanal, 3-methylbutanal, 2-methylpyrazine, 2-ethyl-5-methylpyrazine, and benzaldehyde increased with time, while the contents of benzeneacetaldehyde showed a tendency to first increase and then decrease. This study provides a theoretical basis for flavor quality control during Qidan processing.

The key aroma components of steamed green tea decoded by sensomics and their changes under different withering degree

Steamed green tea has a long history and unique aroma, but little is known about its key aroma components. In this study, 173 volatiles in steamed green tea were identified using solvent-assisted flavor evaporation and headspace-solid phase microextraction plus two chromatographic columns of different polarities. Aroma extract dilution analysis revealed 48 highly aroma-active compounds with flavor dilution factors 64-1024. Internal standards were used to calculate odorant active value (OAV), and 11 OAV > 1 key aroma compounds were determined. Omission test identified eight substances, including dimethyl sulfide, (E)-β-ionone, cis-jasmone, linalool, nonanal, heptanal, isovaleraldehyde and (Z)-3-hexenol, as the key aroma active compounds of steamed green tea. With the increase of withering degree, the content of these substances increased first and then decreased except for heptanal and cis-jasmone. Moreover, the water content of 62 % was suggested to be an appropriate withering degree during the processing of steamed green tea.

New Insights into the Umami and Sweet Taste of Oolong Tea: Formation of Enhancer N-(1-carboxyethyl)-6-(hydroxymethyl) pyridinium-3-ol (Alapyridaine) in Roasting Via Maillard Reaction

Roasting is pivotal for enhancing the flavor of Wuyi rock tea (WRT). A study investigated a novel compound that enhances the umami taste of WRT. Metabolomics of Shuixian tea (SXT) and Rougui tea (RGT) under light roasting (LR), medium roasting (MR), and heavy roasting (HR) revealed significant differences in nonvolatiles compounds. Compared LR reducing sugars and amino acids notably decreased in MR and HR, with l-alanine declining by 69%. Taste-guided fractionation identified fraction II-B as having high umami and sweet intensities. A surprising taste enhancer, N-(1-carboxyethyl)-6-(hydroxymethyl) pyridinium-3-ol (alapyridaine), was discovered and identified. It formed via the Maillard reaction, positively correlated with roasting in SXT and RGT. Alapyridaine levels were highest in SXT among the five oolong teas. Roasting tea with glucose increased alapyridaine levels, while EGCG inhibited its formation. HR-WRT exhibited enhanced umami and sweet taste, highlighting alapyridaine's impact on WRT's flavor profile. The formation of alapyridaine during the roasting process provides new insights into the umami and sweet perception of oolong tea.

Investigation of the effect of over-fired drying on the taste and aroma of Lu'an Guapian tea using metabolomics and sensory histology techniques

Lu'an Guapian (LAGP) tea, a representative Chinese roasted green tea, undergoes significant changes in taste and aroma during over-fired drying. However, limited studies have been conducted on these effects. This study employed metabolomics and sensory histology techniques to analyze non-volatile and volatile compounds the second drying and pulley liquefied gas drying (PLD) samples. The results revealed that after PLD, the samples exhibited lower umami, bitterness, and astringency; whereas floral, sweet, roasted, cooked corn-like, and cooked chestnut-like aromas became stronger. Among them, the content of (-)-epigallocatechin gallate, glutamic acid, and theogallin, which were closely related to taste, decreased by 4.5 %, 12.3 %, and 10.4 %, respectively. Eight key aroma components were identified as the main contributors to the sample aroma changes: (E)-β-ionone, dimethyl sulfide, (E,E)-2,4-heptadienal, geraniol, linalool, benzeneacetaldehyde, 2-ethyl-3,5-dimethylpyrazine, and hexanal. This study provides a theoretical basis for enhancing the quality of LAGP teas.

The role of glutathione in stabilizing aromatic volatile organic compounds in Rougui Oolong tea: A comprehensive study from content to mechanisms

Chinese Oolong tea is widely known for its intricate aroma. However, the degradation of volatile organic compounds (VOCs) poses significant challenges for the tea products. In this study, glutathione (GSH) has an excellent preservation effect on VOCs in both the VOCs extract and the tea infusion during storage, specifically slowing the degradation of hexanal (by 66.39% and 35.09%) and heptanal (by 67.46% and 63.50%). Additionally, the addition of GSH maintained higher levels of active ingredients in tea infusion, including epigallocatechin, procyanidin B1, glutamic acid, and L-(+)-arginine, with respective increases of 184.09, 2.92, 4.10, and 6.35 times. The sulfhydryl group of GSH formed a covalent bond with hexanal and 2-methylbutanal, therefore improving the stability of VOCs. These findings provided a valuable insight for developing effective VOC preservation techniques for water-based tea products.

