Search

Sunlight withering (SWT) enhanced the floral aroma of white tea, whereas withering–tank withering (WWT) enriched its grassy aroma. This study investigated the impact of diverse withering methods on white tea aroma, examining aroma formation mechanisms by analyzing changes in essential compounds, their precursors, and enzyme activities during these treatments. Findings revealed that tea at 50% moisture concentration decreased the concentration of isopentenyl pyrophosphate and dimethylallyl pyrophosphate, leading to increased levels of linalool, geraniol, and β -myrcene. Sunlight spurred non-enzymatic reactions, elevating β -ionone concentration. The breakdown of linoleic and linolenic acids increased the hexanal and (Z)-3-hexenol levels. Enzyme activity analysis indicated that the prominent floral aroma in SWT was linked to heightened geranyl pyrophosphate synthase and alcohol dehydrogenase activity, enriching terpene–catalyzed products and benzeneacetaldehyde concentration, respectively. Elevated levels of lipoxygenase and arogenate dehydratase promoted grassy compounds in WWT. This study underscores how withering methods shape white tea aromas and offers operational insights. [Display omitted] • Mechanism of SWT to enhance floral aroma of white tea was revealed. • Mechanism of aroma improvement by WWT was elucidated. • The dynamics of key aroma precursors and enzyme activity were analyzed. [ABSTRACT FROM AUTHOR]

Fermentation is key to the quality of black tea, and traditional sensory evaluation methods are too slow and subjective for reliable, large-scale evaluation during fermentation. To address this problem, this study formulated a sensing system based on cobalt ions-doped carbon quantum dots emitting bright blue fluorescence. In evaluation experiments, this system was found to work stably for 90 min and accurately determine fermentation quality in real time. In fluorescence spectra results, fluorescence intensity decreased sharply with the concentrations of catechins, theaflavins, and gallic acid and this relationship was highly linear at concentrations from 0.01 to 50 μg/mL; R2 was >97% and the limit of detection was <0.046 μg/mL. A least-squares support vector machine model was established, and it had a 100% accuracy rate in discriminating between degrees of fermentation. Therefore, this quantum-dot-based system serves as an accurate and efficient means of measuring the fermentation quality of black tea. [Display omitted] • A testing technique for in-process black tea products with dynamic quality change. • Co-CQDs respond stably to key substances in black tea fermentation in real time. • The developed model's accuracy in determining the degree of fermentation is 100%. • Stable CQDs provide reliable performance during black tea fermentation. • Standard curves for catechins, theaflavins, and gallic acid were established. [ABSTRACT FROM AUTHOR]

[Display omitted] • White tea shows green, fresh, floral and fruity odors under solar withering (SW). • SW adds the levels of fatty acid-derived volatiles and volatile terpenoids (VTs). • SW decreases the level of amino acid-derived volatiles (AADVs). • Find out 26 key genes enriched in aroma synthesis pathways under SW. • Large VTs and AADVs with fruity and sweet odors increase with SW duration. Withering is a crucial process that determines the quality of white tea (WT). Solar withering (SW) is reported to contribute to the aroma quality of WT. However, the mechanism by which aroma is formed in WT subjected to SW remains unclear. In this study, through headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC–MS) and transcriptomics, we found that 13 key genes enriched in the mevalonic acid and methylerythritol phosphate pathways, such as those of 1-deoxy- D -xylulose-5-phosphate synthase and terpineol synthase, were significantly upregulated, promoting the accumulation of α -terpinolene, geraniol, and nerolidol, which imparted floral and fruity odors to WT subjected to SW. Additionally, the significant upregulation of lipoxygenases enriched in the lipoxygenase pathway promoting the accumulation of hexanol, 1-octen-3-ol, (E , Z)-3,6-nonadien-1-ol, and nonanal, which contributed to the green and fresh odor in WT subjected to SW. This study provided the first comprehensive insight into the effect mechanism of SW on aroma formation in WT. [ABSTRACT FROM AUTHOR]

Black teas harvested during the summer season usually have the defect of low aroma intensity, resulting in unacceptability from consumers. The shaking and standing (SS) process is key to the production of oolong tea and is believed to significantly improve the aroma quality. However, the specific effects of the shaking process on the aroma quality of black tea have not been elucidated. SSBT has a higher aroma intensity than BT, especially floral and sweet odors. By Aroma Extract Dilution Analysis (AEDA), 27 volatiles with flavor dilution factor (FD) above 8 were selected, of which 20 had odor activity values (OAV) values above 1; among them, 9 floral and sweet volatiles with high OAV were linalool (485 for BT, 918 for SSBT), (E)- β -ionone (389, 699), geraniol (315, 493), β -myrcene (25, 62), (E)-2-hexenal (2, 7), phenylacetaldehyde (44, 75), (Z)-3-hexenyl hexanoate (19, 41), 1-hexanol (9, 26), and 2-phenylethanol (2,3). Aroma reconstitution of these 20 volatiles showed reliable results of floral, sweet, fruity, and roasted odors, further validating the aroma and intensity profiles of the key odorants. Overall, our results reveal that the metabolite mechanism of the SS process improves the aroma quality of black tea, providing a theoretical basis and guiding measures for the production of high-aroma black tea. • Shaking and standing improve the sweet and floral aroma of black tea. • Volatiles in tea were detected by SPME- and SAFE-GC–MS. • Nine key odorants responsible for sweet and floral aroma were verified. • A novel processing flow for high-quality black tea was proposed. [ABSTRACT FROM AUTHOR]

