TASTE BUDDIES: The Controversial World Of Taste Science : Short Wave Not much is known about why people experience tastes differently and why some people can detect certain tastes and not others. There also might be other tastes out there to add to the list beyond the five known ones now. In this finale to Short Wave's Taste Buddies series, we're tackling the science of the five tastes, and in this episode, we look at why there is so much more research to be done.

Host Aaron Scott talks to Danielle Reed from the Monell Chemical Senses Center about the controversy in taste science and about what other tastes might exist beyond sweet, sour, bitter, salty and umami.

To listen to more episodes about how we taste, check out our Taste Buddies series: n.pr/3sSOgDB

Email the show at shortwave@npr.org.

TASTE BUDDIES: The Controversial World Of Taste Science

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EMILY KWONG, BYLINE: You're listening to SHORT WAVE from NPR.

AARON SCOTT, HOST:

When Dr. Danielle Reed was a kid, she conducted her own little experiments with taste.

DANIELLE REED: I was very attracted to anything with, like, the tiniest hint of sweetness. And so I started by pulling the tender end of a grass shoot and putting that in my mouth and chewing on it.

SCOTT: And it was sweet. So she started plucking the buds off of bushes and popping flowers into her mouth.

REED: And then, of course, I found the inevitable, which is that most things were bitter, right? And honestly, I ended up feeling perhaps a little unwell afterwards. And so that was a good lesson for me.

SCOTT: So a little inadvertent botanist in training?

REED: Yes. Well, certainly a taste scientist from the early days.

SCOTT: Today, Danielle Reed is the associate director at the Monell Chemical Senses Center in Philadelphia, where she researches taste and smell. In particular, she studies how people differ in their sense of taste and how genetics might influence that.

REED: One of the things I love to do is this classroom demonstration where I take out this little vial of white liquid, and I taste a little bit of it. And to me, it tastes just like water - right? - absolutely, totally fine. But to some of the people that will sample it, it's insanely bitter. Like, it's I-hate-you bitter. People are angry that I've asked them to taste it. And this compound is something that's very similar to what's found in green cruciferous vegetables.

SCOTT: The compound is called PTC. We learned all about it in our episode about bitter. So the next time, say, that your kid spits out their broccoli, remember, we all live in our own sensory world.

REED: Here's what I say - empathize, don't judge. Because you really don't know what the other person's sensory experience is. Most people really take for granted that whatever one person tastes, the other person tastes. But that is not the situation.

SCOTT: And it's not just bitter. Some people are much more sensitive to sweet tastes.

REED: And, of course, salty and sour we know much less about at a biological level.

SCOTT: There is so much yet to learn about the tastes that we know about, and then there are all the tastes we don't know that much about. So today on the show, as part of our TASTE BUDDIES series, we are going to broaden our palate to look at new tastes, like fat and metallic, and the controversies around them, plus how taste differs around the world. I'm Aaron Scott, and you are listening to SHORT WAVE, the daily science podcast from NPR.

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SCOTT: So I want to step back a little bit and just get your basic definition of taste.

REED: So my basic definition of taste is very much oriented to the receptors on the tongue. So my point of view is that there's a receptor on the tongue and a taste cell and that that receptor's hooked up to the brain. And you can give me a word for what you're experiencing. I'd like to call that a taste - so sour, sweet, salty, bitter, umami.

SCOTT: So we have a mechanism on the tongue that is detecting some sort of compound and then recognizing, OK, this is sweet, this is sour, this is umami, which seems super basic and simple. And yet how controversial is the science of taste and what qualifies as a taste?

REED: Well, scientists are typically mild-mannered. But actually, the closest I've ever seen to a fistfight, actually, was in the early days of umami, which is that sort of savory character. There were a lot of people that thought that umami was really just, like, salt or sodium. It was a salty thing, and it wasn't its own thing. And there was a ton of controversy about it. And then when a unique receptor that just responds to this umami taste quality was discovered, it put a little oil on the waters, as it were, and sort of satisfied people's requirement for what constitutes a taste.

But there is a lot of controversy still in what we can call a taste and what we can't. So, for instance, when I just said oil on the water, actually, I was just sort of subconsciously thinking about fat because, you know, fat is beautiful texture, right? You know, you have that sort of, like, lovely salad dressing, butter type of feel. But it seems like it's more than just a feeling, that it might actually be a receptor or receptors on the tongue that help signal the presence of fat in the mouth.

SCOTT: And that was the exact one I was going to ask about because it seems like, a few years ago, there were headlines about fat being the sixth taste. And they called it - and I hope I'm pronouncing this correctly - oleogustus.

REED: Right, oleogustus. Yeah, scientists are not poets. That's what I'm going to say about that. It is an awkward word.

SCOTT: (Laughter) There's not a lot that rhymes with oleogustus...

REED: No.

SCOTT: ...Except maybe disgustus (ph). But what happened there? I mean, is - has there been any sort of agreement?

REED: Yes. So you were not far wrong when you made the joke about disgustus because here's what we know. So fat itself is delicious, right? It's super, has a wonderful quality. But when you break fat into its components - so there's fatty acids on a glycerol backbone. So essentially, there's two things that emerge when fat is broken down. The fatty acid part - that's what the receptors that we know respond to fatty acids. So those are uniformly not liked. They're scratchy. They're irritating. They don't smell good. And so we're sort of left with the receptor on the tongue that responds to this fatty acid - doesn't give you a sensation of yummy, right? It gives you a sensation of disgustus, as you like to say.

So we're still trying to understand, like, is there a unique receptor that responds to the whole fat molecule and that gives the lovely feeling? You know, because that's what everybody wants. They want to understand that with the idea that you could trick that receptor into saying it's lovely when it has no calories. That's one of the impetuses for this kind of work.

