Aroma Formation Research Articles

Sensory-directed flavor analysis reveals the improvement in aroma quality of summer green tea by osmanthus scenting

Flower scenting is an effective way to enhance the aroma of green tea (GT), including those osmanthus scented green tea (OSGT). However, the mechanism of aroma enhancement by scenting is still unclear. Here, the volatiles of GT, OSGT, and osmanthus were detected by GC–MS. The total volatile content of OSGT was significantly increased compared to GT, with the flowery and coconut aromas enhanced. Furthermore, 17 of 139 volatiles were responsible for the enhancement by GC–olfactometry and their absolute odor activity values (OAVs). Aroma recombination, omission and addition experiments showed that dihydro-β-ionone, (E)-β-ionone, (E, E)-2,4-heptadienal, geraniol, linalool, α-ionone, and γ-decalactone were the key aroma volatiles with flowery or coconut aromas. Additionally, the dynamics of the key volatiles (OAVs >1) from different scenting durations were analyzed, proving that the optimal duration was 6–12 h. This study provides new insight into the mechanism of aroma formation during OSGT production.

Molecular Regulatory Mechanisms Affecting Fruit Aroma.

Aroma, an important quality characteristic of plant fruits, is produced by volatile organic compounds (VOCs), mainly terpenes, aldehydes, alcohols, esters, ketones, and other secondary metabolites, in plant cells. There are significant differences in the VOC profile of various fruits. The main pathways involved in the synthesis of VOCs are the terpenoid, phenylalanine, and fatty acid biosynthesis pathways, which involve several key enzyme-encoding genes, transcription factors (TFs), and epigenetic factors. This paper reviews the main synthetic pathways of the main volatile components in fruit, summarizes studies on the regulation of aroma formation by key genes and TFs, summarizes the factors affecting the fruit aroma formation, describes relevant studies on the improvement of fruit flavor quality, and finally proposes potential challenges and prospects for future research directions. This study provides a theoretical basis for the further precise control of fruit aroma quality and variety improvement.

Exploring microbial and moist-heat effects on Pu-erh tea volatiles and understanding the methoxybenzene formation mechanism using molecular sensory science

Piling fermentation (PF) is crucial for Pu-erh tea aroma, yet its microbial and moist-heat impact on aroma quality is poorly understood. Solid-phase microextraction, solvent-assisted flavor evaporation, and gas chromatography–mass spectrometry were used to detected and analyses the samples of sun-green green tea, sterile PF and spontaneous PF. Microbiological action promotes the formation of stale aromas. Moist-heat action promotes the formation of plum-fragrance and sweet aroma. 20 microbial markers and 28 moist-heat markers were screened from 184 volatile components. Combining odor activity values and gas chromatography-olfactometry, 22 aroma-active compounds were screened (1,2,3-trimethoxybenzene, linalool, 1,2,4-trimethoxybenzene …), and analyzed during PF processing. Aroma omission and addition experiments verified its importance. Gallic acid addition experiments successfully verified that microorganisms are the main contributors to the synthesis of methoxybenzenes. Finally, Blastobotrys, Rasamsonia, and Thermomyces showed positive correlation with the synthesis of 1-ethyl-4-methoxybenzene, 1,2,4-trimethoxybenzene, 1,2,3-trimethoxybenzene, and 1,2-dimethoxybenzene. The formation mechanism of Pu-erh tea's aroma was clarified.Exploring microbial and moist-heat effects on Pu-erh tea volatiles and understanding the methoxybenzene formation mechanism using molecular sensory science.

GC-MS Combined with Proteomic Analysis of Volatile Compounds and Formation Mechanisms in Green Teas with Different Aroma Types.

