A taste of physiology

What happens the moment some food hits your tongue? How do you taste it, and how do you know what you've tasted? Hugh Matthews tells us...

If you look at the tongue you can see fleshy bumps called the lingual papillae. Embedded in the upper surfaces of these papillae are the taste buds, which contain the taste receptor cells whose top surfaces are exposed to substances in the mouth at the taste pore.

The human tongue can respond to five basic taste qualities: sweet, bitter, sour, salt and, more recently, we’ve discovered umami, the taste of meaty foods and glutamate. Five is a much smaller number than the many hundreds of different receptor molecules associated with the sense of smell.

These five different tastes can be divided into two categories: sweet,bitter and umami tastes are only triggered by very complex molecules, while salt and sour are triggered by charged atoms, called ions, of sodium and hydrogen respectively. That’s because table salt is sodium chloride, and the amount of hydrogen ions in something is a measure of how acidic (or sour) something is.

The complex molecules require specific receptors formed by joining together two half-receptors in the taste cell membrane, like two puzzle pieces coming together. But while there is just one pairing each to make a sweet or umami receptor, there are some 30 different bitter half-receptors, and so many more possible pairings. This makes sense, as bitter substances are often potentially poisonous plant compounds, so the better early humans were at detecting poisons the better. Agatha Christie’s poison of choice, cyanide, tastes like bitter almonds.

In contrast, the ions responsible for sour and salt taste can be detected more simply because, since they’re charged atoms, they can, either directly or indirectly, change the flow of electrical current across the taste cell membrane.

The receptor cells detecting each taste quality are distributed rather uniformly over the tongue: the notion that different tastes are mapped to different parts of the tongue is a myth, since there are only very subtle differences in sensitivity between different areas. If you want to test this put something bitter on the tip of your tongue, you’ll still taste it even though the bitter region is supposedly at the back of the tongue.

In the brain, taste signals are integrated together with smell to give an overall perception of the food which you are eating, and also to control feeding, appetite and when you’re full.

In addition, there are a number of unconventional tastes which are really other senses. For example, when you eat chilli, a chemical in chillies called capsaicin binds to the same parts of the tongue which respond to painful heat, tricking them into thinking your mouth is on fire.

Mint contains menthol, which does the same thing, but to receptors which respond to cold, and horseradish is stimulating painful heat, painful cold and irritation simultaneously.

But is even this the entire story? There may be other taste sensations too, such as the metallic taste associated with calcium and magnesium ions. There is even some indirect recent evidence for a distinct taste for starchy foods, quite separate from that of the sugars into which it will be broken down. This may go some way to explaining the human craving for bread and chips! So, even now, the story of taste is not over!