1. The 'Kill-Green' Imperative: Defining the Fixation Step

1.1. Introduction to Sha Qing (殺青): The Critical Control Point

In the taxonomy of tea processing, the step known as fixation, or Sha Qing (殺青), represents the single most critical control point. This term, translated from Mandarin as "kill-green," refers to the application of heat to fresh tea leaves. Its primary function is to halt enzymatic oxidation at a precise, predetermined level. This step is what "fixes" the tea's character, locking in its chemical profile and preventing it from progressing toward a more oxidized state.

The function of fixation is best illustrated by the common analogy of a sliced apple. When an apple is cut, its flesh is exposed to oxygen, and endogenous enzymes (the same types found in tea) rapidly cause it to turn brown. However, the apples in a baked pie, which have been subjected to high heat, do not brown. The heat has permanently denatured those enzymes. Sha Qing functions on the same principle; it is the "off-switch" that prevents a fresh green leaf from becoming a brown, oxidized black tea.

1.2. The Biochemical Foundation: Halting Enzymatic Oxidation

The process of Sha Qing is, at its core, a de-enzyming process. Its goal is the permanent thermal denaturation of the oxidative enzymes within the leaf cells. These enzymes, when exposed to oxygen during the withering and rolling stages, are responsible for the oxidative browning of the tea's polyphenols, specifically the catechins.

The preceding step, withering, is often a period of active and desirable enzymatic oxidation. The optimal temperature for these oxidative enzymes is in the range of 30–38 °C. Withering conditions, often controlled at 22–30 °C, place the leaf directly in this active zone. Therefore, the fixation process must be capable of heating the leaf mass rapidly past this optimal temperature zone to a denaturation threshold (approximately 65.5°C / 150°F, and often much higher) to achieve a rapid and complete inactivation.

1.3. The Target Enzymes: Polyphenol Oxidase (PPO) and Peroxidase (POD)

The oxidative browning in tea leaves is a complex process driven primarily by two endogenous enzymes: Polyphenol Oxidase (PPO) and Peroxidase (POD). These enzymes are sourced not only from the tea leaf itself but also from the microbial communities that live on the leaves.

During the manufacturing of oolong and black teas, these enzymes are the "engines" of oxidation. They catalyze the conversion of simple catechins (flavan-3-ols) into the complex polymers that define darker teas: theaflavins (TFs) and thearubigins (TRs). Theaflavins are responsible for the brightness and briskness of black tea, while thearubigins provide its body, dark color, and strength. To produce a green, yellow, or oolong tea, the Sha Qing process must be precisely controlled to inactivate both PPO and POD, halting this conversion.

1.4. Inactivation Kinetics: A Comparative Analysis of PPO and POD Thermal Resistance

A critical, and often overlooked, aspect of fixation is that PPO and POD do not respond to heat identically. Scientific analysis of their thermal inactivation kinetics reveals that POD is significantly more resistant to thermal treatment than PPO.

This differential in heat resistance has profound implications for tea processing. Research demonstrates the following:

• Polyphenol Oxidase (PPO): This enzyme is less heat-resistant. Studies show PPO can be entirely inactivated after 30 minutes at temperatures as low as 40°C and 60°C, and after only 20 minutes at 80°C.
• Peroxidase (POD): This enzyme is far more robust. After 30 minutes at 60°C, POD still retains 65% of its activity (losing only 35%). Even at 80°C, a temperature that rapidly destroys PPO, 6% of POD activity can remain after 45 minutes.

Furthermore, the thermal inactivation process itself is not a simple first-order kinetic. Studies indicate that PPO inactivation, for example, occurs in two phases, suggesting there is a heat-labile form that denatures quickly and a more heat-stable form that requires more energy to inactivate.

Table 1: Comparative Thermal Inactivation of PPO and POD at Various Temperatures

Time (minutes)	Temperature	PPO Activity Remaining	POD Activity Remaining	Source
20	80 °C	0% (Entirely Inactivated)	> 6%
30	60 °C	0% (Entirely Inactivated)	65%
30	40 °C	0% (Entirely Inactivated)	65%
45	80 °C	0%	6%

This data generates two critical conclusions. First, it explains the "residual enzyme" hypothesis for green tea degradation. A fixation process that is too short or at too low a temperature may successfully denature all PPO but leave a significant amount of heat-stable POD intact. This residual POD activity is a primary culprit in the degradation of green tea quality during storage, causing it to slowly brown and lose its fresh flavor. This is why Sha Qing must be aggressive enough to conquer the much-tougher POD enzyme.

Second, this differential resistance provides the biochemical engine for aged teas like Sheng (raw) Pu-erh and white tea. Sheng Pu-erh, for example, undergoes a low-temperature fixation where the leaf temperature is only brought to 60–75°C. White tea is "fixed" not by heat but by slow dehydration, also a low-temperature process. Based on the kinetic data, these temperatures are high enough to denature most of the PPO (preventing the leaf from immediately turning black) but are insufficient to fully denature the heat-stable POD. This intentionally leaves a residual enzymatic engine in the leaf, which, along with microbial fermentation, drives the slow, multi-decade oxidation and transformation of these aged teas. The "kill-green" for these teas is, by design, a partial kill.