Study of the flavor dissipation mechanism of soy-sauce-marinated beef using flavor matrices

Soy sauce-based marinade beef (SSMB) is a traditional Chinese cuisine with a unique flavor. However, pre-precooling and air-cooling tunnels are necessary industrial units in the cold chain for flavor dissipation. Sensory-directed flavor analysis was performed to identify the key aroma-active compounds in SSMB that had just completed (100 ℃), pre-cooled in the brine tanks (45 ℃), and exited the air-cooleded tunnel (10 ℃). We identified 110 aroma-active substances, of which 42 were quantified based on their high flavor dilution factors. Recombination and omission tests identified 29 odorants as the main aroma-active molecules. Additionally, the flavor matrix revealed the relationship between the aroma component expression and sensory attributes. Flavor substances derived from spices, such as eugenol, anethole, and linalool, are enriched during the pre-cooling stage. The different meat attributes of the three samples were primarily related to aldehydes generated from lipid oxidation.

Exploring the visualization of human milk odor profiles: Intuitive characterization and construction of the link between odor compounds and sensory attributes

Studies on odor are not negligible in the human milk (HM) science field because it plays an irreplaceable role in the feeding process of infants. This study aimed to investigate the odor skeleton components (OSCs) in HM and verify and construct an intuitive link between them and the HM odor attributes. A total of 72 odor-active compounds were identified from 32 HMs using the comprehensive two-dimensional gas chromatography-olfactometry-mass spectrometry. Twenty of these compounds were identified as OSCs (average FD ≥ 1 and average OAV ≥ 1), and their actual odor contribution was clarified. Furthermore, the connection of the 20 OSCs with their corresponding eight odor attributes was visualized by constructing a molecular sensory odor wheel. Of them, 2,3-butanedione, (E)-2-decenal, nonanal, (E)-2-nonenal, octanal, 1-octen-3-one, hexanal, methional, and butanoic acid were the most important contributors to dairy-sweet, fishy, dairy-fat, metallic/iron, flour, grassy/green, cooked, and sweaty/rancid odor of HM.

Mechanism underlying the tenderness evolution of stir-fried pork slices with heating rate revealed by infrared thermal imaging assistance

This study aimed to explore the mechanism of cooking intensity on the tenderness of stir-fried pork slices from the perspective of the changes in temperature distribution. Infrared thermal imaging was used to monitor the distribution of temperature. Results showed that the high-level heat (HH) treatment could improve tenderness. When the center temperature increased to 100 °C, the shear force of samples from the low-level heat (LH) group increased by around 3-fold, and HH reduced this upward trend. This result was mainly attributed to the shorter heating time undergone by the HH-treated samples compared to the LH treatment, which resulted in less structural shrinkage and faster passing through the protein denaturation interval of the samples. These changes alleviated temperature fluctuations caused by water loss. This explanation could be confirmed by the results of T2 relaxation time and Fourier transform-infrared spectroscopy (FT-IR). However, the LH treatment caused a slower rise in oil temperature due to more moisture migration, which required the samples to undergo longer thermal denaturation, leading to a deterioration in tenderness. Moreover, histological analysis revealed that the greater integrity of endomysium in the HH group inhibited water loss and oil absorption, which contributed to obtain low-fat meat products with higher tenderness. This study provides support for the industrialization of traditional pork cuisines using oil as the heating medium.

Unveiling the aromatic intricacies of Wuyi Rock Tea: A comparative study on sensory attributes and odor-active compounds of Rougui and Shuixian varieties

The distinctive fragrance of Wuyi Rock Tea (WRT) has garnered high attention in recent years. Herein, we conducted a comprehensive comparison of the sensory attributes and odor-active compounds (OACs) between two quintessential WRTs, namely Rougui (RGT) and Shuixian (SXT). Sensory analysis revealed that RGT exhibited a more pronounced fruity aroma, while SXT had a more complex and intricate sensory profile. By using gas chromatography-olfactory mass spectrometry (GC-O-MS) and two-dimensional gas chromatography time-of-flight mass spectrometry (GC × GC-TOF-MS) analyses, 26 OACs were identified. Among them, 12 compounds with odor activity values > 1 were recognized as key OACs. Noteworthily, eight compounds, including 6-methyl-5-hepten-2-one, 2-ethyl-3,5-dimethylpyrazine, 2,3-diethyl-5-methylpyrazine, linalool, methyl salicylate, geraniol, (E)-β-ionone, and (E)-nerolidol, were shared by both teas. The unique compounds for RGT were (E)-linalool oxide and (Z)-jasmone, while those for SXT were β-cyclocitral and α-ionone. These findings offer valuable insights for better understanding the flavor differences between the two most important types of WRT.

Characterization of odor properties of human milk: Effect of inter-individual nutrient differences on key odor-active compounds and odor attributes

Odor is an important indicator of human milk (HM) quality, with a proven function. Here, the effect of inter-individual nutrient differences on key odor-active compounds (OACs) and odor attributes of HM samples was investigated using flavor analysis techniques and correlation network analysis. A total of ninety-four OACs were identified from 30 HMs, of which 24 key OACs could represent the basic odor characteristics of HMs. Fat content was closely related to the amounts of OACs, with aldehydes being the most abundant species and having the highest correlation with fat content. Of them, nonanal and octanal were the most important OACs in HM, having both high flavor dilution factor (2 ∼ 64, 4 ∼ 128) and odor activity values (<1 ∼ 37, 2 ∼ 36) in most samples. Additionally, different pattern of synergism between key OACs contribute to each odor attribute of HM. These findings will provide insights for subsequent in-depth studies of HM flavor.

