How the Fixation Process Affects the Quality of Green Tea Powder

1.Introduction

Background and Significance

Tea is not just a beverage; it is an essential economic crop cultivated and consumed worldwide for its numerous health benefits. It has been recognized for its ability to delay aging, regulate sugar and lipid metabolism, and promote anti-inflammatory responses, making it an indispensable part of many diets and wellness routines. Among various types of tea, green tea powder, particularly matcha, has gained significant global popularity due to its unique sensory characteristics and associated health benefits. This powdered form of green tea has a vibrant green color, delicate sweetness, and a rich umami flavor, making it highly desirable not only for traditional tea preparation but also for culinary innovations like lattes, pastries, and desserts.

The increasing demand for green tea powder in global markets is driven by its association with health-conscious lifestyles and its integration into modern food and beverage trends. Beyond its sensory appeal, green tea powder’s quality is deeply influenced by its chemical composition, particularly its non-volatile metabolites (NVMs). These compounds play critical roles in defining the flavor, color, and aroma of the tea, thereby influencing both its quality and economic performance.

Quality-Related Components and Their Functions

The quality attributes of green tea powder—such as its astringency, bitterness, umami flavor, and visual appeal—are closely linked to specific non-volatile metabolites. These key components include:

1. Catechins, Flavonoids, and Flavonol Glycosides
   • These are the primary contributors to the astringency of green tea powder. Catechins, a type of flavonoid, are abundant in green tea powder and are known for their antioxidant properties. While beneficial for health, they contribute to the slightly puckering sensation experienced when consuming the tea.
2. Caffeine, Theobromine, and Diketopiperazines
   • These compounds are responsible for the bitterness of green tea powder. Caffeine, well-known for its stimulating effects, is complemented by theobromine, which adds a milder bitterness and a sense of smoothness.
3. L-Theanine and Soluble Sugars
   • These are critical for the umami or savory flavor of green tea powder. L-Theanine, a unique amino acid found in tea, not only provides a rich, brothy taste but also balances the astringency and bitterness, creating a harmonious flavor profile. Soluble sugars, though present in smaller quantities, contribute to the slight sweetness of the tea.
4. Chlorophyll and Carotenoids
   • These fat-soluble pigments are essential for the vibrant green color of green tea powder. They also play a role in the visual appeal and appearance of the tea when prepared, enhancing its attractiveness in culinary applications.
5. Anthocyanins and Water-Soluble Pigments
   • These pigments influence the color of the brewed tea. While less abundant in green tea powder compared to other tea types, they contribute subtle color changes during infusion.

The Role of Non-Volatile Metabolites in Quality

Beyond the immediate sensory attributes, certain non-volatile metabolites serve as precursors to the aromatic compounds in green tea powder. For example:

1. Lipids
   • Linolenic acid and linoleic acid, two essential lipids found in tea leaves, undergo enzymatic reactions during processing to produce aliphatic aromatic compounds such as 1-octen-3-one and hexanal, which contribute to the fresh and grassy aroma of green tea powder.
2. Amino Acids
   • Amino acids, including L-Theanine, not only enhance umami flavors but also participate in the Maillard reaction during heating processes. This reaction produces heterocyclic aromatic compounds like pyrazines and furans, which add roasted and nutty notes to the aroma of green tea powder.

Dynamic Changes in NVMs During Processing

The processing of green tea powder is a highly intricate and carefully controlled series of steps that transform fresh tea leaves into the vibrant green powder we recognize. The processing stages include spreading, fixation (kill-green), shaping, and drying. Among these, the fixation stage is pivotal in shaping the final quality of green tea powder. This step involves applying heat to halt enzymatic activity, preserving the green color and preventing oxidation.

