The metal detector in your mouth

When you taste lemons, you know it because they’re sour. Sugar tastes sweet. Salt tastes, well…salty. Tastes buds on the surface of your tongue help you identify food that you’ve put into your mouth. Until recently, scientists believed there were only a few tastes: salty, sweet, sour, bitter and umami — a meaty taste in Parmesan cheese and portobello mushrooms. That idea may be changing.

At the Nestlé Research Center in Lausanne, Switzerland, scientists are curious about taste. They suspect that there are more taste sensations than the ones we already know about, and they’ve been carrying out experiments to find out how taste works. To test their hypothesis, they have been exploring the taste of metal. You can probably imagine the taste of metal, but can you describe it?

If someone asked you what lemonade tastes like, you may answer that it is both sour and sweet. On the surface of your tongue are taste buds, and in the taste buds are molecules called proteins. Some proteins detect the sourness and others the sweetness. Those proteins help send a message to your brain that tells you what you are tasting.

For scientists like those working in Switzerland, taste is defined by the proteins in taste buds. For instance, people disagreed about whether umami (which means “delicious” in Japanese) was really a taste until scientists discovered proteins that detect it. So in order for metal to qualify as a taste, scientists needed to discover whether specific proteins in taste buds can sense metal.

The Swiss scientists set out to understand the taste of metal by conducting an experiment on mice. These weren’t ordinary mice, however — some of the test mice did not have the special proteins associated with already known tastes. The scientists dissolved different kinds and amounts of metals in water and fed the water to the mice.

If the mice with the missing proteins reacted differently to metal than normal mice, then the scientists would know that the missing proteins must be involved in tasting metal. But if the mice reacted to the metal as usual, then it isn’t a taste or must be sensed by other proteins that the scientists don’t yet know about.

According to the results of the experiment, the taste of metal is connected to three different proteins. Identifying these three proteins helps the scientists figure out how a taste like metal works. The conclusions may surprise you. One of the proteins senses superspicy foods, like hot peppers. Another protein helps detect sweet foods and umami. The third protein helps detect sweet and bitter foods, as well as umami.

“This is the most sophisticated work to date on metallic taste,” says Michael Tordoff of the Monell Chemical Senses Center in Philadelphia.

These three proteins are connected to a metallic taste, but the scientists think there may be more metal-detecting proteins. They don’t yet know all the different proteins involved, but they’re looking. They do know, however, that taste is no simple matter.

 “The idea that there are four or five basic tastes is dying, and this is another nail in that coffin — probably a rusty nail given that it’s metallic taste,” says Tordoff.