Most of the green paste served with sushi is seiyo wasabi, a mix of horseradish, mustard and coloring. This episode is about the real plant, Eutrema japonicum, and its unusual chemistry. The intact plant contains no heat at all. It stores a harmless glucosinolate and the enzyme myrosinase in separate compartments, and only when its cells are torn do they react to form allyl isothiocyanate, a volatile vapor that goes up the nose and hits the TRPA1 receptor.

That chemistry shapes everything in the kitchen. The flavor fades within 15 to 20 minutes, so chefs grate the rhizome on shark skin and tuck it between the rice and the fish. Growing it is just as demanding: sawa wasabi needs shade, air between 8 and 20 degrees Celsius, and cold, fast-flowing, oxygen-rich spring water in terraced tatami-ishi beds. With warming weather cutting Japanese production, growers in China’s Yunnan province, North America and elsewhere now fill the gap.

• How a two-part chemical defense turns wasabi from mild to pungent in an instant
• Why wasabi’s burn fades quickly while chili heat lingers
• The science behind shark skin graters and hiding wasabi inside nigiri
• Tatami-ishi terraces and the Daio Wasabi Farm’s use of upstream rice paddy runoff
• A 2011 wasabi-vapor fire alarm that woke deaf sleepers in under 10 seconds