Aroma is one of the key factors for evaluating the quality of green tea. A tender aroma (NX) and floral-like aroma (HX) are two types of high-quality aroma of green tea. In this work, the different aroma types of baked green tea were classified by sensory evaluation. Then, seven tea samples with a typical tender or floral-like aroma were selected for further volatile component analysis by GC-MS. A total of 43 aroma compounds were identified in two different aroma types of baked green tea samples. The PCA showed that linalool, geraniol, 3-hexenyl butyrate, and 3-hexenyl hexanoate were the major volatiles contributing to the HX. On the other hand, most of the alcohol volatiles, such as 1-octanol, 1-octen-3-ol, 1-dodecanol, 1-hexadecanol, phenylethyl alcohol, benzyl alcohol, aldehydes and some hydrocarbons contributed more to the NX. In addition, the chemical composition analysis showed that the content of free amino acids was higher in NX green tea samples, while the content of catechins was relatively higher in HX tea samples. A proteomic analysis revealed that most of the enzymes involved in VPBs pathways, such as phenylalanine ammonialyase, peroxidase, and shikimate-O-hydroxycinnamoyl transferase, were more abundant in NX than in HX tea samples. These results laid a foundation for the aroma formation mechanism of different aroma types of baked green tea and provided some theoretical guidance for the breeding of specific aroma varieties.

Aroma Substances Generation and Variation of Broth in Pork Braised Processing based on Multi-Instrumental and Multiplex Analysis

The aim was to investigate and reveal the patterns of production and changes of major aroma substances in broth during quantitative repeated cycles of cooking and the effects of meat and spices on the formation of unique aroma in broth. Aroma substances were systematically identified and quantified for three groups (MS, broth cooked with meat and spices; M, broth cooked with meat; and S, broth cooked with spices) by using GC-MS, E-nose and tri-step infrared spectroscopy. The PCA model of M group E-nose showed that the total content of aroma substances reached the maximum value in the broth samples cooked for the third time. Meanwhile, the GC-MS results showed that benzaldehyde in the spices completely disappeared in the 4th brining, esters such as linalyl propionate and isopropyl myristate disappeared after the 5th and 6th brining, ketones basically disappeared in the 6th brining, and the relative content of alcohols in M group was highest in the 4th brining. Successful validation of the generation and variation of aroma substances by E-nose and GC-MS through tri-step infrared spectroscopy technique with increasing resolution. Therefore, the quality of braised meat products can be improved by replenishing the meat and spices after the 3th brining.

Sesame lignans modulate aroma formation in sesame oil through the Maillard reaction and lipid oxidation in model systems

The unknown effect of sesame lignans on aroma formation in sesame oil via the Maillard reaction (MR) and lipid oxidation was investigated. Sesamin, sesamolin, or sesamol was added to 3 models: lysine+glucose (MR), cold-pressed sesame oil (SO), and MR + SO, and were heated at 120 °C for 60 min. All three lignans suppressed SO oxidation while increasing DPPH scavenging ability (p < 0.05). Lignans increased depletions of lysine and glucose and MR browning (p < 0.05). Lignans reduced most aroma-active pyrazines, aldehydes, ketones, alcohols, and esters (p < 0.05). Sesamol and sesamolin increased perceptions of the preferable aromas of nutty, roasted sesame, and popcorn while reducing the undesirable green and rancid aromas (p < 0.05). Sesamol demonstrated a stronger effect on lipid oxidation, MR browning, aroma formation, and sensory perception than sesamin and sesamolin. This study suggests that sesame lignans can modulate aroma formation and sensory perception of sesame oil by interacting with the MR and lipid oxidation pathways.

