In the latest episode of Gastropod, co-hosts Cynthia Graber and Nicola Twilley answer a few of the great questions they’ve received from listeners over the years — like this one from Gino in Berkeley, California.
Gino: I had a question, or many questions, about wasabi. Why does the heat disappear from your palate so quickly, unlike other spicy foods? I’ve always been curious about why that spicy experience is not the same as what you would find in peppers. How do we experience it differently, in the physiological way?
We might describe the effects of a spicy curry and a hot sauce-doused burrito or a forkful of horseradish with smoked fish using the same words — as a burning sensation or heat — but we share your suspicions, Gino. They may both move us to tears, but the way wasabi sets your sinuses on fire, then dissipates quickly, seems so different from the long-lasting full oral cavity burn of a ghost pepper — surely they can’t be the same sensation?
“No, they’re not,” Pam Dalton, a scientist at the Monell Chemical Senses Center in Philadelphia, told us. “They are produced by two different chemical compounds and they bind to two different types of channels in our airways, and so the way we experience them is and should be different.”
After validating our sensory truth, Dalton explained what on earth is going on in our mouths and brains when we feel the burn. For all of these foods — chile peppers, horseradish, wasabi, mustard, and even spicy radishes — the feeling of “heat” is sensed by things called transient receptor potential (TRP) receptors. These receptors are basically channels that respond to the presence of external stimuli — pressure, light, temperature, or irritating chemicals — by moving around charged particles called ions. When lots of ions move around, they stimulate electricity to travel through nerves that signal different sensations to the brain.
Beyond our eyes, ears, and noses, these electrically active TRP channels are a major part of how our bodies sense what’s going on in the world around us. Scientists have only been aware of their existence for half a century, but you have TRP channels to thank when you feel pressure on your skin or light hitting your eyes, whether that cup of tea in your hands feels cozily warm or dangerously hot — and, importantly for our purposes, whether the food that you just put in your mouth has something irritating in it.
Among those irritants are capsaicin, which makes chiles spicy, and a second set of chemicals called isothiocyanates in wasabi, mustard, horseradish, garlic, and radishes. But there are a bunch of different TRP channels, and it turns out that, while the capsaicin in chiles sets the TRPV1 channel firing, the irritating isothiocyanate molecules in wasabi mostly activate the TRPA1 receptor.
The molecules in these foods literally communicate your brain through different channels, so that’s one reason their burn feels different. But their size also changes the way that you experience them. Isothiocyanate is small and volatile, and, in the warmth of your mouth, it instantly vaporizes and whooshes up the back of your throat into your nose and sinuses. As it just so happens, there are lots and lots of TRPA1 receptors in your nose, which is why you can feel the burn of a big mouthful of wasabi in your nasal passages right away.
Capsaicin, meanwhile, is a bigger, heavier molecule, which means that it isn’t as volatile and doesn’t vaporize unless you really heat it. (If you’ve ever sauted spicy chiles in hot oil and accidentally created pepper spray, you know what we mean.) As a result, the spiciness from chiles tends to stay on your tongue and mouth rather than getting up in your sinuses. However, there are also TRPV1 receptors on your lips, skin, and inside all of your mucous membranes, like your eyes and nose. If you’re unlucky enough to rub your eyes after cutting a pepper, the TRPV1 receptors there will be the first to let you know.
This leads us to the final element of Gino’s question: In those highly unfortunate cases, why does the heat from chiles stick around, while TRPA1-activating foods like wasabi are just a flash in the pan, palate-wise? This comes down to other differences in the molecules themselves. All of those deliciously irritating isothiocyanates are water-soluble, and so they wash away quickly. Meanwhile, capsaicin only dissolves in oil and doesn’t get washed off by saliva, which is why it hangs around so long in your mouth — and why a yogurt- or milk-based drink goes so well with a spicy curry!
For more answers to listeners’ pressing food questions, including whether or not white chocolate is really chocolate, why asparagus makes your pee so stinky, and whether calling chocolate sprinkles “jimmies” is racist, check out “Ask Gastropod.”