Black Tea Quality Research Articles

Unveiling the effect of leaf maturity on biochemical constituents and quality of CTC black tea: insights from Northeast India’s commercial cultivars

Unveiling the effect of leaf maturity on biochemical constituents and quality of CTC black tea: insights from Northeast India’s commercial cultivars

A black tea quality testing method for scale production using CV and NIRS with TCN for spectral feature extraction

To rigorously assess black tea quality in large-scale production, this study introduces a multi-modal fusion approach integrating computer vision (CV) with Near-Infrared Spectroscopy (NIRS). CV technology is first applied to evaluate the tea's appearance quality, while NIRS quantifies key chemical components, including tea polyphenols (TP), free amino acids (FAA), and caffeine (CAF). Additionally, different methods are employed to extract potential quality features from NIR spectra. The information are then fused, and a classifier is utilized to accurately identify tea quality. Results show that the Temporal Convolutional Network (TCN) fused model achieves a 98.2 % accuracy rate, surpassing both the Convolutional Neural Network (CNN) fused model and traditional methods. This study demonstrates that TCNs effectively extract spectral features and that data fusion significantly enhances tea quality testing, offering valuable insights for production optimization.

Metabolomics reveals a differential attitude in phytochemical profile of black tea (Camellia Sinensis Var. assamica) during processing

Black tea's quality and flavor are largely influenced by its processing stages, which affect its volatile and non-volatile phytochemicals. This study aimed to optimized black tea manufacturing by investigating withering time, fermentation time, and temperature's impact on sensory quality. Using a U*15 (157) uniform design, optimal conditions were determined: 14 h of withering, 5.6 h of fermentation, and a 34 °C temperature. A verification experiment analyzed the volatile and non-volatile profiles. HPLC, GC–MS, and LC-MS revealed dynamic changes in phytochemicals. Among 157 VOCs and 2642 metabolites, 19 VOCs (VIP > 1.5) were crucial for aroma, while 50 (VIP > 1.5, p < 0.01) characteristic metabolites were identified. During processing, fragrant volatile compounds like linalool oxides, geraniol, benzeneacetaldehyde, benzaldehyde, methyl salicylate, and linalyl acetate increased, contributing to rose and honey like aromas. These changes were crucial in developing the characteristic flavor and color of black tea. Twenty-four new compounds formed, while 80 grassy odor volatiles decreased. Non-volatile metabolites changed notably, with decreased catechins and increased gallic acid. Theaflavin compounds rose initially but declined later. This study outlines metabolite changes in Yunkang 10 black tea, crucial for flavor enhancement and quality control.

Dynamic Change of Volatile Fatty Acid Derivatives (VFADs) and Their Related Genes Analysis during Innovative Black Tea Processing

Volatile fatty acid derivatives (VFADs) play a significant role in contributing to flowery–fruity flavor black tea. Innovative black tea is typically crafted from aroma-intensive tea cultivars, such as Jinmudan, using defined production methodologies. In this study, the during-processing tea leaves of innovative black tea were applied as materials, and we selected a total of 45 VFADs, comprising 11 derived aldehydes, nine derived alcohols, and 25 derived esters. Furthermore, the dynamic variations of these VFADs were uncovered. Transcriptome analysis was performed to identify genes involved in the LOX (lipoxygenase) pathway, resulting in the identification of 17 CsLOX genes, one hydrogen peroxide lyase (CsHPL) gene, 11 alcohol dehydrogenases (CsADH) genes, 11 genes as acyl CoA oxidase (CsACOX) genes, and three allene oxide synthase (CsAOS) genes. Additionally, the expression levels of these genes were measured, indicating that the processing treatments of innovative black tea, particularly turn-over and fermentation, had a stimulation effect on most genes. Finally, qRT-PCR verification and correlation analysis were conducted to explain the relationship between VFADs and candidate genes. This study aims to provide a reference for illuminating the formation mechanisms of aroma compounds in innovative black tea, thereby inspiring the optimization of innovative processing techniques and enhancing the overall quality of black tea.