Characterisation of aroma-active compounds in dried Dendrobium spp. stems (Shihu) using GC-Olfactometry and a modified NIF-SNIF method

Dendrobium orchid stems (Chinese name Shihu) are becoming popular ingredients in food and drinks such as wine, teas and desserts, however, its aroma chemistry is not currently understood. Gas Chromatography-Mass Spectrometry (GC-MS) and Gas Chromatography-Olfactometry (GC-O) were used to analyse two Dendrobium species. The Nasal Impact Frequency-Surface of Nasal Impact Frequency (NIF-SNIF) method was employed on thirteen untrained panellists using a newly built open-source software (“nif-simple”). An expert annotated odour descriptions and intensities of NIF-SNIF peaks. Twenty-four volatile aroma compounds were found using GC-MS, with 22 shared compounds and 17 reported for the first time. GC-O NIF-SNIF indicated nine aroma compounds were aroma-active in D. nobile and D. officinale. Five were found in both species suggesting that they are species-independent markers. Aroma-active compounds were characterised as brown, mushroom, metallic, nutty and potato aromas. Methional (NIF 83 %) and 1-octen-3-one (NIF 81 %) were most frequently detected in D. nobile and D. officinale respectively, and this study proposed aroma formation mechanisms in dried Dendrobium stems. Modified Frequencies demonstrated that NIF-SNIF is an effective GC-O method for identifying key aroma-active compounds. Understanding the flavour properties of Shihu could enhance its global appeal and encourage NIF-SNIF to be used in future studies.

Dynamical changes of volatile metabolites and identification of core fungi associated with aroma formation in Fu Brick tea during the fungal fermentation

Fu Brick Tea (FBT), a distinguished dark tea, derives its unique sensory identity from a specialized fungal fermentation process. However, the effect of core functional fungi on sensory attributes during FBT fungal fermentation still needs to be systematically explored. Employing a synergistic blend of electronic sensory evaluation, microbiomics, and volatile metabolomics, this study delves into the dynamic evolution of fungi and volatile metabolites (VMs) during fungal fermentation. Utilizing fungal sequencing and Gas Chromatography-Mass Spectrometry (GC-MS) techniques, we unveiled 234 OTUs, with g_Eurotium emerging as a dominant fungus. Additionally, we identified 288 contributing VMs, 35 of which exhibited notable fluctuations during fermentation. These associated fungi influenced crucial metabolic pathways, enhancing the synthesis of sweet-scented components like acetophenone, while repressing woody and fecal aromas, exemplified by nerolidol, skatole, and indole. This study reveals the evolution of fungi and VMs during FBT fungal fermentation, and suggests 4 possible pathways of substance synthesis in which fungi may be involved, delivering valuable insights into the nuanced processes governing the quality formation of FBT.

Jasmonates Play an Important Role in Differential Accumulation of Key Oolong Tea Aromas in Two Tea Varieties (Camellia sinensis)

Aroma is an important factor in the measurement of the quality and market value of oolong tea. However, it is hard to develop an oolong tea with good aroma quality using unsuitable tea varieties. To explore the key factors of tea varieties in the formation of oolong tea aromas, the fresh leaves of the Chungui variety (CG, suitable for oolong tea, Camellia sinensis (L.) O. Kuntze) and the Fuyun No. 6 variety (F6, unsuitable for oolong tea, Camellia sinensis (L.) O. Kuntze) were harvested and treated by withering and mechanical stress in order. Then, aroma, transcriptome, and jasmonate (JA) contents, and weighted gene co-expression network analysis (WGCNA), of samples were investigated. The contents of characteristic oolong tea aromas, including indole, (E)-β-ocimene, (E)-nerolidol, α-farnesene, and jasmine lactone, were all accumulated in much higher quantities in the CG variety after withering and mechanical stress. Accordingly, the coding genes of aroma formation synthases TSB2, OCS, NES, AFS, and LOX1, and related genes in MVA, MEP, and ALA pathways, were all much more highly activated. These differential reactions are mainly caused by the higher accumulation of jasmonates, especially methyl jasmonate, a type of important plant signal chemical, in CG after mechanical stress. WGCNA analysis indicated 34 different transcription factors from different families are predicted to be involved in this jasmonate-responsive reaction.

