Black Tea Processing Research Articles

Characterization of the key differential volatile components in different grades of Dianhong Congou tea infusions by the combination of sensory evaluation, comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry, and odor activity value

Aroma is an important factor in evaluating tea infusion quality. In this study, the aroma quality of 44 Dianhong Congou tea infusions was classified into high-quality group and non-high-quality group according to the sensory evaluation. The volatile components were investigated by using headspace solid phase microextraction combined with comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry. A total of 258 volatile components were identified, of which the three component types with the highest relative percentages were alcohols (17.05%), aldehydes (15.12%), and heterocycles (15.12%). Clear discrimination was achieved by orthogonal partial least squares discrimination analysis (R2Y = 0.94, Q2 = 0.82). In addition, 69 important differential compounds were identified based on variable importance in the projection value (VIP >1.0) and one-way ANOVA (p < 0.05). Moreover, seven common key odorants with odor activity values (OAVs) > 1 were screened out, including (E, E)-2,4-heptadienal, linalool, 1-octene-3-one, geraniol, β-myrcene, (Z)-4-heptenal, and nonanal. Aroma character impact value indicated that linalool performed a positively supportive role in the aroma quality of Dianhong Congou tea infusions, while (E, E)-2,4-heptadienal negatively affected the aroma quality. These results contribute to providing a theoretical support for directional processing of high-quality black tea.

Assessment of Occupational Noise Generation and Associated Noise-Induced Hearing Loss among Employees of a Black Tea Processing Factory in Darjeeling District, India.

Exposure to high levels of noise is a problem among tea factory workers worldwide, but it is poorly studied in India. This study aimed to assess noise generation in various parts of a black tea factory and find out prevalence of occupational noise-induced hearing loss (ONIHL) and its determinants among employees. An observational cross-sectional study was performed in a black tea processing factory of Darjeeling District. Noise levels and exposure data at different parts of the factory were measured using a sound level meter. Participant characteristics were obtained using a questionnaire and noise-induced hearing loss data obtained by audiometry. A sound map was generated based on noise exposure data and a multivariable logistic regression performed to assess determinants of ONIHL. Sound pressure levels ranged from 58.7 to 90.3 dBA, with the highest levels in the crushing-tearing-curling (CTC) room. Noise exposure of workers was the highest during curling process and the lowest during packaging in the packing room. The prevalence of ONIHL was found to be 28.3%, most of which were of moderate degree, and the highest prevalence was among CTC room workers. Multivariable analysis showed significant association only between daily noise exposure and the presence of ONIHL (AOR 1.68, P value = 0.018). None of the study participants used any hearing protection equipment during work. Generation of high levels of noise is a pertinent problem in the black tea factory, which, coupled with non-use of personal protective equipment, led to a high risk and prevalence of ONIHL.

Targeted metabolomics and DIA proteomics-based analyses of proteinaceous amino acids and driving proteins in black tea during withering

Withering is a necessary step to improve the flavors during the manufacturing of black tea. Fresh tea leaves of Fuding Dabai were subjected to withering (kept for 0–28 h at room temperature of 25 °C) to observe corresponding morphological changes. In addition, dynamic changes in proteinaceous amino acids, especially taste-related proteinaceous amino acids and their corresponding driving proteins, were investigated using targeted metabolomics and data-independent acquisition (DIA) proteomics. The results indicated that tea leaves gradually withered and shrunk, and their color deepened with the extension of withering time. Furthermore, the contents of umami amino acids were always higher during the withering process. Sweet and bitter amino acids exhibited a significant cumulative effect at the middle and late withering stages. A total of 31 driving proteins were detected that were involved in the metabolic regulation of Ser, Thr, Phe, Trp, and His amino acids. The functional analysis of 776 differential expressed proteins revealed that the tea leaf cells maintained the maximum possible cellular activity during the withering process, and the toxins were decomposed at the end of withering. These findings can provide the metabolic basis for room temperature withering during black tea processing to further enhance the quality of tea.

