tea finishing fire (hi-ire)

Firing (hiire) is the finishing heat treatment that turns crude tea into finished tea, developing the aroma and taste distinctive of fired tea while drying it. Technical information from the Kochi Prefectural Agricultural Technology Center Tea Experiment Station gives the sequence for finishing locally produced normally steamed sencha: grading, sorting, firing, then blending. It describes firing grade A crude tea to preserve its superior appearance and inner quality (about 90°C for 30 minutes in a rotary-drum firing machine, or about 100°C for 20–30 minutes for a light roast aroma). For grades B and C, it describes firing that adds a medium to strong roast aroma (about 100°C for 30–40 minutes) to emphasize the taste and aroma distinctive of fired tea. Excessive firing is described as generating a burnt smell that masks sweet aroma.

The center evaluated grade A crude tea fired in a small rotating-drum firing machine adapted from a Collins roaster at combinations of tea temperature 90–120°C and 10–40 minutes. At 90°C for 10 minutes, its aroma score (on a 9-point scale) was 4, roast aroma 0, and burnt smell 0; at 120°C for 40 minutes, aroma was 9, roast aroma 4, burnt smell 1, sweetness of aroma 2, and sweetness of taste 3. Roast aroma and burnt smell both intensified with higher temperature and longer time. Grade C tea showed a similar trend, but its aroma score at 90°C for 10 minutes was only 2, below grade A (4) under the same conditions. The evaluation addressed sweetness of aroma, sweetness of taste, roast aroma, and burnt smell; the consulted source did not show a scale for bitterness itself.

In a separate 2014 trial, when honcha (the main sieved tea leaves) and powdered tea were fired separately, sensory judging (liquor color, aroma, and taste each out of 20 points, by consensus of 5 tea professionals) rated 100°C for 30 minutes in a shelf dryer highest at 60 points, above 90°C for 30 minutes (57) and 110°C for 30 minutes (54). When honcha and powdered tea (10% by weight) were fired together, 90°C for 30 minutes ranked highest at 58 points, above 100°C for 30 minutes (54). Comparison of the 2 tables thus shows a different temperature for the top sensory rating depending on whether honcha is fired alone or together with powdered tea.

On heat sources, a tea merchant's explanatory article says from experience that charcoal gives 「香りが素直」 (in translation: "a straightforward aroma") and is 「面で温めやすい」 (in translation: "easy to heat across a surface"), but requires ventilation and ash management. The article also says that each 10°C rise in firing temperature increases aroma release and the likelihood of astringency. This is an experiential rule without measured data, and concerns astringency rather than bitterness itself. In hand-rolled tea making, registered as an Intangible Cultural Property in 2024, a tool called jotan (a wooden frame with washi on its underside) is placed over a drying furnace called a hoiro to transfer heat, and the leaves are dried while shaped with various hand movements. The Agency for Cultural Affairs account does not specify whether charcoal fuels the hoiro.