Descriptions of hojicha talk about how strong the fire is. Over a strong flame. High heat, all at once. Few of them give a number. The numbers are in the research, in a report that roasted stem tea at three temperatures.
What follows looks in turn at what was established, where the temperature was taken, the sample and conditions, the taste compounds, and how such numbers can be compared.
The hotter, the toastier
Inside the sound of a turning drum, the leaf shifts colour little by little. Within the range tested, the higher the heat, the more toasty compounds formed, and the colour moved from yellow-green towards yellow-red.
The toasty compounds all came out higher at the higher temperatures. The colour drifted from the green side towards the red. What was not established is how hot the leaf itself became. Where that temperature was taken is the next question.
What was hot was the inside of the drum
The inside of a turning drum gets too hot to hold a hand near. The report's temperature belongs to the inner surface of the drum, not to the leaf.
The researchers read the drum's inner surface with an infrared thermometer. Other reports measured in other places, and some do not say where at all. How the measuring points differ is set out in this section's table.
The sources tell us which part of the machine was measured, and that is where they stop. Next, the stems that were roasted and how.
Figure 5-4-1Roasting and hiire temperatures are measured in different places from source to source
| Source (year) | Temperature | What the temperature was measured on | Time |
|---|---|---|---|
| Shiono 1974, tray type | 65°C | exhaust temperature | 5 hours |
| Japan Tea Central Association, Green tea labelling standard, appendix | hiire 70–130°C / roasting 140°C or above | not stated | not stated |
| Shiono 1974, air-through type | 90–110°C | internal temperature | 20–25 minutes |
| Takahashi 2015 | around 100°C (80°C for higher grades) | not stated | 30–40 minutes |
| Kochi Prefecture (Table 1, 2013) | exhaust temperature 100°C at the second roasting (the same for light, medium and dark) | exhaust temperature | not stated |
| NARO 2015 | 100 / 115 / 130°C | leaf surface temperature | not stated |
| Kawakami 1999 | 150 / 130 / 100°C | not stated | 12 min / 10 min / 15 min |
| Shiono 1974, drum-over-fire type | 140–150°C | exhaust temperature | 40–60 minutes |
| Hara 1970 | 160–180°C | drum inner wall temperature | not stated |
| Sasaki et al. 2020 | 180 / 200 / 220°C | drum inner surface temperature (infrared thermometer) | 40–420 seconds |
| GJTA (an industry body) | 200°C | not stated | 10 minutes or more |