Lightweight CNN combined with knowledge distillation for the accurate determination of black tea fermentation degree

Black tea is the second most common type of tea in China. Fermentation is one of the most critical processes in its production, and it affects the quality of the finished product, whether it is insufficient or excessive. At present, the determination of black tea fermentation degree completely relies on artificial experience. It leads to inconsistent quality of black tea. To solve this problem, we use machine vision technology to distinguish the degree of fermentation of black tea based on images, this paper proposes a lightweight convolutional neural network (CNN) combined with knowledge distillation to discriminate the degree of fermentation of black tea. After comparing 12 kinds of CNN models, taking into account the size of the model and the performance of discrimination, as well as the selection principle of teacher models, Shufflenet_v2_x1.0 is selected as the student model, and Efficientnet_v2 is selected as the teacher model. Then, CrossEntropy Loss is replaced by Focal Loss. Finally, for Distillation Loss ratios of 0.6, 0.7, 0.8, 0.9, Soft Target Knowledge Distillation (ST), Masked Generative Distillation (MGD), Similarity-Preserving Knowledge Distillation (SPKD), and Attention Transfer (AT) four knowledge distillation methods are tested for their performance in distilling knowledge from the Shufflenet_v2_x1.0 model. The results show that the model discrimination performance after distillation is the best when the Distillation Loss ratio is 0.8 and the MGD method is used. This setup effectively improves the discrimination performance without increasing the number of parameters and computation volume. The model’s P, R and F1 values reach 0.9208, 0.9190 and 0.9192, respectively. It achieves precise discrimination of the fermentation degree of black tea. This meets the requirements of objective black tea fermentation judgment and provides technical support for the intelligent processing of black tea.

Revealing novel insights into the enhancement of quality in black tea processing through microbial intervention

Black tea is highly favored by consumers worldwide, with enzymatic reactions being recognized as a pivotal factor influencing tea quality. The role of microorganisms in shaping the composition of black tea has emerged as a focus of research due to their involvement in enzyme catalysis and metabolic processes. In this study, full-length amplicon sequencing combined with qPCR more accurately reflected microbial profile, and Pantoea, Pseudomonas, Paucibacter, and Cladosporium were identified as the main microbial genera. Moreover, by comprehensively analyzing color, aroma, and taste components over time in black tea samples, correlations were established between the dominant genus and various quality factors. Notably, peroxidase activity levels, total soluble sugar content, and tea pigments concentration exhibited significant associations with the dominant genus. Consequently, this microbiological perspective facilitated the exploration of driving factors for improving black tea quality while establishing a theoretical foundation for quality control in industrial production.

An improved 3D-SwinT-CNN network to evaluate the fermentation degree of black tea

Fermentation is a key process in forming the flavor quality of black tea. Evaluating the degree of fermentation during black tea processing is difficult. This paper proposes an improved 3D-SwinT-CNN network for the end-to-end processing of black tea's hyperspectral images to evaluate the degree of fermentation. The model incorporates dilated convolution and a shifted window self-attention mechanism, expanding the network's receptive field and capturing both local spatial-spectral and global features of black tea hyperspectral images. The accuracy is 98.13% on the test set. Compared to manual feature extraction methods, the accuracy improved by 3.45%. Compared to baseline 3D-CNNs, 3D-SwinT-CNN demonstrates superior spatial-spectral feature extraction, achieving an average accuracy improvement of 10.17%. Ablation experiments were conducted to further verify the effectiveness of the introduced modules. The proposed 3D-SwinT-CNN aims to establish a research foundation for online evaluation of black tea's fermentation degree.

Effect of osmanthus hydrolat on the aroma quality and volatile components of osmanthus black tea

Osmanthus fragrans is an evergreen shrub with a pleasant fragrance and a wide range of applications in many fields. The condensed hydrolat obtained during the drying process of its fresh flowers was collected in a low-temperature vacuum environment and its sensory evaluation and volatile components were studied. The main aroma compounds in Osmanthus fragrans were dihydro-β-ionone, nonanal, β-cyclocitral, β-ionone, benzaldehyde, α-ionone, and 6-methyl-5-hepten-2-one, whose contents were used as the main evaluation criteria, and the hydrolats obtained under different scenting and drying times were compared. This process can effectively collect the aroma components in Osmanthus fragrans and the optimal drying conditions were 50 °C for 5 h. The hydrolat was used to provide the scent of osmanthus black tea, which had a fresher and mellower taste, while the fragrance of osmanthus was abundant. These results show that osmanthus hydrolat can be used to provide the scent of floral black tea. Chemical compounds studied in this article(−)-Catechin (PubChem CID: 1203); (−)-epigallocatechin gallate (PubChem CID: 65064); (−)-epicatechin gallate (PubChem CID: 367141); (−)-epigallocatechin (PubChem CID: 72277); (−)-epicatechin (PubChem CID: 72276); (−)-gallocatechin gallate (PubChem CID: 199472); (−)-catechin gallate (PubChem CID: 6419835); (−)-gallocatechin (PubChem CID: 9882981).