2. The Classic Dichotomy: A Comparative Analysis of Pan-Firing and Steaming

The two most traditional and culturally significant methods of fixation are pan-firing, championed by China, and steaming, which defines Japanese green tea processing. The choice between these two methods creates the fundamental taste and aroma divide in green tea.

2.1. Pan-Firing (Chinese Method): Technique, Parameters, and Sensory Impact

Pan-firing is the quintessential Chinese Sha Qing method. It involves heating the leaves in a large, hot wok or a rotating, heated drum. This is fundamentally a dry heat method of fixation.

2.1.1. The Maillard Reaction: Generating Nutty, Toasty, and Roasted Aromas

The defining characteristic of pan-firing is its promotion of the Maillard reaction. This is a form of non-enzymatic browning that occurs under dry heat, a chemical reaction between the amino acids and reducing sugars naturally present in the tea leaf. This reaction is the same one responsible for the flavors of seared steak, toasted bread, and roasted coffee.

In tea processing, the Maillard reaction is creative. It transforms the leaf's chemical profile, breaking down some compounds to build new, complex flavor compounds. This process is responsible for the characteristic nutty, toasty, roasted chestnut, and caramel-like flavors that define Chinese green teas. The heat also breaks down L-theanine, a key amino acid, contributing to these caramel tones. This method simultaneously volatilizes and removes low-boiling compounds associated with "grassy" aromas, while creating and locking in the more desirable high-boiling, "chestnut-like" aromas.

2.1.2. Process Parameters: Wok vs. Drum Temperatures and Durations

The parameters for pan-firing are highly variable, but they generally involve very high temperatures for a short duration.

• Artisanal Wok: For Sha Qing, wok temperatures can range from 250–300°C, with the process lasting 3–5 minutes.
• Mechanized Drum: Parameters are highly specific. For example, Longjing (Dragon Well) may require a leaf temperature of 150–160°C, while Biluochun is processed at 140–150°C.

A critical operational principle is: "young leaves with high temperature and faster, old leaves with lower temperature and slower". This is a matter of thermal physics. Young, tender leaves can be fixed quickly at high heat. Older, thicker leaves require a lower temperature and slower speed; if the heat is too high, the outside of the leaf will scorch and burn long before the heat can penetrate to the center to denature the enzymes, resulting in a useless, "case-hardened" leaf that is burnt on the outside and raw on the inside.

2.1.3. Artisanal Hand-Firing vs. Mechanized Drum Firing: A Cost-Benefit Analysis

The choice between traditional hand-firing and modern machine-firing represents a classic quality vs. scalability trade-off.

• Artisanal Hand-Firing: Capable of producing the highest quality green tea with a "more fragrant" taste. However, it requires immense, "guru-level" skill and is highly inconsistent, as temperature is difficult to control and quality degrades as the processor fatigues.
• Mechanized Drum Firing: Offers precise control over temperature and rotation speed, resulting in high consistency and significant labor cost savings. However, large-scale, continuous-feed drums, while efficient, tend to produce a lower quality of tea.

2.2. Steaming (Japanese Method): Technique, Parameters, and Sensory Impact

Steaming is the predominant method used in Japan and is the defining feature of most Japanese green teas. This process involves exposing the leaves to high-heat steam, typically at or near 100°C. This is a wet heat method of fixation.

2.2.1. Preservation of Chemistry: The "Grassy" and "Marine" Profile

The steaming process is defined by its speed. As a wet heat method, steam transfers thermal energy to the leaves much more quickly and efficiently than the dry heat of a wok. The leaf temperature reaches the denaturation threshold very rapidly.

Benefit: This extreme speed prevents the Maillard reaction from occurring. Unlike pan-firing, steaming is a preservative process. It does not create new flavors; it locks in the chemical profile of the fresh leaf.

Result:

• Color: Chlorophyll is preserved, resulting in a vibrant, bright green leaf and a brilliant green liquor.
• Aroma: The original volatile compounds are preserved, including dimethyl sulphide, which imparts the characteristic grassy, vegetal, and marine (seaweed) aromas of Japanese tea.
• Health: This rapid, efficient inactivation is excellent at preserving a high percentage of the leaf's original catechin (antioxidant) content.

Disadvantage: The resulting tea lacks the "diverse aroma" and deep, roasted notes created by pan-firing. The flavor profile is preserved, not created.

2.2.2. A Spectrum of Steam: Analyzing Asamushi, Chumushi, and Fukamushi

In Japanese tea, the single most important processing variable is not temperature (which is constant at ~100°C) but duration. The length of the steam blast fundamentally alters the tea's character, creating a spectrum of styles.

• Asamushi (Light-Steamed): A very short steaming, typically 20–40 seconds. The leaves remain whole and intact, often appearing as long, glossy, dark green needles. The liquor is a clear, light-yellow-green, and the taste is light, grassy, and often more astringent.
• Chumushi (Middle-Steamed): The middle ground, with steaming from 40–80 seconds. It provides a balance of the two extremes.
• Fukamushi (Deep-Steamed): A long steaming, ranging from 80–200 seconds, or more commonly 60–90 seconds. This extended steam time physically breaks down the leaf's cellular structure, causing the leaves to become soft, brittle, and break apart into small, matte-textured particles.