• Changes in aroma characteristic of Keemun black tea during ripening were analyzed. • Fatty acid degradation products cause the rich aroma of Keemun Congou black tea. • The heat degradation pattern of major fatty acids in the tea was identified. Ripening refers to the process of chemical change during the refinement of Keemun black tea (KBT) and is crucial in the formation of Keemun Congou black tea's quality. In this study, the aroma composition of KBT during the ripening was analyzed. Sensomics indicated that ripening strengthened the coconut and fatty aroma of KBT and contributed to the decrease of green aroma substances, resulting in a shift of the overall aroma type of KBT to an integrated aroma profile, which was consistent with sensory evaluation. Changes in fatty acid content and the results of in vitro addition simulation tests confirmed that heat causes highly degradation of fatty acids into fatty aroma volatiles, which is a key driver of the formation of "Keemun aroma" quality. This study revealed the mechanism behind the formation of KBT's integrated "Keemun aroma" quality and the mode of thermal degradation of major fatty acids. [ABSTRACT FROM AUTHOR]

Grain Rain Period (GRP), one of the 24 solar terms in China, signifies a crucial phase for the transformation of tea quality, especially for Lu'an Guapian (LAGP) tea. During GRP, LAGP teas showed 3 distinct aroma profiles, each spanning 3–4 days. Specifically, the sensory evaluation result revealed that LAGP tea exhibited stronger flowery and fresh aromas in the early phase, with the soybean-like aroma significantly intensifying as the harvest period progressed during GRP. Furthermore, the key contributors to the aroma profile and its variation during GRP were identified as indole, δ-decalactone, geraniol, linalool, decanal, jasmone, (E) - β -ionone, benzeneacetaldehyde, dihydroactinolide, nonanal, octanal, (E) -isoeugenol, (E,E) -2,4-nonadienal, 4-ketoisophorone, (E,Z) -2,6-nonadienal, and 1-octen-3-one. Additionally, we proposed a binary blending strategy using sensory evaluation with the methods of triangle test and normal distribution fitting to predict the blending threshold accurately. This study elucidated the dynamics of LAGP tea aroma during GRP and offered insights for tea blending optimization. • During GRP, LAGP teas showed 3 distinct aroma profiles, each spanning 3–4 days. • LAGP teas mainly presented flowery and fresh aroma in the early phase of GRP. • Soybean-like aroma notably intensified as the harvest period progressed during GRP. • 16 active key volatile compounds were identified in LAGP teas during GRP. • A strategy was proposed to accurately predict binary blending thresholds. [ABSTRACT FROM AUTHOR]

The intensity of green tea's floral and sweet flavors was enhanced after being scented by osmanthus (OSGT). However, the mechanism of flavor enhancement by key volatiles remains unknown. Here, the role of key volatiles in OSGT on aroma and taste was explored by sensory experiment-guided flavor analysis. Binary mixed models of (E)- β -ionone, dihydro- β -ionone, and α -ionone showed additive interactions on floral aroma enhancement, the interactions were increased with increasing concentrations. At the concentration in OSGT, binary mixed models of (E)- β -ionone, geraniol, linalool, and γ -decalactone showed additive interactions on sweet aroma enhancement. (E)- β -ionone, geraniol, linalool, and γ -decalactone all significantly increased the perceived intensity of sweetness of sucrose solutions. Additionally, molecular docking revealed the perception mechanism of olfactory and taste receptors to the above characterized volatiles, with hydrogen bonding and hydrophobic interactions being the main interactions. This study highlights the importance of characteristic volatiles in enhancing the flavor of OSGT. • Binary mixed models of ionones showed additive interactions on floral enhancement. • Four volatiles showed additive interactions on sweetness taste enhancement. • The sweetness enhancement of OSGT was induced by volatiles, not soluble sugars. • Hydrogen bonding and hydrophobic interactions occur between volatiles and receptors. [ABSTRACT FROM AUTHOR]

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. • Tea stems and leaves significantly differed in aromatic compositions. • Retaining tea stems enhanced the roasty aroma of large-leaf yellow tea. • Roasted stems were rich in pyrazines (2-ethyl-3,5-dimethylpyrazine). • High concentrations of monosaccharides and amino acids occurred in the stems. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effects of processing stages on LAGP aroma constitutions were clarified. • Spreading and deep baking affected the aroma quality of LAGP the most. • Low-temperature spreading (4 °C) could increase the aroma content in tea leaves. • Low-temperature with long baking time was favorable to the formation of tea aroma. 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. [ABSTRACT FROM AUTHOR]