Insight into the aroma and taste enrichment pattern in Chinese traditional braised soup based on HS-SPME-GC-MS and HS-GC-IMS

Sauce-braised products are Chinese specialty delicacies with a long history, and the repeatedly stewed-braised soup is the decisive factor in the formation of aroma and taste in sauce-braised products. In this study, duck two-jointed wings were employed as a stewing model to investigate the effects of stewing cycles (0, 1, 5, 10, 15 and 20 cycles) on Chinese traditional braised soup. The result showed that the aroma and taste components of the braised soup were gradually enriched with cyclic stewing and stabilized after 15 cycles. The total aroma content of the braised soup increased from 11 431.09 μg/kg (C0) to 16,710.40 μg/kg (C15). And the contents of total FAAs and 5′-nucleotides in C15 increased to 2.65-fold and 1.30-fold compared to C0, respectively. Terpenoids and aldehydes were identified as the major aroma compounds in the braised soup. The GC-MS PLSDA model has screened 10 characteristic compounds: anethole, eugenol, terpinyl acetate, alpha-pinene, l-caryophyllene, acetyl eugenol, d-limonene, furfural, L-borneol acetate and (Z)-cinnamaldehyde. 3-Methylbutyraldehyde and furfural were identified with the highest VIP values in the GC-IMS PLSDA model. 5′-IMP and 5′-GMP were observed to significantly contribute to the enrichment of umami taste in the braised soup. Furthermore, we have observed significant correlations between the aroma compounds 3-methylbutyraldehyde, benzaldehyde and 2,3-diethylpyrazine with the amino acids Leu, Phe and Ser, respectively. This study provides insight into the enrichment patterns of aroma and taste during cyclic stewing, offering valuable information for the industrial production and cost-saving strategies of sauce-braised products.

Aroma formation in single- and double-petal jasmines (Jasminum sambac) during flowering via volatile metabolome and transcriptome

Jasminum sambac (L.) Aiton exists in single-petal (SP), double-petal (DP), and multi-petal (MP) phenotypes, which have a sweet floral scent and release aroma when the flowers open. A total of 103 volatiles were discovered from SP and DP jasmines. 16 volatiles were identified as their key odorants. Caryophyllene, geraniol, linalool, α-farnesene, β-ocimene, methyl anthranilate, methyl salicylate, benzeneacetaldehyde and hexanal may form the overall aroma of those two jasmines during flowering. High-resolution transcriptional profiles of those two jasmines at four flowering stages were generated from our recently reported chromosome-level genome assembly, from which we identified 35 differentially expressed genes (DEGs) involved in terpenoid metabolic pathways and 10 DEGs involved in the phenylpropane/benzene metabolic pathway, which may shape the characteristic aroma of those two jasmines at the molecular level. Through co-expression network analysis of key odorants, we found that bZIP8, bHLH6, MYB4, and AP2/ERF3 may contribute to the regulation of aroma volatiles in those two jasmines. These results offer an important theoretical foundation for studying the regulation of aroma formation for SP and DP jasmines.

Effect of tea green leafhopper (Empoasca onukii Matsuda) sucking on the quality of Oriental Beauty

AbstractThis study aimed to investigate the effects of tea green leafhopper damage on the sensory quality and metabolite composition of Oriental Beauty by using tea leaves from different varieties of tea plants and processing factories. The results indicated that tea green leafhopper damage could reduce bitterness and increase the level of fruity aroma, which was related to the content changes of catechin components and alcohol substances. Furthermore, the tea green leafhopper salivary treatment experiment revealed that the decrease of catechin content and increase of theaflavin content were mainly influenced by the action of saliva and mechanical injury, and changes in volatile components such as linalool oxidation were responsible for the alteration in aroma. These findings provide a theoretical reference for the aroma formation and quality improvement of Oriental Beauty.