2.2.3. The Fukamushi Effect: Impact on Infusion, Umami, and Commercial Strategy

The Fukamushi style is not just a chemical change; it is a physical one. This physical breakdown leads to three cascading effects:

1. Faster Infusion: The broken, smaller particles have a much greater surface area, allowing them to release their contents almost instantly.
2. Cloudy Liquor: The fine leaf particles become suspended in the water, creating a "murky," opaque, deep green liquor.
3. Altered Taste Profile: Because the cell structure is thoroughly broken down, more chemical compounds are released into the cup. This results in a bolder, richer, and sweeter taste with more umami (from amino acids) but less astringency (as the umami and sweetness mask the bitterness).

This "Fukamushi effect" has a significant commercial application. Producers can apply this deep-steaming technique to later-harvest, lower-grade, or more robust cultivars, which would normally be unpleasantly bitter. The fukamushi process breaks down the leaf, masks the astringency, and creates a highly palatable, rich, and umami-forward brew. It is a processing technology that can "fix" or elevate less-desirable terroir, which explains why its production volume in Japan now reportedly exceeds that of asamushi.

Table 2: Comparison of Traditional Fixation Methods: Pan-Firing vs. Steaming

Feature	Pan-Firing (e.g., Chinese)	Steaming (e.g., Japanese)
Heat Type	Dry Heat	Wet Heat
Key Reaction	Maillard Reaction (Creative)	Preservation (Inhibitive)
Parameters	High Temp (e.g., 150-300°C), Short Duration	100°C, Variable Duration (20-200s)
Final Color	Yellow-green leaf, yellow liquor	Bright green leaf, vibrant green liquor
Aroma Profile	Toasty, nutty, roasted, chestnut	Grassy, vegetal, marine, seaweed
Taste Profile	Roasted, mellow, sweet	Umami, bright, astringent (if asamushi)

Table 3: Japanese Steaming Parameters and Resulting Profiles

Style	Steaming Time (seconds)	Leaf Integrity	Liquor Appearance	Key Taste Profile
Asamushi (Light)	20–40	Whole, intact, glossy needles	Clear, light yellow-green	Light, grassy, vegetal, tannic, astringent
Chumushi (Middle)	40–80	Partially broken	-	Balanced
Fukamushi (Deep)	80–200	Broken, small, matte particles	Opaque, "murky," deep green	Rich, bold, sweet, "full throttle," high umami, low astringency

3. Modern and Industrial Fixation Methodologies: Beyond Tradition

Beyond the classic wok and steamer, modern industrial tea processing employs a range of technologies to achieve fixation, each with a different balance of scalability, cost, and impact on the final product.

3.1. Hot Air and Heated Tumbling

This is a common, highly scalable industrial method that uses controlled hot-air systems, oven-like machines, or large, heated, rotating tumblers to apply dry heat. This method is very frequently used to halt the oxidation of oolong teas at the precise desired level.

• Advantages: This method is highly scalable, controllable, and adaptable to large volumes of tea.
• Disadvantages: It can suffer from uneven heating. Hot air fixation can "make the edge of the leaves dry". This creates the same problem seen in pan-firing older leaves: the outside of the leaf can scorch or dry before the heat has fully penetrated the center, leading to an inconsistent product that is both burnt and incompletely fixed.

3.2. Microwave Fixation: An Analysis of Speed, Efficiency, and Quality

Microwave fixation is a modern, high-tech solution that uses dielectric heating. Unlike conventional methods that heat from the outside-in (conduction/convection), microwaves are a form of non-ionizing radiation that agitates water molecules within the leaf, generating heat volumetrically—both inside and outside simultaneously.

• Advantages: This method is extremely rapid and provides a more consistent and uniform heating of the entire leaf mass. It is also automated, reducing labor costs, and is often cited as more environmentally friendly.
• Impact on Chemistry and Taste: Microwave fixation has been found to raise the amino acid content and lower the phenol/ammonia ratio, resulting in a less bitter and more umami finished product, with some research suggesting "superior aroma and taste outcomes".
• Disadvantages: This technology presents a major strategic conflict. Tea polyphenols (catechins) are prone to thermal degradation. One study found that microwave-fixed green tea had the lowest presence of these polyphenols when compared to steamed, wok-fired, or tumbler-heated teas. For the producer, this is a near-perfect solution (cheaper, faster, better taste). For the health-conscious consumer, it may fail to deliver on the perceived antioxidant benefit.

3.3. Emerging Dielectric Methods: Radio-Frequency (RF) Heating

Similar to microwave heating, radio-frequency (RF) heating is another rapid, uniform, volumetric heating method. It is noted in research as being more cost-effective and energy-efficient. RF treatment has been shown to efficiently inactivate enzymes (like POD in lettuce) with less cell damage and better preservation of physiochemical properties like color and vitamin C. This suggests RF could be a superior future technology, potentially offering the speed and cost-savings of dielectric heating without the significant polyphenol degradation observed in microwave fixation.

