A nontargeted and targeted metabolomics study on the dynamic changes in metabolite levels during the anaerobic treatment of γ-aminobutyric acid (GABA) tea

Integrative analysis of the impact of N2/CO2 on gabaron oolong tea aroma

This study aimed to investigate the effect of the combination of shaking and various anaerobic treatments on the aroma quality of gabaron oolong tea (GAOT) by chemical and sensory evaluation. The results showed that elevated anaerobic treatment harmed GAOT aroma, emphasizing undesirable attributes such as earthy, fatty, etc. A total of 85 volatiles were identified by gas chromatography-ion mobility spectrometry (GC-IMS), and the relationship between aroma attributes and volatiles were revealed by PLS regression projection and correlation network. Hexanal and octanal at inappropriate concentrations were main causes to the earthy attribute, while nonanal exhibited a potential masking effect against unpleasant attributes. Addition experiments and σ-τ plot analysis verified these associations. Furthermore, observing dynamic patterns of content changes of these three aldehydes in fresh leaves prior to tea thermal processing, providing references for future process optimization. These results provide a new direction for enhancing the quality of GAOT.

HS-SPME-GC-MS combined with relative odor activity value identify the key aroma components of flowery and fruity aroma in different types of GABA tea

Anaerobic processing is a crucial factor influencing the formation of flavor quality in Gamma-aminobutyric acid (GABA) tea. In this study, headspace solid-phase microextraction combined with gas chromatography and mass spectrometry was employed to explore the flavor characteristics of different types of GABA tea. We utilized multivariate analyses to identify at least 146 volatile components (VOCs) across 12 functional groups in the GABA tea samples via principal component analysis (PCA). At least 40 differential VOCs were screened from the GABA tea samples via orthogonal partial least squares-discriminant analysis. Subsequently, a minimum of four VOCs were identified in the GABA tea samples via the Pearson correlation coefficient combined with relative odor activity values as potential markers for flowery and fruity aromas, clarifying the impact of the VOCs on these characteristics. The results of this study provide a theoretical basis for understanding the formation of flowery and fruity flavor characteristics in GABA tea.

GABA, epigallocatechin gallate, tea, and the gut-brain axis

Our investigations on GABA-enriched tea and the reduction of stress in a student cohort have shown that more than just GABA may be involved. The effects of other constituents that are changed in the enrichment process are likely to be important. We have concentrated on GABA as well as the major tea flavonoid, epigallocatechin gallate. While this flavonoid is known to get to the brain on oral administration, it is far from clear that GABA does the same. GABA may act primarily on the gut and influence brain function via the gut-brain axis and the gut microbiome. In addition, there may be a microbiome in the brain that has a role. The situation is complex and not clearly understood. Mixtures of bioactive compounds are always difficult to investigate, but even the precise mechanisms of how pure oral GABA acts as a neuro-nutraceutical is unclear.

Effects of anaerobic treatment on the non-volatile components and angiotensin-converting enzyme (ACE) inhibitory activity of purple-colored leaf tea

This study investigated the effect of anaerobic treatment on the non-volatile components and angiotensin-converting enzyme (ACE) inhibitory activity in purple-colored leaf tea. Results showed that after 8 h of anaerobic treatment, the γ-aminobutyric acid (GABA) content significantly increased from 0.02 mg/g to 1.72 mg/g (p < 0.05), while lactic acid content gradually rose from non-detectable levels to 3.56 mg/g. Notably, certain flavonols like quercetin and myricetin exhibited significant increments, whereas the total anthocyanins (1.01 mg/g) and epigallocatechin-3-(3''-O-methyl) gallate (13.47 mg/g) contents remained almost unchanged. Furthermore, the ACE inhibition rate of purple-colored leaf tea increased significantly from 42.16% to 49.20% (p < 0.05) at a concentration of 2 mg/mL. Moreover, galloylated catechins showed stronger ACE inhibitory activity than non-galloylated catechins in both in vitro ACE inhibitory activity and molecular docking analysis. These findings might contribute to the development of special purple-colored leaf tea products with potential therapy for hypertension.

