How do we sense taste?

How does our tongue recognise a taste we like, like yummy chocolate, or dislike? Georgia Mills was joined by Rebecca Ford, an Assistant Professor of Sensory Science at the University of Nottingham.

Rebecca - It’s not surprise that it’s all happening in your mouth, but it probably will be quite surprising to you that there only just five basic tastes. For example, with your chocolate it will be the sweetness and the bitterness that are probably the most apparent. And if you look at your tongue in the mirror you can see lots of little tiny bumps along your tongue. The less red in colour to the rest of your tongue you’ve got a lot of them at the very tongue tip, the anterior part of the tongue, and these papillae, they house taste buds.

The taste buds are the term that we commonly know of and within those taste buds we have lots of taste receptor cells. As you chew your chocolate the tastants, so the sweetness from sugars and the bitter compounds they become dissolved in the saliva as you chew. These different compounds, they enter the taste pore of the taste bud on the tongue. There’s a little taste pore so as they get dissolved in the saliva they have to enter that taste pore, and then there’s a cascade of different reactions then that happens within the taste cell itself. It then sends a neural impulse up the nerves and then finally to the brain, to the gustatory cortex. The gustatory is the scientific name for taste essentially.

Georgia - Right. So there’s the things we can see when we go yuhh in front of the mirror and they house these taste bud, which then themselves house these little cells which respond to the various five tastes?

Rebecca - That’s exactly right.

Georgia - So what’s the difference between tasting something you like or you don’t like?

Rebecca - We’re actually all born with innate preferences to sense out things that are nutritive and non-nutritive; for example things that might be poisonous. But essentially, as we grow up and we learn these different things, what we like and what we dislike is very much about the brain. We detect things in the mouth but our brain then interprets those responses in terms of what we like and what we dislike.

Georgia - You mentioned the tasting part is very much happening on the tongue but the nose is involved as well, so what role does smell play?

Rebecca - It’s so important actually, and more important than people probably realise. It’s what we call taste-aroma interactions and normally, when you’re consuming some food you very very rarely will just have taste in isolation unless you were just adding a teaspoon of sugar into water, for example.

If we take the example of a strawberry. As you consume that strawberry, as you’re chewing it, you’re getting sugars released, you’re getting acids being released and they’re a sense of course in the mouth. But what’s happening at exactly the same time as you’re chewing, the aroma compounds are becoming airborne and they’re getting transported up to your olfactory bulb, which is very high up in your nose, at the base of your brain. The olfactory bulb senses the different aromas, and this is all happening at exactly the same time so your brain tells you it’s all happening in your mouth - it’s something that’s calle oral referral.

This is one of the reasons why when you’ve got a cold, nothing tastes quite right because it’s all happening in the brain. It’s exactly the same thing when you have one of those beautiful smelling fruit teas. they smell absolutely amazing  they’ve got this lovely strawberry aroma; when you drink it it’s just not quite right, and that’s because the sugar and the acids aren’t there.

Georgia - I’ve always wondered that. Fruit tea always smells so good and then you taste it it’s just a bit bland, isn’t it?

Rebecca - Of course, you’ve got all the aroma there, you’ve just not got the taste. So they’re so important; they go hand in hand.

Georgia - How do we tell one taste from another?

Rebecca - The majority of the evidence shows that each taste receptor cell is specifically tuned so it has one taste receptor on it’s membrane. Some are capable of detecting sweetness, those that are capable of detecting bitterness, umami, salty and sourness. So contrary to what you might have been taught at school where there are distinct parts of the tongue that are responsible for responding to different tastes, that is completely incorrect.

Within each of our taste buds we have taste receptor cells that are specifically tuned to each one of the tastes, but we have all of those taste receptor cells within a taste bud, and so essentially there isn’t that localisation that we were taught when we were at school. That was just down to a miscommunication of information unfortunately, from a paper that was published that was miscommunicating some findings.

Georgia - I remember that taste map well, and thinking it was a bit strange when you tasted sweets at different spots on your tongue and thinking this doesn’t seem quite right.

What’s in something to make your tongue respond and say this is bitter? Is it a different chemical, is it the shape of them; what’s different about them?

Rebecca - Exactly, it’s different chemical compounds; for example caffeine that’s in coffee, quinine that’s in tonic water, and then we have things like (10.39) acids that are  in beer. They are different chemical compounds and we have different bitter receptors all capable of receiving that information from those different chemical compounds, and then sending those signals to the brain telling us that it’s a bitter compound.

Whereas we tend to use tastes that are termed to describe everything that we are experiencing in our mouth when we consume things, which is a very complex scenario of lots of different signals that are being sent to the brain concurrently. We have taste being one of them; we have all those aromas that we were talking about being sent to the olfactory bulb in the nose. We also have all these texture receptors that are innovated all over the tongue and soft palate as well. They’re sending lots of different signals about the creaminess, the thickness, maybe the spiciness. And, of course, we have temperature receptors as well that give us information about the temperature of  the food or drink.

Georgia - Those five tastes you mentioned: bitter, salty, sweet, sour and umami - umami’s a brilliant word. But are those the only five tastes there are, is that everything?

Rebecca - At the moment… yes. But for something to be classified as a taste it’s got to be distinct from the other tastes, so it’s got to have stimuli that’s responsible from a taste that’s very different to the others. There’s got to be taste receptors that are able to send a signal to the brain, and there’s got to be perceptual independence from all the other taste qualities. So it’s got to be clearly identifiable essentially and all of those five basic tastes tick all of those boxes.

Now there are some other ones that have got a lot of evidence to suggest that they would be candidates for the sixth basic taste; for example fatty acids, carbohydrates, and this wonderful sounding taste that we know called kokumi.

Georgia - And another great word?

Rebecca - It is. And that really is an example of something that people describe as kind of a rich taste. It’s almost the way people describe it as like a mouth film, but it’s not very easily defined. And because it’s not clearly identifiable, that’s one of the reasons why we’re struggling at the moment to have enough data to say that it really ticks all of those boxes. Fatty acids, carbohydrates again, none of them quite meet all this criterial  although there’s a lot of scientific evidence that ticks some of those boxes.