During fixation, the dynamic changes in NVMs are particularly pronounced. Key transformations include:

1. Amino Acids
   • Amino acids tend to increase during the early stages of fixation, enhancing the umami and sweetness of the tea. Their conversion into aromatic compounds during heat application is a critical factor influencing the aroma profile.
2. Lipids
   • Lipid oxidation pathways are activated during fixation, leading to the generation of volatile aromatic compounds. These changes contribute to the fresh and vegetal aroma of green tea powder.
3. Flavonoids and Phenolic Compounds
   • These compounds undergo significant transformations during fixation. The stabilization of catechins and other polyphenols prevents oxidation, preserving the astringency and antioxidant properties of the tea.
4. Chlorophyll and Carotenoids
   • Heat during fixation stabilizes chlorophyll, ensuring the retention of the bright green color. Carotenoids also remain stable, contributing to the overall appearance of the powder.
5. Sugars and Organic Acids
   • The levels of soluble sugars and organic acids are influenced by the heat treatment, impacting the tea’s sweetness and acidity.

NVMs as Precursors to Aroma

In addition to their direct contribution to flavor and color, NVMs serve as precursors to the volatile compounds responsible for the aroma of green tea powder. For instance, lipids such as linolenic acid are precursors to green and fresh-smelling aldehydes, while amino acids like L-Theanine contribute to the formation of roasted and nutty aromas through heat-driven reactions.

The dynamic interplay between enzymatic and thermal chemical reactions during the fixation stage determines the composition and abundance of these non-volatile metabolites, which in turn influence the sensory qualities of the final product.

Importance of Studying NVM Dynamics

The changes in NVMs during the processing of green tea powder are critical for understanding and optimizing its quality. Previous studies have highlighted the importance of fixation techniques in influencing the sensory characteristics of tea. For instance:

• Drum Hot-Air Fixation is associated with producing a roasted chestnut-like aroma.
• High-Temperature Fixation results in richer green tea powder aroma profiles.
• Combined Fixation Methods yield higher levels of amino acids and soluble sugars compared to single fixation techniques.

However, most studies focus on the finished tea product, leaving a gap in understanding the transformation of NVMs during the fixation stage itself. Further research is essential to elucidate the metabolic pathways and mechanisms behind these changes, providing a foundation for fine-tuning processing techniques and enhancing the quality of green tea powder.

In summary, green tea powder’s quality and sensory attributes are closely linked to its non-volatile metabolites, which undergo significant transformations during processing, particularly during fixation. These changes influence the tea’s flavor, aroma, color, and overall appeal. Understanding the dynamic changes and roles of NVMs during this stage offers valuable insights for improving processing techniques, enhancing product quality, and meeting consumer preferences in the global market. Future studies integrating NVMs with volatile aroma compounds will further advance the knowledge of green tea powder processing and quality control.

2. The Critical Role of Fixation in the Quality of Green Tea Powder

Introduction to the Fixation Process

The production of green tea powder involves a series of meticulous processing stages, including spreading, fixation (kill-green), shaping, and drying. Among these steps, fixation is considered a pivotal stage that significantly determines the final quality of the tea powder. Fixation, also referred to as “kill-green,” involves the application of heat to freshly harvested tea leaves, with the primary aim of deactivating oxidative enzymes such as polyphenol oxidase (PPO) and peroxidase (POD). This halts enzymatic oxidation, preserving the characteristic green color, fresh aroma, and delicate flavor of green tea powder.

Fixation serves multiple essential purposes in green tea powder processing:

1. Preservation of Color
   • By preventing enzymatic browning, fixation retains the vibrant green color of the tea leaves. Chlorophyll remains intact during this process, ensuring the characteristic appearance of the final powdered product.
2. Development of Aroma
   • Heat application triggers biochemical transformations that result in the generation of aroma compounds. These include fresh and grassy aldehydes, nutty pyrazines, and sweet-smelling lactones, which contribute to the appealing fragrance of green tea powder.
3. Enhancement of Flavor
   • Fixation stabilizes amino acids and soluble sugars, which are key contributors to the tea’s umami and sweetness. Additionally, it prevents the degradation of catechins and polyphenols, preserving the tea’s astringency and antioxidant properties.
4. Control of Moisture Content
   • Moisture reduction during fixation prepares the tea leaves for subsequent shaping and drying steps, ensuring a consistent texture and preventing microbial growth.

