In addition to forming N-ethyl-2-pyrrolidinone-substituted flavan-3-ols (EPSFs) with catechins, the Strecker degradation products of theanine can also combine with flavonol glycosides (FGs) to form EPSFGs during tea processing. In the present study, a novel adduct of theanine and rutin was identified and named as EPS-rutin. The effects of reaction temperature, time, initial reactant ratio, and water content on the generation of EPS-rutin in thermal reaction models were also analyzed. When the initial mass ratio of rutin to theanine was 1:5 and heating under 140 degrees C for 60 min, the content of EPS-rutin was the highest. Meanwhile, the content of EPS-rutin in green tea during processing was determined. Results showed EPS-rutin was formed during the first drying stage, and its level significantly increased after final drying. Furthermore, the bitterness and astringency thresholds of EPSrutin were determined to be higher than those of rutin. This study further improved the understanding of the transformation pathways of theanine and polyphenols during tea processing, as well as contributed to exploring the flavor characteristics and health benefits of EPS-rutin.

A color-deepening effect of theaflavins on the theanine-glucose thermal reaction model was revealed. Generated chromogenic intermediates in the initial stage and an accelerated browning rate through the promoted degradation of theanine-glucose Amadori rearrangement product in the intermediate and final stages are responsible for the color-deepening effect. Four pink-to-red theaflavin-theanine intermediates were verified as theaflavinies referencing the nuclear magnetic resonance and liquid chromatography-mass spectrometry information on theaflavins and l-theanine, including one accurately identified as theaflavinie 4. Theaflavinie 4 showed two maximum absorption peaks at 401 and 506 nm with parallel intensities, which resulted in a significant dichromic color change from pale pink to orange and red. Theaflavinies also could undergo further thermal reactions to yield brown polymers under higher temperatures (130 and 140 degrees C). This research provided new insight into realizing thermally formed polymers during black tea processing, which may be formed by oxidation products and amino acids or proteins through non-enzymatic thermal reactions.

Black tea is frequently consumed worldwide and is renowned for having a distinctive scent. The volatile chemicals in tea are responsible for its aroma, which is important for sensory quality. The enzymatic and non-enzymatic processes that produce the compounds endowing black tea with its distinctive aroma are complex. Black tea is well known for its robust and complex aroma, which can vary based on the type of tea leaves used and processing technologies used. During the production of black tea, several intricate biological and chemical processes contribute to the aroma’s development. Different volatile chemicals are generated during the processing of black tea, which includes withering, rolling, fermentation (enzymatic oxidation), and drying. Various methods have been used to analyze and describe the aroma of black tea. Different methods, such as gas chromatography-mass spectrometry, olfactometry, and solid phase extraction, have been used to assess the fragrance of black tea. These methods evaluate characteristics including fruity, flowery, woody, malty, spicy, and smoky flavors. Phenyl ethyl alcohol, one-octen-3-ol, trans-linalool oxide (furanoid), geraniol, and nonanal are major aroma-active compounds in black tea. Sensory analytic techniques are used to assess black tea’s flavor and scent qualities. This assessment helps figure out the tea’s quality, unique characteristics, and even some unpleasant attributes. Black tea is mostly made from Camellia sinensis and Camellia assamica tea varieties. These two varieties, members of the Camellia genus, differ in their development patterns, leaf sizes, and chemical makeup, impacting how black tea’s scent is formed. When evaluating black tea made from Camellia sinensis and assamica, sensory analysis involved assessing the aroma when the tea was dry and, after brewing, noting any differences from other teas. This review focused on how key aromatic compounds are formed during the tea manufacturing process by Camellia sinensis and Camellia assamica black tea.

Different from post-fermented teas, pickled tea is a unique processed tea of fresh leaves of Camellia sinensis var. assamica under anaerobic fermentation. It is mainly consumed in ethnic minority areas in southwest China. The chemical characteristics of pickled tea during processing were studied by mass spectrometry-based metabolomics and quantitative determination. Metabolomics results showed that a total of 44 marker compounds, including flavan-3-ols, organic acids, glycosides, and flavonoids were identified in pickled tea during fermentation. Thereinto, galloylated catechins were mainly degraded or oxidated into the corresponding nongalloylated cat-echins and theaflavins. Caffeine and theobromine slightly varied, while organic acids markedly varied during the whole processing. In addition, butanedioic, lactic, and gallic acid were found as the main organic acids in pickled tea and their contents were 220 +/- 9, 177 +/- 21, and 172 +/- 6 mu g/g, respectively. The inhibitory effects of pickled tea samples on alpha-amylase and alpha-glucosidase were also investigated and subsequently analyzed with respect to chemical-biological activities correlation. The results showed that glycosides and most organic acids were key alpha-amylase inhibitors, while catechins and flavonoids were key alpha-glucosidase inhibitors. The present study aimed to comprehensively explore the chemical variation of pickled tea during processing, and key inhibitors of alpha-glucosidase and alpha-amylase.

