Harold McGee talks about how our sense of smell affects taste, why things smell the way they do and the ways different chemicals combine to create surprising (and sometimes distasteful) odors.



This is FRESH AIR. I'm Terry Gross. The COVID pandemic has made us more conscious of smell. The loss of the sense of smell is a common symptom of COVID. And wearing masks to prevent the spread of COVID has made many of us more conscious than we'd like to be of the smell of our own breath, which gets trapped and amplified by the mask. My guest, Harold McGee, is the author of the new book "Nose Dive: A Field Guide To The World's Smells." It helps explain everything from bad breath to why the beef from grass-fed cows tastes different from that of cows fed grain, and why sweaty feet can smell kind of cheesy. Harold McGee is famous in the food world for his books and articles on the science of cooking. His book, "On Food And Cooking: The Science And Lore Of The Kitchen," has been described as a Bible for home chefs around the world. His writings about food and science have also inspired restaurant chefs in the food avant garde. McGee is a visiting lecturer in Harvard University's course Science and Cooking: From Haute Cuisine To Soft Matters Science.

Harold McGee, welcome back to FRESH AIR.

HAROLD MCGEE: Thank you so much, Terry. It's great to be back.

GROSS: It's a pleasure to have you back. So let's start with mask breath. The mask creates a feedback loop with your breath. You're like - you're smelling your breath. And then you're talking, and there's more breath trapped in the mask. So why do we get bad breath?

MCGEE: Well, we get bad breath because we have a thriving microbiome in our mouth. In fact, our skin, our innards are all homes to microbes that have learned to live in those environments. And the microbes in our mouth thrive by consuming the remains of what we've been eating, as well as the shed cells and things like that, proteins in our saliva and that kind of thing. And when they do that, they produce small molecules, molecules that are small enough to be volatile - that is to say, to fly out into the air where we can smell them. And so what we're smelling when we smell our breath is the smell of the microbes that we're hosts to and feeding.

GROSS: Have you spoken with people who have had COVID and lost the sense of smell and what that experience was like for them?

MCGEE: I have not. I don't really know anyone who's had COVID so far, but I have myself had the experience of losing my sense of smell a couple of times, once while actually writing this book, which was scary.

GROSS: What happened?

MCGEE: It was just apparently a viral infection, a sinus infection. And I woke up one morning and made my coffee as usual and then after a couple of minutes realized that I wasn't tasting it, wasn't smelling it. It was just not there. And I have friends in the kind of clinical side of taste and smell research. And so I immediately contacted them to find out what I could do and why this had happened and so on. And they basically said, you're going to have to wait and see. It's something that we don't fully understand why it is that a virus will have this kind of effect. Sometimes it wears off very quickly, sometimes it never does. And you just - you've lost it forever. So it was an anxious few months. It took a few months before it came back very gradually.

GROSS: Well, how did you deal with it during that time?

MCGEE: Not very well because I was writing about it every day and paying much more attention to it than I had in years past. And the thing is - the thing about it is that it just removes this dimension from your life that at least I was kind of taking for granted. You know, it's the kind of thing where you don't notice something until it's gone. And so just the ambient smell of my house and the air outside and food and drink became much less interesting, much, much less appealing. I, you know, spent less and less time cooking. There was no point in going out to restaurants because I wasn't really going to enjoy it, so it was not fun.

GROSS: Why is smell so important in tasting food?

MCGEE: It turns out to be the dominant sense in our appreciation and perception of food. So we have what we call flavor, the overall quality of a food - sensory quality of the food is built of several different sensations. One of them is taste on the tongue, which is limited to about a half-dozen to a dozen different sensations - sweet, sour, salty, bitter, savory, things like that. And then the sense of smell is actually able to distinguish among hundreds, thousands, maybe many more than thousands of molecules. And so the distinctiveness of particular foods really comes from our perception of their aromas, their smells, and not so much their taste.

GROSS: Smell is sometimes so subjective. You know, as you point out in the book, like, some cheese smells very similar to vomit. We love the cheese, but the smell of vomit makes you want to puke yourself. So why do they smell kind of similar? And why is our reaction to that smell so different, you know, depending on whether it's puke or, you know, ripe cheese?

