Cooling and heating the tongue arouse the perception of tastes, even in the absence of food.
THE PHYSIOLOGY OF FLAVOR is riding a wave of fresh discoveries. In recent years physiologists have elucidated the molecular bases of the sensation of spiciness, described the mechanisms in the papillary cells of the mouth responsible for taste perception, and at long last, in April 2000, identified the first receptor for a taste molecule. It was expected that the analytical methods that led to this last discovery would help researchers find other receptors, but a great surprise lay in store: thermal tastes. Variations in the temperature of the tongue alone are enough to cause tastes to be perceived.
When we eat, various molecules stimulate the olfactory cells of the nose, the papillary cells of the mouth, and receptors that register levels of spiciness and a set of mechanical and thermal sensors. How do these different perceptions interact to form the synthetic experience of flavor? It was long supposed that the pieces of information obtained by the various sensory cells traveled upward up from neuron to neuron until they reached the higher centers of the brain, which then gave them a joint interpretation, thus creating the sensation of flavor. A few years ago, however, physiologists came to realize that this account was inadequate: Sensory information has already been combined by the time it reaches the first neuronal relay station, which is to say that sensory integration begins in the tongue.
Heating the Tongue
To determine the physiological consequences of this integration, Ernesto Cruz and Barry Green at Yale University investigated the effect of the temperature of foods on taste perception. Using small thermodes (devices whose temperature is regulated by means of electric currents) to stimulate the tongues of subjects, they discovered—rediscovered, actually—the “thermal tastes” aroused by heating and cooling the tongue. This effect had been discovered in 1964 by G. von Békésy, but the observation was part of a theory that was later discredited and so failed to receive the attention it deserved.
The reactions were not identical for all subjects, but for a large proportion of them, heating the tip of the tongue up to 35°C (95°F) produced a slight sweet sensation, and cooling it down to 5°C (41°F) elicited a sour sensation or, in the case of one person, a salt taste. Heating the back of the tongue produced only a weak sensation of sweetness, but cooling this region often gave rise to more distinct sensations, variously described as bitter or sour. A few subjects tested perceived none of these thermal flavors.
Plainly this curious phenomenon called for further scrutiny. The Yale physiologists sought first to quantify the relationship between temperature and the intensity of sensations: The intensity of the sweet flavor, at the front of the tongue, increases as the temperature rises, and the sour taste caused by cooling becomes saltier as the temperature falls. Because the subjects often reported differences in taste perception between the center and the sides of the tongue, Cruz and Green sought to make these impressions more precise. They found that for all subjects the perception of thermal sweetness was greatest in the tip of the tongue and that thermal sourness was most clearly perceived on the sides of the tongue.
Paradox Elucidated
How are these results to be interpreted? We know that the nerve fibers of two cranial nerves innervate the gustatory papillae. The fibers of the chorda tympani are ramified in the papillae and transmit information about tastes to the brain, whereas the fibers of the lingual branch of the trigeminal nerve terminate in the papillary epithelium, near the taste buds, and transmit information about temperature, pain, irritation, and pressure. Note that thermal sensors and taste receptors are found near one another in the mouth.
It seems plausible to suppose that changes in temperature activate the receptors responsible for the normal coding of the perception of tastes. In this hypothesis, it ought to be possible to block thermal tastes by stimulating taste receptors. Conversely, if the perception of thermal flavors is not caused by the normal taste receptors, inactivating these receptors will have no effect on the perception of thermal taste. Tests are under way.
What are cooks to make of this discovery? One does not taste with the tip of the tongue alone, so the effect of thermal tastes is weak in ordinary eating situations. On the other hand, it is a simple matter to determine whether you are capable of perceiving such flavors: Just place an ice cube against the tip of your tongue or stick your tongue in a glass of warm water.