3.4. Atypical Thermal Methods: The Impact of Hot Water Blanching

Blanching, or scalding the leaves with boiling water, is an uncommon but studied method of fixation. A 2024 study comparing steam-roasting, pan-firing, and blanching (B-2) produced a "most significant" finding: the blanching process resulted in a 47–52% reduction in caffeine levels compared to the other methods.

• Mechanism: This is a simple leaching effect. Caffeine is highly water-soluble, and the hot water bath effectively "washes" it out of the leaf.
• Key Finding: This dramatic reduction in caffeine was achieved without affecting the total catechin content or antioxidant potential of the tea.

This discovery has profound commercial implications. This research shows that a simple, low-cost blanching step can function as a "natural" decaffeination method while preserving the catechins (like EGCG) that are the primary selling point for green tea's health benefits.

4. Fixation as the Defining Variable: Role and Timing Across Tea Categories

The application, timing, and degree of fixation is the primary processing variable that defines the six major categories of tea.

4.1. Green and Yellow Tea: Immediate and Total Enzymatic Deactivation

For green tea, the goal is to prevent all oxidation. Fixation is therefore applied immediately after a very brief wither. The goal is 100% and immediate enzyme inactivation to lock in the "green" compounds like chlorophyll and the full spectrum of catechins. Yellow tea is processed similarly, but a slightly less-thorough fixation is followed by a "piling" or "sweltering" step, which allows a mild, slow, non-enzymatic oxidation to occur, yellowing the leaves and mellowing the taste.

4.2. Oolong Tea: Halting a Controlled, Partial Oxidation

Oolong tea's identity is defined by partial oxidation, with levels ranging from 8% to 85%. To achieve this, the leaves are withered and then "bruised" or "shaken"—a process that intentionally breaks cell walls to initiate enzymatic oxidation. The leaves are then left to oxidize. Fixation (typically in a heated tumbler) is applied at the precise moment the tea master determines the desired oxidation level has been reached. In this context, fixation is not an immediate "stop" but a carefully timed "brake" applied in the middle of a controlled reaction.

4.3. Black Tea: Fixation via Drying (The Absence of Sha Qing)

Black tea has no "kill-green" step. The goal is full enzymatic oxidation. After withering, the leaves are aggressively macerated (e.g., in a Crush, Tear, Curl or "CTC" machine) to maximize the exposure of enzymes to oxygen and catechins. The leaves are then left to oxidize completely, transforming their catechins into theaflavins and thearubigins. The oxidative enzymes are only "killed" (denatured) at the very end of the process, during the final drying or firing stage.

4.4. White Tea: Atypical Deactivation by Dehydration

White tea, like black tea, does not have a distinct Sha Qing step. It is considered the "least processed" tea. The leaves are simply withered for a long period and then dried. The enzymes are not denatured by a blast of heat. Instead, enzyme activity is slowly inhibited and eventually halted by the gradual removal of moisture (dehydration) during the final drying process. As established in section 1.4, this gentle, slow dehydration is an incomplete fixation, leaving residual enzymes (like POD) active, which is why white tea can be aged and will slowly darken over many years.

4.5. Pu-erh Tea: The Criticality of Incomplete Fixation

Pu-erh tea presents the most complex and nuanced application of Sha Qing.

4.5.1. Sheng (Raw) Pu-erh: Low-Temperature Sha Qing to Enable Long-Term Aging

Sheng Pu-erh does undergo a Sha Qing step, typically pan-firing. However, the temperature is deliberately lower than for green tea.

• Green Tea: Wok temperature ≈ 300°C (leaf temperature > 80°C).
• Sheng Pu-erh: Wok temperature ≈ 200°C (leaf temperature 60–75°C).

This temperature difference is the entire point. A leaf temperature of 60–75°C is insufficient to fully denature the highly heat-stable POD enzyme. This partial fixation deactivates enough PPO to keep the leaf "green" (preventing it from becoming black tea) but intentionally leaves the POD "engine" intact. This residual enzymatic activity, combined with a rich microbial flora, allows the tea to undergo slow microbial fermentation and oxidation for decades, transforming its flavor from vegetal and astringent to smooth, earthy, and complex.

This process is combined with sun-drying (rigan). This gentle drying method (versus the hot-air drying for green tea) is crucial for preserving the enzymes and microbes that are necessary for the aging process.

4.5.2. Shou (Ripe) Pu-erh: Steaming as a Post-Fermentation Step

Shou Pu-erh starts as maocha (the partially-fixed Sheng leaf). It then undergoes Wo Dui (Wet Piling), an accelerated microbial fermentation where leaves are piled, moistened, and covered for 45–65 days. After this fermentation is complete, the leaves are steamed. This final steaming acts as a second fixation, this time to kill the active microbial culture, halt the fermentation, and stabilize the final, dark, earthy product before it is compressed into cakes.

