Published online 24 February 2000 | Nature | doi:10.1038/news000224-12

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Questionable taste

Sour ice cream or salty ice cubes might not tickle many people's taste buds but apparently frozen foods can trigger the sensation of these unlikely flavours, explains David Adam.

Sour ice cream and salty ice cubes might not tickle many people's taste buds, but new research published in Nature1this week reveals that frozen foods can trigger the sensation of these unlikely flavours. Researchers have found that changing the temperature of the tongue induces sensations of sweet, sour, salt and bitterness.

Alberto Cruz and Barry Green of the Yale School of Medicine, Connecticut, report that they have found the first evidence that warming and cooling regions of the tongue can produce 'tastes', even in the absence of any other stimuli. Nerve cells are often sensitive to temperature, but researchers had thought that this effect was somehow dampened in the gustatory system.

The human tongue contains up to 5,000 taste buds, each housing receptor cells that respond to the basic chemical ingredients of food and drink. These taste signals, combined with others monitoring smell, temperature and texture, form the sensation of flavour.

Because these sensory systems operate independently of each other, researchers had thought that the higher, cognitive levels of the brain piece them together. Demonstrating that a 'taste' sensation can be produced by a direct change of temperature -- which is, of course, tasteless and odourless -- raises questions about the where and when these flavour-forming signals arise.

"It is a very interesting result", says Marion Frank, a neuroscientist and head of the Taste and Smell Centre at the University of Connecticut, Connecticut. "It will be of great significance in helping understand the transduction of taste."

All the four primary taste qualities of sweet, salty, bitter and sour, Cruz and Green found, can be triggered by temperature. Cooling the tip of the tongue down to 20 °C evoked sour and bitter tastes. Similarly bitter tastes were produced when a region at the back of the tongue was cooled. Warming the tip of the tongue, on the other hand, from 20 °C to 35 °C gave volunteers the sensation of a sweet taste.

Many of those who took part in these experiments were surprised at the "clarity and strength" of the tastes they experienced, some tasted only a few and others nothing at all. These differences reflect other variations in taste sensitivity, most famously that three in ten people cannot taste the bitter substance 'phenylthiocarbamide' (PTC).

Green's group tested other regions of the tongue and turned up results that could see school biology teachers scrabbling for their old textbooks. Sensitivity to different thermally induced tastes, the team found, varies with location. "Most of us were taught at school that different parts of the tongue are sensitive to sweet and salt etc", says Frank. "Recent studies have suggested this is not the case and that sensitivity to specific tastes is spread over the tongue, but I think that argument is seriously weakened by these results", she adds.

But if cold temperatures can produce salt and sour tastes, why don't ice cream sundaes evoke the lip-curling grimace that lemon juice does? The answer is probably that thermal tastes are weaker than the conventional tastes of food and drink, and easily masked. Food is also rarely placed only on the exact positions on the tongue that Cruz and Green tested. 

  • References

    1. Cruz,A. & Green, B. G. Thermal stimulation of taste. Nature 403, 889 2000.