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Chili burns mouths, wasabi burns noses
Capsaicin, the compound in chili peppers, and piperine, the compound in black pepper, are made up of larger, heavier molecules called alkylamides. Those molecules mostly stay in your mouth, so that's where the burn is.
Mustard, horseradish, and wasabi are made up of smaller molecules called isothiocyanates. They easily float up into your sinuses, so the burn lands in your nose.
Not all spicy foods are spicy in the same way, and the difference lies in which compounds are involved. Both kinds act on the heat-sensing neurons found in the mouth and nose, and which group the food belongs to decides where they get triggered.
Why does grating wasabi release its heat?
In an intact wasabi plant, compounds called glucosinolates are stored apart from an enzyme called myrosinase. Grating crushes the cells and lets the two mix. The enzyme converts the glucosinolates into isothiocyanates within seconds. Freshly grated wasabi loses most of its pungency within a few minutes as those molecules escape into the air.
Why does the nasal burn fade so quickly?
Isothiocyanates are volatile, which means they evaporate readily at room temperature. When food containing them is eaten, the vapor travels from the back of the mouth up into the nasal passages. Once the vapor clears with the next few breaths, the receptors there stop firing. The sensation is short compared with a chili burn, which sits on the tissue of the mouth.
Why do plants make mustard-type compounds?
Plants in the cabbage family produce glucosinolates as a defense against insects and grazing animals. Crushing the plant tissue triggers the release of isothiocyanates, which are irritating and toxic to many pests. Mustard, horseradish, and wasabi all belong to this family. Broccoli, cabbage, and kale also contain glucosinolates.
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