MATCHA CODEX — Part 14 of 30
110°C — The Golden Intersection of Fire and Flavor
Maillard reaction pyrazines, the pheophytinization curve, leaf vs stem thermal capacity, and why the sweet spot is exactly 110°C
Before the stone mill touches the dried tencha leaf, there is an optional but profoundly consequential step: hi-ire — finishing fire. A secondary heat treatment, typically conducted at around 110°C, that transforms the aromatic profile of the final matcha without — if executed correctly — sacrificing its defining green color. The temperature is not approximate. It sits at the exact intersection of two chemical curves moving in opposite directions, and understanding why requires mapping both.
1. The Maillard Reaction: Building Depth from Sugar and Amino Acid
The Maillard reaction is a cascade of non-enzymatic browning reactions between reducing sugars and free amino acids, initiated by heat. It is responsible for the flavor complexity of bread crust, roasted coffee, seared meat, and — at the temperatures encountered in hi-ire — the toasty, nutty, slightly caramel notes that distinguish fired matcha from unfired.
Pyrazine Formation at 110°C
Among the hundreds of compounds produced by Maillard chemistry, alkylpyrazines are the most significant for matcha's aromatic profile. These nitrogen-containing heterocyclic compounds contribute roasted, nutty, and warm aromatic notes. At 110°C, the early stages of the Maillard reaction generate methylpyrazine, 2,5-dimethylpyrazine, and 2,3,5-trimethylpyrazine — compounds that add aromatic depth without overwhelming the leaf's inherent vegetal and marine character.
Below 100°C, pyrazine generation is negligible. The activation energy for the Strecker degradation pathway — the Maillard sub-reaction most directly responsible for pyrazine precursors — is not reached. Above 120°C, pyrazine production accelerates dramatically, but so do Maillard browning products (melanoidins) and a competing degradation reaction that threatens matcha's most important visual characteristic.
2. Pheophytinization Returns: The Color Penalty Curve
Chlorophyll degradation — the conversion of vivid green chlorophyll to olive-brown pheophytin via displacement of the central Mg2+ ion — was the primary risk during steaming (Part 12). It returns during hi-ire with different kinetics.
In steaming, the reaction was driven by a combination of heat and the leaf's own organic acids in a high-moisture environment. In hi-ire, moisture content is approximately 5%, which changes the reaction dynamics. Low moisture reduces the mobility of H+ ions, slowing the Mg2+ displacement reaction. However, the thermal energy at 110°C compensates: above this temperature, even in dry conditions, pheophytinization resumes at analytically significant rates.
The Crossing Point
If you plot pyrazine generation rate on one axis and chlorophyll degradation rate on the other, both as functions of temperature, the curves cross near 110°C. Below this point, pyrazine generation climbs steeply while pheophytinization remains flat. Above it, pheophytinization begins an exponential ascent while additional pyrazine yield shows diminishing returns. The 110°C threshold is not merely optimal — it is the last temperature at which the aroma benefit of Maillard chemistry exceeds the color cost of chlorophyll loss.
This crossing point is what producers call the "golden intersection." It is the temperature where fire gives the most and takes the least.
3. Leaf vs Stem: Why Uniform Thermal Mass Matters
The hi-ire step is performed on tencha that has already been de-stemmed and de-veined during the raking and sifting process. This is not merely a purity measure — it is a thermal engineering requirement.
Thermal Capacity Differences
Stem tissue is denser and more fibrous than laminar leaf tissue. It has a higher specific heat capacity and lower thermal conductivity, meaning it absorbs heat more slowly and retains it longer. In a mixed batch of leaf and stem, the thin leaf fragments reach 110°C quickly and respond to the applied temperature in near-real-time. Stem fragments lag behind, reaching target temperature later and cooling more slowly after removal from the firing chamber.
The consequence is non-uniform thermal exposure. While leaf tissue spends the correct duration at 110°C, stem fragments spend a shorter time at target temperature but a longer time in the cooling phase, during which they continue to transfer heat to adjacent leaf fragments. The result: localized over-firing zones around stem material, producing patchy pheophytinization that appears as dull spots in the final powder.
De-Stemming as Thermal Quality Control
By removing stems before hi-ire, the producer ensures that the material entering the firing chamber has a uniform thermal mass. Every particle responds to the 110°C environment at the same rate, reaches the same peak temperature, and cools at the same speed. The pyrazine-to-pheophytin ratio is consistent throughout the batch. This is why de-stemming is not a cosmetic step but a prerequisite for precise firing.
