Exogenous stimulation-induced biosynthesis of volatile compounds: Aroma formation of oolong tea at postharvest stage

Lipidomic profiles of shaking-withering black tea and their metabolic conversion during tea processing

Shaking-withering is a new technique in black tea manufacturing, which enables promoting aroma quality. Lipids are important tea aroma precursors. However, the lipids metabolism and its contribution to aroma formation of shaking-withering black tea (SBT) remain unknown. Herein, 436 lipids and 45 fatty acid-derived volatiles (FADVs) and their dynamic changes during SBT processing were investigated. Among them, 113 lipids and 29 FADVs (mainly floral/fruity fatty aldehydes and esters) were screened as critical compounds associated with shaking-withering. Key enzymes PLA, LOX and HPL showed enhanced activity/expression in SBT. Degradation of glycerophospholipids, glycoglycerolipids, and fatty acids were annotated as potential lipid metabolism pathways. Particularly, glycoglycerolipids containing 18:2, 18:3 fatty acyls, e.g., MGDG(18:2/18:3), DGDG(18:3/18:3), showed most predominate degradation after shaking and negative correlation with FADVs, and were highlighted as key potential aroma precursors in SBT. These results demonstrated that shaking may trigger greater glycoglycerolipids degradation and FADVs formation, contributing to SBT aroma improvement.

Revealing the roles of solar withering and shaking processes on oolong tea manufacturing from transcriptome and volatile profile analysis

Solar and indoor withering in the manufacturing process of semi-fermented oolong tea are crucial for aroma formation. While the processes have been established through accumulated experience, the underlying mechanisms remain largely unknown. This study identified pairs of gene and volatile organic compound (VOC) that were significantly correlated and up-regulated during solar withering and the first shaking, including lipoxygenase 8 (LOX8) with 3-hexenyl iso-butyrate, terpene synthase 2 (TPS2) with β-ocimene and linalool, as well as tryptophan synthase β-subunit 2 (TSB2) with indole. Besides, two β-glucosidases (β-GH), β-GH1_1 and β-GH3_1, were up-regulated by more than 30-fold in these stages. When comparing the three manufacturing procedures, indole, nerolidol, β-ocimene, benzyl nitrile, and jasmine lactone, were largely accumulated only in the normal process, where both solar withering and shaking were included. These findings provide insights into the regulation of VOC accumulation under stresses during withering, and highlight the importance of specific manufacturing processes in the formation of oolong tea characteristic aroma.

Effects of microbial biocontrol agents on tea plantation microecology and tea plant metabolism: a review

The quality of fresh tea leaves is crucial to the final product, and maintaining microbial stability in tea plantations is essential for optimal plant growth. Unique microbial communities play a critical role in shaping tea flavor and enhancing plant resilience against biotic stressors. Tea production is frequently challenged by pests and diseases, which can compromise both yield and quality. While biotic stress generally has detrimental effects on plants, it also activates defense metabolic pathways, leading to shifts in microbial communities. Microbial biocontrol agents (MBCAs), including entomopathogenic and antagonistic microorganisms, present a promising alternative to synthetic pesticides for mitigating these stresses. In addition to controlling pests and diseases, MBCAs can influence the composition of tea plant microbial communities, potentially enhancing plant health and resilience. However, despite significant advances in laboratory research, the field-level impacts of MBCAs on tea plant microecology remain insufficiently explored. This review provides insights into the interactions among tea plants, insects, and microorganisms, offering strategies to improve pest and disease management in tea plantations.

Study on dynamic alterations of volatile organic compounds reveals aroma development over enzymatic-catalyzed process of Tieguanyin oolong tea production

To elucidate the formation of characteristic aroma over enzymatic-catalyzed processes (ECP), GC-MS-based volatile-metabolomic combined with desorption-electrospray-ionization coupled mass-spectrometry-imaging (DESI-MSI) were employed to analyze the changes of volatile organic compounds (VOCs) in Tieguanyin tea. A total of 579 VOCs were obtained, from which 24 components involved in five pathways were identified as biomarkers. Among these, four VOCs including 2-furancarboxylic acid, 4-methylbenzaldehyde, N-benzylformamide, cuminaldehyde, were detected in both DESI-MSI and GC-MS analysis, exhibiting dynamic changes along processing steps. RNA-sequencing analysis indicated the genes referring to stress response were activated during tea processing, facilitating the accumulation of flora-fruity aroma in tea leaf. Metabolic pathways analysis revealed that the increase in floral-fruity related components such as volatile terpenoids, phenylpropanoids/benzenoids, indole, alongside a decrease in green leaf volatiles including (E)-2-Hexenal, (Z)-3-Hexenol, played a crucial role in development of characteristic aroma, which could be a feasible index for evaluating processing techniques or quality of oolong tea.