The quality of crush–tear–curl black tea (CTC-BT) varies greatly by geographic origin. Origin traceability is crucial for consumer interest protection, market order regulation, and food safety monitoring. This paper proposes a fast and accurate method for qualitative discrimination of CTC-BT origins and quantitative detection of its key taste-presenting substances. The method involves a simple colorimetric sensor array and ultraviolet–visible spectroscopy. The effects of various variable screening methods on modeling results were compared. A particle swarm optimization–based support vector machine achieved the highest performance in qualitative discrimination, with a correct classification rate of 99.48%. Based on their origin-distinguishing contributions and dose-over-thresholds, seven key taste-presenting substances were screened, namely, theaflavin, caffine, vitexin-2- O -rhamnoside, rutin, epigallocatechin gallate, epicatechin gallate, gallic acid. A least squares-support vector regression model achieved accurate quantification of the seven aforementioned compounds (square root of determination coefficient of prediction >0.9698, residual prediction deviation >2). [Display omitted] • A simple method for CTC black tea origin tracing and taste evaluation was developed. • Stable and simplified solution phase-colorimetric sensing array was built. • SG-PSO-SVM model achieved best CCR of 99.48% in origin discrimination. • Taste compounds were screened by origin-distinguishing contributions and thresholds. • Accurate quantification (Rp > 0.9698, RPD>2) of 7 taste compounds were achieved. [ABSTRACT FROM AUTHOR]

Abstract: Near-infrared spectroscopy (NIRS) combined with chemometric tools was utilized as a rapid analysis method to assess quality and to differentiate geographical origins of black tea. A partial least squares (PLS) algorithm was employed for the calibration of models predicting the levels of caffeine, water extract, total polyphenols, and free amino acids, while a factorization method was proposed to trace black tea from different geographical origins. In the calibration set, the root mean squared error of cross validation (%) and the correlation coefficient (R) for caffeine, water extracts, total polyphenols and free amino acids were 0.102%, 0.654%, 0.552%, and 0.248% and 0.983, 0.977, 0.975, and 0.943, respectively. In the prediction set, the root mean squared error of prediction and R for the corresponding constituents were 0.160%, 0.685%, 0.594%, and 0.273% and 0.955, 0.962, 0.954, and 0.927, respectively. The identification accuracy for black tea from different geographical origins reached 94.3%. This study demonstrated that NIR spectroscopy can be successfully applied to rapidly determine the main chemical compositions and geographical origins of black tea. [Copyright &y& Elsevier]

Piling fermentation (PF) is the key to the formation of Pu-erh tea quality; however, the traditional PF process limits the digital and intelligent production of Pu-erh tea. To establish qualitative and quantitative prediction models for the PF degree of Pu-erh tea, hyperspectral imaging technology and chemometric analysis were utilized. A qualitative model that uses least-squares support-vector-machine effectively distinguished the PF degree with an accuracy of 98.63%. Moreover, the chemical contents of quality-affecting components, namely total catechin, free amino acids, and chlorophyll a , were accurately quantified using raw spectral data with residual prediction deviations of 11.26, 4.34, and 3.89, respectively. The spatial distribution of these components during PF was mapped through chemical imaging, and the PF was deemed complete when the model predicted that the total catechin, amino acid and chlorophyll a content were less than 0.48, 11.21 and 1.29 mg/g, respectively. These findings provide a theoretical foundation for digital processing. [Display omitted] • Hyperspectral image technology (HIS) is used for quality monitoring of Pu-erh tea. • The prediction of the degree of Pu-erh tea piling fermentation (PF) was achieved. • Quality components was accurately quantified by raw spectral data (RPD>3.0). • Spatial distribution of quality composition during PF was mapped by HSI. [ABSTRACT FROM AUTHOR]

[Display omitted] • Over-fired drying can significantly improve the quality of LAGP tea. • LC–MS, HPLC and GC–MS revealed the taste and aroma compounds due to TSD and PLD. • A total of 5 compounds related to taste were selected by DOT > 1. • Concentrations of volatiles were 45.46% higher after PLD treatment. 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • The optimized yellowing process enhanced the sweet and floral aromas of yellow tea. • The optimized yellowing process increased the content of sweet and floral odorants. • β -Damascenone was the main odorant for sweet aroma enhancement. • β -Ionone was the main odorant for floral aroma enhancement. An optimized yellowing process for yellow tea (YT) was recently developed. The study found that the optimized yellowing process caused a significant increase in sweet and floral aromas by 31.3% and 24.0%, respectively. A total of 21 aroma-active compounds were identified using gas chromatography-mass spectrometry (GC–MS) and gas chromatography–olfactometry (GC-O) combined with sensomics analysis. Quantification of the 15 aroma-active compounds and calculation of odor activity values (OAVs) showed that the OAVs of sweet and floral aroma compounds increased significantly by 986.2% and 46.4%, respectively, after the optimized yellowing process. Sensory-directed aroma reconstitution and omission experiments confirmed that dimethyl sulfide, 3-methylbutanal, β -ionone, β -damascenone, geraniol, phenylacetaldehyde, and linalool were the key odorants in YT after the optimized yellowing process. Odorant addition tests further demonstrated that β -damascenone (OAV 590.4) was the main odorant for YT sweet aroma enhancement, while β -ionone (OAV 884.6) was the main odorant for YT floral aroma enhancement. [ABSTRACT FROM AUTHOR]