Rapid fluorescence detection of black tea fermentation degree based on cobalt ion mediated carbon quantum dots

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.

Evaluation of the quality grade of Congou black tea by the fusion of GC-E-Nose, E-tongue, and E-eye

In the present study, the comprehensive quality of Congou black tea (CBT) including aroma, taste, and liquid color was investigated by a combination of gas chromatography electronic nose (GC-E-Nose), electronic tongue (E-tongue), and electronic eye (E-eye). An excellent discrimination of different quality grades of CBT was accomplished through the fusion of GC-E-Nose, E-tongue, and E-eye combined with orthogonal partial least squares discriminant analysis, with parameters of R2Y = 0.803 and Q2 = 0.740. Moreover, the quantitative evaluation of CBT quality was successfully achieved by partial least squares regression analysis, with the absolute error within 1.39 point, and the relative error within 1.62%. Additionally, 12 key variables were screened out by stepwise multiple linear regression analysis, which significantly contributed to the comprehensive quality score of CBT. Our results suggest that the fusion of multiple intelligent sensory technologies offers great potential and practicability in the quality evaluation of black tea.

The shaking and standing processing improve the aroma quality of summer black tea

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.

Mechanisms and quality variations of non-volatile and volatile metabolites in black tea from various ages of tea trees: Insights from metabolomics analysis

The sensory quality of black tea (BT) influenced by various factors, among which tree age is particularly significant. People prefer BT produced by fresh leaves from old tea trees, yet the correlation between tree age and tea quality has not been thoroughly investigated. In this study, we analyzed the quality of BT from young trees (H-JYH) and old trees (H-OJYH) using e-tongue technology and sensory evaluation. Our findings revealed that H-OJYH had stronger sweetness and sourness, richer flavor, and diminished bitter-astringency compared to H-JYH. 1231 non-volatile metabolites and 504 volatile metabolites were discovered by ultra-performance liquid chromatography (UPLC) and gas chromatography–mass spectrometry (GC–MS). L-tartaric acid and trans-citridic acid were found to contribute to increase acidity, and 7,8-dihydroxy-6-methoxycoumarin and d-fructose 6-phosphate were associated with enhanced sweetness in H-OJYH. Additionally, lower levels of octyl gallate and vanillic acid in H-OJYH contributed to the diminished bitter-astringency. β-ionone, 2-phenylethanol and phenylacetaldehyde merged as characteristic compounds of older tree BT with stronger floral and sweet aroma. Our study serves as a guideline to explore the relationship between tree age and tea quality.

Mixed processing improved the theaflavin, theanine and aroma components of Congou black tea

SummaryCongou black tea is mainly processed from single variety leaves, with obvious variety characteristics. This study investigated the effects of mixed processing withered leaves varieties on sensory quality, the volatile and non‐volatile compounds of seven Congou black tea samples. Sensory quality evaluation revealed that the Congou black tea with three leaves varieties were fresher, more floral and had coordinate taste. The total quality score was in the order: MPH &gt; H &gt; M&amp;H &gt; M &gt; M&amp;P &gt; H&amp;P &gt; P. However, samples MPH and M&amp;H have a higher content of theaflavins and amino acids, especially theanine. Furthermore, 130 volatile substances were identified in seven black teas by using HS‐SPME/GC–MS. M&amp;H and MPH have more aroma types and total amount than that of M, P and H. The mixed processing of withered leaves may fully use the catalysis of endogenous enzymes to increase the 19 kinds of black tea key aromas including Benzyl alcohol, trans‐2‐hexenyl hexanoate, Hexyl hexanoate and Methyl jasmonate. Thus, blended processing can enhance the taste, aroma and LC quality of black tea.

