When you consider the tongue, what leaps to mind are the five canonical tastes – sweet, salt, bitter, sour, and umami. These sensations arise when receptors on the surface of taste bud cells are activated by your food, triggering nerve fibres that run to your brain and help generate the experience of a savoury roast or a fresh strawberry. But your tongue is more versatile than that. It's also sensitive to temperature, pressure, and chemicals that mimic both of these things, which turn up in a number of foods. This peculiar latter group of sensations is called chemesthesis, and you probably experience some flavour of it every day.
One of the strangest examples is the Szechuan peppercorn, a staple of Asian cooking. You know when it's been sprinkled over a dish because suddenly your mouth begins to tingle gently, while going curiously numb. A compound known as “sanshool” is responsible. It turns out that sanshool binds to channels in the membranes of neurons in the tongue that respond to touch, producing a kind of tactile mirage.
In fact, in a 2013 paper, researchers rubbed the substance on volunteers' lips and then tapped their fingertips with devices that can vibrate at a variety of frequencies, asking them to say which frequency most resembled the throbbing caused by the peppercorn. Consistently the answer was about 50 Hz, which suggested to researchers that a certain type of nerve is responsible for the sensation, one that is sensitive to that particular frequency.
Another “non-taste” taste is the burning of capsaicin, the molecule that gives hot peppers their kick. Capsaicin binds to a receptor on cells that detect temperature and cells that send messages of pain. The same receptor and related ones are activated by piperine, a compound in black pepper that you might shake on your eggs in the morning, and allyl isothiocynanate, the burning compound in mustard and radishes.
It feels hot when you eat these foods because the receptors they trigger are usually switched on at temperatures higher than 42C or by acid, presumably to warn us that whatever we've put in our mouths is bad news. However, capsaicin and other hot foods won't damage your tongue – eat as much as you want.
Look at the tongue closely, and there's a lot going on (SPL)
You may notice, in fact, after you've eaten a lot of spicy food, that the burn won't affect you as much, as the receptors eventually stop responding so strongly to the compound. The phenomenon is called capsaicin desensitisation and has long fascinated scientists because it suggests that capsaicin is able to alleviate pain. Capsaicin creams are now available for treating arthritis pain, for instance. But a drug that turns off the receptor in an attempt to treat pain was deep-sixed, because it made subjects feel unusually hot.
Ethanol can lower the temperature at which the capsaicin receptor is activated, which has been suggested to be the reason why a shot of alcohol burns. It may also be why spicy food can taste spicier if you're drinking warm booze, and why that chilled drink is so satisfying. Ethanol has other peculiar properties: after ethanol has been applied to the tongue, your mouth hurts more easily, perhaps because of the connection between this receptor and pain perception.
And on the other side of the spectrum, the chill of menthol in peppermint also arises from an odd coincidence. A receptor that triggers when the temperature in your mouth takes a dive is also set off when menthol is around.
Of course, all this isn't limited to your mouth. The same receptors are present in your skin. If you were to bathe in Szechuan peppercorns, you'd get the same effect, and rubbing your eyes after chopping a hot pepper is an unforgettable experience. But we grow so used to the riot of sensations caused by food that it's worth drawing attention to these odd wonders. Next time you eat Chinese or feel the burn of mustard, consider the non-taste receptors inside your mouth.