Metabolomics applications for plant-based foods origin tracing, cultivars identification and processing: Feasibility and future aspects

Metabolomics, the systematic study of metabolites, is dedicated to a comprehensive analysis of all aspects of plant-based food research and plays a pivotal role in the nutritional composition and quality control of plant-based foods. The diverse chemical compositions of plant-based foods lead to variations in sensory characteristics and nutritional value. This review explores the application of the metabolomics method to plant-based food origin tracing, cultivar identification, and processing methods. It also addresses the challenges encountered and outlines future directions. Typically, when combined with other omics or techniques, synergistic and complementary information is uncovered, enhancing the classification and prediction capabilities of models. Future research should aim to evaluate all factors affecting food quality comprehensively, and this necessitates advanced research into influence mechanisms, metabolic pathways, and gene expression.

Dynamic changes in the non-volatile and flavour compounds in withered tea leaves of three different colour cultivars based on multi-omics

In this study, metabolomics and proteomics were performed to investigate the fluctuations of non-volatile compounds and proteins in tea leaves from three tea cultivars with varying colours during withering. A total of 2798 compounds were detected, exhibiting considerable variations in amino acids, phenylpropanoids, and flavonoids. The ZH1 cultivar displayed increased levels of amino acids but decreased levels of polyphenols, which might be associated with the up-regulation of enzymes responsible for protein degradation and subsequent amino acid production, as well as the down-regulation of enzymes involved in phenylpropanoid and flavonoid biosynthesis. The FUD and ZH1 cultivars had elevated levels of flavanols and flavanol-O-glycosides, which were regulated by the upregulation of FLS. The ZJ and ZH1 cultivars displayed elevated levels of theaflavin and peroxidase. This work presents a novel investigation into the alterations of metabolites and proteins between tea cultivars during withering, and helps with the tea cultivar selection and manufacturing development.

Milestone review: GABA, from chemistry, conformations, ionotropic receptors, modulators, epilepsy, flavonoids, and stress to neuro-nutraceuticals

Arising out of a PhD project more than 50 years ago to synthesise analogues of the neurotransmitter GABA, a series of new chemical entities were found to have selective actions on ionotropic GABA receptors. Several of these neurochemicals are now commercially available. A new subtype of these receptors was discovered that could be a target for the treatment of myopia, the facilitation of learning and memory, and the improvement of post-stroke motor recovery. The development of these new chemical entities over many years demonstrates the importance of neurochemicals with which to investigate selective aspects of GABA receptors and illustrates the significance of collaboration between chemists and biologists in neurochemistry. Vital were the improvements in synthetic organic chemistry and the use of functional human receptors expressed in oocytes. Current interest in ionotropic GABA receptors includes the clinical development of subtype-specific agents and the role of gain-of-function receptor variants in epilepsy. Dietary flavonoids were found to cross the blood-brain barrier to influence brain function. Natural and synthetic flavonoids had a range of effects on GABA receptors, ranging from positive, silent, and negative allosteric modulators, to even second-order modulation of first-order modulators. Flavonoids have been called "a new family of benzodiazepines." Like benzodiazepines, flavonoids reduce stress. Stress produces changes in GABA receptors in the brain that may be because of changes in endogenous modulators, such as neurosteroids and corticosteroids. GABA also occurs naturally in the diet leading to studies of the effects of oral GABA on brain function. This finding has resulted in studies of GABA and related neurochemicals as neuro-nutraceuticals. GABA systems in the gut microbiome are essential to such studies. The actions of oral GABA and of GABA-enriched beverages and foodstuffs are now an area of considerable scientific and commercial interest. GABA is a deceptively simple chemical that can take up many shapes, which may underlie its complex functions. The need for new chemical entities with selective actions for further studies highlights the need for continuing collaboration between chemists and biologists.

Combined multi-omics approach to analyze the flavor characteristics and formation mechanism of gabaron green tea

Gabaron green tea (GAGT) has unique flavor and health benefits through the special anaerobic treatment. However, how this composite processing affects the aroma formation of GAGT and the regulatory mechanism was rarely reported. This study used nontargeted metabolomics and molecular sensory science to overlay screen differential metabolites and key aroma contributors. The potential regulatory mechanism of anaerobic treatment on the aroma formation of GAGT was investigated by transcriptomics and correlation analyses. Five volatiles: benzeneacetaldehyde, nonanal, geraniol, linalool, and linalool oxide III, were screened as target metabolites. Through the transcriptional-level differential genes screening and analysis, some CsERF transcription factors in the ethylene signaling pathway were proposed might participate the response to the anaerobic treatment. They might regulate the expression of related genes in the metabolic pathway of the target metabolites thus affecting the GAGT flavor. The findings of this study provide novel information on the flavor and its formation of GAGT.