[Display omitted] • Increasing the yellowing temperature and relative humidity can shorten the time. • The optimal yellowing process of yellow tea (YT) improved its quality. • Hydrolysis of catechins and flavonoid glycosides enhanced the sweetness of YT. • The oxidative polymerization of catechins contributed to the yellowness of YT. In this study, an optimized yellowing process for yellow tea (YT) was developed by response surface methodology. The results showed that increasing the yellowing temperature from 20 °C to 34 °C, increasing the relative humidity from 55% to 67%, and reducing the yellowing time from 48 h to 16 h, caused a 40.5% and 43.2% increase in the yellowness and sweetness of YT, respectively, and improved the consumer acceptability by 36.8%. Moreover, metabolomics was used to explore the involved mechanisms that resulted in the improved YT quality. The optimized yellowing promoted the hydrolysis of 5 gallated catechins, 6 flavonoid glycosides, theogallin and digalloylglucose, resulting in the accumulation of 5 soluble sugars and gallic acid. Meanwhile, it promoted the oxidative polymerization of catechins (e.g., theaflagallin, δ-type dehydrodicatechin and theasinensin A), but decelerated the degradation of chlorophylls. Overall, this optimized yellowing process could serve as a guide to a shorter yellowing cycle. [ABSTRACT FROM AUTHOR]

• IDA sensor showed high sensitivity for catechin detection. • IDA elements were composed of natural and edible materials. • IDA sensor provided accurate authentication of PDO Longjing tea. • IDA has the advantage of being hazardous-free and cost-efficient. Traceability and authentication of protected designation of origin (PDO) tea is an important prerequisite to safeguard its production and distribution system. Here, indicator displacement array (IDA) sensors consisting of natural anthocyanidins and edible metal ions were developed to authenticate PDO and non-PDO Longjing from different origins. Five IDA elements were selected for constructing sensors, achieved by an indicator displacement reaction after adding epigallocatechin gallate solution. The obtained sensors were subsequently used for real tea samples. Unsupervised algorithms were used for data exploration among PDO and non-PDO teas. The supervised support vector machine (SVM) model further achieved accurate authentication of PDO and non-PDO Longjing with a correct classification rate of 100% for the 26 validated samples. The developed IDA sensor thus achieves accurate authentication of PDO tea in a hazard-free and cost-efficient way, providing a useful tool for origin authentication of other agricultural products. [ABSTRACT FROM AUTHOR]

• Volatile and non-volatile metabolites were characterized and analyzed. • Different data fusion and feature extraction methods were investigated. • Common dimension was used to analyze block weights and visualize data. • Mid-level data fusion in the RBF-SVM model achieved 93.33% test set accuracy. • HS-SPME-GC–MS and NMR achieved synergy to obtain higher classification accuracy. The narrow geographical traceability of green tea is both important and challenging. This study aimed to establish multi-technology metabolomic and chemometric approaches to finely discriminate the geographic origins of green teas. Taiping Houkui green tea samples were analyzed by headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry and 1H NMR of polar (D 2 O) and non-polar (CDCl 3). Common dimension, low-level and mid-level data fusion approaches were tested to verify if the combination of several analytical sources can improve the classification ability of samples from different origins. In assessments of tea from six origins, the single instrument data test set results in 40.00% to 80.00% accuracy. Data fusion improved single-instrument performance classification with mid-level data fusion to obtain 93.33% accuracy in the test set. These results provide comprehensive metabolomic insights into the origin of TPHK fingerprinting and open up new metabolomic approaches for quality control in the tea industry. [ABSTRACT FROM AUTHOR]

[Display omitted] • Over-fired drying methods affect the aroma quality of Lu'an Guapian (LAGP). • (E)- β -ionone is the most crucial aroma active compound in LAGP. • Pulley charcoal drying significantly enhanced the floral and fruity aroma of LAGP. 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Contributing volatiles for HX and LX aromas were decoded by GC–MS and GC–IMS. • Differential volatiles among HX, LX, QX were confirmed by VIP of PLS-DA and relative OAV. • Alcohols were the main contributing volatiles for floral-like aroma in TH green teas. Aroma types of green teas associate with their commercial prices and consumer acceptance, mainly including floral-like (HX), chestnut-like (LX), and fresh (QX) aromas. However, the volatile differences and specificities in these aroma types are still unclear. Herein, Taiping Houkui green teas with HX, LX, and QX aromas were processed separately with the same fresh tea leaves. Ninety-four and seventy-eight volatiles were detected and identified by headspace solid-phase microextraction gas chromatography–mass spectrometry (HS–SPME–GC–MS) and headspace gas chromatography–ion mobility spectrometry (HS–GC–IMS), respectively. Candidate differential volatiles among the tea samples were determined by the variable importance in projection (VIP) of the partial least squares–discriminant analysis (PLS–DA) and were further confirmed by the relative odor activity value (ROAV) and odor description. The volatiles 1-hexanol, linalool oxide (furanoid), linalool, geraniol, (E)- β -ionone, isoamyl acetate, and 2-methylpropanal enriched in HX and contributed to the floral-like aroma, while 3-methylbutanal, 2-ethyl-1-hexanol, indole, β -damascone, and cedrol enriched in LX and contributed to the chestnut-like aroma. This study reveals the specificities and contributions of volatiles in green teas with different aromas, thus providing new insights into the molecular basis of different flavored teas, benefiting for their precision processing and targeted quality control. [ABSTRACT FROM AUTHOR]

