The Science of Taste I

How you perceive taste is a very intricate and complicated relationship...
07 October 2012

Interview with 

Barry Smith, University of London


A human tongue


Chris -   Now most people regard taste as a very simple sense.  You put something in your mouth and your tongue tells you the flavour, right or wrong?  Well not quite because it turns out that how you perceive taste is a very intricate and complicated relationship that combines several different senses.  Apart from just your tongue's taste buds, food texture, smell, sight, and even the sounds around you affect an eating experience.  And to explain exactly how, with us is Professor Barry Smith from the University of London.  Hello, Barry.

Barry -   Hello.

Chris -   So, begin by taking us on a flavour trip if you like, of what actually happens when we eat something.

Barry -   Well, as you just said, people think they're getting most of the experience of flavour from their tongue, but in fact, the tongue gives you very little.  It just gives you salt, sweet, sour, bitter, savoury, now we think metallic as well.  You can taste blood in your mouth, that metal taste.  But think of all the things that you are used to experiencing.  You can taste strawberry or onion, or melon, or cinnamon, and we don't have receptors for those.  So that tells you that you're getting it from somewhere else and most of it is coming from smell.  But as well as smell, you've also got touch.  That's the creaminess of something in the mouth, there's how stale it is if it crumbles rather differently.  But think of something like pepper.  We like to put black pepper on our food, but we don't have taste receptors for paper.  In fact, what that is doing is irritating the trigeminal nerve and it's the one that becomes very sore at the bridge of the nose.  It starts to sort of ring bells and be stingy when your mustard is too hot.

Chris -   Horseradish...

Barry -   Horseradish, right and it also makes peppermint or spearmint taste cool in the mouth even though there's no temperature change at all.  So you have at least got touch, taste and smell combining always to give you what you get as flavour.

Chris -   So, when we put something in there, the heat from your mouth imparts some energy to the molecules of food you've put in, so some of them are going to drift off and go up your nose up the back of your throat then and so, you'll get this retro aroma experience.

Barry -   Actually, you have 2 senses of smell - so you can smell something externally from the environment and then when you put something in your mouth, and those volatiles go up through the nasal pharynx and start to activate the very same receptors in the nose, you will maybe get a very different experience.  So, the brain pays attention to the difference between airflow coming in or going out, coming from the mouth or coming from the world.  So, think of a smelly chess like epoisses.  It smells really rather disgusting and yet, when you put it in your mouth, delicious, lovely.  And that shows you the aroma is behaving differently depending on direction of flow.

Chris -   A relative of mine said to me the other day - there was one cheese that he did actually have to throw away.  He bought some stinking bishop and he said, "I could not bring myself to eat it."

Barry -   But if he had, maybe...

Chris -   But it tastes fantastic, doesn't it?

Barry -   Yeah, exactly.

Chris -   Why is that then, why do we get this very nauseating affront when you open the packet and you think, "Am I really going to put that in my mouth?"  But then when it goes in, it is actually gorgeous.

Barry -   Well, it is because of the difference between the way the very same odours are treated, depending on whether they came retronasaly from the mouth or orthonasaly from the nose.  And there's another way in which they can mismatch.  Think of coffee.  A smell of freshly brewed coffee is wonderful, but aren't yours just a little disappointed that the taste in your mouth is not quite the same?

Chris -   It's certainly a disparity.  That's true.

Barry -   Yeah, it is a disparity, but there's one food where we have exactly the same sniffing from the outside and experiencing from the inside, and that's chocolate.  And that's probably why it's a wonder food because you get what you expect, and you're very satisfied

Chris -   To think about chocolate is that it does fulfil or tick all those boxes that you mentioned isn't it because you have the texture.  It melts in the mouth, so it changes from being something hard with that rough texture into something smooth, that easily glides over your tongue.  It's got an intense sugar punch that it packs, the same for fat.  So there's all those experiences plus those aromas going at the back of your throat.

Barry -   Yeah, I think everything as you said ticks all the boxes.  And also, you'll notice when you're eating chocolate truffles, so full of butter that the very smooth feel actually bluffs the brain into thinking there's something cool and sensuous in the mouth, and actually because it's rubbing the somatosensory receptors in a slightly different way.  It's more like being stroked than being rubbed.

Chris -   Well, they do say that chocolate does play a part in love and that kind of thing, don't they?  What about actually using that knowledge then?  Are food manufacturers are saying, "Well, we know how chocolate does it and we know it's irresistible."  So, can we confer that experience on other things so that people will embrace a pot noodle even, something like that with the same lure as in the same alacrity as a chocolate bar?

Barry -   Well, that's right.  I mean, people in the food industry know this stuff very well and they know that say, the creaminess of something can affect how sweet it tastes.  So perhaps you can put less sugar in by making something taste creamier.  And equally, when you've got low fat yogurts, people didn't want to eat them because they thought they just don't have that substance to them.  So, if you can have something which is still giving you the same stroking, the same feeling, maybe that will slightly improve the flavour of what you're eating, although we do, I think now believe there are fat receptors in the mouth or at least fatty acid receptors.  So, the brain is only bluffed so long and then eventually, it wants the real fat content.