From heat to flavor: Unlocking new chemical signatures to discriminate Wuyi rock tea under light and moderate roasting

Roasting is crucial for the distinct flavor of Wuyi rock tea (WRT). This study applied untargeted metabolomics to investigate the effects of roasting on 139 WRT samples roasted at light fire (LF) or moderate fire (MF) intensities. Compared to LF, MF roasting led to a decrease in the cis/trans flavanol ratio by 56% and theanine by 85%, while increasing the levels of N-ethyl-2-pyrrolidione-substituted flavanols (EPSFs), flavonol aglycones and flavone C-glycosides. Two new roast markers, 3-p-coumaroyl 1,5-lactone and 4-p-coumaroyl 1,5-lactone, were identified in WRT and their formation increased with roasting temperature. MF roasting facilitated the formation of diverse heterocycles (e.g., pyrazines) and aldehydes (e.g., (Z)-4-heptenal and (E,E)-2.4-decadienal) that contributed to the augmented roasted and fatty odors in WRT. Additionally, the Maillard product furfuryl methyl ether was solely detected in MF samples. These findings provide novel insights into roast markers in WRT with implications for improving quality control measures during tea roasting.

The Characteristic Aroma Compounds of GABA Sun-Dried Green Tea and Raw Pu-Erh Tea Determined by Headspace Solid-Phase Microextraction Gas Chromatography-Mass Spectrometry and Relative Odor Activity Value

We aim to improve the product quality of GABA raw Pu-erh tea during development and processing. In this study, headspace solid-phase microextraction gas chromatography-mass spectrometry technology combined with relative odor activity evaluations was used to compare the volatile compounds of GABA sun-dried green tea and GABA raw Pu-erh tea. Sensory evaluation showed a higher aroma score of GABA raw Pu-erh tea than that of GABA sun-dried green tea, with significant differences in aroma type and purity. A total of 147 volatile compounds of 13 categories were detected, which differed in composition and quantity between the two teas. 2-Buten-1-one,1-(2,6,6-trimethyl-1,3-cyclohexadien-1-yl)-,(E)- and beta.-myrcene largely contributed to the aroma formation of both teas. Five volatile compounds were screened as potential markers for tea aroma. Metabolic pathway analysis showed that monoterpenoid biosynthesis may be beneficial to the formation of flowery and fruity aromas in the teas. We suggest that the findings of this study may provide important guidance for the processing and optimization of GABA tea.

Characterization of the odor compounds in human milk by DHS/GC × GC-O-MS: A feasible and efficient method

Odor analysis of human milk (HM) is often challenging. Here, a new strategy for the analysis of odorants in HM using dynamic headspace sampling combined with comprehensive two-dimensional gas chromatography-olfactometry-mass spectrometry (DHS/GC × GC-O-MS) was established based on the comparison of different extraction methods and instrument modes. Overall, DHS/GC × GC-O-MS was proved to be able to provide higher extraction efficiency and better analytical results of the odor-active compounds (OACs) in HM, meanwhile, the salt addition during the extraction further promoted the release of the odorants. Twenty key OACs in HM were identified by flavor dilution analysis and odor activity calculation, of which 1-octen-3-one, 2,3-butanedione, (E)-2-nonenal, and nonanal contributed significantly to the odor of HM. In addition, 2,3-pentanedione was detected as a key OAC in HM for the first time. This study provided a powerful analytical strategy for the comprehensive odor analysis of HM.

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.

Effects of different over-fired drying methods on the aroma of Lu'an Guapian tea

Over-fired drying, a crucial process in the production of Lu'an Guapian (LAGP) tea, greatly enriches the tea's aroma. In this study, the aroma compounds of LAGP tea processed through pulley charcoal drying (PCD), roller drying (RD), roller-conveyor drying (RCD), and hot air drying (HD) were analyzed using gas chromatography-mass spectrometry. A subsequent analysis of aroma extraction dilution analysis and odor activity values revealed that (E)-β-ionone, dimethyl sulfide, (E,E)-2,4-heptadienal, geraniol, linalool, benzeneacetaldehyde, coumarin, 2-ethyl-3,5-dimethyl-pyrazine, indole, hexanal, (Z)-jasmone, and (Z)-3-hexen-1-ol were the key contributors to the samples' aroma variation. Moreover, a quantitative descriptive analysis and aroma recombination and omission experiments analysis revealed that (E)-β-ionone is the most critical contributor to the formation of floral aroma in tea processed using PCD, whereas (E,E)-2,4-heptadienal is responsible for the more pronounced fresh aroma in tea processed using HD. In addition, 2-ethyl-3,5-dimethyl-pyrazine contributes to the formation of a roasted aroma in tea processed using RD and RCD. The study results provide a theoretical basis for choosing the processing method, especially for drying, to obtain high-quality LAGP tea.