Table 4: The Role and Timing of Fixation Across Six Tea Categories

Tea Category	Distinct Fixation (Sha Qing) Step?	Timing of Fixation	Goal of Fixation
Green Tea	Yes	Immediate (post-wither)	Total and immediate enzyme inactivation
Yellow Tea	Yes	Immediate	Partial enzyme inactivation, followed by sweltering
Oolong Tea	Yes	Delayed (post-oxidation)	Total inactivation, timed to stop partial oxidation
Black Tea	No	N/A (Final drying)	Full oxidation; enzymes killed during final drying
White Tea	No	N/A (Final drying)	Mild oxidation; enzymes killed by slow dehydration
Sheng Pu-erh	Yes	Immediate	Incomplete inactivation (low temp) to allow aging

5. Non-Thermal and Specialized Processes: The Case of GABA Tea

A common misconception exists regarding "non-thermal fixation," often citing the production of GABA tea. However, a closer analysis of the process reveals this is a misinterpretation.

5.1. Anaerobic (Nitrogen) Treatment for Gamma-Aminobutyric Acid (GABA) Accumulation

GABA (Gamma-Aminobutyric Acid) tea is a product developed in Japan in 1987. Its production involves a unique additional step, not a replacement for fixation.

• Mechanism: After withering, fresh leaves are placed in an anaerobic (oxygen-free) environment, typically a stainless steel drum that has been flushed with nitrogen gas.
• Biochemistry: The leaves are held in this anaerobic state for 6–10 hours. This oxygen-deprived stress causes a metabolic shift within the still-living leaf, triggering the conversion of its natural glutamic acid into GABA. This process, which can also be done with carbon dioxide, also increases other amino acids like alanine.

5.2. The Overlooked Role of Heat in Finalizing GABA Tea Production

The anaerobic treatment is not fixation; it is an anaerobic fermentation or metabolic treatment. The leaves are still "live" and are metabolically reacting to their environment. After the desired GABA level is reached, the leaves are still "green" and full of active enzymes; they must be stabilized.

The full GABA tea process is: Wither → Anaerobic (N2) Treatment → Panning (Thermal Fixation) → Rolling → Drying (Thermal). In fact, a final "firing" (roasting) at 110–120°C is often required to remove undesirable "anaerobic" odors that are created during the nitrogen treatment. Therefore, GABA tea is not an example of non-thermal fixation. It is a prime example of a pre-fixation anaerobic treatment used to accumulate specific compounds, which is then followed by a traditional thermal fixation.

6. A Comparative Synthesis: The Chemical and Sensory Consequences of Fixation

The choice of fixation method is a decision with cascading, irreversible consequences for the tea's final chemical and sensory profile.

6.1. Impact on Non-Volatile Compounds: A Comparative Study

• Catechins (e.g., EGCG): Steaming is excellent at preserving catechins. Pan-firing can cause some degradation. Blanching successfully preserves catechin content. Microwave appears to cause the most thermal degradation of polyphenols.
• Amino Acids (e.g., L-Theanine, Umami): Steaming (especially Fukamushi) is associated with high levels of theanine. Pan-firing degrades some L-theanine as part of the Maillard reaction. Microwave methods excel at creating or preserving amino acids, leading to higher umami and sweet tastes.
• Caffeine: Caffeine is relatively heat-stable and is largely preserved by both steaming and pan-firing. Blanching is the exception, as the hot water leaching drastically reduces caffeine content by 47–52%.

Table 5: Comparative Chemical Composition of Teas by Fixation Method

Fixation Method	Catechin / Polyphenol Content	Amino Acid / Umami Content	Caffeine Content	Key Sources
Pan-Firing	High (some epimerization)	Medium (some degradation via Maillard)	High
Steaming (Asamushi)	Very High (Preserved)	High (Preserved)	High
Steaming (Fukamushi)	High (some degradation)	Very High (High release)	High
Microwave	Lowest (High thermal degradation)	Highest (High protein degradation)	High
Blanching (Hot Water)	High (Preserved)	High (Preserved)	Significantly Reduced (47-52%)

6.2. Impact on Volatile Aroma Compounds (VACs) and Final Sensory Profile

• Pan-firing: This is a creative process. It actively destroys low-boiling "green" volatiles while simultaneously creating new, complex, high-boiling "toasty" and "nutty" volatiles via the Maillard reaction.
• Steaming: This is a preservative process. It retains the original chemical profile, leading to "green," "grassy," and "marine" aromas.
• Light-wave: This experimental method is reported to produce a "clear aroma" and a "fresh, mellow taste".
• Microwave: Research suggests "superior aroma and taste outcomes," likely by preserving the umami-rich amino acids while avoiding the "scorched" or "burnt" notes that can result from improper pan-firing.

6.3. Concluding Analysis: Fixation as a Transformative, Not Destructive, Process

The analysis of Sha Qing reveals that "fixation" is a misnomer. It is not merely an "off-switch" or a destructive step to halt oxidation. It is, in fact, a high-temperature, transformative biochemical event that is one of the most creative and defining stages in tea manufacturing.

The choice of fixation method (wet, dry, dielectric) and its parameters (temperature, duration) is a deliberate, creative decision that directly determines the tea's final character.

• The choice of how to fix (e.g., pan-fire vs. steam) determines the fundamental aroma and flavor profile.
• The choice of when to fix (e.g., immediate for green vs. delayed for oolong) determines the tea's entire category.
• The choice of if to fix (e.g., green vs. black) defines the tea.
• And finally, the choice of how completely to fix (e.g., the total kill for green tea vs. the partial, incomplete kill for Sheng Pu-erh) determines its entire future: a short life of freshness followed by decay, or a long life of managed transformation and aging.

