Mustard and chillies 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?
鈥� The chemical mainly responsible for the burning spice in chilli peppers is , 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 鈥� watery saliva doesn鈥檛 do the trick.
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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
鈥� The hotness of mustard comes from , which is formed when myrosinase and sinigrin (in mustard seeds) react together in water. It dissolves well in most organic compounds, and to an extent in water, and is also volatile, so will quickly disperse.
On the other hand, capsaicin, the hot ingredient of chillies, is not very water-soluble. So its heat tends to stay. It is soluble, however, in alcohol, which raises the question: which came first, the lager or the vindaloo?
Matt Billingham, Switzerland