Characterizing and Decoding the Effects of Different Fermentation Levels on Key Aroma Substances of Congou Black Tea by Sensomics

Fermentation is the key technology for black tea aroma formation. The key aroma substances of black tea at different fermentation stages (unfermented (WDY), underfermented (F1H), fully fermented (F4H), and overfermented (F8H)) were characterized by the methodology of Sensomics. Aroma extract dilution analysis was performed on volatile fractions extracted by using solvent-assisted flavor evaporation and solid-phase microextraction, yielding 93 odor-active areas. Internal standard method plus stable isotope dilution analysis was used for quantitative analysis. The omission experiment identified 23 aroma substances. Further reduction and addition experiments revealed phenylacetaldehyde, (E,E)-2,4-heptadienal, geraniol, linalool, β-damascenone, 2-methylbutyraldehyde, dimethyl sulfide, and isovaleraldehyde with odor activity values (OAV) > 100 as the characteristic aroma components of F4H and also as the main contributors to aroma differences between different fermentation degrees. The green odor of (E,E)-2,4-heptadienal was highlighted in WDY and F1H relative to that in F4H due to the lower contribution of phenylacetaldehyde and β-damascenone in the former two samples. Additionally, excessive OAV increase of fatty aldehydes in F8H masked its similar floral and fruity aroma.

Enantiomeric separation and precise quantification of chiral volatiles in Wuyi rock teas using an efficient enantioselective GC × GC-TOFMS approach

Chiral volatiles play important roles in the formation of aroma quality of foods. To date, enantiomeric characteristics of chiral volatiles in Wuyi rock tea (WRT) and their aroma contributions are still unclear. In this study, an efficient enantioselective comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (Es-GC × GC-TOFMS) approach to separate and precisely quantitate 24 pairs of chiral volatiles in WRTs was established, and the enantiomeric distribution and aroma contribution of chiral volatiles among WRTs from four representative cultivars were investigated. Enantiomeric ratio (ER) of R-α-ionone (80%) in Dahongpao (DHP), ER of S-α-terpineol (57%) in Jinfo (JF), ERs of R-γ-heptanolactone (69%), S-γ-nonanolactone (55%), (2R, 5S)-theaspirane B (91%), concentration of S-(E)-nerolidol (313.37 ng/mL) in Rougui (RG) and concentration of R-α-ionone (33.01 ng/mL) in Shuixian (SX) were unique from other types of WRTs, which were considered as the potential chemical markers to distinguish WRT cultivars. The OAV assessment determined 7 volatile enantiomers as the aroma-active compounds, especially R-α-ionone and R-δ-octanolactone in SX, as well as S-(E)-nerolidol and (1R, 2R)-methyl jasmonate in RG contribute much to aroma formation of the corresponding WRTs. The above results provide scientific references for discrimination of tea cultivars and directed improvement of the aroma quality of WRT.

Aroma Identification and Classification in 18 Kinds of Teas (Camellia sinensis) by Sensory Evaluation, HS-SPME-GC-IMS/GC × GC-MS, and Chemometrics

Tea (Camellia sinensis) is one of the most popular beverages worldwide. Many types of tea products continuously emerge in an endless stream; so, the classification of tea becomes more difficult. Aroma is a vital indicator of tea quality. The present study deals with the identification of aroma compounds in 18 different kinds of tea belonging to three typical tea varieties, including green tea, oolong tea, and black tea, using GC-IMS and GC × GC-O-MS. Moreover, the clustering of all 18 tea samples and the in depth correlation analysis between sensory evaluation and instrumental data were performed using the PCA and OPLS-DA. The results revealed that in all 18 kinds of tea, a total of 85 aroma compounds were detected by GC-IMS, whereas 318 were detected by GC × GC-O-MS. The PCA result revealed that green tea, oolong tea, and black tea could be clearly separated based on their peak areas. The OPLS-DA result showed that a total of 49 aroma compounds with VIP value > 1.0 could be considered as the potential indicators to quickly classify or verify tea types. This study not only compared the aroma differences across different types of teas, but also provided ideas for the rapid monitoring of tea quality and variety.

Effects of Sun Withering Degree on Black Tea Quality Revealed via Non-Targeted Metabolomics

In this study, the effects of different sun withering degrees (75% (CK), 69% (S69), 66% (S66), 63% (S63), and 60% (S60) water content in the withered leaves) on black tea sensory quality were investigated by means of sensory evaluation plus metabolomics analysis. Sensory evaluation results showed higher sensory quality scores for the black tea in S69-S66, due to better freshness, sweeter taste, and a sweet and even floral and fruity aroma. Additionally, 65 non-volatile components were identified using Ultra Performance Liquid Chromatography-Quadrupole-Time of Flight-Mass Spectrometry (UPLC-Q-TOF/MS). Among them, the content increase of amino acids and theaflavins was found to promote the freshness and sweetness of black tea. The aroma of tea was analyzed using combined Solvent Assisted Flavor Evaporation-Gas Chromatography-Mass Spectrometry (SAFE-GC-MS) and Headspace-Solid Phase Micro Extract-Gas Chromatography-Mass Spectrometry (HS-SPME-GC-MS), and 180 volatiles were identified, including 38 variable importance in projection (VIP) > 1 (p < 0.05) and 25 Odor Activity Value (OAV) > 1 volatiles. Statistical analysis revealed 11 volatiles as potential major aroma differential metabolites in black tea with a different sun withering degree, such as volatile terpenoids (linalool, geraniol, (E)-citral, and β-myrcene), amino-acid-derived volatiles (benzeneethanol, benzeneacetaldehyde, and methyl salicylate), carotenoid-derived volatiles (jasmone and β-damascenone), and fatty-acid-derived volatiles ((Z)-3-hexen-1-ol and (E)-2-hexenal). Among them, volatile terpenoids and amino acid derived volatiles mainly contributed to the floral and fruity aroma quality of sun-withered black tea.