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Works Cited

1. Fixation (Kill-Green) in Tea Production - Taiwanleaftea.com, accessed November 12, 2025, https://taiwanleaftea.com/info/kill-green-in-tea-production
2. Biochemical Components Associated With Microbial Community Shift During the Pile-Fermentation of Primary Dark Tea - Frontiers, accessed November 12, 2025, https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2018.01509/full
3. Kill-Green in Tea Production - Tea Epicure, accessed November 12, 2025, https://teaepicure.com/killing-the-green/
4. Extraction of ingredients from tea leaves using oxidative enzymatic ..., accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC7892889/
5. Pan-Fired vs Steamed Green Tea - Sugimoto Tea Company, accessed November 12, 2025, https://www.sugimotousa.com/blog/pan-fired-vs-steamed-green-tea
6. Asamushi, Fukamushi, Chumushi: what's the difference between these sencha?, accessed November 12, 2025, https://camellia-sinensis.com/en/blog/asamushi-fukamushi-chumushi-whats-the-difference-b
7. How Tea Is Made: Fixing - Killing the Green - Tea for Me Please, accessed November 12, 2025, https://www.teaformeplease.com/how-tea-is-made-fixing/
8. Microwave Fixation - The Future of Green Tea? – One River Tea, accessed November 12, 2025, https://onerivertea.com/blogs/one-river-tea/microwave-fixation-the-future-of-green-tea
9. RF heating treatment of food and their reductions in microorganisms - ResearchGate, accessed November 12, 2025, https://www.researchgate.net/figure/RF-heating-treatment-of-food-and-their-reductions-in-microorganisms_tbl2_379726515
10. Comparative evaluation of the impact of processing methods in ..., accessed November 12, 2025, https://maxapress.com/article/doi/10.48130/bpr-0024-0016
11. Tea Processing Steps: Eli Tea Processing Chart | ELITEA | Loose Leaf | Chai, accessed November 12, 2025, https://www.eliteabar.com/blogs/tea-education/7780719-tea-processing
12. GABA tea - Wikipedia, accessed November 12, 2025, https://en.wikipedia.org/wiki/GABA_tea
13. How Is White Tea Made? A Look At The Production Process - ICHA TEA, accessed November 12, 2025, https://ichateashop.com/blogs/news/how-is-white-tea-made-production-process
14. Drying process that affects the flavor of raw pu-erh tea - HOJO, accessed November 12, 2025, https://hojotea.com/en/posts-112/
15. Exploring Flavour 2 : Why different teas have the flavours they do - Comins Tea, accessed November 12, 2025, https://cominstea.com/blogs/news/exploring-flavour-2-why-different-teas-have-the-flavours-they-do
16. Stress-Relieving Effects of Japanese Green Tea: Evaluation Using ..., accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC11720457/
17. The manufacturing process provides green teas with differentiated nonvolatile profiles and influences the deterioration of flavor during storage at room temperature - NIH, accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC11021834/
18. Chemical Composition of Green Teas According to Processing Methods and Extraction Conditions | Request PDF - ResearchGate, accessed November 12, 2025, https://www.researchgate.net/publication/279710487_Chemical_Composition_of_Green_Teas_According_to_Processing_Methods_and_Extraction_Conditions
19. Several Methods Of Tea Fixation Processing, accessed November 12, 2025, https://www.delijx.com/faqs/several-methods-of-tea-fixation-processing.html
20. Raw Pu-erh Tea vs Green Tea: Unveiling Differences - Path of Cha, accessed November 12, 2025, https://pathofcha.com/blogs/all-about-tea/raw-pu-erh-tea-vs-green-tea-unveiling-differences
21. When Does Green Tea Become Sheng Pu'erh? : r/puer - Reddit, accessed November 12, 2025, https://www.reddit.com/r/puer/comments/11g6873/when_does_green_tea_become_sheng_puerh/
22. Tea Processing Step: Oxidation, accessed November 12, 2025, https://teaepicure.com/tea-leaves-oxidation/
23. “Kill Green”: A Fascinating Tea Process You Should Know About, accessed November 12, 2025, https://blog.denstea.com/kill-green-a-fascinating-tea-process-you-should-know-about/
24. How Is Oolong Tea Made? From Leaf to Cup - Tealeavz, accessed November 12, 2025, https://tealeavz.com/blogs/tea-talk/how-is-oolong-tea-made
25. Tea processing - Wikipedia, accessed November 12, 2025, https://en.wikipedia.org/wiki/Tea_processing
26. Recent Advances Regarding Polyphenol Oxidase in Camellia sinensis: Extraction, Purification, Characterization, and Application - NIH, accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC10887689/
27. (PDF) Impact of moisture loss and temperature on biochemical changes during withering stage of black tea processing on four Tocklai released clones - ResearchGate, accessed November 12, 2025, https://www.researchgate.net/publication/275038498_Impact_of_moisture_loss_and_temperature_on_biochemical_changes_during_withering_stage_of_black_tea_processing_on_four_Tocklai_released_clones