• Volatile compounds during the storage of An tea by SPME and SPE extraction. • One hundred and fifty-seven volatile compounds were detected. • The woody, stale and medicinal aromas are revealed during the storage of An tea. • Key aroma compounds related to stale and woody aromas were screened. The aim of this study was to reveal the molecular basis of aroma changes during storage of An tea (AT). The key volatile compounds in AT were screened using SPME-GC–MS and SPE-GC–MS analytical techniques in combination with odor activity value (OAV) and flavor dilution factor (FD). The results showed that with the increase of storage time the stale and woody aromas were revealed. Esters, acids and hydrocarbons are the main types of volatile compounds in AT, and their content accounts for 52.69 %–61.29 % of the total volatile compounds. The key volatile compounds with stale and woody aromas during AT storage were obtained by OAV value and FD value, namely ketoisophorone (flavor dilution factor, FD = 64), linalool oxide C (FD = 64), 1-octen-3-ol (OAV > 1, FD = 32), 1,2-dimethoxybenzene (FD = 16), naphthalene (OAV > 1, FD = 32), 3,4-dimethoxytoluene (FD = 16), and 1,2,3-trimethoxybenzene (FD = 8). Our research provides a scientific basis and insights for the improvement of quality during the storage of dark tea. [ABSTRACT FROM AUTHOR]

[Display omitted] • IDA sensor showed high sensitivity for EGCG detection. • IDA elements were composed of low-cost accessible pH dyes and metal ions. • IDA provided qualitative discrimination of green tea freshness. • IDA provided quantitative prediction of catechins in green tea during storage. Aging of green tea leads to reductions in its flavor and health value, yet in situ testing methods for green tea freshness are lacking. A novel sensitive indicator displacement assay (IDA) sensor was constructed and applied for monitoring of green tea freshness during storage. Low-cost pH dyes and metal ions were used as indicators and receptors, respectively, for the targeted detection of catechins in tea samples. The feasibility of the IDA reaction was verified using images and UV–vis spectroscopy, respectively. IDA combined with supervised algorithms achieved accurate identification of green tea freshness with an accuracy of 86.67%, and acceptable accuracies in the prediction of catechin monomers and total catechins with ratio of prediction to deviation values over 1.5. Thus, the developed IDA sensor is capable of qualitative and quantitative monitoring of the green tea freshness during storage, providing a new option for quality evaluation and control of green teas. [ABSTRACT FROM AUTHOR]

[Display omitted] • IDA was composed of phenylboronic acids, pyrocatechol violet, and alizarin red S. • TP, TC, and EGCG were key quality indicators of black tea fermentation. • SVM model achieved accurate qualitative evaluation of fermentation degrees. • SCARS-SVR model achieved accurate prediction of TP, TC, and EGCG. • In-situ quality monitoring of black tea fermentation was realized. Herein, a new indicator-displacement array (IDA) sensor was developed for the quality evaluation of black tea fermentation. On the principle of the reversible covalent binding of phenylboronic acid and catechol, phenylboronic acids were selected as acceptors for targeted binding to polyphenols. Pyrocatechol violet and alizarin red S were used as indicators of the reaction. The IDA sensors have sensitive differential responses to fermented tea samples, achieving an assessment of the fermentation degree with accuracies of 80.39–88.00% by support vector machine (SVM). In addition, the key polyphenol components of the fermentation process were accurately predicted by the IDA and SVM regression with ratio of prediction to deviation values of 1.55–1.72, 2.03–2.21, and 2.03–2.08 for total polyphenols, total catechins, and epigallocatechin-3-O-gallate, respectively. In conclusion, the developed IDA sensor is capable of the in-situ quality monitoring of black tea fermentation, with the advantages being cost-effectiveness, sensitivity, and rapidity. [ABSTRACT FROM AUTHOR]