Effects of Fermentation Temperature and Time on the Color Attributes and Tea Pigments of Yunnan Congou Black Tea.

Yunnan Congou black tea (YCBT) is a typical black tea in China, and is rich in theaflavins (TFs), thearubigins (TRs), and theabrownins (TBs). However, the influence of the fermentation temperature and time on the liquor and appearance color and the correlation between the tea pigments and its color attributes remain unclear. We investigated the effects of the fermentation temperature and time on the color attributes and tea pigments of YCBT. A low fermentation temperature was beneficial to maintain a bright orange-red liquor color and promote the accumulation of TFs and TRs. In contrast, a high temperature gave the liquor a glossy appearance and was beneficial for the formation of TBs. A correlation analysis showed that the 10TFRB index best represented the contribution of tea pigments to the quality of black tea. Moreover, TRs and TBs content prediction models were established based on the liquor L and H values, where the former value can be used as an important index to judge the fermentation process. This study will further enrich the theory of black tea processing chemistry and provide technical support for the precise and directional production of black tea.

Characterization of the key aroma compounds in black teas with different aroma types by using gas chromatography electronic nose, gas chromatography-ion mobility spectrometry, and odor activity value analysis

Aroma is one of the key factors in evaluating tea quality. However, the dominant aroma components of black teas with different aroma characteristics are still unknown. In this study, the key aroma compounds in black teas with floral aroma, sweet aroma, and sweet & floral aroma were systematically characterized by the combination of gas chromatography electronic nose (GC-E-Nose), gas chromatography-ion mobility spectrometry (GC-IMS), and odor activity value (OAV) analysis. A satisfactory discrimination was obtained by using partial least squares-discriminant analysis, based on the volatile fingerprints from the GC-E-Nose and GC-IMS, with robustness model parameters (R2Y = 0.981, and Q2 = 0.967) and (R2Y = 0.893, and Q2 = 0.86), respectively. Among the 38 identified aroma compounds, 15 volatiles were simultaneously identified as important odorants with OAVs greater than 1 in three fragrant black teas, namely (E)-2-nonenal, linalool, nonanal, benzene acetaldehyde, octanal, benzaldehyde, oct-1-en-3-ol, heptanal, ethyl 2-methylpentanoate, (E)-2-hexen-1-ol, hexanal, 2-methylbutanal, 3-methylbutanal, (E)-3-penten-2-one, and butanal, with ethyl 2-methylpentanoate (176.44≤OAV≤1453.47), 3-methylbutanal (19.59≤OAV≤76.16), (E)-2-nonenal (37.43≤OAV≤56.46) and linalool (3.29≤OAV≤56.72) being the most notable. The results provide a theoretical support for flavor quality improvement and oriented processing of black tea.

Determination of Five Phthalate Esters in Tea and Their Dynamic Characteristics during Black Tea Processing.

A highly specific and high extraction-rate method for the analysis of dimethyl phthalate (DMP), diethyl phthalate (DEP), di-n-butyl phthalate (DBP), diisobutyl phthalate (DiBP), and di-(2-ethyl) hexyl phthalate (DEHP) in tea samples was developed. Based on three-factor Box–Behnken response surface design, solid-phase extraction (SPE) of five phthalate ester (PAE) residues in tea was optimized. Optimal extraction conditions were found for extraction temperature (40 °C), extraction time (12 h), and ratio of tea to n-hexane (1:20). The dynamic distribution of PAEs at each stage of black tea processing was also analyzed, and it was found that the baking process was the main stage of PAE emission, indicating that traditional processing of black tea significantly degrades PAEs. Further, principal component analysis of the physicochemical properties and processing factors of the five PAEs identified the main processing stages affecting the release of PAEs, and it was found that the degradation of PAEs during black tea processing is also related to its own physicochemical properties, especially the octanol–water partition coefficient. These results can provide important references for the detection, determination of processing losses, and control of maximum residue limits (MRLs) of PAEs to ensure the quality and safety of black tea.