Analysis of tea quality of large-leaf black tea with different harvesting tenderness based on metabolomics

The harvesting tenderness of tea fresh leaves has a significant effect on the flavor and aroma quality of black tea (BT), which is one of the important standards for BT grading. In this study, sensory evaluation, electronic tongue technique and metabolomics were performed to evaluate the flavor and aroma qualities of three different tenderness of large-leaf BTs that were manufactured from the fresh leaves of single-bud, one-bud and one-bud two-leaf teas, respectively. The results of the electronic tongue and sensory evaluation indicated that the increase in the tenderness of the fresh leaf material enhanced the sweetness, freshness and fruity flavor of the tea and reduced the bitterness of the tea. A total of 519 differential flavor components and 84 differential aroma components were identified by metabolomics. Among them, elevated levels of maltitol, cyclic leucine and l-methionine and l-aspartic acid led to elevated sweetness and freshness in more tender tea leaves. Reduced levels of 4-aminosalicylic acid, tyrosol, uracil, inosine, benzamide, and l-tyrosine led to reduced bitterness in BT. Moreover, the content of volatiles such as (Z)-2-Decenal, Phenylacetic acid propyl ester, Acetophenone, and 2-phenylethyl ester were increased, which contributed to the fruity aroma in the high-tenderness tea. This study provides a theoretical basis for grading the flavor quality of BT with different tenderness.

Microbiological quality of different tea grades produced in diverse agro-climatic regions in Sri Lanka

In Sri Lanka, several tea grades are produced in factories located in different agro-climatic regions within three geographical elevations. The study aimed to determine the microbial quality of different tea grades and composite tea samples obtained from factories situated at diverse locations. The average APC, yeast & mould counts and coliforms in different tea grades ranged from 3.4 × 103 to 2.0 × 104 cfu/g, 4.8 × 102 to 2.5 × 103 cfu/g and 0.005 to 3.9 × 101 Most Probable Number (MPN)/g respectively. The tea samples collected from different factories had mean values of APC and yeast & mould as 5.3 × 103±1.3 × 103 cfu/g and 9.7 × 102±1.9 × 102 cfu/g. Escherichia coli (E.coli) and Salmonella were not detected either in tea grades or in composite samples. The identified microorganisms in tea samples belong to phyla Firmicutes, Proteobacteria, Ascomycota, Basidiomycota and Zygomycota.The samples collected from the mid country elevation had the highest counts of APC and yeast & mould counts were high in the low country elevation. More than 70 % of the tested samples comply with the SLTB guidelines given for the microbiological quality of black tea. The distribution of bacterial, yeast and mould and coliform densities of tea were significantly variable with respect to geographic areas.

Development of Assessment Criteria for Managing the Quality of Taishan Black Tea

In order to obtain the quality information of Taishan black tea and provide data support for the establishment of assessment criteria for quality control of Taishan black tea, in this study, 45 samples of Taishan black tea were collected during the summer to investigate their quality characteristics. The results showed that the Taishan black tea typically exhibited a dark-brown, curly appearance and a uniform texture. The tea soup displayed a bright orange-red color. The tea was mainly described as having a fruity aroma, followed by a caramel flavor, sweet aroma, flowery flavor, and clean aroma, with a strong and enduring fragrance. The taste profile was mostly heavy and mellow or heavy and strong, with fewer samples exhibiting slight bitterness and astringency. The tea leaves were characterized by a reddish-auburn color, a soft texture, and an even appearance. The main biochemical components of the Taishan black tea fell within the following ranges: water extracts (38.65–43.35%), free amino acids (1.41–3.45%), tea polyphenols (9.80–15.05%), catechins (6.11–9.03%), and caffeine (1.65–3.05%). The phenolic acid/amino acid ratio was 2.31–6.65%. Catechins and tea polyphenols emerged as critical indicators influencing taste quality, followed by amino acids, water extracts, and caffeine. The aroma analysis identified common compounds, such as 3-methyl-butanal, 2-methyl-butanal, decanal, 2-methyl-propanal, 1-octen-3-ol, and β-ionone, in most samples. These compounds exhibited relatively high contents and high odor activity values, making them the primary contributors to the tea’s aroma. This investigation into the quality of Taishan black tea offers valuable scientific insights, providing a foundation for the standardization of Taishan black tea’s quality.

Effects of fermentation duration on the flavour quality of large leaf black tea based on metabolomics

Fermentation durations are crucial in determining the quality of black tea flavour. The mechanism underlying the degradation of black tea flavour caused by inappropriate fermentation duration remains unclear. In this study, the taste of black teas with different fermentation durations (BTFs) was analysed using sensory evaluation, electronic tongue, and metabolomics. The results revealed significant differences in 46 flavour profile components within the BTFs. Notably, metabolites such as gallocatechin gallate, gallocatechin, and epigallocatechin were found to be primarily reduced during fermentation, leading to a reduction in the astringency of black tea. Conversely, an increase in d-mandelic acid and guanine among others was observed to enhance the bitter flavour of black tea, while 3-Hydroxy-5-methylphenol nucleotides were found to contribute to sweetness. Furthermore, succinic acid and cyclic-3′,5′-adenine nucleotides were associated with diminished freshness. This study offers a theoretical foundation for the regulation of flavour quality in large leaf black tea.