Fixation is the key step of green tea processing, usually with moisture content as the quality evaluation indicator. Near-infrared (NIR) spectroscopy has been widely used in tea moisture detection but is mostly realized in the laboratory, and online detection in tea processing is limited. This study explores the possibility of calibration transfer from lab-based hyperspectral imaging (HSI) to online NIR to enable the rapid detection of moisture during the fixation step. Raw spectral data were preprocessed, and direct standardization (DS) and partial least-squares regression (PLS) were used for data calibration and modeling, respectively. The results showed that there were observable differences in the spectral data of the same sample obtained by different instruments. The prediction model based on HSI failed to be transferred directly to the online NIR and has a large root mean square error of prediction (RMSEP) value. Spectral preprocessing with a standard normal variate (SNV) transformation reduced the differences between the instruments. When 40 standard samples were used, the established transferable SNV-DS-PLS model based on HSI achieved the optimal prediction performance with a correlation coefficient of prediction (Rp), RMSEP, and ratio of prediction to deviation (RPD) of 0.94, 2.24%, and 2.76, respectively, for the 50 validated tea samples obtained from online NIR. In conclusion, the model transfer across instruments was successfully realized, and tea moisture detection was brought from the laboratory to the tea factory, which provided a reference model for online moisture detection for other agricultural products. • Calibration transfer from HSI to online NIR for tea processing was optimized. • Online detection of moisture during green tea fixation was achieved. • Model built based on HSI fails to be transferred directly to the online NIR. • Transferable SNV-DS-PLS model gave the optimal performance for moisture. [ABSTRACT FROM AUTHOR]

• Data of fermented tea samples was collected at-line by an industrial camera. • Catechins, theaflavins and chlorophylls were chosen as key quality indicators. • Correlations between color variables and quality indicators were established. • Spatial and temporal distribution of indicators was mapped and visualized. Fermentation is a key black tea processing step and makes an important contribution to quality formation. Current approaches to fermentation monitoring are costly or laboratory-based. Here, we first evaluated the potential of at-line computer vision for detecting fermentation quality in a tea factory. A self-built industrial camera was used to collect tea samples at various fermentation durations. The correlations of color variables that were extracted from the images with key quality indicators in the tea samples were verified. Subsequently, partial least-squares regression models based on the color variables showed high prediction accuracy with residual prediction deviation values of 4.13, 3.53, and 3.39 for catechins, theaflavins and chlorophylls, respectively. Finally, the spatial and temporal distributions of indicators during fermentation were mapped to visualize the fermentation quality. This study realized low-cost, at-line and real-time detection for black tea fermentation, which provides technical support for the industrial and intelligent production of black tea. [ABSTRACT FROM AUTHOR]

• A reproducible high sensitivity hemp spherical SERS substrate was prepared. • A simple and rapid method for colorants detection without pretreatment was proposed. • The CARS-PLS model achieved accurate quantification for the 4 adulterated colorants. • A rapid qualitative detection of multiple colorants mixed was achieved. This study synthesized stable and sensitive hemp spherical AgNPs as the SERS substrate for the simultaneous and rapid detection of sunset yellow, lemon yellow, carmine and erythrosine adulteration in black tea. With R6G as the probe molecule, the AgNPs were determined to have satisfactory stability over 60 days with an enhancement factor of 108. The effects of three variable screening methods on model performance were compared. Among them, CARS-PLS exhibited superior performance for the quantification of all the four colorants, with prediction set correlation coefficients of 0.95, 0.97, 0.99 and 0.88, respectively. The differentiation of the mixed colorants was also achieved, with recoveries ranging from 91.87 % to 106.5 % with RSD value <1.97 %, demonstrating the high accuracy and precision of the proposed method. The results indicate that AgNPs-based SERS is an effective method and has substantial potential for application in the identification and quantification of colorant in tea. [ABSTRACT FROM AUTHOR]

[Display omitted] • Strip green tea (SGT) volatile metabolite variations during process were clarified. • Fixation and rubbing were the key processes responsible for SGT aroma formation. • Lipid and glycoside derivatization were crucial in the volatile conversion of SGT. • Thirty-four differential volatiles were identified at different rubbing degrees. • Moderate rubbing was beneficial to the shape and aroma formation of SGT. Strip green tea (SGT) is widely distributed in China owing to its unique appearance and aroma but the evolution and formation mechanisms of volatile metabolites (VMs) during SGT processing, and especially in the unique process of rubbing, remain unclear. In this study, based on untargeted metabolomics, 217 VMs (8 categories) were identified, and fixation and rubbing processes were found to be key for SGT aroma formation. Moreover, targeted metabolomics was applied to obtain 38 differential VMs and their related substances, of which fatty acid-derived volatiles (14 VMs) and glycoside-derived volatiles (8 VMs) showed significant contributions to SGT aroma, and their derivation laws during SGT manufacturing were clarified. Furthermore, the effect of rubbing degree on volatile metabolite formation was explored, and 11 key differential VMs were screened by variable importance in projection, and odor activity value analyses. Appropriate rubbing promoted the loss of grassy VMs (such as 1-octanol and 2-pentyl-furan) and enrichment of floral/fruity VMs (such as trans -β-ionone, nonanal, geraniol, citral, (Z)-3,7-dimethyl-2,6-octadien-1-ol, and (Z)-hexanoic acid, 3-hexenyl ester). Our study not only enriches the chemical theory of green tea processing but also provides technical support for the precision directional processing of high-quality SGT. [ABSTRACT FROM AUTHOR]

