Stone-Grinding — How Matcha Is Made
Traditional granite matcha millstones grind tencha at 30–40 grams per hour — a rate so slow that it prevents the frictional heat that would degrade L-theanine and chlorophyll. One millstone produces ~40g of matcha per hour at standard RPM.
| Measure | Value | Unit | Notes |
|---|---|---|---|
| Grinding rate (traditional granite millstone) | 30–40 | g/hour | At ~70–100 RPM; increasing speed degrades heat-sensitive compounds |
| Typical millstone diameter | 25–30 | cm | Heavier upper stone; granite preferred for hardness and low iron content |
| Target particle size | 5–10 | microns (μm) | Fine enough to suspend without settling; coarser particles produce gritty texture |
| Time to produce 100g of matcha (one millstone) | 2.5–3.3 | hours | 400g ceremonial batch = ~10–13 hours of grinding |
| Maximum safe grinding temperature | <40 | °C | Higher temperatures degrade L-theanine and chlorophyll; granite's thermal mass helps |
| Industrial ball mill comparison | 10–100× | faster | Ball mills grind faster but generate more heat; some quality loss for high-heat-sensitive compounds |
The stone grinding step is what makes matcha fundamentally different from any other form of green tea. It is also one of the slowest deliberate processes in food production — a single traditional millstone produces just 30–40 grams of matcha per hour, making it roughly 100× slower per gram than commercial wheat milling.
The Millstone Design
A traditional matcha millstone (usu) consists of two circular granite discs:
- Upper stone (uwa-ishi): Rotates, driven by a motor (historically by hand or water power)
- Lower stone (shita-ishi): Fixed; has a collection tray around the edge
Radial furrows carved into both surfaces direct the ground powder outward as the stones rotate. The gap between the stones is calibrated to approximately 10 microns — tight enough to achieve the target particle size (5–10μm) in a single pass.
Heat Management
Granite’s specific heat capacity and thermal conductivity are calibrated for this application by tradition and testing. The stone absorbs friction heat slowly and dissipates it into the surrounding air. Commercial operations sometimes climate-control the grinding room to 15–18°C to provide additional thermal buffering.
Ball Mill vs. Stone Mill
Industrial ball mills (ceramic balls in a rotating cylinder) can grind matcha 10–100× faster than stone mills. Modern food-safe ceramic ball mills can achieve particle sizes of 5–10μm with reasonable heat management using water-cooled jackets. Some commercial culinary-grade matcha is produced this way. The trade-off is: faster grinding means shorter time for particulate size distribution to equilibrate, and even with cooling, the high mechanical energy input produces more heat than stone grinding. Premium ceremonial matcha is exclusively stone-ground.
Related Pages
Sources
- Uchida S et al. (2012) — Stone grinding parameters for matcha quality. J Tea Science
- Japanese Ministry of Agriculture — Traditional Food Processing Techniques
- Yamamoto T et al. (2000) — Chemistry and applications of green tea. CRC Press
Frequently Asked Questions
Why is stone grinding so slow?
The 30–40 gram per hour grinding rate is not a limitation of tradition — it is a deliberate quality requirement. Faster grinding generates frictional heat that degrades L-theanine (which breaks down above 50°C), causes chlorophyll to convert to dull pheophytin (reducing color vibrancy), and produces irregular particle sizes. Granite's thermal mass absorbs heat slowly; the combination of stone material and low RPM keeps the grinding surface below 40°C.
Does the stone material matter?
Yes. Granite is preferred because it is extremely hard (resistant to wear that would contaminate the matcha), has low iron content (iron catalyzes oxidation), and has sufficient thermal mass to absorb frictional heat. Some producers use ceramic or synthetic millstones for cost reasons, but granite is the traditional and widely considered superior material. The pattern carved into the grinding surface (the furrows or 'fumi') also affects grinding efficiency and particle size distribution.
Can you make matcha at home with a stone grinder?
Yes, but home stone grinders are small (typically 15–20cm diameter) and slow — producing 10–20g per hour. Commercial grade matcha mills are larger and more precisely engineered. Home-ground matcha from good tencha is genuinely fresh and high quality, but the time investment (1 hour for 10–20g) makes it impractical for regular use. The primary benefit of home grinding is immediate freshness — matcha's aromatic volatile compounds are most concentrated right after grinding.