Changes of some bioactive and physicochemical properties during the black tea processing

In this research, some physical and bioactive properties of tea samples collected from every processing stage of black tea production were investigated. The processing stages were significantly effective on the investigated properties (moisture content, color parameters [L* , a* , and b* ], and pH values the total phenolic substance, antioxidant activity) (p<0.01). The antioxidant activities were found to be at a high level in fresh tea but decreased during the processing stages. Furthermore, the lowest activity was observed in the drying stage for extracts prepared with water and in the drying and sorting stages for extracts prepared with ethanol. In addition, it was determined that palmitic acid, myristic acid, and capric acid as saturated fatty acids, oleic acid, heptadecenoic acid, and eicosenoic acid as unsaturated fatty acids formed the fatty acid profile. Palmitic acid was determined to be the dominant fatty acid in tea samples. PRACTICAL APPLICATION: Tea is a beverage known to have positive health effects. The chemical structure of tea is very effective in providing these positive effects on health. Black tea production takes place in five stages in general terms (withering, rolling, oxidation, drying, and sorting stages) after the harvesting. These processes are very effective in the quality properties of tea. This research provides information about effects of processing stages on black tea. Our results demonstrate that processing was significantly effective on the physicochemical and bioactive properties of tea. Functionality of bioactive compounds could be maintained by optimization of the applied processes.

Quantitative prediction and visual detection of the moisture content of withering leaves in black tea (Camellia sinensis) with hyperspectral image

In actual production, the rapid and non-destructive detection of moisture content in withering leaves is still an unsolved problem due to the randomness of withering leaves on the conveyor belt and the limitation of detection range using near infrared detection equipment. To solve this problem, the hyperspectral images of accumulated withering leaves were obtained and the moisture prediction model was established in the range of 400–1000 nm. Different pretreatment and effective bands selection methods were used to optimize the model. The results showed that the performance of nonlinear model was better than that of linear model and the SNV-Si-CARS-ELM model had the best performance. The Rc2, RMSEC, Rp2, RMSEP and RPD were 0.9940, 0.0074, 0.9942, 0.0078 and 13.0907, respectively. Furthermore, the moisture distribution maps of accumulated withering leaves in different withering degrees were developed. This study provides a theoretical basis for the on-line application of hyperspectral image technology in black tea processing.

Free, soluble conjugated and insoluble bonded phenolic acids in Keemun black tea: From UPLC-QQQ-MS/MS method development to chemical shifts monitoring during processing

Phenolic acids, including benzoic acid and hydroxycinnamic acid derivatives, are the main compounds of black tea. An efficient and accurate analytical method to quantify ten phenolic acids was established and validated by ultra-performance liquid chromatography coupled with triple quadrupole mass spectrometry (UPLC-QQQ-MS/MS). The chemical shifts during the processing of Keemun black tea were analyzed and the phenolic acids were quantified. Compared with fresh tea leaves, after processing, the contents of free phenolic acids, including gallic acid, salicylic acid, p-coumaric acid, and ferulic acid, increased markedly. Still, the contents of soluble conjugated phenolic acids, including p-coumaroylquinic acid isomers and chlorogenic acid isomers, decreased. Furthermore, the total contents of lignin, and insoluble bonded phenolic acids decreased. The adduct of (-)-epigallocatechin gallate and 3-caffeoylquinic acid was detected in tea samples, and its content increased highly after fermentation. The developed and validated analytical method can be used to monitor the manufacturing process of black tea.