28. Effect Of Withering Temperature and Time on Biochemical Properties of Vegetatively Propagated and Seedling Tea - Juniper Publishers, accessed November 12, 2025, https://juniperpublishers.com/nfsij/pdf/NFSIJ.MS.ID.555814.pdf
29. Tea Harvesting and Processing Techniques and Its Effect on Phytochemical Profile and Final Quality of Black Tea: A Review - PMC, accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC10743253/
30. Thermal inactivation kinetics of polyphenol oxidase in Tea (Camellia ..., accessed November 12, 2025, https://teascience.in/index.php/ijts/article/download/287/228
31. White Tea Processing Techniques - esgreen, accessed November 12, 2025, https://esgreen.com/blogs/info/white-tea-processing-techniques
32. White tea processing : r/tea - Reddit, accessed November 12, 2025, https://www.reddit.com/r/tea/comments/j8nxcf/white_tea_processing/
33. What to Look for When Aging Puerh Tea, accessed November 12, 2025, https://www.teaformeplease.com/aging-puerh-tea/
34. accessed November 12, 2025, https://www.reddit.com/r/tea/comments/4phthh/what_does_the_aging_process_do_to_puerh/#:~:text=all%20the%20things%20happening%20in,continues%20to%2imperfect%20oxidation.
35. Behind the scenes of Oolong tea taste and aroma - Path of Cha, accessed November 12, 2025, https://pathofcha.com/blogs/all-about-tea/behind-the-scenes-of-oolong-tea-taste-and-aroma
36. Aroma-active compounds related to Maillard reaction during roasting in Wuyi Rock tea, accessed November 12, 2025, https://www.researchgate.net/publication/364078977_Aroma-active_compounds_related_to_Maillard_reaction_during_roasting_in_Wuyi_Rock_tea
37. Steamed vs. Pan-Fired Green Tea: How Processing Shapes Flavor and Quality - NAM SON, accessed November 12, 2025, https://nam-son.com/steamed-vs-pan-fired-green-tea-how-processing-shapes-flavor-and-quality/
38. Insights into the effects of fixation methods on the sensory quality of ..., accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC10740047/
39. Traditional Chinese Method of Green Tea Manufacture: A Step-by-Step Guide, accessed November 12, 2025, https://agriculture.institute/post-harvest-mgt-value-addition/traditional-chinese-green-tea-manufacture-guide/
40. How Fixation Time Shapes Green Tea Quality, accessed November 12, 2025, https://www.delijx.com/new/How-Fixation-Time-Shapes-Green-Tea-Quality-A-Practical-Guide-for-Tea-Lovers.html
41. Hand Processing vs. Machine Processing - Mauna Kea Tea, accessed November 12, 2025, https://maunakeatea.com/hand-processing-vs-machine-processing/
42. Green Tea Blog | Steaming - KEIKO, accessed November 12, 2025, https://www.keiko.de/en/blog/steaming-of-green-tea-how-is-it-done-and-why
43. What is the difference between pan-fired and steamed green teas? - Fusion Teas Blog, accessed November 12, 2025, https://blog.fusionteas.com/what-is-the-difference-between-pan-fired-and-steamed-green-teas/
44. Steaming vs fermenting vs Roasting for Japanese tea, accessed November 12, 2025, https://www.shizuokatea.com/blog/steaming-vs-fermenting-vs-roasting-for-japanese-tea/
45. The Green Tea Steaming Process, accessed November 12, 2025, https://www.japanesegreenteain.com/blogs/green-tea-and-health/what-is-the-green-tea-steaming-process
46. The Importance of Steaming in Japanese Green Tea - Mizuba Tea Co., accessed November 12, 2025, https://mizubatea.com/blogs/news-1/the-importance-of-green-tea-steaming
47. Effects of Brewing Conditions on the Antioxidant Capacity of Twenty-four Commercial Green Tea Varieties - PMC - NIH, accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC4549831/
48. Dynamic Changes in Non-Volatile Components during Steamed ..., accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC10094149/
49. Do you prefer light, medium, or deep steamed sencha? • TeaForum.org, accessed November 12, 2025, https://www.teaforum.org/viewtopic.php?t=951
50. Taste profiles of various sencha steam levels - Steepster, accessed November 12, 2025, https://steepster.com/discuss/5201-taste-profiles-of-various-sencha-steam-levels
51. Japanese Tea Steeping Techniques - Yunomi.life, accessed November 12, 2025, https://yunomi.life/pages/japanese-tea-steeping-techniques
52. Fukamushi sencha and how it differs from a regular sencha - Yunomi.life, accessed November 12, 2025, https://yunomi.life/blogs/japanese-tea-guide/fukamushi-sencha-and-how-it-differs-from-a-regular-sencha
53. Effect of Fixation Methods on Biochemical Characteristics of Green Teas and Their Lipid-Lowering Effects in a Zebrafish Larvae Model - PMC - NIH, accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC9180411/