Lipidomics, transcription analysis, and hormone profiling unveil the role of CsLOX6 in MeJA biosynthesis during black tea processing.

Jasmonates, such as jasmonic acid (JA) and methyl jasmonate (MeJA), are crucial aspect of black tea quality. However, lipids species, hormones, and genes regulated mechanism in the jasmonate biosynthesis during black tea processing are lacking. In this study, we employed lipidomics, hormone metabolism analysis, and transcriptome profiling of genes associated with the MeJA biosynthesis pathway to investigate these factors. The contents of lipids GLs, PLs, and TAG are decreased, accompanied by the main lipids species reduced during black tea processing. Galactolipids, primarily 34:3/36:6/36:3 DGDG and 36:6/36:5/36:4 MGDG, are transformed into massive MeJA and JA in black tea processing, accompanied by the decreased SA, MeSA, IAA, and BA and increased zeatin. Additionally, the transcriptional activity of the primary genes in MeJA biosynthesis pathway exhibited downregulated trends except for AOS and OPR and non-primary genes tend to be a little high or have fluctuation of expression. Coordinated expression of main CsHPL (TEA008699), CsAOS (TEA001041), and CsJMT (TEA015791) control the flow of lipids degradation and MeJA production. A strong infected reduction of a key lipoxygenase gene, CsLOX6 (TEA009423), in tea buds significantly reduced the level of jasmonates and expression of downstream genes, accompanied by SA, MeSA level rising, and ABA declining. We have identified a key CsLOX6, as well as established galactolipids, mainly 34:3/36:6/36:3 DGDG and 36:6/36:5/36:4 MGDG, sources for MeJA biosynthesis regulated by dynamics hormone and controlled by coordinated expressed CsHPL (TEA008699), CsAOS (TEA001041), and CsJMT (TEA015791). Our findings provide a theoretical basis for breeding high-quality black tea and offer valuable insights for improving processing methods.

Re-rolling treatment in the fermentation process improves the taste and liquor color qualities of black tea

Fermentation is a vital process occurred under the premise of rolling affecting black tea quality. Theoretically, re-rolling during fermentation will remodel the biochemical conditions of tea leaves, and thus influence black tea quality. Herein, we studied the effect of re-rolling on black tea taste and liquor color. Sensory evaluation showed that re-rolling significantly weakened the astringency taste and improved the redness and luminance of liquor. With re-rolling, the color attributes of a* and L* and the contents of theaflavins and thearubigins were significantly improved. Metabolomics analysis showed that the contents of 110 non-volatile compounds were significantly different among black teas with different rolling treatments. In summary, re-rolling accelerated the oxidation of polyphenols into pigments, the hydrolysis of proteins into amino acids, and the metabolism of alkaloids, organic acids, glycosidically-bound volatiles, and lipids during the fermentation period. Our study provided a novel and simple way to improve black tea quality.

Non-Targeted Metabolomics Reveals the Effects of Different Rolling Methods on Black Tea Quality.

A non-targeted metabolomics approach and sensory evaluation, coupled with multivariate statistical analysis, systematically uncover the impact of the rolling time on the quality parameters of black tea. GC-MS analysis reveals that a moderate extension of rolling time favorably contributes to the accumulation of characteristic aroma components in black tea. The volatile components reach their highest concentration in black tea samples processed during an 80-min rolling period. UHPLC-Q-TOF/MS analysis demonstrates a substantial decrease in the contents of catechins and flavonoids with an increase in rolling time. Simultaneously, the production of theaflavins, coupled with the degradation of green bitterness volatiles (GBVs), significantly contributes to the formation of endogenous aroma components in black tea. These findings underscore the close relationship between rolling time control and black tea quality, emphasizing that a moderate extension of the rolling time fosters the development of improved black tea flavor quality. The comprehensive quality evaluation indicates that the optimal duration is 80 min. However, the initial 0 to 20 min of rolling is a crucial phase for the genesis and transformation of black tea quality. This study offers valuable insights into the influence of rolling time on black tea quality, potentially enhancing future studies of rolling technology. It provides theoretical guidelines for optimizing the processing of Gongfu black tea.