The fixation process is highly sensitive to variables such as temperature, duration, and heating method. Common fixation techniques include steam fixation, pan-firing, and hot-air fixation, each of which imparts unique sensory characteristics to the green tea powder. For example, pan-firing often results in a nutty or roasted aroma, while steam fixation emphasizes the fresh and vegetal qualities of the tea.

Limitations of Previous Research

Despite its importance, much remains to be understood about the fixation process and its role in shaping the quality of green tea powder. Past studies have largely focused on the final product, providing insights into its chemical composition and sensory attributes. However, these studies fall short in analyzing the dynamic changes that occur during the fixation stage itself. This gap in understanding leaves several critical questions unanswered, particularly regarding the role of non-volatile metabolites (NVMs).

1. Limited Understanding of NVM Dynamics
   • Non-volatile metabolites, including amino acids, lipids, flavonoids, and phenolic compounds, undergo complex transformations during fixation. These changes directly influence the sensory properties of green tea powder, such as its flavor, aroma, and color. While it is known that fixation stabilizes many of these compounds, the specific pathways and mechanisms of their transformation remain unclear. For instance, how amino acids like L-Theanine are preserved or converted into aroma precursors during heat treatment is not well-documented.
2. Lack of Focus on Enzymatic and Non-Enzymatic Interactions
   • Fixation involves a delicate interplay between enzymatic reactions (driven by residual enzymatic activity) and non-enzymatic thermal chemical reactions. During the early stages of fixation, residual enzyme activity may still contribute to the breakdown or transformation of key compounds. As temperature increases, thermal reactions dominate, leading to the formation of Maillard reaction products and other aroma compounds. The precise interaction between these two processes is poorly understood and has not been adequately explored in previous research.
3. Inconsistent Attention to Process Variables
   • While the effects of specific fixation methods on the final product have been studied, there is limited investigation into how process variables such as temperature gradients, heating duration, and moisture levels influence NVM transformations during fixation. Understanding these variables is crucial for optimizing the process and achieving consistent quality in green tea powder production.
4. Overemphasis on Volatile Compounds
   • Many studies on tea processing have focused on the volatile aroma compounds that contribute to the fragrance of green tea powder. While these compounds are undoubtedly important, the non-volatile metabolites that serve as precursors to these volatiles are often overlooked. For example, lipids such as linolenic acid are critical precursors to aliphatic aldehydes and ketones, which are responsible for the tea’s fresh aroma. A deeper understanding of these precursor compounds is essential for controlling the development of aroma during fixation.

Key Research Gaps

To address these limitations, further research is needed to provide a comprehensive understanding of the fixation process. Specifically, studies should focus on:

1. Dynamic Tracking of NVMs
   • Advanced analytical techniques such as ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) can be employed to monitor the real-time changes in NVMs during fixation. These data can help elucidate the specific metabolic pathways involved in the transformation and stabilization of key compounds.
2. Integration of Enzymatic and Thermal Analyses
   • Measuring enzyme activity during the early stages of fixation can reveal the critical points at which enzymatic reactions cease and thermal reactions take over. Combining this information with chemical analyses can provide insights into the interplay between enzymatic and non-enzymatic processes.
3. Exploration of Process Optimization
   • Investigating the effects of different fixation methods (e.g., steam, pan-firing, or hybrid techniques) and process parameters (e.g., temperature, duration, and airflow) can help identify optimal conditions for preserving or enhancing specific NVMs.
4. Linking NVMs to Sensory Properties
   • Correlating the concentrations of NVMs with sensory evaluations of green tea powder can provide a clearer understanding of how chemical changes during fixation influence flavor, aroma, and color. This knowledge can be used to develop targeted processing strategies for achieving desired sensory profiles.
5. Mechanistic Studies on Precursor-Aroma Relationships
   • Investigating how specific NVMs, such as lipids and amino acids, contribute to the formation of aroma compounds during fixation can shed light on the biochemical mechanisms behind aroma development.