MCGEE: Yes. Yeah. No, it's absolutely true. And the reason that there is that kind of crossover is that both of those materials, the cheese and the vomit, contain a number of different volatile molecules that we can smell. But one of the most prominent in both of them is butyric acid, which is a breakdown product, a small fragment of ordinary fats that we find in foods of all kinds, especially cheese, which is very rich in animal fats. And so when cheeses are fermented by microbes, those microbes in the process of the fermentation break down some of the animal fats, the B fats, milk fats into butyric acid. And that's an element of the flavor which actually becomes stronger and stronger with time.

There's also butyric acid in vomit because the stuff that we have upchucked has been in our stomach. We've been breaking it down into its components. And butyric acid is a prominent component in almost everything we have. So there's - that echo is from that molecule and maybe a few others. And our reaction to that molecule is dependent on the context. You know, if we're sitting down at a table and cutting ourselves a piece of cheese, it's a nice kind of accent in the flavor of that cheese. If we're feeling really sick and, you know, staring at a toilet, then it's a very different context.


MCGEE: OK. Let me reintroduce you here. If you're just joining us, my guest is Harold McGee, who's famous for his books on the science of cooking. His new book, "Nose Dive," is about the science of smell and how smell affects taste. We'll be right back after a short break. This is FRESH AIR.


GROSS: This is FRESH AIR. Let's get back to my interview with Harold McGee, who's famous in the food world for his books on the science of food and cooking. His new book is called "Nose Dive." And it's about the science of smell and how smells affect how we taste food.

One of the incidents that got you started on the science of smell was eating grouse on one of your, you know, food trips abroad. And grouse is a wild game bird. I can't say I've ever tasted it. So tell us a little bit about what grouse is and why tasting it left you absolutely speechless.

MCGEE: (Laughter) Yeah. It was an experience. So grouse are - they're kind of like bluefish in this country or matsutake mushrooms or morels in the springtime. They're very much a seasonal food. You can only get this bird for a few months in the late summer, early fall. I happened to be in England during that period. I never tasted grouse before. I knew it existed. It had this kind of, you know, Wodehouse, Jeevesy (ph) kind of aura around it - for me, anyway - because it is so English.

And so I'd made sure to get a taste of it. I went to lunch at a restaurant that was serving it and was talking with the people with me at the table and took my first bite. And I was actually momentarily paralyzed. I was in the middle of saying something. I stopped. I just couldn't - I was really confused. I'd never felt that way (laughter) eating anything. And the people I was with looked at me and thought, you know - and told me afterwards they thought maybe I was having a stroke or something.

GROSS: (Laughter) I shouldn't be laughing. Yeah.

MCGEE: Yeah. It was a really striking experience. And what it got me to thinking about was, of course, why could a bite of food have that kind of effect on me? What was going on? And so I learned about grouse. I should, though, first of all, say that the flavor that knocked me out like that was the meatiest, most intense, most kind of animal-like flavor that I'd ever experienced before. I'd had, you know, things like duck, which is a rich bird, but - and was kind of expecting something along those lines.

But this was really funky. It was almost as if it were slightly off. And I think that's part of the reason my brain decided to pay all of its attention to the bird and not to my companions. They wanted to make sure that it was OK to swallow. So I decided to learn about grouse. And it turns out that they are wild birds. They live on the heath in the north of England. They eat scrubby, very tannic, to us, unpleasant heath vegetation. They're prone to get parasites, which allow animals that hunt them to find them more easily.

When they're shot, they're actually kept in a cool room for several days to mature them with the guts intact, sometimes those guts including parasites. So the flavor of the whole bird becomes kind of imbued with the stuff that's in the digestive system, maybe the parasites that are in the digestive system. All that contributes to the aroma of the bird. And then the bird is cooked very briefly. So it's very rare and just, as I say, extremely strong, funky flavor. That's what got my attention and kind of pushed me off on this trek to find out about smells.

GROSS: Not only does that sound kind of disgusting to me, it sounds unhealthy.