Dynamic changes and mechanisms of organic acids during black tea manufacturing process

Sourness usually reduces the sensory quality of black tea, and organic acids are considered as one of major contributors. A UPLC-MRM-MS method was successfully developed to analyze organic acids during the processing of black tea made of “Yingshuang” tea variety. The contents of quinic acid and citric acid increased first and then decreased during withering, but didn't significantly changed after the whole processing. Malic acid content obviously dropped in early withering, and then stayed at low level. These three acids at withering stage were regulated by quinate dehydrogenase (QDH), citrate synthase (CS), phosphoenolpyruvate carboxylase (PEPC) and malic enzyme (ME). Gallic acid sharply increased after rolling, resulted from the hydrolysis of epigallocatechin gallate (EGCG) by endogenous tannase. This work revealed the dynamic changes and regulatory mechanisms of organic acids during the black tea manufacturing process, which is helpful to improve the processing technology and quality of black tea. • A UPLC-MRM-MS method was developed to analyze organic acids in black tea. • The contents of quinic acid, citric acid and malic acid changed during withering. • The three acids were regulated by four enzymes including QDH, CS, PEPC and ME. • Gallic acid increased after rolling, due to the hydrolysis of EGCG by tannase. • Fermentation is not the major processing step for organic acid accumulation.

PH indicator-based sensor array in combination with hyperspectral imaging for intelligent evaluation of withering degree during processing of black tea

Withering is one of the most critical steps in the processing of black tea. The degree of withering affects the aroma quality of the finished tea. In this study, we used a pH indicator-based colorimetric sensor array in combination with hyperspectral imaging to intelligently evaluate the withering degree. After analyzing the difference between images taken before and after the reaction of pH indicators with withered leaves, six pH indicators were selected to build a sensor array. Then, the hyperspectral image of each pH indicator was obtained at wavelengths between 400 and 1000 nm. Nonlinear support vector machine (SVM) and least-squares (LS) SVM models were established to determine the degree of withering. Results revealed that the spectral information from single pH indicator failed to accurately evaluate the withering degree. The LS-SVM model achieved satisfactory discriminant results with the low-level data fusion of six pH indicators followed by principal component analysis for dimensionality reduction. The optimal model yielded accuracies of 93.75% and 90.00% for the calibration and prediction sets, respectively. The results indicated that colorimetric sensor array in combination with hyperspectral imaging can effectively determine the withering degree, thus providing a novel method for the intelligent processing of food and tea.

Rapid monitoring of black tea fermentation quality based on a solution-phase sensor array combined with UV-visible spectroscopy

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.

Profiling of dynamic changes in non-volatile metabolites of shaken black tea during the manufacturing process using targeted and non-targeted metabolomics analysis

Shaken black tea (SBT) is a new type of black tea that combines the traditional black tea processing technology with the shaking process of oolong tea. In this study, non-targeted metabolomics analysis based on ultra-high-pressure liquid chromatography quadrupole time-of-flight mass spectrometry (UHPLC-QTOF-MS) was applied to comprehensively analyze the non-volatile metabolites of SBT during the manufacturing process. Using multivariate statistical analysis, 93 compounds were screened as differential compounds. During the whole process of SBT, amino acids, dipeptides, theaflavins and lipids contents showed significant increases, while that of catechins and procyanidin C1 significantly decreased. Compared with fresh leaves, the content of proteinaceous amino acids (PAAs) in the dried black tea increased 1.61-fold, and total catechins decreased to 7.25%. The results of hierarchical clustering analysis (HCA) showed that fermentation is the key process that causes the variations of these metabolites, followed by drying and shaking. The biosynthesis metabolism of amino acids, flavonoids and linoleic acid are abundant, mainly due to the stress response of fresh tea leaves to dehydration and mechanical damage during shaking processing. It is beneficial to explore the quality formation during the production of SBT through comprehensive analysis of the changes in metabolites.