Characterization of the key odorants in floral aroma green tea based on GC-E-Nose, GC-IMS, GC-MS and aroma recombination and investigation of the dynamic changes and aroma formation during processing

To characterize the key odorants of floral aroma green tea (FAGT) and reveal its dynamic evolution during processing, the volatile metabolites in FAGT during the whole processing were analyzed by integrated volatolomics techniques, relative odor activity value (rOAV), aroma recombination, and multivariate statistical analysis. The volatile profiles undergone significant changes during processing, especially in the withering and fixation stages. A total of 184 volatile compounds were identified (∼53.26% by GC-MS). Among them, 7 volatiles with rOAV > 1 were identified as characteristic odorants of FAGT, and most of these compounds reached the highest in withering stage. According to the formation pathways, these key odorants could be divided into four categories: fatty acid-derived volatiles, glycoside-derived volatiles, amino acid-derived volatiles, and carotenoid-derived volatiles. Our study provides a comprehensive strategy to elucidate changes in volatile profiles during processing and lays a theoretical foundation for the targeted processing of high-quality green tea.

Biosynthesis and the Transcriptional Regulation of Terpenoids in Tea Plants (Camellia sinensis)

Terpenes, especially volatile terpenes, are important components of tea aroma due to their unique scents. They are also widely used in the cosmetic and medical industries. In addition, terpene emission can be induced by herbivory, wounding, light, low temperature, and other stress conditions, leading to plant defense responses and plant-plant interactions. The transcriptional levels of important core genes (including HMGR, DXS, and TPS) involved in terpenoid biosynthesis are up- or downregulated by the MYB, MYC, NAC, ERF, WRKY, and bHLH transcription factors. These regulators can bind to corresponding cis-elements in the promoter regions of the corresponding genes, and some of them interact with other transcription factors to form a complex. Recently, several key terpene synthesis genes and important transcription factors involved in terpene biosynthesis have been isolated and functionally identified from tea plants. In this work, we focus on the research progress on the transcriptional regulation of terpenes in tea plants (Camellia sinensis) and thoroughly detail the biosynthesis of terpene compounds, the terpene biosynthesis-related genes, the transcription factors involved in terpene biosynthesis, and their importance. Furthermore, we review the potential strategies used in studying the specific transcriptional regulation functions of candidate transcription factors that have been discriminated to date.

Sensomics-Assisted Characterization of Key Flowery Aroma Compounds in Lu'an Guapian Green Tea Infusion (Camellia sinensis)

The volatile fraction was isolated from the premium and common grade of Lu'an Guapian green tea infusion by solvent-assisted flavor evaporation distillation. With the application of aroma extract dilution analysis, a total of 52 aroma-active compounds were revealed in the flavor dilution (FD) factor area of 32-8192. Besides, five additional odorants with higher volatility were identified using solid-phase microextraction. The aroma profiles, FD factors, and quantitative data of premium Guapian (PGP) and common Guapian (CGP) showed apparent differences. The intensity of the flowery attribute was significantly higher in PGP than in CGP, while cooked vegetable-like was the most outstanding odor note in CGP. The recombination experiment and the omission test of PGP clarified that dimethyl sulfide, (E,E)-2,4-heptadienal, (E)-β-ionone, (E,Z)-2,6-nonadienal, 2-methylbutanal, indole, 6-methyl-5-hepten-2-one, hexanal, 3-methylbutanal, γ-hexalactone, methyl epijasmonate, linalool, geraniol, and (Z)-3-hexen-1-ol were the key odorants of PGP tea infusion. The omission and addition tests of flowery odorants manifested that (E)-β-ionone, geraniol, and (E,E)-2,4-heptadienal, with higher odor activity values in PGP than in CGP, contributed to the flowery attribute most. The difference in the concentration of the abovementioned odorants with flowery odor notes could be one of the main factors which led to the difference in aroma quality between the two grades of Lu'an Guapian.

Metabolomics integrated with machine learning to discriminate the geographic origin of Rougui Wuyi rock tea

The geographic origin of agri-food products contributes greatly to their quality and market value. Here, we developed a robust method combining metabolomics and machine learning (ML) to authenticate the geographic origin of Wuyi rock tea, a premium oolong tea. The volatiles of 333 tea samples (174 from the core region and 159 from the non-core region) were profiled using gas chromatography time-of-flight mass spectrometry and a series of ML algorithms were tested. Wuyi rock tea from the two regions featured distinct aroma profiles. Multilayer Perceptron achieved the best performance with an average accuracy of 92.7% on the training data using 176 volatile features. The model was benchmarked with two independent test sets, showing over 90% accuracy. Gradient Boosting algorithm yielded the best accuracy (89.6%) when using only 30 volatile features. The proposed methodology holds great promise for its broader applications in identifying the geographic origins of other valuable agri-food products.