The fixation stage plays a decisive role in determining the quality of green tea powder by influencing its color, flavor, and aroma. However, current research has yet to fully uncover the complex biochemical and thermal processes that occur during this critical stage. Addressing the identified research gaps will not only enhance our understanding of green tea powder processing but also provide valuable insights for optimizing production techniques to meet growing consumer demands for high-quality, sensory-rich products. By focusing on the dynamic changes in non-volatile metabolites and their interactions during fixation, future studies can pave the way for more precise and efficient production methods, ensuring the consistent quality of green tea powder in global markets.

3. Research Methods

Sample Collection

To comprehensively analyze the transformation of non-volatile metabolites (NVMs) during the fixation process, green tea powder samples were collected at seven distinct time points throughout the fixation stage, labeled as FTS1 to FTS7. These time points were carefully chosen to represent different phases of the fixation process, capturing the dynamic changes in both enzymatic and thermal reactions. By dividing the fixation stage into discrete time intervals, researchers were able to pinpoint specific moments of metabolic activity and transformation, enabling a detailed investigation into the progression of NVM changes.

Additionally, a control sample, referred t

o as STS (spreading stage sample), was collected prior to fixation to serve as the baseline. This allowed the comparison of metabolite concentrations and enzyme activity levels before and after fixation, facilitating a better understanding of how the fixation process alters the biochemical composition of green tea powder.

Real-Time Monitoring

Dynamic changes in leaf temperature and moisture content were continuously monitored throughout the fixation process to capture the environmental conditions that drive enzymatic and thermal reactions.

1. Leaf Temperature
   • The temperature of the tea leaves was measured in real time during the fixation process. This helped determine the critical temperature ranges where enzymatic activity was inhibited and thermal reactions began to dominate. Understanding these transitions is essential for optimizing the fixation process to preserve desired qualities in green tea powder.
2. Moisture Content
   • The water content in the tea leaves was monitored during the fixation process. Moisture plays a significant role in the thermal conductivity of the leaves and influences the rate of biochemical reactions. Observing moisture levels provided insights into how water loss impacts the stabilization of pigments, amino acids, and other metabolites during fixation.
3. Enzyme Activity Analysis
   • The activities of key oxidative enzymes, including polyphenol oxidase (PPO) and peroxidase (POD), were measured to identify the exact time points at which these enzymes became inactivated. This is a critical factor in determining the effectiveness of the fixation process, as enzyme inactivation prevents oxidation and preserves the bright green color and fresh aroma of green tea powder.

Analytical Techniques

To comprehensively profile the biochemical changes occurring during fixation, advanced analytical tools and statistical methods were employed:

1. UPLC-MS/MS-Based Broad-Spectrum Targeted Metabolomics
   • Ultra-Performance Liquid Chromatography-Tandem Mass Spectrometry (UPLC-MS/MS) was used to detect and quantify a wide range of NVMs with high sensitivity and accuracy. This technique enabled the identification of 1,196 NVMs across ten major categories, including lipids, amino acids, flavonoids, phenolic acids, and nucleotides. The broad-spectrum metabolomics approach provided a detailed snapshot of the metabolic landscape during each stage of fixation.
2. PLS-DA and Clustering Analyses
   • Partial Least Squares Discriminant Analysis (PLS-DA) was applied to evaluate the differences in metabolite profiles across the seven fixation time points. This statistical method allowed researchers to identify the most significant metabolites contributing to changes in green tea powder quality during fixation.
   • Heatmaps and K-means clustering were used to group metabolites based on their dynamic changes over time. These visual tools highlighted distinct patterns of metabolite transformation and revealed key clusters of NVMs with similar trends.
3. Visualization Tools
   • Volcano plots were employed to illustrate the differential metabolites between fixation stages, highlighting significant increases or decreases in metabolite concentrations.
   • Bubble charts were used to display the enrichment of metabolic pathways related to the transformation of NVMs. These charts provided insights into the biochemical processes most affected by fixation.
   • Venn diagrams were used to identify shared and unique metabolites across different time points, helping to elucidate the key stages where significant changes occurred.
4. KEGG Pathway Annotation and Enrichment Analysis
   • Using the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, researchers annotated and enriched the metabolic pathways associated with the observed NVMs. This analysis provided a detailed understanding of the biochemical pathways most active during fixation, such as unsaturated fatty acid metabolism, phenylpropanoid biosynthesis, and flavonoid biosynthesis. These pathways are closely linked to the aroma, flavor, and color of green tea powder.