MCGEE: (Laughter) Well, yeah, it's been done long enough that I didn't worry too much about the health consequences. It's not the most hygienic of preparations. And, in fact, I think that the disgust that you're registering, I think, was actually part of my experience then. It was a mixture of interest and disgust. And, I think, afterwards, the way I interpreted it was that it's - it was really my first experience with what it means to eat animal flesh. You know, we - when we buy meat these days, it's been prepackaged. And we don't really know anything about where it came from. It smells like nothing. But this is animal flesh as our ancestors would've eaten it. And it was an encounter with, really, the necessity that for us to eat meat, an animal has to die. That's kind of what it came down to. And that's kind of what I was tasting in that moment.

GROSS: So we've been talking about the, like, shocking taste of grouse (laughter). And you say that part of the reason why grouse tastes different than, say, like, you know, duck or, you know, other birds or poultry has to do with what the grouse eats before it's killed. And that's apparently true of a lot of the food that we eat, that the taste depends on what the animal or the bird has eaten. So what's the difference between how grass-fed meat - the meat from grass-fed cows compares to cows that have been fed grain?

MCGEE: Yeah. So it's absolutely true that the foods that animals eat in order to grow affect the way they taste when we, in turn, eat them as food. And in the case of grass and grain-fed animals, the difference is in the kinds of fat that they take in. So it's not that we're actually tasting grass or tasting grain when we detect the difference between the two. It's actually the fact that the fats, the oils in grass, are very irregular molecules. And they tend to be broken down in the animal into particular fragments that are very characteristic of those original fats and oils. When we feed animals with grain, we give them a very different diet. The fats and oils in grains are very different from the fats and oils in the grass.

The other thing about a grain diet is that it's a much richer diet, of course, than pasture feeding. And so the tissue is much fattier, the meat is much fattier. And we're tasting, predominantly, the fats from the grains. When we have grass-fed beef, it's much leaner. They're just different. And different people in different countries have different preferences. We in America tend to prefer the flavor of grain-fed beef because that's what we've been used to for decades now. But in other parts of the world, in South America, where a lot of beef is raised, and in Europe, the flavor of grass-fed beef is much preferred.

GROSS: Yeah. You write that fish farmers are experimenting with what to feed the fish to make the fish taste better. What are some of those experiments like?

MCGEE: Well, it's interesting. They're, in fact, trying out better ways to make fish, in a sense, taste fishier (laughter) because aquaculture is becoming more and more important for sparing the wild populations in the oceans. Many of the animals, many of the fish in the oceans eat other fish. And so it doesn't really do that much good if you're going to culture fish, raise fish outside of the oceans or in confined areas in the oceans, but then go ahead and feed them all the fish that they would've been eating from the rest of the ocean. So they're trying to find feeds that will be more sustainable in aquaculture. And that usually means plant-based materials. And so the trick is to feed the fish with materials that will end up giving the fish the same fragments that we can smell and taste that having a wild diet would. And that turns out to be a challenge.

GROSS: We've been talking about taste, but your book is about the sense of smell. So where does smell figure into this?

MCGEE: Well, the difference is actually a difference in smells and not in taste. So - and this is where the English language kind of does us wrong because we talk about taste and flavor as being kind of the same thing in some circumstances. But if we distinguish between taste, which is the sensations we get on our tongue, and smell, which are the sensations we get in the nose, then it's actually the smells of foods that make them as distinctive as they are.

So animals, plants, all the things around us that are alive that we end up eating in one form or another, they're made up, for the most part, of very large molecules - proteins, fats, starches, things like that - which, themselves, have no smell. They are large molecules. And for a molecule to be smellable, it has to be small, small enough to escape whatever it's in, fly through the air, into our noses, where we can detect them. The diet of the animals provides the animals with large molecules, which the animals then break down into smaller ones. And it's those particular groups of smaller ones that constitute the aroma of the food that we're eating.

GROSS: Why does cooking bring out the aroma of food?