Aroma characteristics of Wuyi rock tea prepared from 16 different tea plant varieties

Wuyi rock tea (WRT) is famous for its long history and unique characteristic of floral, fruity and nutty flavors. This study investigated the aroma characteristics of WRTs prepared from 16 different oolong tea plant varieties. The sensory evaluation results showed that all WRTs had an 'Yan flavor' taste, and the odor was strong and lasting. Roasted, floral and fruity odors were the prime aroma profiles for WRTs. Furthermore, a total of 368 volatile compounds were detected using HS-SPME-GC-MS and analyzed with OPLS-DA and HCA methods. The volatile compounds heterocyclic compounds, esters, hydrocarbons, terpenoids and ketones were the major aromatic components of the WRTs. Specifically, the volatile profiles among newly selected cultivars were comparatively analyzed, and 205 differential volatile compounds were found with variable importance in the projection (VIP) values above 1.0. These results indicated that the aroma profiles of WRTs were mainly dependent on the cultivar specificities of volatile compounds.

Insights into Characteristic Volatiles in Wuyi Rock Teas with Different Cultivars by Chemometrics and Gas Chromatography Olfactometry/Mass Spectrometry

Wuyi rock tea (WRT) is one of the most famous subcategories of oolong tea, exhibiting distinct aroma characteristics with the application of different cultivars. However, a comprehensive comparison of the characteristic volatiles among WRTs with different cultivars has rarely been carried out. In this study, non-targeted analyses of volatile fragrant compounds (VFCs) and targeted aroma-active compounds in WRTs from four different cultivars were performed using chemometrics and gas chromatography olfactometry/mass spectrometry (GC-O/MS). A total of 166, 169, 166, and 169 VFCs were identified for Dahongpao (DHP), Rougui (RG), Shuixian (SX), and Jinfo (JF), respectively; and 40 components were considered as the key differential VFCs among WRTs by multivariate statistical analysis. Furthermore, 56 aroma-active compounds were recognized with predominant performances in "floral & fruity", "green & fresh", "roasted and caramel", "sweet", and "herbal" attributes. The comprehensive analysis of the chemometrics and GC-O/MS results indicated that methyl salicylate, p-cymene, 2,5-dimethylpyrazine, and 1-furfurylpyrrole in DHP; phenylethyl alcohol, phenethyl acetate, indole, and (E)-β-famesene in RG; linalool, phenethyl butyrate, hexyl hexanoate, and dihydroactinidiolide in JF; and naphthalene in SX were the characteristic volatiles for each type of WRT. The obtained results provide a fundamental basis for distinguishing tea cultivars, recombination, and simulation of the WRT aroma.

Characterization of Key Odor-Active Off-Flavor Compounds in Aged Pasteurized Yogurt by Sensory-Directed Flavor Analysis

The purpose of this study was to determine the key odor-active compounds contributing to the off-flavor of aged pasteurized yogurt (APY) using sensory-directed flavor analysis. Additionally, different extraction methods were compared to determine their effects on the volatile compounds, including dynamic headspace sampling (DHS), solid-phase microextraction, and stir bar sorptive extraction, and DHS was found to be suitable for this study. The results showed that comprehensive two-dimensional gas chromatography-olfactometry-mass spectrometry analysis (GC × GC-O-MS) had more advantages in separating and identifying the volatile compounds than the traditional GC-O-MS. A total of 17 odor-active compounds were determined in the fresh pasteurized yogurt and APY samples by DHS coupled with GC × GC-O-MS. The dynamic headspace dilution analysis demonstrated that 2-heptanone and hexanal were the most vital components in APY with the highest flavor dilution factor. Furthermore, the spiking and omission experimental results revealed that the odor-active compounds, such as 2-heptanone, butanoic acid, pentanoic acid, hexanal, and (E)-2-heptenal, were the key odor-active off-flavor contributors in APY. Therefore, these compounds could be used as potential indicators to determine the freshness of pasteurized yogurt.

Characterization of Key Odorants in Lingtou Dancong Oolong Tea and Their Differences Induced by Environmental Conditions from Different Altitudes

Lingtou Dancong oolong tea is a famous Chinese oolong tea due to its special honey-like aroma. However, little is known about its specific aroma profile and key contributors. Furthermore, whether the aroma characteristics of Lingtou Dancong oolong tea are affected by the environmental conditions at different altitudes is unknown. In this study, the aromas in Lingtou Dancong oolong tea were extracted and analyzed by stir-bar sorptive extraction (SBSE) combined with gas chromatography-olfactometry (GC-O) and GC-mass spectrometry (GC-MS), and the aroma profiles of tea plants grown at different altitudes were compared. We detected 59 odor compounds in Lingtou Dancong oolong tea. Eight compounds with honey and floral odors were identified as key components on the basis of GC-O, GC-MS, odor activity value, and flavor dilution analyses. Differences in the contents of precursor geranyl diphosphate and transcript levels of structural genes were found to be responsible for the differential accumulation of linalool and hotrienol among plants grown at different altitudes. This is the first report on the aroma characteristics and key contributors of Lingtou Dancong oolong tea and their differences, as affected by altitude. These results provide details of the chemical basis of the aroma quality of Lingtou Dancong oolong tea.