Chris -   What about sound?  Because there was a study I read a number of years ago where I don't know if this is apocryphal or if it's true - a supermarket playing different music in the wine section felt they could influence people's choices of wine.  People are presumably imagining what that wine would taste like in the context of that music, which is why they were changing their minds about what they wanted to buy when they played French music or Italian music.

Barry -   They didn't even have to imagine.  I mean, the beautiful thing in this study was that you had Oom-pa-pa German music and French accordion music, and the percentage of French and German wine varied according to the time at which different music was played.  But when you ask people on the checkout, "Why did you buy this wine?"  "Oh!  I just thought this would be a nice thing to drink."  "Was it the music that influenced you?"  "What music?"  So, sound can have a big clue, but the best sound effect on flavours actually, collaborator and friend of mine, the psychologist Charles Spence who works on this, he won the Ig Nobel Prize for his work on potato chips.  So, if you leave them out for a couple of days and they go stale, you put them in your mouth, not so fresh, but if you put headphones on people, and you amplify the high frequency sound of their own crunching back to them, the crisps tastes fresh.

Chris -   Oh wow! So you get people to eat them and play the sounds back, and they say that the crisps are fresh.  They can't tell that they're stale.

Barry -   They can't tell that they're stale which shows you and he wants to know whether that makes tasting stale more of a sound experience than a taste experience.  It's also the reason why the manufacturers make the bag so noisy, that awful rustling noise.  It's because it's bluffing the brain.  It's getting the brain to think fresh, fresh, fresh.

Chris -   So, is that reason why crisp bags have become cracklier or food packaging in general I think has become cracklier in recent years?

Barry -   Cracklier and in fact, there was one bag that was so noisy that the decibel level was banned.  It was just too intrusive.

Chris -   What was in the bag?

Barry -   That was crisps as well.  That was in Latin America, but it was eventually outlawed because it was just far too loud.

Chris -   We had a Question of the Week a few years ago where someone said there were crunch in their breakfast cereal and they would listen to our show as a podcast ( incidentally).  If you have these ear bud earphones in, he said, his breakfast cereal was amplified because, this is something called the occlusion effect where if you create a sort of resonant cavity in your ear between the ear bud and your eardrum, things get louder, especially sounds coming from within your own head.  So based on what you've just said then, do you think his breakfast cereal was becoming a more intense experience for him because he could hear it more loudly?

Barry -   It was more intense in one way, but less discernible in another because we know that when you're hearing white noise as on an aircraft, it actually reduces your ability to discriminate flavours.  So therefore, they give people noise-reducing headphones in business class and the food seems to taste better.  But it's also why Heston Blumenthal who's been doing a lot of work with British Airways is starting to put more spice into the food because as I said before, spice is irritating the trigeminal nerve.  It's not got from the tongue and that's not affected by noise in the ears, and so, people are beginning to say that the food is tastier.

Chris -   And it's all down to it being a bit more 'zingy'.

Barry -   It's more 'zingy' and if you boost one of the senses, it tends to have a knock-on effect on the other.  So, you can modulate the overall experience by affecting one or other of the sensory inputs.

Chris -   So, if you give people a spicy curry in the air, do they report that the aircraft is less noisy?

Barry -   No.  The noise will still be there, but they'll actually enjoy the taste a lot more.  And I think that's actually important for the elderly as well because we know that as smell diminishes when you get older, it's going to be increasingly difficult to get people to enjoy their food.  But if you give them more trigeminal stimulation or you give them other sensations at the same time that they're having the input in their mouth, then they'll still have flavours they enjoy.

Chris -   You've brought in some individually wrapped sweets in a cracky bag I'm pleased to say, so they must be extremely fresh or at least my brain will be fooled into thinking they are.  What are you going to do with these?

Barry -   Okay, so I'm going to ask Martha if she'll demonstrate this.  So what I want her to do is I want her to hold her nose, so she talks as though she's got a bad cold, and if she keeps her nose tightly pinched like that, so she's really not taking in any air, I'm going to ask her to put one of these jelly beans now into her mouth, keeping her nose held tight.  So, put the jelly bean in the mouth, keep the nose tightly closed, that's it and chew and chew, and just chew.  And what do you taste, probably some sweetness, a little bit of sourness maybe, but certainly sweetness?

Martha -   Yeah.

Barry -   Now, let go of the nose.

Martha -   Yeah, I was getting sort of jelly texture, but really not much taste at all, but since I let my nose go, you get all that berry flavour, that was a black currant flavour coming through.

Barry -   That's right.  So, that shows you that the fruity flavour that you now get is actually coming from the nose and not from the tongue at all.  So, it's a very easy way that everybody can try this and demonstrate for themselves what's actually happening.  Smell is a huge part.  Maybe 80% of what we call, what we ordinarily call 'taste'.

Chris -   We've got some more tricks that Barry can show us later in the programme.  I discovered that when I was very little and I was trying to eat Brussels sprouts actually and hated them, and I found if I pinch my nose, I could pretty much swallow anything.  


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