SCOTT: And, I mean, is it kind of like umami, where it's going to be debated until we can find the biological receptor? Is that, in our Western scientific mind, the evidence that we need that something is real?

REED: Yeah. So what we like to do is we like to find the receptor and then remove it and show that the taste is not there anymore. That's kind of the minimum evidence that seems to satisfy everyone. But until you find the receptor, you can't do those kinds of experiments to prove it to people that that's actually what's going on.

SCOTT: Along with fat, what are some of the other tastes that are controversial right now, or what are the other things that scientists are debating?

REED: Two things are really important right now. One is this metallic taste. And the metallic taste really comes up a lot, actually, at the minute because some of the COVID medicines are super metallic-y (ph). And that is a very strange - some people say if you lick a penny, but it's actually more noxious than that, you know? But whether it's an actual taste or not - it probably is. But that is really a hotbed issue at the moment because a lot of people are very put off from taking these antivirals because they really don't like what happens in their mouth when they do it.

And then the other thing that's really interesting at the minute is calcium taste. So when you think about calcium taste, you want it to taste like milk, right? That's what everybody goes to and thinks about. But, actually, it's probably more of a mineral-y (ph) taste. But it seems like there are specialized receptors on the tongue that respond to it.

SCOTT: And where do you come down on in your umpiring? Are calcium and metallic flavors?

REED: I think that metallic bitter is probably its own thing, and there's a very good candidate receptor for it. And with calcium, I think it probably also is a proper taste. But one of the problems with calcium, though, is this, like - we put sugar in the mouth, we say sweet - right? - very straightforward. With calcium, there's more hemming and hawing. So I think there's still room to debate.

SCOTT: One that we didn't talk about is kokumi.

REED: Yeah. So we've been studying kokumi. And when I've tried to get some of my colleagues to explain to me what kokumi is, they just smile and smack their lips, and they just say, delicious. And sometimes people say, it's mom soup. You know, it's that sort of delicious, mouthfeel-y (ph), full feeling we get from a very savory broth. And I've tried to study kokumi, and I don't know if I'm blind to it. I can't detect it. I can't label it. I can't guess if it's there or not, but clearly other people can. So I think it's going to be a mystery until we can actually find the receptors and understand why so many people are blind to it and why other people are able to taste it.

SCOTT: I feel like, you know, we all learn about the five basic tastes in school, and it just seems so kind of cut and dry. And we know what there is to know about it. And yet, looking at your work and the work of your lab and other taste researchers, it makes me realize just how little we actually know about taste.

REED: Right. So there's a lot of undiscovered country here. The thing that's the most puzzling and the thing people are chasing the hardest right now is to try to understand, really, how salt taste works. So we understand how a part of salt works for, like, the lower concentrations, but we really don't understand why salt tastes good, what the receptors are. That's undiscovered country for sure. And there can be many different kinds of taste receptors that we're completely unaware of. And so one of the things we're doing now is looking at all the receptors in the taste cells and trying to figure out what their jobs are.

SCOTT: And then also, it seems some of your research is looking at how taste plays out across the world.

REED: Absolutely. This is what - I'm really all about this at the minute. So we have a lot of beliefs - like, everybody loves sweet, right? That's - sugar is universal. But right now we're doing a wonderful project where we're looking at people worldwide to see, are we all sort of one family when it comes to taste, or is there a lot of particularity that's geographically based and ancestry-based in the sense of taste?

So you might imagine that the peoples that lived in Greenland, who are adapting to a fish diet with very little plant material, are going to have a lot fewer and different kinds of challenges than somebody that grows up and lives for hundreds of thousands of years in sort of a forest, very vegetative environment, where there's a lot of plants that have a lot of bitter and poisonous compounds. And so what we see is that some people, you know, due to their ancestry and the evolution of their bitter receptors, are really tuned up to certain things to protect them. Whereas other people, they seem to have bitter receptors that have kind of broken over the evolutionary timescale.

SCOTT: And it gets at kind of a bigger question and a basic one. But I'm going to ask, just as we kind of near the end, why do we need taste?

REED: I think because we've got to know what we're putting in our mouth. The body needs to be situated to receive the sugar. We need to release insulin. There's some things we've got to do when sugar is coming our way. Likewise, there's things we've got to do when we're eating bitter things. Like, we've got to be very careful when we're having a bitter thing, that we're not overdoing it. With salt, we really don't want to chug a gallon of salt water, right? We really need to be very judicious in how we do it. So that's sort of the protective, the safety issues.

But also, taste is really a - is the pleasure sense. And I don't think there's very many people in the world who would really voluntarily give that up. I mean, putting something sweet in your mouth - that apple, that - I don't know, tiramisu is coming to mind - that's a beautiful thing.

SCOTT: I love tiramisu.

REED: Yeah. And, you know, we just don't want to give up. It's what makes life worth living.

SCOTT: Excellent. Well, thank you so much for speaking with us, Danielle Reed. It has been a joy to talk to you about taste.

REED: My pleasure.

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SCOTT: This episode was produced by Eva Tesfaye, who is flying the SHORT WAVE nest, out into the great wide world. Is it too much to say we're bereft? I don't think it's too much to say we are bereft. We are going to miss you, Eva, but we are going to be listening to everything you do. You know we are fans. Our editor was Gisele Grayson, who is also our senior supervising editor, fact-checking by Margaret Cirino and audio engineering by Hannah Gluvna. Andrea Kissack is the head of the science desk. Edith Chapin and Terence Samuel are the executive editors and vice presidents of news. And Nancy Barnes is our senior vice president of news. Thank you, as always, for listening to SHORT WAVE, the daily science podcast from NPR.

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