Analysis of differences in microorganisms and aroma profiles between normal and off-flavor pit mud in Chinese strong-flavor Baijiu

The unique cellar fermentation process of CFSB is the reason for its characteristic “cellar aroma” flavor. The types, abundance, community structure and metabolic activity of microorganisms in the pit mud directly affect the microbial balance in the white spirit production environment, promoting the formation of typical aromas and influencing the quality of CFSB. During the production process, the production of off-flavor in the cellar may occur. The aim of this study is to elucidate the differences in microbiota and flavor between normal pit mud and abnormal pit mud (pit mud with off-flavor). A total of 46 major volatile compounds were identified, and 24 bacterial genera and 21 fungal genera were screened. The esters, acids, and alcohols in the abnormal pit mud were lower than those in the normal pit mud, while the aldehydes were higher. Indole, 3-methyl-, which have been proven to be responsible for the muddy and musty flavors, were detected in both types of pit mud, and for the first time, high levels of 4-Methylanisole were detected in the pit mud. The microbial composition of the two types of pit mud showed significant differences in the bacterial genera of Sporosarcina, Lactobacillus, Garciella, Anaerosalibacter, Lentimicrobium, HN-HF0106, Petrimonas, Clostridium_sensu_stricto_12 and Bacillus, and the fungal genera of Millerozyma, Penicillium, Mortierella, Monascus, Saccharomyces, Issatchenkia, Pithoascus, Pseudallescheria, and Wickerhamomyces. While we speculate that Sporosarcina is the predominant bacterial genus responsible for the imbalance of microbiota in pit mud.

A review on fermented vegetables: Microbial community and potential upgrading strategy via inoculated fermentation.

Fermentation is a traditional method utilized for vegetable preservation, with microorganisms playing a crucial role in the process. Nowadays, traditional spontaneous fermentation methods are widely employed, which excessively depend on the microorganisms attached to the surface of raw materials, resulting in great difficulties in ideal control over the fermentation process. To achieve standardized production and improve product quality, it is essential to promote inoculated fermentation. In this way, starter cultures can dominate the fermentation processes successfully. Unfortunately, inoculated fermentation has not been thoroughly studied and applied. Therefore, this paper provides a systematic review of the potential upgrading strategy of vegetable fermentation technology. First, we disclose the microbial community structures and succession rules in some typical spontaneously fermented vegetables to comprehend the microbial fermentation processes well. Then, internal and external factors affecting microorganisms are explored to provide references for the selection of fermented materials and conditions. Besides, we widely summarize the potential starter candidates with various characteristics isolated from spontaneously fermented products. Subsequently, we exhibited the inoculated fermentation strategies with those isolations. To optimize the product quality, not only lactic acid bacteria that lead the fermentation, but also yeasts that contribute to aroma formation should be combined for inoculation. The inoculation order of the starter cultures also affects the microbial fermentation. It is equally important to choose a proper processing method to guarantee the activity and convenience of starter cultures. Only in this way can we achieve the transition from traditional spontaneous fermentation to modern inoculated fermentation.

Key Odorants Forming Aroma of Polish Mead: Influence of the Raw Material and Manufacturing Processes.