Novel insight into the effect of fermentation time on quality of Yunnan Congou black tea

Fermentation is a crucial process in black tea processing; however, the effect of fermentation time on the quality of Yunnan Congou black tea has never been assessed. This study aimed to evaluate the quality of Yunnan Congou black tea by detecting quality components, artificial sensory evaluation, and objective attributes under different fermentation times (2, 3, 4, and 5 h). Partial least squares discriminant, odor active value, and least-significant difference analyses were used to identify key differences in its compounds. Overall, 36 non-volatile and 142 volatile compounds were detected. We found that 3 h was the optimal fermentation time for retaining several simple catechins, allowing formation of theaflavins (TFs) and thearubigins (TRs) and promoting formation and conversion of sugars and degradation of flavonoid glycosides. The volatile compounds were most abundant, and sweet and umami comprehensive sensor responses were stronger in the 3 h-fermented samples with the best sensory quality. Nine non-volatile (including TFs, TRs, theabrownins, caffeine, and glucose) and eight volatile (including linalool, phenylacetaldehyde, methyl salicylate, benzaldehyde, 2-pentyl-furan, and trans-β-ionone) markers were identified using multivariate statistical analysis. This study provides technical guidance for directed and standardized production and lays a theoretical foundation for the in-depth study of Yunnan Congou black tea.

Qualitative and quantitative quality evaluation of black tea fermentation through noncontact chemical imaging

Fermentation is the most critical step in black tea processing and contributes to the taste and appearance of black tea. Conventionally, the degree of fermentation is evaluated through sensory reviews by experts. However, this method has low accuracy and is subject to reviewers’ experience. In this study, the fermentation quality of black tea was qualitatively and quantitatively evaluated through near-infrared hyperspectral imaging (HSI). The spectral information of samples of four fermentation degrees was collected and preprocessed using standard normal variate. The developed qualitative model could discriminate fermentation degrees, with a correct classification rate of 83.33 % for the prediction set. The quantitative prediction model achieved excellent prediction accuracy for the catechin content, with a residual prediction deviation of 2.18. Finally, the spatial distribution of catechins during fermentation was mapped through chemical imaging. Our study confirmed the feasibility of HSI for monitoring the fermentation quality of black tea.

Metabolomics Provides A Novel Interpretation of the Changes in Main Compounds during Black Tea Processing through Different Drying Methods.

This study aimed to compare the effect of hot roller (HR) drying and hot air (HA) drying on the sensory evaluation, chemical quality, antioxidant activity, and metabolic profile of Yihong Congou black tea processed from E’cha NO1. The Yihong Congou black tea dried with HA obtained higher sensory scores and better chemical qualities such as the hue of tea brew color (a and b), content of theaflavins, thearubigins, water extract, free amino acids, tea polyphenol, and the ratio of polyphenol to amino acids as well as higher antioxidant capacities compared to that dried with HR. The HA drying tea increased the contents of volatile compounds that had positive correlation with sweet and flowery flavor, while the HR drying tea increased the contents of volatile compounds related to fruity flavor. Moreover, non-targeted metabolomics data indicated that the levels of most free amino acids significantly increased, while the levels of most soluble sugars reduced in the HA drying method compared to the HR drying method. The metabolic analysis was also consistent with the above results and revealed that D-ribose and gallic acid were the main characteristic metabolites of HA drying. Our results could provide a technical reference and theoretical guide to processing a high quality of Yihong Congou black tea.

Dissipation behavior and risk assessment of tolfenpyrad from tea bushes to consuming

The dissipation behavior of tolfenpyrad, a widely used pyrazole insecticide in tea plantations, was investigated during tea bushes growing, manufacturing and brewing. Tolfenpyrad dissipated fast on the tea bushes with the half-lives of 1.8–2.3 days. Manufacturing processes of green tea and black tea further reduced the tolfenpyrad residue by 3.5%–36.4%. The average processing factors (PFs) of tolfenpyrad ranged from 0.68 to 1.40 and 0.84 to 1.30 during the processing of green tea and black tea, respectively. Then a low infusion factor of 9.8%–19.9% was observed during the brewing of made tea, as the water solubility of tolfenpyrad was only 0.087 mg/L. Therefore, more than 96% of the initial deposition of tolfenpyrad was dissipated and not accessible for consuming. Results of the risk quotient (RQ) assessment also indicated a negligible health risk by tea consumption. Results from this study indicated that the residue of tolfenpyrad can be reduced by proper field management, manufacturing and brewing processes, where field dissipation and brewing were key steps to minimize its risks. Data of this study could also provide guidance for rational application of tolfenpyrad in tea plantations.