Integration of Data

By integrating the data collected from real-time monitoring and advanced metabolomic techniques, the study aimed to uncover the intricate relationship between fixation conditions and NVM dynamics. The combined use of statistical analyses and visualization tools allowed researchers to identify key metabolites and pathways responsible for the sensory and quality attributes of green tea powder. Moreover, this approach provided a systematic framework for optimizing the fixation process to enhance the overall quality of the final product.

Significance of the Methodology

The methodological framework employed in this study represents a significant advancement in understanding the fixation process. By capturing real-time data and combining it with metabolomic profiling, this study provides a comprehensive view of the biochemical transformations that occur during fixation. This holistic approach not only enhances our knowledge of green tea powder processing but also lays the groundwork for improving production techniques through evidence-based interventions.Next Steps in Research

Future studies will aim to expand this methodological fram

ework by integrating volatile metabolite profiling with the current analysis of NVMs. This will provide a more complete picture of how fixation influences both aroma compounds and their non-volatile precursors, further refining our understanding of green tea powder’s sensory properties and quality determinants. Additionally, exploring how different fixation methods and parameters impact these metabolic changes will help identify optimal processing strategies for producing high-quality green tea powder tailored to diverse consumer preferences.

4. Results and Discussion

Segmentation of the Fixation Stages

The fixation process in green tea powder production was divided into two distinct phases based on the dynamic changes observed in enzyme activity and thermal conditions:

1. The “Enzyme Activity-Thermal” Phase (FTS1 to FTS3)
   • This phase represents the early stages of fixation when enzymatic activity is still partially active but begins to decline due to rising leaf temperatures. The interaction between residual enzymatic activity and increasing thermal effects leads to significant biochemical transformations, particularly in the stabilization of non-volatile metabolites (NVMs).
   • Key oxidative enzymes, such as polyphenol oxidase (PPO) and peroxidase (POD), are gradually inactivated during this phase, resulting in the preservation of chlorophyll and the prevention of browning reactions. The gradual inactivation of enzymes also plays a critical role in stabilizing key quality-related metabolites such as amino acids and polyphenols.
2. The “Non-Enzymatic Thermal” Phase (FTS4 to FTS7)
   • During this phase, enzymatic activity has been completely suppressed due to the high temperature of the tea leaves, and the metabolic changes are primarily driven by non-enzymatic thermal chemical reactions.
   • These reactions include lipid oxidation and the Maillard reaction, which generate aromatic compounds that contribute to the characteristic flavor and aroma of green tea powder. This phase is crucial for developing the sensory qualities of the final product, as it allows for the formation of nutty, roasted, and sweet notes.

The segmentation into these two phases provides a clearer understanding of how different processes dominate at various points in the fixation stage, highlighting the interplay between enzymatic and non-enzymatic reactions.

Dynamic Changes in NVMs

Through broad-spectrum targeted metabolomics, 1,196 NVMs were identified during the fixation process, which were further classified into 10 major categories: lipids, amino acids and derivatives, phenolic acids, flavonoids, organic acids, tannins, nucleotides and derivatives, lignans and coumarins, alkaloids, and others. The results revealed dynamic transformations of NVMs across the fixation stages, with significant trends observed in their concentrations and compositions.