MCGEE: Well, it does so in fact by kind of doing what it is that microbes are doing in fermented foods and inside us. It's breaking down these large, complicated molecules, like proteins and fats, which are too big to have an aroma because they're not able to fly through the air and reach our nose. Cooking using heat energy breaks apart those large molecules into smaller ones. And those smaller ones are what we experience as the typical aromas of cooking foods.

GROSS: Let me reintroduce you here again. If you're just joining us, my guest is Harold McGee. He's famous for his books on the science of cooking. His new book "Nose Dive" is about the science of smell and how smell affects taste. We'll be right back after we take a short break. I'm Terry Gross. And this is FRESH AIR.


GROSS: This is FRESH AIR. I'm Terry Gross. Let's get back to my interview with Harold McGee, who's famous in the food world for his books on the science of food and cooking. His book "On Food And Cooking: The Science And Lore Of The Kitchen" has been described as a Bible for home chefs around the world. His new book, "Nose Dive," is about the world's smells and how what we smell affects what we taste.

You know, there's so many, like, really strange commonalities that you found between things whose smells we consider kind of repulsive and unpleasant and things whose odors we find, you know, delightful and pleasing. And so let's get to cat pee. Some of the volatiles, the molecules that release the smell in cat pee are also found in certain foods. And there's something called cat ketones that I guess is a part of cat pee that's actually used as a flavor enhancer. So explain that for us. Why is an element that's actually found in cat pee used as a flavor enhancer?

MCGEE: Yeah. This was one of the great pleasures of delving into smells in general was the discovering over and over again that things that we enjoy in foods are actually found elsewhere in the world and as in as unlikely places as cat pee and human sweat, for example. So in the case of cats, animals use small molecules, smells to communicate with each other to attract or to repel. And in the case of cats, male cats in particular, to mark their territories, they'll put into their urine a particular molecule that in and of itself is too large to have a smell. But once it's released, microbes attack that larger molecule, break it up into smaller molecules. And it's those smaller molecules that have the effect of really making cat urine smell as strong as it does.

So it's a kind of natural time-release system. The cat releases the precursor to the smells, and then the microbes release the smells from the precursors. And it turns out that sulfur atoms are really distinctive, give molecules very strong smells, at least as we mammals perceive them. And so these molecules in cat urine are rich in sulfur. And it turns out that sulfur molecules are also important in many, many foods, meats in particular, roasted meats especially, but also tropical fruits, fruits that we think of as having kind of exotic flavors, exotic aromas, things like passionfruit, for example, and less exotic than passionfruit, blackcurrants.

And you can try this at home. You know, get some blackcurrant jelly and then taste it and see if you don't get a kind of - not a catty smell, but an unusual smell for a fruit. And so these sulfur compounds found in cats, also in human sweat, give another layer of aroma, another layer of interest to a number of foods that we would never associate with those things.

GROSS: Yeah. So I'm really getting a sense that what we smell affects how we taste, and what we smell is a function of a unique combination of a whole bunch of different molecules. So it sounds like there must be an infinite variety of these molecules, which is what contributes to so many different aromas and tastes.

MCGEE: Yeah. It's - I don't know that it's infinite, but it's a very large number for sure. And that's something just in general worth knowing about smells is that we think, for example, of the smell of blackcurrant jelly or of cat pee as being a specific smell. But, in fact, it's more like a chord in music. There are many, many notes that are contributing to that overall sensation that we get. And it's - we generally - we pay attention only to the chord. We just, you know, recognize that's cat pee, and I'm going to go in the other direction. But it's possible with some effort to sit there and kind of dissect a few notes out so that you can begin to appreciate the complexity of that overall chord.

GROSS: Let's get to fish. Everybody knows that some fish get too fishy. And sometimes you find that it's too fishy when you get it home and before you put it in the refrigerator or the freezer and you realize, wow, this fish really smells. I mean, I've brought home some fish where I've just thrown it right out because it smelled so much like ammonia. And I thought, this is not going to be good. What is it about fish that gives it that fishy smell either when it's fresh or, you know, if you've - sometimes after you - as you're cooking, it just smells too fishy, even if it smelled fresh when you brought it home.