Enhancement of Black Tea Aroma by Adding the β-Glucosidase Enzyme during Fermentation on Black Tea Processing.

Black tea aroma is one of the essential attributes in determining the quality of black tea. β-Glucosidases were investigated for their ability to enhance the aroma of black tea by hydrolyzing the glycoside compound. The addition of β-glucosidase was done by dissolving the enzyme on a sodium citrate buffer (pH 5.0), which was then sprayed on tea leaves during black tea processing. The β-glucosidase treatment significantly increases the volatile compound from glycoside precursors such as linalool, geraniol, and methyl salicylate. Moreover, the volatile compound from carotenoid and lipid precursors (nerolidol and β-cyclocitral) was also increased with β-glucosidase treatment.

An internet of things (IoT)-based optimum tea fermentation detection model using convolutional neural networks (CNNs) and majority voting techniques

Abstract. Tea (Camellia sinensis) is one of the most consumed drinks across the world. Based on processing techniques, there are more than 15 000 categories of tea, but the main categories include yellow tea, Oolong tea, Illex tea, black tea, matcha tea, green tea, and sencha tea, among others. Black tea is the most popular among the categories worldwide. During black tea processing, the following stages occur: plucking, withering, cutting, tearing, curling, fermentation, drying, and sorting. Although all these stages affect the quality of the processed tea, fermentation is the most vital as it directly defines the quality. Fermentation is a time-bound process, and its optimum is currently manually detected by tea tasters monitoring colour change, smelling the tea, and tasting the tea as fermentation progresses. This paper explores the use of the internet of things (IoT), deep convolutional neural networks, and image processing with majority voting techniques in detecting the optimum fermentation of black tea. The prototype was made up of Raspberry Pi 3 models with a Pi camera to take real-time images of tea as fermentation progresses. We deployed the prototype in the Sisibo Tea Factory for training, validation, and evaluation. When the deep learner was evaluated on offline images, it had a perfect precision and accuracy of 1.0 each. The deep learner recorded the highest precision and accuracy of 0.9589 and 0.8646, respectively, when evaluated on real-time images. Additionally, the deep learner recorded an average precision and accuracy of 0.9737 and 0.8953, respectively, when a majority voting technique was applied in decision-making. From the results, it is evident that the prototype can be used to monitor the fermentation of various categories of tea that undergo fermentation, including Oolong and black tea, among others. Additionally, the prototype can also be scaled up by retraining it for use in monitoring the fermentation of other crops, including coffee and cocoa.

Black Tea Quality is Highly Affected during Processing by its Leaf Surface Microbiome.

Microbiomes can greatly affect the quality of fermented food and beverages, including tea. In this study, microbial populations were characterized during black and green tea manufacturing, revealing that tea processing steps can drive both the bacterial and fungal community structure. Tea leaves were found to mostly harbor Proteobacteria, Bacteriodetes, Firmicutes, and Actinobacteria among bacteria and Ascomycetes among fungi. During processing, tea microbial populations changed especially between sterilized and unsterilized samples. The surface sterilization of fresh leaves before processing can remove many microbes, especially the bacteria of the genera Sphingomonas and Methylobacteria, indicating that these are mostly phylloplane microbes on tea leaves. The surface sterilization removed most fungi, except the Debaryomyces. We also observed a fluctuation in the content of several tea quality-related metabolites during processing. Caffeine and theanine were found in the same quantities in green tea with or without leaf surface sterilization. However, the sterilization process dramatically decreased the content of total catechins and theanine in black tea, indicating that microbes on the surface of tea leaf may be involved in maintaining the formation of these important metabolites during black tea processing.