4. Duration and Intensity: The Firing Spectrum
Hi-ire is not binary (fired or unfired). It exists on a spectrum of intensity, controlled by the combination of temperature and duration. Producers calibrate along this spectrum to match the intended character of the final matcha.
Light Fire (Asa-ire)
Brief exposure at 100–110°C produces minimal pyrazines. The leaf retains its raw, vegetal, intensely green character. This approach is typical for single-origin ceremonial matcha where the cultivar's intrinsic flavor is paramount and any roasted note would be considered an interference.
Medium Fire (Chu-ire)
Standard duration at 110°C produces a balanced pyrazine contribution. The matcha acquires a subtle warmth and nuttiness beneath its vegetal and marine notes, adding complexity without masking origin character. This is the most common treatment for premium ceremonial grades.
Strong Fire (Fuka-ire)
Extended duration or temperatures approaching 120°C produce pronounced roasty, toasty character. Pheophytinization becomes visible: the powder shifts from vivid green toward a warmer, slightly yellowed green. This treatment is used for specific blend components or for matcha intended for culinary applications where roasted notes complement the recipe context.
5. Interaction with DMS: The Furnace Legacy
DMS, the oceanic aroma compound generated in the tencha furnace (Part 13), has a boiling point of 37.3°C. Hi-ire at 110°C would seem to guarantee its total loss. However, the reality is more nuanced.
By the time tencha enters hi-ire, DMS is adsorbed within the dry cell wall matrix at approximately 5% moisture. The adsorption bonds — primarily van der Waals interactions between DMS and cell wall polysaccharides — require activation energy to break. At 110°C, some DMS is liberated, but the exposure is brief (typically 30–90 seconds at target temperature), and the dry matrix retains a significant fraction.
The DMS loss during hi-ire is a known and accepted trade-off. Producers calibrate firing intensity partly based on how much oceanic aroma they can afford to sacrifice in exchange for pyrazine depth. Matcha with minimal hi-ire retains maximum DMS; strongly fired matcha trades DMS for toast. The golden intersection at 110°C minimizes this loss while maximizing pyrazine yield — another dimension of the optimization that makes this temperature remarkable.
6. Fire as Final Signature
Hi-ire is the last transformation the tencha leaf undergoes before it enters the stone mill. It is the producer's final opportunity to shape the aromatic identity of the matcha. Everything after this point — milling, packaging, preparation — preserves or releases what already exists. Hi-ire creates.
The discipline of holding at 110°C, neither higher nor lower, reflects a broader principle that runs through every stage of matcha production: the best outcomes occur at the intersection of competing constraints. Steaming balances PPO deactivation against chlorophyll preservation. The furnace balances DMS generation against volatilization loss. And hi-ire balances Maillard complexity against chlorophyll integrity. At each step, the optimal solution is not a compromise but a precise point where maximum benefit from one reaction intersects minimum cost from another.
Frequently Asked Questions
Does all matcha undergo hi-ire firing?
No. Hi-ire is optional and its application depends on the producer's intent. Some single-origin ceremonial matchas skip hi-ire entirely (or use only a very light treatment) to preserve the pure cultivar character and maximum DMS aroma. Blended matchas more commonly use medium hi-ire to harmonize diverse cultivar contributions. Culinary grades may receive stronger firing to develop roasted notes suited to cooking applications.
Why are stems removed before firing rather than after?
Stems have higher density and thermal mass than leaf tissue, meaning they heat and cool at different rates. If stems remain during hi-ire, they create thermal inconsistencies: surrounding leaf fragments are over-exposed to heat during the cooling phase when stems continue radiating stored energy. This causes uneven pheophytinization — patchy color degradation visible in the final powder. Removing stems before firing ensures uniform thermal response across the entire batch.
What are pyrazines and how do they affect matcha's taste?
Pyrazines are aromatic compounds formed through the Maillard reaction between amino acids and reducing sugars under heat. In matcha, they contribute nutty, toasty, and subtly caramelized notes that add complexity and depth to the base vegetal and marine character. At the 110°C firing temperature, pyrazines round the raw green edge of steamed tencha without overpowering its inherent flavor, creating a more layered aromatic experience in the bowl.
