Mustard and chillis are both hot, but the burning sensation from a chilli stays in the mouth for ages while the sensation from hot mustard disappears in a few seconds. Why is this?
The chemical mainly responsible for the burning spice in chilli peppers is capsaicin, a complicated organic compound that binds to receptors in your mouth and throat, producing the desired (or dreaded) sensation.
Capsaicin is an oil, almost completely insoluble in water. This is why you need a fat-containing substance like milk to wash it away - watery saliva doesn't do the trick.
On the other hand, the compound responsible for the hotness of mustard (as well as horseradish and wasabi) is called allyl isothiocyanate. This chemical is slightly water-soluble, and can be more readily washed away into the stomach by saliva.
Further, the chemical in mustard is more volatile than capsaicin so it evaporates more readily, allowing its fumes to enter the nasal passages (explaining why the burning sensation from mustard is often felt in the nose). These fumes can be easily removed by breathing deeply, a useful strategy if the sensation becomes overwhelming.
Zachary Vernon, Toronto, Canada
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