Manipulating the tongue's temperature can result in the same salty, sweet or sour taste caused by sugars, acids or other chemicals, a Yale study finds, furthering understanding of the physiology of taste.
"We've discovered that specific tastes can be produced by temperature stimulation, just as certain chemicals can evoke only certain taste qualities," said Barry G. Green, principal investigator on the study, which will appear in the February 24 issue of Nature.
Green and his colleagues call this temperature stimulation of taste "thermal taste." It has been known since the first electrical recordings of taste nerves that they are sensitive to temperature as well as to chemicals, but it was not known how the brain interprets this thermal stimulation. Thermal taste shows that it is interpreted as taste, not temperature.
The close relationship between temperature and taste qualities suggests that receptors in the tongue that respond to chemicals have certain properties that make them vulnerable to specific kinds of temperature change. This information may provide clues to understanding the nature of these receptor processes.
Thermal taste is different on different parts of the tongue, indicating that taste receptors that are sensitive to temperature are not uniformly distributed throughout the tongue. For example, sweetness is more readily perceived on the tip, sourness on the side and bitterness in the back.
Not everyone experiences thermal taste, Green says. About two out of every three people tested in the study experienced at least one taste quality -- sweetness is the most common thermal taste and saltiness is the least common. Green notes that these individual differences are consistent with other evidence that taste physiology and taste experiences vary substantially from person to person.
"Thermal taste probably does not affect the taste of most foods and beverages because the temperature conditions that produce it are rarely encountered during eating or drinking, and when they are, the chemical tastes of foods and beverages tend to mask thermal tastes," Green said. "However, it is possible that frozen desserts may taste somewhat different to individuals sensitive to cold-induced sensations of thermal sourness and saltiness."
Individuals sensitive to salt will notice that an ice cube touched to the very tip of the tongue for a few seconds will begin to taste salty. Unfortunately, Green says, saltiness is the least common of the thermal tastes, and this is not a reliable way to demonstrate the phenomenon.
Green's research grew out of an accidental observation made while preparing to study the temperature sensitivity of the tongue. As his then research assistant, Alberto Cruz, was experimenting with a small thermal stimulator on his own tongue, he noticed that cooling the tongue tip and then rewarming it caused a weak sweet taste.
"We conducted informal tests on each other to confirm his observation," Green said. "While doing so, we noticed a sour taste when the tongue tip was cooled to about 15C below normal mouth temperature, and a salty taste when it was cooled to about 25C below mouth temperature. This paper reports formal experiments we conducted to quantify the phenomenon and to study its relationship to chemical taste."
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