[Display omitted] • Solar withering (SW) increases the bitterness and astringency of white tea (WT). • SW affects the upregulation of genes involved in flavonoid biosynthesis. • 27 genes responding to light and heat stress are up-regulated in SW. • Metabolism and taste presentation are related to stress response. • A comprehensive view on WT quality formation by metabolomics and transcriptomics. The mechanism through which solar withering (SW) affects the quality of white tea is unclear. To address this gap in the literature, in this study, we used metabolomics and transcriptomics to investigate the effect of SW on the quality of WT. WT that underwent SW was slightly more bitter and astringent than WT that underwent natural withering (control group). Specifically, SW considerably increased the concentration of astringent flavonoids and flavone glycosides in WT. This increase was mainly attributed to the upregulated expression of key genes in the shikimic acid, phenylpropanoid, and flavonoid biosynthesis pathways, such as shikimate kinase, chalcone synthase, and flavonol synthase. In addition, SW experienced considerable heat and light stress. The levels of glycerophosphatidylcholine and carbohydrates increased in response to the stress, which also affected the taste of WT. The results of this study indicate the mechanism through which SW affects the quality of WT. [ABSTRACT FROM AUTHOR]

• Different withering methods affect the aroma quality of Keemun black tea. • Sun withering significantly enhanced the floral and fruity of Keemun black tea. • Methional, geraniol and linalool are the most active-aroma in Keemum black tea. • A more comprehensive profile of Keemun aroma was obtained. Withering is a key process that affects the aroma of Keemun black tea (KBT). In this study, the aroma composition of KBT through natural withering, sun withering, and warm-air withering was analysed using gas chromatography–mass spectrometry. The results revealed significant differences in the three samples. Gas chromatography–olfactometry and aroma extract dilution analysis were performed with screening through a relative odour activity value (rOAV) > 1. In total, 11 aroma-active compounds (geraniol, (Z)-4-heptenal, 1-octen-3-ol, (E)- β -ionone, 3-methylbutanal, linalool, β -damascenone, (E , E)-2,4-decadienal, methional, (E , E)-2,4-nonadienal, and (E)-2-nonenal) were found to be responsible for the differences in aroma caused by different withering methods. Linalool (rOAV, 161) and geraniol (rOAV, 785) were responsible for the higher flowery and fruity aromas when sun withering was applied, whereas methional (rOAV, 124) contributed to the intense roasty aroma when warm-air withering was employed. Moreover, our results were verified by quantitative descriptive analysis and addition experiments. [ABSTRACT FROM AUTHOR]

[Display omitted] • The yellowing process improves the flavor of SGT. • Yellowing significantly decreases gallated catechins. • Benzene acetaldehyde and β-ionone enhance the sweetness of SGT. • 1-octen-3-ol enhances the umami of SGT. Summer green tea (SGT) has poor flavor due to its high levels of bitterness and astringency. The present study aimed to improve the flavor of SGT using the yellowing process. The results showed that after the yellowing process, the sweetness and overall acceptability increased, and the content of gallated catechins and flavonol glycosides decreased by 30.2% and 27.4%, respectively, as did the bitterness and astringency of SGT. Yellowing caused a decrease in the concentration of some aroma compounds, such as (z)-3-hexen-1-ol, 1-hexanol, pentanal, heptanal and 1-octanol, which caused grassy, floral and fruity aromas. In contrast, the concentrations of 1-octen-3-ol, benzene acetaldehyde and β-ionone increased, which have mushroom and sweet aromas. Meanwhile, the sweetness and umami of SGT were enhanced by the addition of selected aroma compounds (1-octen-3-ol, benzene acetaldehyde and β-ionone), demonstrating that the yellowing process improves the flavor of SGT through odor–taste interactions. [ABSTRACT FROM AUTHOR]

• A novel solution phase CSA was used for monitoring black tea fermentation. • UV-visible spectroscopy was employed to obtain CSA features in real time. • The accuracy of CARS-SVM model for discriminating fermentation degree was 100%. • Accurate prediction models for catechins and theaflavins were obtained. Rapid monitoring of fermentation quality has been the key to realizing the intelligent processing of black tea. In our study, mixing ratios, sensing array components and reaction times were optimized before an optimal solution phase colorimetric sensor array was constructed. The characteristic spectral information of the array was obtained by UV–visible spectroscopy and subsequently combined with machine learning algorithms to construct a black tea fermentation quality evaluation model. The competitive adaptive reweighting algorithms (CARS)-support vector machine model discriminated the black tea fermentation degree with 100% accuracy. For quantification of catechins and four theaflavins (TF, TFDG, TF-3-G, and TF-3′-G), the correlation coefficients of the CARS least square support vector machine model prediction set were 0.91, 0.86, 0.76, 0.72 and 0.79, respectively. The results obtained within 2 min enabled accurate monitoring of the fermentation quality of black tea, which provides a new method and idea for intelligent black tea processing. [ABSTRACT FROM AUTHOR]