Mead was analyzed by using the concept of molecular sensory science for the identification of key odorants. A total of 29 odor-active compounds were identified in mead by using gas chromatography olfactometry (GCO). Flavor dilution (FD) factors of identified compounds ranged from 1 to 16,384, compounds with FD factors ≥32 were quantitated by using stable isotopically substituted odorants as internal standards or external standard method, and odor activity values (OAVs) were calculated. Fifteen compounds showed OAVs ≥1: aldehydes (2-phenylacetaldehyde, 3-(methylsulfanyl)propanal), 4-hydroxy-3-methoxybenzaldehyde), esters (ethyl 3-methylbutanoate, ethyl propanoate, ethyl octanoate), alcohols (2-phenylethan-1-ol, 3- and 2-methylbutan-1-ol, 3-(methylsulyfanyl)propan-1-ol), furanons (4-hydroxy-2,5-dimethylfuran-3(2H)-one, 3-hydroxy-4,5-dimethylfuran-2(5H)-one), acids (3- and 2-methylbutanoic acid, acetic acid), 1,1-diethoxyethane, and 4-methylphenol. 2-Phenylacetaldehyde (OAV, 3100) was suggested as the compound with the biggest influence on the aroma of mead, followed by 4-hydroxy-2,5-dimethylfuran-3(2H)-one (OAV, 1900), 3-(methylsulfanyl)propanal (OAV, 890), and 2-phenylethan-1-ol (OAV, 680). Quantitative olfactory profile analysis revealed strong honey, malty, and alcoholic impressions. Omission experiments revealed that 3-(methylsulfanyl)propanal, 2-phenylethan-1-ol, 4-hydroxy-2,5-dimethylfuran-3(2H)-one, ethyl propanoate, ethyl 3-methylbutanoate, 2-phenylacetaldehyde, 3- and 2-methylbutanoic acid, 3-hydroxy-4,5-dimethylfuran-2(5H)-one, and 4-hydroxy-3-methoxybenzaldehyde were the key odorants in the mead. Determining concentrations of key odorants in important production steps showed that the fermentation and maturation stages had the strongest effect on the formation of mead aroma.

Golden-flower fungus (Eurotiwm Cristatum) presents fungal flower aroma as well as accelerates the aging of white tea (Shoumei)

Golden-flower fungus (Eurotiwm Cristatum, EC) is widely inoculated in dark tea to endow a typical fungal floral aroma. Recently, Golden Flower White Tea (GFWT), prepared by transplanting EC-mediated fermentation to white tea (Shoumei, SM) to reform the roughness and coarseness, has attracted much attention attributed to coordinated flavor. However, the bio-chemistry reactions between EC and SM, along with origination of composited aroma are still unclear. Thus, the rejected EC, GFWT leaves and stems after EC removal were separated by layer-by-layer stripping following sensory evaluation, volatiles and microstructure analysis to uncover aroma formation mechanism. In GFWT, EC presents fungal flower aroma rather than contribution of extracellular enzymes secreted by fungus in Fu brick tea. Moreover, the short “flowering process” (7 days) endows SM with a stale, jujube, and sweet aroma, which is regarded as the typical characteristic of aged white tea. This inspires EC-mediated fermentation as a promising rapid aging process.

The interaction between lipid oxidation and the Maillard reaction model of lysine-glucose on aroma formation in fragrant sesame oil

The formation mechanism behind the sophisticated aromas of sesame oil (SO) has not been elucidated. The interaction effects of the Maillard reaction (MR) and lipid oxidation on the aroma formation of fragrant sesame oil were investigated in model reaction systems made of l-lysine (Lys) and d-glucose (Glc) with or without fresh SO (FSO) or oxidized SO (OSO). The addition of OSO to the Lys-Glc model increased the MR browning at 294 nm and 420 nm and enhanced the DPPH radical scavenging activity greater than the addition of FSO (p < 0.05). The presence of lysine and glucose inhibited the oxidation of sesame oil, reduced the loss of γ-tocopherol, and facilitated the formation of sesamol (p < 0.05). The Maillard-lipid interaction led to the increased concentrations of some of the alkylpyrazines, alkylfurans, and MR-derived ketones and acids (p < 0.05) while reducing the concentrations of other pyrazines, lipid-derived furans, aliphatic aldehydes, ketones, alcohols, and acids (p < 0.05). The addition of FSO to the MR model enhanced the characteristic roasted, nutty, sweet, and fatty aromas in sesame oil (p < 0.05), while excessive lipid oxidation (OSO) brought about an unpleasant oxidized odor and reduced the characteristic aromas. This study helps to understand the sophisticated aroma formation mechanism in sesame oil and provides scientific instruction for precise flavor control in the production of sesame oil.