Gamma-aminobutyric acid (GABA): a comprehensive review of dietary sources, enrichment technologies, processing effects, health benefits, and its applications

Gamma-aminobutyric acid (GABA) is a naturally occurring potential bioactive compound present in plants, microorganisms, animals, and humans. Especially, as a main inhibitory neurotransmitter in the central nervous system, GABA possesses a broad spectrum of promising bioactivities. Thus, functional foods enriched with GABA have been widely sought after by consumers. However, the GABA levels in natural foods are usually low, which cannot meet people's demand for health effects. With the increasing public awareness on the food securities and naturally occurring processes, using enrichment technologies to elevate the GABA contents in foods instead of exogenous addition can enhance the acceptability of health-conscious consumers. Herein, this review provides a comprehensive insight on the dietary sources, enrichment technologies, processing effects of GABA, and its applications in food industry. Furthermore, the various health benefits of GABA-enriched foods, mainly including neuroprotection, anti-insomnia, anti-depression, anti-hypertensive, anti-diabetes, and anti-inflammatory are also summarized. The main challenges for future research on GABA are related to exploring high GABA producing strains, enhancing the stability of GABA during storage, and developing emerging enrichment technologies without affecting food quality and other active ingredients. A better understanding of GABA may introduce new windows for its application in developing functional foods.

Special tea products featuring functional components: Health benefits and processing strategies

The functional components in tea confer various potential health benefits to humans. To date, several special tea products featuring functional components (STPFCs) have been successfully developed, such as O-methylated catechin-rich tea, γ-aminobutyric acid-rich tea, low-caffeine tea, and selenium-rich tea products. STPFCs have some unique and enhanced health benefits when compared with conventional tea products, which can meet the specific needs and preferences of different groups and have huge market potential. The processing strategies to improve the health benefits of tea products by regulating the functional component content have been an active area of research in food science. The fresh leaves of some specific tea varieties rich in functional components are used as raw materials, and special processing technologies are employed to prepare STPFCs. Huge progress has been achieved in the research and development of these STPFCs. However, the current status of these STPFCs has not yet been systematically reviewed. Here, studies on STPFCs have been comprehensively reviewed with a focus on their potential health benefits and processing strategies. Additionally, other chemical components with the potential to be developed into special teas and the application of tea functional components in the food industry have been discussed. Finally, suggestions on the promises and challenges for the future study of these STPFCs have been provided. This paper might shed light on the current status of the research and development of these STPFCs. Future studies on STPFCs should focus on screening specific tea varieties, identifying new functional components, evaluating health-promoting effects, improving flavor quality, and elucidating the interactions between functional components.

Characterization of triterpenoids as possible bitter-tasting compounds in teas infected with bird’s eye spot disease

Tea infected with bird's eye spot disease generally imparts a long-lasting bitter taste, which is unacceptable to most consumers. This study has comprehensively evaluated the taste profiles of infected and healthy teas and investigated their known bitter compounds previously reported in tea. Quantification analyses and calculation of dose-over-threshold (DoT) factors revealed that no obvious difference was visualized in catechins, caffeine, bitter amino acids, and flavonols and their glycosides between infected and healthy tea samples, which was also verified by principal component analysis (PCA) and hierarchical cluster analysis (HCA). Therefore, these known bitter compounds have been ruled out as critical contributors to the long-lasting bitterness of infected teas. Furthermore, Gel permeation chromatography, sensory analysis, and UPLC-Q-TOF-MS were employed and identified 13 substances from the target bitter fractions, including caffeine, ten triterpenoids, and two oxylipins. The higher triterpenoid levels were supposed to be the reason causing the long-lasting bitterness. This study has provided a research direction for the molecular basis of the long-lasting bitterness of infected tea leaves with bird's eye spot disease.

Ultrasound accelerated γ-aminobutyric acid accumulation in coffee leaves through influencing the microstructure, enzyme activity, and metabolites

Gamma-aminobutyric acid (GABA) is a non-protein amino acid that possesses various physiological functions. Our previous study has shown that ultrasound increased GABA accumulation in coffee leaves. In this study, we aimed to uncover the GABA enrichment mechanism by investigating the surface microstructure, cellular permeability, enzyme activities, and metabolomics of coffee leaves under ultrasound treatment. The results showed that ultrasound increased the electrical conductivity and the activities of glutamate decarboxylase, γ-aminoaldehyde dehydrogenase, and diamine oxidase by 12.0%, 265.9%, 124.1%, 46.8%, respectively. Environmental scanning electron microscope analysis demonstrated an increased opening of stomata and the rougher surface in the leaves after ultrasound treatment. UPLC-qTOF-MS/MS-based untargeted metabolomics analysis identified 82 differential metabolites involved in various metabolism pathways. Our results indicated that ultrasound changed the surface microstructure of coffee leaves, thereby accelerating the migration of glutamate into the cells; activated related enzymes; regulated C/N metabolism pathways, which led to an increase of GABA.