1. The “Enzyme Activity-Thermal” Phase (FTS1 to FTS3)
   • During this phas
   • e, a significant increase in NVM concentrations was observed, particularly in lipids, amino acids, and phenolic acids. This is attributed to the inactivation of oxidative enzymes and the stabilization of these metabolites under controlled thermal conditions.
   • Key findings:
     • Lipids: 60 different lipid compounds were identified, showing a notable increase during the early stages of fixation. These compounds play a critical role in the development of fresh and grassy aroma notes through lipid oxidation pathways.
     • Amino Acids: 29 amino acids and their derivatives exhibited an upward trend, with L-Theanine being particularly prominent. Aminoacids contribute to the umami flavor of green tea powder and act as precursors for aroma compounds formed during the Maillard reaction.
     • Phenolic Acids: 17 phenolic acids demonstrated significant changes, enhancing the astringency and antioxidant properties of green tea powder.
2. The “Non-Enzymatic Thermal” Phase (FTS4 to FTS7)
   • As enzymatic activity ceased, the levels of most NVMs stabilized or slightly decreased due to thermal degradation. However, certain compounds, such as Maillard reaction products, continued to form, contributing to the development of roasted and nutty aromas.
   • Notably, lipid-derived aldehydes and ketones (e.g., hexanal and 1-octen-3-one) were produced during this phase, enhancing the fresh and vegetal aroma of green tea powder.

Key Differential NVMs

Using VIP ≥ 1 and fold change ≥ 2 or ≤ 0.5 as selection criteria, 160 differential NVMs were identified across the fixation stages, with the most significant changes observed during the “Enzyme Activity-Thermal” phase. These differential metabolites primarily included:

• Lipids (60 species): Responsible for aroma and flavor precursors.
• Amino Acids (29 species): Critical for umami flavor and aroma compound formation.
• Phenolic Acids (17 species): Important for astringency and antioxidant capacity.

Key Metabolic Pathway Analysis

The metabolic pathways associated with the dynamic changes in NVMs during fixation were analyzed using the KEGG database. Two pathways were identified as particularly significant for influencing the sensory qualities of green tea powder:

1. Unsaturated Fatty Acid and Linoleic Acid Metabolism
   • These pathways were highly active during fixation and are directly linked to the production of aroma compounds such as aldehydes and ketones. For example:
     • Hexanal: A product of linoleic acid metabolism that contributes to the fresh, green aroma.
     • 1-Octen-3-one: A lipid-derived compound that imparts a mild mushroom-like scent.
   • The increased activity in these pathways during the “Non-Enzymatic Thermal” phase highlights the importance of lipid oxidation in aroma formation.
2. Phenylpropanoid and Flavonoid Biosynthesis
   • These pathways were primarily active during the “Enzyme Activity-Thermal” phase, stabilizing polyphenols and contributing to the vibrant green color and antioxidant properties of green tea powder. Key compounds include:
     • Catechins: Responsible for astringency and health benefits.
     • Quercetin and Kaempferol Derivatives: Important for antioxidant activity and flavor complexity.

The results of the KEGG analysis provide insights into how metabolic pathways contribute to the quality attributes of green tea powder, offering a theoretical basis for optimizing the fixation process.

Implications for Green Tea Powder Quality

1. Flavor and Aroma Development
   • The combination of lipid oxidation and amino acid transformations during fixation plays a critical role in shaping the unique flavor and aroma of green tea powder. Managing the balance between enzymatic inactivation and thermal reactions is key to achieving desirable sensory qualities.
2. Color Retention
   • The stabilization of chlorophyll and prevention of enzymatic browning during the early stages of fixation ensure the vibrant green color of green tea powder, which is a critical quality attribute for consumer acceptance.
3. Health Benefits
   • The preservation of phenolic acids, flavonoids, and other bioactive compounds during fixation enhances the antioxidant properties of green tea powder, reinforcing its health-related appeal.

The results demonstrate the critical role of fixation in determining the quality of green tea powder through dynamic changes in non-volatile metabolites. The segmentation of the fixation process into enzymatic and thermal phases provides a framework for understanding the interplay between enzymatic and chemical reactions. By linking NVM dynamics to key metabolic pathways, this study offers valuable insights for optimizing fixation techniques to enhance the flavor, aroma, color, and health benefits of green tea powder. Future research should explore the integration of volatile metabolite profiling to provide a more comprehensive understanding of how fixation shapes the sensory and quality attributes of green tea powder.