Comprehensive investigation on the dynamic changes of volatile metabolites in fresh scent green tea during processing by GC-E-Nose, GC–MS, and GC × GC-TOFMS

Processing technology plays a crucial role in the formation of tea aroma. The dynamic variations in volatile metabolites across different processing stages of fresh scent green tea (FSGT) were meticulously tracked utilizing advanced analytical techniques such as GC-E-Nose, GC–MS, and GC × GC-TOFMS. A total of 244 volatile metabolites were identified by GC–MS and GC × GC-TOFMS, among which 37 volatile compounds were concurrently detected by both methods. Spreading and fixation stages were deemed as pivotal processes for shaping the volatile profiles in FSGT. Notably, linalool, heptanal, 2-pentylfuran, nonanal, β-myrcene, hexanal, 2-heptanone, pentanal, 1-octen-3-ol, and 1-octanol were highlighted as primary contributors to the aroma profiles of FSGT by combining odor activity value assessment. Furthermore, lipid degradation and glycoside hydrolysis were the main pathways for aroma formation of FSGT. The results not only elucidate the intricate variations in volatile metabolites but also offer valuable insights into enhancing the processing techniques for improved aroma quality of green tea.

Aroma formation mechanism by the drying step during Congou black tea processing: Analyses by HP-SPME and SAFE with GC-MS

Drying plays a key role in the formation of the final aroma of black tea. However, the contribution to its aroma remains unclear. Herein, headspace solid-phase microextraction (HP-SPME) and solvent-assisted flavor evaporation (SAFE) coupled with GC-MS were used to detect the volatiles of tea samples (before or after drying) from three cultivars. We found that the drying step caused increase in sweet and fruity aromas and decrease in mint-like and green aromas. 89 Differential volatiles were identified with variable importance in projection (VIP) over 1. Furthermore, 25 decreased aroma-active volatiles were obtained with relative odor activity values (rOAVs) over 1, showing that green, flowery, fruity, sweet, and mint-like decreased significantly by 62.27%, 64.88%, 67.56%, 58.26%, and 69.72% after drying step. (Z)-3-Hexen-1-ol (rOAV = 36.93), linalool (rOAV = 301.56) and methyl salicylate (rOAV = 5.50) were the odorants for the green, flowery, fruity, and mint-like odor. Moreover, other 3 increased aroma-active volatiles showed that flowery, fruity, and sweet volatiles increased significantly after drying step. β-Damascenone (rOAV = 43.49) was the main odorant for fruity and sweet aroma. The 28 differential volatiles were associated with carotenoid degradation, lipid degradation, and glycoside hydrolysis. This study reveals the contribution of drying to the aroma formation of black tea.

UPLC-ESI-MS/MS analysis revealed the dynamic changes and conversion mechanism of non-volatile metabolites during green tea fixation

Fixation is crucial for green tea quality; previous studies were conducted on finished teas and focused on the volatile metabolites of green tea. However, non-volatile metabolites’ (NVMs) evolution and formation mechanisms during fixation remain unclear. Here, fixated tea samples were collected at seven different time points during the green tea fixation stage, the dynamic changes and formation mechanisms of NVMs investigated via ultra-performance liquid chromatography-tandem mass spectrometry and enzymatic activity analyses. The results showed that fixation contained enzymatic activity thermal and non-enzymatic thermal stages. Moreover, 1196 NVMs (10 classes) were detected, and significant NVM variations were observed during the "enzymatic activity thermal" fixation stages, with increasing trends in lipids and amino acids, except alkaloids. Furthermore, 160 common differential NVMs were identified. KEGG analysis revealed that unsaturated fatty acid, linoleic acid phenylpropanoid, and flavonoid metabolism pathways were essential during fixation and influenced green tea aroma, taste, and color formation. These results provide a theoretical basis to regulate green tea fixation technology and enrich the chemical theory of green tea processing.