Effect of red light on the composition of metabolites in tea leaves during the withering process using untargeted metabolomics

BACKGROUND

Red light withering significantly improves the sensory flavor qualities of tea, although changes in metabolites during this process have not been systematically studied until now. The present study comprehensively analyzes metabolites in withered tea leaves at 2-h intervals up to 12 h under red light (630 nm) and dark conditions using ultra performance liquid chromatography-high resolution mass spectrometry (untargeted metabolomics).

RESULTS

Ninety-four non-volatile compounds are identified and relatively quantified, including amino acids, catechins, dimeric catechins, flavonol glycosides, glycosidically-bound volatiles, phenolic acids and nucleosides. The results show that amino acids, catechins and dimeric catechins are most affected by red light treatment. Ten free amino acids, theaflavins and theasinensin A increase after red light irradiation, whereas epigallocatechin gallate and catechin fall.

CONCLUSION

The present study provides a comprehensive and systematic profile of the dynamic effects of red light on withering tea and a rationale for its use in tea processing quality control. © 2021 Society of Chemical Industry.

New insights into the influences of baking and storage on the nonvolatile compounds in oolong tea: A nontargeted and targeted metabolomics study

A nontargeted and targeted metabolomics method was applied to comprehensively investigate the influences of baking and storage on chemical constituents in fresh-, strong-, and aged-scent types of Foshou oolong teas. The contents of N-ethyl-2-pyrrolidone-substituted flavanols (EPSFs), flavone C-glycosides, gallic acid, and most lipids increased after baking and storage, while the contents of cis-flavanols, alkaloids, flavonol O-glycosides, and most amino acids decreased. Degradation, epimerization, and interaction with theanine were main pathways for the decrease in cis-flavanols. Approximately 20.7%, 12.8%, and 11.6% of epigallocatechin gallate were degraded, epimerized, and interacted with theanine after baking, respectively; 22.5% and 8.71% of epigallocatechin gallate were degraded and interacted with theanine after 10-year storage, respectively. Simulated reactions confirmed that the increases in EPSFs and apigenin C-glycosides were caused by interactions between theanine and flavanols and between apigenin aglycone and glucose, respectively. This study offers novel insights into chemical changes during baking and storage of oolong tea.

GABA-enriched teas as neuro-nutraceuticals

Teas enriched in GABA are consumed for their beneficial effects on blood pressure, stress and anxiety. These effects may involve actions of GABA on the central and peripheral nervous systems. The anaerobic procedures for the production of GABA-enriched teas increase GABA levels by 10-20 times. They also significantly alter the levels of other constituents that may interact with the actions of GABA. These include epigallocatechin gallate, caffeine and theanine. The possible interactions of these active constituents make the understanding of the effects of GABA-enriched teas complex. More data is needed to establish where and how GABA is acting following consumption of GABA-enriched teas. While there is considerable evidence that such GABA is acting on GABA receptors in the periphery, there is rather less evidence that is acting directly in the brain. Certainly, there is more to the action of GABA-enriched teas than GABA itself.

Metabolomics combined with proteomics provides a novel interpretation of the changes in nonvolatile compounds during white tea processing

In this study, metabolomics and proteomics were employed to investigate the change mechanism of nonvolatile compounds during white tea processing. A total of 99 nonvolatile compounds were identified, among which the contents of 13 free amino acids, caffeine, theaflavins, 7 nucleosides and nucleotides, and 5 flavone glycosides increased significantly, while the contents of theanine, catechins, theasinesins, 3 proanthocyanidins, and phenolic acids decreased significantly during the withering period. The results of proteomics indicated that the degradation of proteins accounted for the increase in free amino acid levels; the weakened biosynthesis, in addition to oxidation, also contributed to the decrease in flavonoid levels; the degradation of ribonucleic acids contributed to the increase in nucleoside and nucleotide levels during the withering period. In addition, the drying process was found to slightly promote the formation of white tea taste. Our study provides a novel characterization of white tea taste formation during processing.