54. Tea 102: Tea Processing - The Whistling Kettle, accessed November 12, 2025, https://www.thewhistlingkettle.com/blogs/tea-university/tea-processing
55. The Beginner's Guide To Tea Processing Methods - Fellow, accessed November 12, 2025, https://fellowproducts.com/blogs/learn/beginners-guide-to-tea-processing-methods
56. Domestic Microwave Versus Conventional Tissue Processing: A Quantitative and Qualitative Analysis - PMC - NIH, accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC3681050/
57. Microwave-assisted tissue processing, fixation and staining in tissues of different thicknesses: A comparative study - PMC, accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC7269310/
58. Inactivation of Endogenous Pectin Methylesterases by Radio Frequency Heating during the Fermentation of Fruit Wines - MDPI, accessed November 12, 2025, https://www.mdpi.com/2311-5637/8/6/265
59. Effects of Radio Frequency Heating on the Stability and Antioxidant Properties of Rice Bran, accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC8069604/
60. Radio frequency energy inactivates peroxidase in stem lettuce at different heating rates and associate changes in physiochemical properties and cell morphology - PubMed, accessed November 12, 2025, https://pubmed.ncbi.nlm.nih.gov/33127230/
61. Turning green tea into black tea? : r/tea - Reddit, accessed November 12, 2025, https://www.reddit.com/r/tea/comments/q4undc/turning_green_tea_into_black_tea/
62. A Complete Guideline for Oolong Tea, accessed November 12, 2025, https://muayiktea.com/news/oolong-tea-guideline
63. What is Fixation or Kill-Green in Tea Processing? - The Path of Tea, accessed November 12, 2025, https://thepathoftea.com/what-is-fixation-or-kill-green-in-tea-processing/
64. Effects of Three Different Withering Treatments on the Aroma of White Tea - PMC - NIH, accessed November 12, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC9407123/
65. What Is White Tea Drying?, accessed November 12, 2025, https://www.delijx.com/new/what-is-white-tea-drying.html
66. Dark teas and their Most Common Production Processes - alveus.eu, accessed November 12, 2025, https://www.alveus.eu/blog/dark-tea-process
67. Pu-erh Cake Tea process – teavivre, accessed November 12, 2025, https://www.teavivre.com/info/puerh-cake-tea-process.html
68. Pu Erh Tea: From Yunnan to the whole world - Teewald, accessed November 12, 2025, https://teewald.com/blogs/tee-kultur/pu-erh-tee-von-yunnan-in-die-ganze-welt
69. What does the aging process do to pu-erh? : r/tea - Reddit, accessed November 12, 2025, https://www.reddit.com/r/tea/comments/4phthh/what_does_the_aging_process_do_to_puerh/
70. Effect of drying on the quality of raw tea of Pu'er tea - ResearchGate, accessed November 12, 2025, https://www.researchgate.net/publication/353058875_Effect_of_drying_on_the_quality_of_raw_tea_of_Pu'er_tea
71. The Art of Pu Erh Tea Processing: An Insiders Guide | Teasenz.eu, accessed November 12, 2025, https://teasenz.eu/blogs/pu-erh-tea-101/processing
72. Understanding the Aging Process of Pu Erh Tea: How Time Changes the Flavor, accessed November 12, 2025, https://cspuerh.com/blogs/tea-101/understanding-the-aging-process-of-pu-erh-tea-how-time-changes-the-flavor
73. What is Pu-erh Tea? Part 3 - The science of tea fermentation - Happy Earth Tea, accessed November 12, 2025, https://happyearthtea.com/blogs/tea-101-puerh-tea/what-is-pu-erh-tea-part-3
74. GABA Oolong Tea - Teavivre, accessed November 12, 2025, https://www.teavivre.com/info/gaba-oolong.html
75. Anaerobically Treated Tea and Its Hypotensive Effects, accessed November 12, 2025, https://www.jircas.go.jp/sites/default/files/publication/jarq/24-2-105-110_0.pdf
76. Comparative Metabolic Responses and Adaptive Strategies of Tea Leaves ( Camellia sinensis) to N2 and CO2 Anaerobic Treatment by a Nontargeted Metabolomics Approach - PubMed, accessed November 12, 2025, https://pubmed.ncbi.nlm.nih.gov/30133278/
77. Enhancement of Bioactive Components Content and the Antioxidant Activity of Green Tea after Continuous Anaerobic Incubation - CABI Digital Library, accessed November 12, 2025, https://www.cabidigitallibrary.org/doi/pdf/10.5555/20123246135
78. Taiwan GABA Oolong - Tea and Whisk, accessed November 12, 2025, https://teaandwhisk.com/products/gaba-oolong
79. Effects of anaerobic fermentation in a nitrogen atmosphere on bioactive compound content in Vietnamese GABA tea - ResearchGate, accessed November 12, 2025, https://www.researchgate.net/publication/327703175_Effects_of_anaerobic_fermentation_in_a_nitrogen_atmosphere_on_bioactive_compound_content_in_Vietnamese_GABA_tea
80. Effect of fixation methods and various clones of Camellia sinensis var. sinensis (L) properties and antioxidant activity of, accessed November 12, 2025, https://www.cabidigitallibrary.org/doi/pdf/10.5555/20230008069