Another inner truth of shaking: Water migration and transformation-advanced physicochemical alterations in tea leaves

Shaking, essential in oolong tea production, is becoming an innovative method to impart floral fragrance. Research on shaking primarily concentrates on biological underpinnings, including modifications in gene expression and stress-triggered enzymatic catalysis, and consequent physicochemical properties. Water phase and distribution, reshaped by shaking and affected the biological and physicochemical alterations of tea leaves, is always ignored. This work utilized TEM, LF-NMR, UPLC-QqQ-MS, and GC-TOF-MS to explore physicochemical alterations during shaking. Results revealed shaking induced stomatal opening, water migration from stems to leaf veins, and a reduction in free water, transformed into bound water. Mechanical stimulation disrupted cell microstructures, including vacuoles, chloroplasts, and cell walls, releasing precursors and enzyme substrates. Shaking triggered intracellular physicochemical reactions that decreased polyphenols, amino acids, chlorophyll, and carotenoids, while increasing organic acids and sugars. Also catalyzed the synthesis of aromatic compounds like (E)-nerolidol, β-ionone epoxide, and α-farnesene, shaping the floral-fruity aroma and mellow taste of tea.

A Gas Sensors Detection System for Real-Time Monitoring of Changes in Volatile Organic Compounds during Oolong Tea Processing

The oxidation step in Oolong tea processing significantly influences its final flavor and aroma. In this study, a gas sensors detection system based on 13 metal oxide semiconductors with strong stability and sensitivity to the aroma during the Oolong tea oxidation production is proposed. The gas sensors detection system consists of a gas path, a signal acquisition module, and a signal processing module. The characteristic response signals of the sensor exhibit rapid release of volatile organic compounds (VOCs) such as aldehydes, alcohols, and olefins during oxidative production. Furthermore, principal component analysis (PCA) is used to extract the features of the collected signals. Then, three classical recognition models and two convolutional neural network (CNN) deep learning models were established, including linear discriminant analysis (LDA), k-nearest neighbors (KNN), back-propagation neural network (BP-ANN), LeNet5, and AlexNet. The results indicate that the BP-ANN model achieved optimal recognition performance with a 3-4-1 topology at pc = 3 with accuracy rates for the calibration and prediction of 94.16% and 94.11%, respectively. Therefore, the proposed gas sensors detection system can effectively differentiate between the distinct stages of the Oolong tea oxidation process. This work can improve the stability of Oolong tea products and facilitate the automation of the oxidation process. The detection system is capable of long-term online real-time monitoring of the processing process.

Effect of Mechanical Damage in Green-Making Process on Aroma of Rougui Tea

Rougui Tea (RGT) is a typical Wuyi Rock Tea (WRT) that is favored by consumers for its rich taste and varied aroma. The aroma of RGT is greatly affected by the process of green-making, but its mechanism is not clear. Therefore, in this study, fresh leaves of RGT in spring were picked, and green-making (including shaking and spreading) and spreading (unshaken) were, respectively, applied after sun withering. Then, they were analyzed by GC-TOF-MS, which showed that the abundance of volatile compounds with flowery and fruity aromas, such as nerolidol, jasmine lactone, jasmone, indole, hexyl hexanoate, (E)-3-hexenyl butyrate and 1-hexyl acetate, in green-making leaves, was significantly higher than that in spreading leaves. Transcriptomic and proteomic studies showed that long-term mechanical injury and dehydration could activate the upregulated expression of genes related to the formation pathways of the aroma, but the regulation of protein expression was not completely consistent. Mechanical injury in the process of green-making was more conducive to the positive regulation of the allene oxide synthase (AOS) branch of the α-linolenic acid metabolism pathway, followed by the mevalonate (MVA) pathway of terpenoid backbone biosynthesis, thus promoting the synthesis of jasmonic acid derivatives and sesquiterpene products. Protein interaction analysis revealed that the key proteins of the synthesis pathway of jasmonic acid derivatives were acyl-CoA oxidase (ACX), enoyl-CoA hydratase (MFP2), OPC-8:0 CoA ligase 1 (OPCL1) and so on. This study provides a theoretical basis for the further explanation of the formation mechanism of the aroma substances in WRT during the manufacturing process.