5.Conclusion

Key Findings

This study provides a comprehensive analysis of the fixation process and its influence on the quality of green tea powder. By integrating real-time monitoring, broad-spectrum targeted metabolomics, and pathway analysis, several key findings were revealed:

1. Two Distinct Phases in the Fixation Process
   • The fixation stage can be divided into the “Enzyme Activity-Thermal” phase (FTS1 to FTS3) and the “Non-Enzymatic Thermal” phase (FTS4 to FTS7).
   • In the early stages, enzymatic reactions gradually decline, leading to the stabilization of key non-volatile metabolites (NVMs). As temperature increases, thermal reactions dominate, driving the development of aroma, flavor, and other sensory qualities.
2. Dynamic Changes in NVMs and Their Impact on Quality
   • A total of 1,196 NVMs were identified during the fixation process, spanning ten major categories such as lipids, amino acids, phenolic acids, and flavonoids. The dynamic transformations of these compounds were closely tied to critical quality attributes like flavor, aroma, and color.
   • The most significant changes occurred during the “Enzyme Activity-Thermal” phase, where lipids, amino acids, and phenolic acids showed notable increases, laying the foundation for the flavor and aroma of green tea powder.
3. Metabolic Pathways and Their Roles in Processing
   • Pathway analysis revealed that unsaturated fatty acid metabolism and linoleic acid pathways were key contributors to the formation of aroma compounds, while phenylpropanoid biosynthesis and flavonoid biosynthesis pathways were vital for maintaining the green color and antioxidant properties.
   • These findings highlight the central role of metabolic pathways in determining the sensory and functional qualities of green tea powder.

Significance and Outlook

This study offers valuable theoretical and practical insights into the fixation process, paving the way for future advancements in green tea powder production:

1. Theoretical Contributions
   • The findings enrich the fundamental understanding of tea processing chemistry, particularly the dynamic changes in NVMs and their relationship to key quality attributes.
   • By elucidating the interplay between enzymatic and thermal reactions, this study provides a detailed mechanistic explanation of the fixation process, which can serve as a foundation for further research in tea chemistry.
2. Practical Implications for Precision Processing
   • The identification of key metabolites and pathways offers practical guidance for refining fixation techniques. For example:
     • Temperature and timing adjustments can be optimized to maximize the retention of amino acids and lipids during the “Enzyme Activity-Thermal” phase.
     • Tailored fixation methods (e.g., steam fixation, hot-air fixation, or hybrid methods) can be employed to achieve specific sensory profiles based on consumer preferences.
   • This precision in processing can help manufacturers consistently produce high-quality green tea powder with desirable sensory and functional characteristics.
3. Future Research Directions
   • While this study provides a detailed analysis of non-volatile metabolites, future research should integrate volatile metabolite profiling to understand how aroma compounds are formed during fixation.
   • Combining the analysis of volatile and non-volatile metabolites will provide a holistic understanding of the biochemical transformations during fixation, enabling a more comprehensive optimization of the green tea powder production process.
   • Additionally, exploring the impact of different fixation techniques and parameters on metabolic pathways will further refine our ability to control and enhance green tea powder quality.
   • Lastly, the role of environmental factors such as tea cultivar, growing conditions, and harvest time on fixation outcomes should be examined to provide broader insights into the processing of green tea powder.

In conclusion, this study underscores the critical role of the fixation process in shaping the quality of green tea powder. By delineating the dynamic changes in NVMs, identifying key metabolic pathways, and linking these transformations to sensory and functional attributes, the findings provide a robust theoretical framework for understanding and optimizing green tea powder production. Future research, integrating volatile and non-volatile metabolite analysis and exploring the effects of process variables, will further enhance our ability to produce high-quality green tea powder that meets the demands of global consumers. This study not only advances the science of tea processing but also contributes to the growing body of knowledge needed to support the continued development of the green tea industry.