MadSci Network: Biochemistry Query:

The material in question is called ALLYL ISOTHIOCYANATE. Roughly, it looks like this: CH2==CH-CH2-N==C==S It is usually found in "hot" mustard and in horseradish. It is an oily compound that vaporizes rather easily (vapor pressure 5 mm Hg at 25 deg C). Like the "hot oils" found in peppers, which are generally capsaicins and related compounds, allyl isothiocyanate is very irritating to mucous membranes, and for some reason many of us like it. The difference, though, is in its volatility, which is to say, how fast it vaporizes on your tongue. When you chew spicy food the oil is released. Capsaicins are not very volatile and thus tend to stick to your tongue. That is why Mexican food and other pepper-spicy foods mostly burn the tongue and throat. Allyl isothiocyanate evaporates more readily on your tongue and floats its way up into your sinuses (yes, as a gas), which is why both your mouth and nose burn when you eat something with horseradish or mustard in it. It is also why the burn from these foods tends to fade after a few seconds, while the burn from Mexican food can stay with you for a good long time. And that is why I'll take Japanese wasabi over jalapenos any day!

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