Comparison of Different Volatile Extraction Methods for the Identification of Fishy Off-Odor in Fish By-Products

This study was conducted to analyze volatile odor compounds and key odor-active compounds in the fish soup using fish scarp and bone. Five extraction methods, including solid-phase microextraction (SPME), dynamic headspace sampling (DHS), solvent-assisted flavor evaporation (SAFE), stir bar sorptive extraction (SBSE), liquid-liquid extraction (LLE), were compared and SPME was finally selected as the best extraction method for further study. The volatile odor compounds were analyzed by gas chromatography-olfactometry-mass spectrometry (GC-O-MS) and comprehensive two-dimensional gas chromatography-olfactometry-mass spectrometry (GC × GC-O-MS) techniques, and the key odor-active compounds were identified via aroma extract dilution analysis (AEDA) and relative odor activity value (r-OAV) calculation. A total of 38 volatile compounds were identified by GC-O-MS, among which 10 were declared as odor-active compounds. Whereas 39 volatile compounds were identified by GC × GC-O-MS, among which 12 were declared as odor-active compounds. The study results revealed that 1-octen-3-one, 2-pentylfuran, (E)-2-octenal, 1-octen-3-one, hexanal, 1-octen-3-ol, 6-methylhept-5-en-2-one, (E,Z)-2,6-nondienal and 2-ethyl-3,5-dimethylpyrazine were the key odor-active compounds in the fish soup.

Study on the Suitability of Tea Cultivars for Processing Oolong Tea from the Perspective of Aroma Based on Olfactory Sensory, Electronic Nose, and GC-MS Data Correlation Analysis

The oolong tea aroma is shown to consist of cultivar aroma and technical aroma in this study based on the aroma differences between oolong tea products of cultivars of different suitability, as determined by correlation analysis of olfactory, sensory, electronic nose, and GC-MS data. Human senses were significantly affected by the aroma components, which included eight terpene metabolites (β-Ocimene, (Z)-Furan linalool oxide, linalool, (3E)-4,8-Dimethyl-1,3,7-nonatriene, (E)-Pyranoid linalool oxide, γ-Elemene, Humulene, (Z,E)-α-Farnesene), three carotenoid metabolites (β-Ionone, (Z)-Geranylacetone and 6-methyl-5-Hepten -2-one), three lipid metabolites ((Z)-3-Hexenyl (Z)-3-hexenoate, Butanoic acid hexyl ester, and (Z)-Jasmone), four amino acid metabolites (Methyl salicylate, Geranyl isovalerate, indole, and Phenylethyl alcohol), and six thermal reaction products (2-Pentylfuran, Octanal, Decanal, (E,E)-2,4-Nonadienal, (Z)-2-Decenal, and (E)-2-Undecenal). Meanwhile, several aroma compounds (such as (E)-Nerolidol and α-Farnesene), mainly comprising the “technical aroma” formed in the processing mode, were noted to be less closely related to cultivar suitability. This study sheds light on the aroma characteristics of different tea cultivars for oolong tea processing.

Effects of Lactic Acid Bacteria Additives on the Quality, Volatile Chemicals and Microbial Community of Leymus chinensis Silage During Aerobic Exposure

Silage exposed to air is prone to deterioration and production of unpleasant volatile chemicals that can seriously affect livestock intake and health. The aim of this study was to investigate the effects of Lactobacillus plantarum (LP), Lactobacillus buchneri (LB), and a combination of LP and LB (PB) on the quality, microbial community and volatile chemicals of Leymus chinensis silage at 0, 4, and 8 days after aerobic exposure. During aerobic exposure, LP had higher WSC and LA contents but had the least aerobic stability, with more harmful microorganisms such as Penicillium and Monascus and produced more volatile chemicals such as Isospathulenol and 2-Furancarbinol. LB slowed down the rise in pH, produced more acetic acid and effectively improved aerobic stability, while the effect of these two additives combined was intermediate between that of each additive alone. Correlation analysis showed that Actinomyces, Sphingomonas, Penicillium, and Monascus were associated with aerobic deterioration, and Weissella, Pediococcus, Botryosphaeria, and Monascus were associated with volatile chemicals. In conclusion, LB preserved the quality of L. chinensis silage during aerobic exposure, while LP accelerated aerobic deterioration.