Transcriptomic Insights into the Enhanced Aroma of Guangdong Oolong Dry Tea (Camellia sinensis cv. Yashixiang Dancong) in Winter

Winter dry tea (WDT) exhibits a more intense and lasting aroma compared to dry tea from other seasons; however, this conclusion is solely based on sensory outcomes and lacks corroborative theoretical evidence. Our study aimed to analyze the aroma compounds in WDT and investigate the causes behind the formation of WDT's aroma by analyzing the volatile organic compounds (VOCs) in WDT, spring dry tea (SDT), winter fresh leaves (WFLs) and spring fresh leaves (SFLs) by gas chromatography-mass spectrometry (GC-MS), complemented by an analysis of gene expression pertinent to WFLs and SFLs by using transcriptomic analysis. The results revealed a significant increase in total VOCs in WDT compared to SDT, with WDT exhibiting distinct woody aromas as indicated by a higher α-muurolene content. In WFL, the contents of aldehydes and ketones were richer than those in SFL. Notably, the study found that UDP-glycosyltransferase genes in WFLs were significantly up-regulated, potentially promoting the synthesis of terpene glycosides. These terpene glycosides can release terpene aroma compounds during processing, contributing significantly to the intense and lasting aroma of WDT. Overall, this research provides valuable insights into the mechanism behind aroma formation in Guangdong oolong tea harvested during winter.

The Cultivar Effect on the Taste and Aroma Substances of Hakka Stir-Fried Green Tea from Guangdong

The flavor and quality of tea largely depends on the cultivar from which it is processed; however, the cultivar effect on the taste and aroma characteristics of Hakka stir-fried green tea (HSGT) has received little attention. High-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), and sensory evaluations were used to detect and predict the essential taste and aroma-contributing substances of HSGTs made from Huangdan (HD), Meizhan (MZ) and Qingliang Mountain (QL) cultivars. Orthogonal partial least squares data analysis (OPLS-DA) ranked four substances that putatively distinguished the tastes of the HSGTs, epigallocatechin gallate (EGCG) > theanine > epigallocatechin (EGC) > epicatechin gallate (ECG). Ten substances with variable importance in projections (VIPs) ≥ 1 and odor activation values (OAVs) ≥ 1 contributed to their overall aromas, with geranylacetone having the most significant effect on HD (OAV 1841), MZ (OAV 4402), and QL (OAV 1211). Additionally, sensory evaluations found that HD was relatively equivalent to QL in quality, and both were superior to MZ. HD had a distinct floral aroma, MZ had a distinct fried rice aroma, and QL had a balance of fried rice and fresh aromas. The results provide a theoretical framework for evaluating the cultivar effect on the quality of HSGT and put forward ideas for future HSGT cultivar development.

Tea: From Historical Documents to Modern Technology

Tea is among the most important beverages globally. The spread of tea from the East to West has not only affected lifestyles, but also promoted the exchange of exchange between the East and West. Tea processing, which is critical for the development of tea flavor, includes multiple steps, such as withering, deactivation, rolling, fermentation (enzymatic oxidation) and post-fermentation. With the development of mechanical processing, tea has now become widely produced, both by hand-crafting and mechanical processing. Multiple components of tea, such as tea polyphenols, theanine, tea pigments and caffeine, have also been acquired by modern separation techniques. In traditional Chinese medicine, tea has long been documented as beneficial to health. Modern medical and nutritional studies have demonstrated that tea has many health benefits, acting to lower blood lipids, blood sugars, anti-inflammation, and anti-oxidation. To some extent, the activities of tea verified by modern medicine are consistent with the recordings in traditional medicine. Interdisciplinary theories, methods and techniques will contribute bridging knowledge contained within historical documents on tea and modern technology and science.

The Impact of Different Withering Approaches on the Metabolism of Flavor Compounds in Oolong Tea Leaves

In this study, complementary metabolomic and proteomic analyses were conducted on the solar- and indoor-withered oolong tea leaves, and freshly plucked leaves as the control, for the purpose to reveal the mechanisms underlying the initial formation of some flavor determinants during the early stage of oolong tea processing. As a result, a total of 978 non-volatile compounds and 152 volatile compounds were identified, the flavonoids and several esters were differently accumulated in various tea samples. In total, 7048 proteins were qualitatively and quantitatively determined, the analysis on pathway enrichment showed that phenylpropanoid, flavonoid metabolisms, and protein processing in endoplasmic reticulum were the major pathways discriminating the different tea samples. The joint protein-metabolite analysis showed that the multiple stresses such as dehydration, heat, and ultra-violet irradiation occurred during the withering step induced the dynamic and distinct changes in the biochemical network in the treated leaves compared to fresh leaves. The significant decreases in flavonoids, xanthine alkaloids, and several amino acids contributed to the alleviation of bitter or astringent taste of withered leaves, although the decomposition of L-theanine resulted in the loss of umami flavor over the solar-withering step. Moreover, the fruity or floral aromas, especially volatile terpenoids and phenylpropanoids/benzenoids, were retained or accumulated in the solar withered leaves, potentially aiding the formation of a better characteristic flavor of oolong tea made by indoor withered tea leaves. Distinct effects of solar- and indoor-withering methods on the flavor determinant formation provide a novel insight into the relationship between the metabolite accumulation and flavor formation during the withering step of oolong tea production.