Not only do flavan-3-ols participate in the formation of chromogenic oxidation products such as theaflavins, but chlorogenic acid (3-caffeoylquinic acid, CQA) is also involved in the enzymatic oxidation during black tea processing. The critical oxidation product of CQA and (-)-epigallocatechin (EGC) were identified as an adduct containing benzobicyclo[3.2.2]nonenone structure, which was named as the dichlorogeniccatechin (DCGC) oligomer. It was composed of two molecules of CQA and one molecule of EGC. The effects of the initial reactant ratio and reaction time on the generation of DCGC were also analyzed. A high proportion of CQA promoted the production of DCGC, but a high proportion of EGC inhibited the DCGC formation. In addition, the content of DCGC in Keemun black tea during processing was determined. The content of DCGC highly increased after withering but decreased after drying. This study provides a new perspective for the investigation of other oxidation oligomers in black tea.

N-ethyl-2-pyrrolidinone-substituted flavan-3-ols (EPSFs) were prepared by an in vitro model reaction, and the taste thresholds of EPSFs and their dose-over-threshold factors in large-leaf yellow tea (LYT) were investigated. The effects of initial reactant ratios, reaction temperatures and time, pH values, and water addition on the yield of EPSFs were explored. The contents of EPSFs during roasting were determined by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-Q-TOF-MS). When the initial ratio of (-)-epigallocatechin gallate (EGCG) to theanine was 1:2 and roasted under 120 °C for 120 min, the contents of EPSFs were the highest. The bitterness and astringency thresholds of four EPSF isomers were measured by the half-tongue method, of which EPSF2 and EPSF3 had higher thresholds than EGCG. In LYT, four EPSFs had lower bitterness and astringency dose-over-threshold factors than EGCG. This study suggested that the reduction of bitterness and astringency of tea after roasting may be mainly due to the formation of EPSFs.

Amadori rearrangement products (ARPs) derived from the Maillard reaction between theanine and glucose (ARP 1), as well as pyroglutamic acid and glucose (ARP 2), were identified by liquid chromatograph tandem mass spectroscopy methods. The effects of initial reactant ratio, temperature, pH, and heating time on ARP generation were analyzed. The formation of both ARPs was most favored under 100 °C, while an alkaline environment slightly promoted the generation of ARP 1 and acidic conditions contributed more to ARP 2 formation. The decomposition of ARP 1 was suggested to be the predominant formation mechanism of ARP 2. Preparation, purification, and structure identification of ARP 1 were conducted, with its structure confirmed as 1-deoxy-1-l-theanino-d-fructose. The contents of ARP 1 in green, black, dark, white, yellow, and Oolong teas were quantitatively determined, of which black teas contained the highest levels of ARP 1, possibly due to the high glucose content and processing techniques.

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.

Nine black tea samples with different color intensity were firstly determined by chromatic difference analyzer. The color characteristics were secondly quantitatively described by UV-visible spectroscopy. Thirdly, liquid chromatography tandem mass spectrometry (LC-MS) based metabolomics analysis was applied in low-molecular weight compounds. Finally, the color contributors were identified by the correlation analysis of color, spectrometry and mass data. UV-visible based metabolomics analysis revealed that the wavelength at 380-520 nm (VIP > 1.50) was the critical absorbance band for distinguishing different color of BT infusions, while LC-MS based metabolomics analysis indicated that there were 48 main marker compounds responsible for the classification of different BT infusions. Correlation analysis results showed that the coefficients of theaflavins, thearubigins, theabrownins, flavonoid glycosides, and some hydroxycinnamoyl acids were > 0.7, which suggested they were main color contributors of BT infusion. The present study expanded a new vision on the color analysis of BT infusion.

Black tea stems, the significant by-products of the black tea industry, always possess great potential for utilization. The chemical characterization of black tea stems and leaves from the same manufacturing process was compared based on multiple-spectral analysis of 1H nuclear magnetic resonance (1H NMR) analysis, ultraviolet-visible spectroscopy (UV-Vis) and Fourier transform infrared spectroscopy (FTIR) analysis. Furthermore, the multivariate statistics method orthogonal projection on latent structure-discriminant analysis (OPLS-DA) was employed for screening chemical differences among leaves and stems. Meanwhile, the taste quality and chromatic difference of their infusions were evaluated. The color of the black tea stem and leaf infusions showed no significant differences (P > 0.05). Compared with leaves, higher contents of theanine and sugars were found in stems with richer umami and sweet flavors. While higher contents of polyphenols and caffeine were present in leaves, corresponding to more pungent bitter and astringent flavors.

During tea processing, roasting significantly affects the transformation pathway of catechins. When (-)-epigallocatechin gallate (EGCG) and glucose were roasted at different pH values, the degree of degradation and isomerization of EGCG was the lowest at pH 7 and the highest at pH 8. Thirty-five products were found in the model reaction of EGCG and glucose under high temperatures, of which four EGCG-glucose adducts were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and nuclear magnetic resonance (NMR). In addition, catechins, gallic acid, and theanine in tea with added glucose were significantly reduced during roasting. The contents of four EGCG-glucose adducts were increased significantly at 150 °C after 30 min and dropped gradually after 60 min. Therefore, based on the present study, EGCG could form crosslinks with glucose under high temperatures in a short time, which provides insight for tea processing and synthesis of catechin-sugar adducts.