Flavor of tea (Camellia sinensis): A review on odorants and analytical techniques

Tea is among the most consumed nonalcoholic beverages worldwide. Understanding tea flavor, in terms of both sensory aspects and chemical properties, is essential for manufacturers and consumers to maintain high quality of tea products and to correctly distinguish acceptable or unacceptable products. This article gives a comprehensive review on the aroma and off-flavor characteristics associated with 184 odorants. Although many efforts have been made toward the characterization of flavor compounds in different types of tea, modern flavor analytical techniques that affect the results of flavor analysis have not been compared and summarized systematically up to now. Thus, the overview mainly provides the instrumental flavor analytical techniques for both aroma and taste of tea (i.e., extraction and enrichment, qualitative, quantitative, and chemometric approaches) as well as descriptive sensory analytical methodologies for tea, which is helpful for tea flavor researchers. Flavor developments of tea evolved toward time-saving, portability, real-time monitoring, and visualization are also prospected to get a deeper insight into the influences of different processing techniques on the formation and changes of flavor compounds, especially desired flavor compounds and off-flavor substances present at (ultra)trace amounts in tea and tea products.

Decoding the Specific Roasty Aroma Wuyi Rock Tea (Camellia sinensis: Dahongpao) by the Sensomics Approach

Aroma extract dilution analysis was performed on volatile fractions extracted from a freshly prepared Dahongpao (DHP) tea infusion using solvent-assisted flavor evaporation, yielding 65 odor-active domains with flavor dilution factors ranging between 32 and 32,768. In addition, six aromatic substances were captured by headspace analysis. Quantitation of 54 compounds by an internal standard method and stable isotope dilution assays revealed that the concentrations of 32 odorants exceeded their respective orthonasal odor threshold values in tea infusion. The results of odor activity values (OAVs) suggested that 2-metylbutanal (malty) and γ-hexalactone (coconut-like) had the highest OAVs (248 and 154). Eight odorants including γ-hexalactone (OAV 154), methyl 2-methylbutanoate (59), phenylacetic acid (7.2), acetylpyrazine (5.7), 2-methoxyphenol (3.4), p-cresol (2.7), 2,6-diethylpyrazine (2.7), and vanillin (1.8) were newly identified as key odorants in DHP tea infusion. An aroma recombination model in a non-volatile matrix extracted from tea infusion satisfactorily mimicked the overall aroma of DHP tea infusion, thereby confirming the identification and quantitative experiments. Omission experiments verified the obvious significance of 6-methyl-5-hepten-2-one (OAV 91), 2-ethyl-3,5-dimethylpyrazine (19), 4-hydroxy-2,5-dimethylfuran-3(2H)-one (13), and acetylpyrazine (5.7) as key odorants for the special roasty and caramel-like aroma of DHP tea.

Digital Evaluation of Aroma Intensity and Odor Characteristics of Tea with Different Types-Based on OAV-Splitting Method

Aroma is one of the most important quality indicators of tea. However, this evaluation method is a subjective one. In this study, the volatiles of tea with 5 types were determined by headspace solid-phase micro-extraction (HS-SPME) combined with gas chromatography mass spectrometry (GC-MS). The aroma intensity and odor characteristics of teas were comparatively analyzed based on the OAV-splitting method. The results showed that OAV were green tea (492.02), red tea (471.88), oolong tea (302.74), white tea (68.10), and dark tea (55.98). The odor index I(o) indicated that green tea was strong-flavor tea with highlight green accompanied by fruity, woody and fatty odors; oolong tea was strong-flavor tea with fruity and fatty accompanied by woody, floral and green odors; red tea was strong-flavor tea with highlight fruity accompanied by woody, green and floral odors; white tea was a light-flavor tea with floral, woody and green odors; and dark tea was light-flavor tea with woody and floral notes accompanied by fatty and green odors. These results fitted perfectly with the people’s consensus on these teas, and proved that the OAV-splitting method is feasible to evaluate the aroma intensity and odor characteristics of tea aroma. We suggest that the digital evaluation of tea aroma can facilitate people’s communication.

Comparative Analysis of Volatile Compounds in Tieguanyin with Different Types Based on HS-SPME-GC-MS

Tieguanyin (TGY) is one kind of oolong tea that is widely appreciated for its aroma and taste. To study the difference of volatile compounds among different types of TGY and other oolong teas, solid-phase microextraction−gas chromatography−mass spectrometry and chemometrics analysis were conducted in this experiment. Based on variable importance in projection > 1 and aroma character impact > 1, the contents of heptanal (1.60−2.79 μg/L), (E,E)-2,4-heptadienal (34.15−70.68 μg/L), (E)-2-octenal (1.57−2.94 μg/L), indole (48.44−122.21 μg/L), and (E)-nerolidol (32.64−96.63 μg/L) in TGY were higher than in other varieties. With the increase in tea fermentation, the total content of volatile compounds decreased slightly, mainly losing floral compounds. Heavily fermented tea contained a higher content of monoterpenoids, whereas low-fermentation tea contained higher contents of sesquiterpenes and indole, which could well distinguish the degree of TGY fermentation. Besides, the volatiles analysis of different grades of TGY showed that the special-grade tea contained more aroma compounds, mainly alcohols (28%). (E,E)-2,4-Heptadienal, (E)-2-octenal, benzeneacetaldehyde, and (E)-nerolidol were the key volatile compounds to distinguish different grades of TGY. The results obtained in this study could help enrich the theoretical basis of aroma substances in TGY.