• Low-cost, portable instruments were used to evaluate the tea processing quality. • A satisfactory classification rate (89.19%) was achieved using data fusion. • Quantitative model showed high prediction performance for the theaflavin content. • Our results provide templates for at-line monitoring of tea processing quality. Intelligent identification of black tea fermentation quality is becoming a bottleneck to industrial automation. This study presents at-line rapid detection of black tea fermentation quality at industrial scale based on low-cost micro-near-infrared spectroscopy (NIRS) and laboratory-made computer vision system (CVS). High-performance liquid chromatography and a spectrophotometer were used for determining the content of catechins and theaflavins, and the color of tea samples, respectively. Hierarchical cluster analysis combined with sensory evaluation was used to group samples through different fermentation degrees. A principal component analysis–support vector machine (SVM) model was developed to discriminate the black tea fermentation degree using color, spectral, and data fusion information; high accuracy (calibration = 95.89%, prediction = 89.19%) was achieved using mid-level data fusion. In addition, SVM model for theaflavins content prediction was established. The results indicated that the micro-NIRS combined with CVS proved a portable and low-cost tool for evaluating the black tea fermentation quality. [ABSTRACT FROM AUTHOR]

Polyphenol and catechin are key components in black tea processing, contributing to both taste and color quality. However, the rapid detection methods that are applicable throughout the processing stages are lacking. Here, we explored the potential of miniature near-infrared spectroscopy and self-built computer vision. Fresh tea leaves, and the samples from withering, rolling, fermentation, and drying steps were collected for in-situ data acquisition in a tea factory. Data from two sensors were fused, competitive adaptive reweighted sampling and Pearson correlation analysis were employed to select effective variables from spectral and color variables, respectively. And the linear partial least squares (PLS) were used for modeling. The results showed that PLS models based on low-level data fusion could not effectively improve the prediction accuracies compared to single data. By contrast, middle-level data fusion achieved the best prediction accuracies for both polyphenol and catechin, with average root mean square error of prediction of 0.66 ± 0.12 and 1.06 ± 0.11 g/100 g, and residual prediction deviations of 5.41 ± 0.99 and 4.03 ± 0.38, respectively. Overall, this study demonstrated the enhanced predictive capability of fused spectral and imaging systems for polyphenols, overcoming the low predictive accuracy of single sensors. • NIRS and computer vision were jointly used to monitor quality of tea processing. • Low- and mid-level data fusions were employed and compared for prediction. • Predictive accuracy of mid-level fusion outperformed that of single data. • Accurate predictions of polyphenol and catechin during processing were realized. • It offers in-situ and low-cost method for quality monitoring of tea processing. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pu'er tea (RPT) volatiles were identified by metabolomics and molecular sensory science. • Potential markers of RPTs stored in dry-cold and wet-hot environments were found. • Markers may explain the corresponding floral, sweet, fruit and woody, stale aromas. The aroma profile of raw pu'er tea (RPT) depends on its storage duration (2–10 years) and storage conditions (wet-hot or dry-cold environment). We analyzed the major odorants of RPT samples by performing metabolomic analysis and by using the molecular sensory science approach. Under dry-cold storage conditions, tea leaves had more carotenoid derivatives, glycoside-derived volatiles, and phenolic volatiles, resulting in "fresh," "floral," and "sweet" aroma. Under wet-hot storage conditions, tea leaves had more methoxybenzenes, which contributed considerably to their "stale" and "woody" aroma. We identified 11 and 4 compounds as the odor markers of RPTs when stored in dry-cold and wet-hot environments, respectively. Our findings provide a scientific basis for optimal storage that yields the desired aroma profile. [ABSTRACT FROM AUTHOR]

Withering of leaves is an important step in the processing of black tea and determines the taste and aroma of the tea. Currently, the withering degree of the leaves is mainly determined via sensory evaluation by the tea maker, and this is time-consuming and laborious. In this study, near-infrared spectroscopy, electronic eye, and colorimetric sensing array technologies were combined to evaluate the extent of withering. A low-cost micro-spectrometer, a self-built machine vision system, and a homemade colorimetric array were used to capture relevant information regarding the withered tea leaf samples in situ. Additionally, low- and middle-level data fusion strategies were modeled and compared using a support vector machine (SVM). The SVM model, which combined the information obtained using the three technologies, performed substantially better than a single technology. Low-level fusion achieved acceptable discriminant performance, with an accuracy of 90.00% for both the SVM and principal component analysis–SVM models for the prediction set. The middle-level fusion strategy achieved better performance than the low-level fusion strategy, with an optimal discriminant accuracy of 97.50% for SVM model. Hence, this study demonstrated that in situ and low-cost quality assessment and intelligent control of the withering process of black tea leaves are possible. [Display omitted] • E-nose, E-eye and NIRS were jointly used to monitor withering degree of black tea. • Low- and middle-level data fusion were critically used and compared. • Performance of data fusion in middle-level outperformed that of independent data. • It provides a low-cost, in situ method for quality monitoring of tea processing. [ABSTRACT FROM AUTHOR]