MCGEE: Yeah. Fish are complicated, interesting creatures. And part of the reason that they have that particular smell is the fact that they live in the oceans. They live in water, saltwater. And they live for the most part in a very cold environment compared to what we're used to. And so those two factors actually help determine what it is that is in a fish that can then be broken down either by microbes or by cooking into fragments that we can smell.

And there are two different things that contribute to fishiness. One is a molecule that many fish - ocean fish make and keep in their tissues in order to balance the salt in the salt water so that they - the saltwater doesn't just dehydrate them. And when the fish are caught and the fish's own muscles and organs stop working, then microbes can get to work on those molecules. And they break that molecule down into something called trimethylamine, which is really fishy.

And so when you smell that strong ammonia smell from the fish right after you'd bought it, what you could have done was just wash it off because the microbes are generally working only on the surface of the fish. That's where most of that smell is coming from. And you can usually get rid of a lot of it simply by rinsing it off and then patting it dry. The other thing you can do if there's still a residue of that smell, just a little bit of it, what you can do is use any kind of acid. Lemon juice, of course, is the usual one.

But you could also use vinegar. You could use cream of tartar, which is an acid. And that turns out to change the chemical environment of the surface of the fish in such a way that the smelly molecule, trimethylamine, is still there, but it's no longer volatile. It's not able to fly through the air and into our nose. So even though it's there, we don't detect it, and it doesn't bother us.

MCGEE: Let's take another break here, and then we'll talk some more. If you're just joining us, my guest is Harold McGee. And his new book, "Nose Dive," is about the sense of smell and how smell contributes to taste. We'll be right back. This is FRESH AIR.


GROSS: This is FRESH AIR. Let's get back to my interview with Harold McGee, who's famous in the food world for his books on the science of food and cooking. His new book, "Nose Dive," is about the science of smell and how smells affect how foods taste.

Part of what you write about is how rain releases certain smells on soil or even on pavement. You know, one of my favorite musicals is "Guys And Dolls." And one of my favorite songs from the show is called "My Time Of Day." And there's a line from it - when the smell of the fresh-washed pavement comes up cool and fresh and clean. And I think you had to, like, grow up in the city to know, like, what the smell is of, like, fresh-washed pavement. And this dates back to the days when there used to be, like, washing trucks that would come and, like, spray water onto the streets and, of course, you know, rain. But they literally, like, washed the streets. What is it about, you know, rain or water that releases a different set of smells on pavement or soil?

MCGEE: Well, it has to do with the chemical difference between water and the things that have smells. So it turns out that these small molecules that trigger our sense of smell are, by and large, not friendly to water and vice versa. So you know how when you make an oil and vinegar dressing, you have to work hard to integrate the oil in the water because they naturally separate? Smell molecules are, generally speaking, much more like oil than like water. So they - if there's water around, they tend to go in the other direction.

And what's happening with what the scientists call wet up in nature or on a pavement or actually even in our mouths when we have something dry, when we eat something dry, including chocolate, is that the moisture actually drives the volatile molecules, the smell molecules out of the dry material that it's coming into contact with. And so we get this burst of aroma that simply wasn't evident before.

GROSS: Now that you've written a book that's a field guide to the world's smells, are you eating and cooking differently than you used to?

MCGEE: Well, I'm not really eating and cooking differently because of what I've been writing. I'm eating and cooking differently because the world has changed in the way that it has.

GROSS: You're talking about the pandemic?

MCGEE: Yeah. Yeah. I'm eating - I'm growing my own vegetables much more. I'm cooking more and, of course, going out much less and just enjoying the variety of sensations that foods can give us. You know, when we're locked up in our houses and not getting out into the world much, foods and the aromas of foods are one way to kind of get that sensory stimulation that we're missing.

GROSS: I've been cooking more since the pandemic because I can't go out to eat, although I do order out sometimes. But I think one of my many flaws as a cook is that I'm always in a hurry. So I'm really impatient. So I took - I cook everything on. It's like really high flame, just like get it done. Am I killing the food?

MCGEE: No. No, not necessarily. I mean, if you - it really is important to control the heat. And so just paying attention to what's happening, to what's going on. And actually, smell is a really important aspect of that. You know, we do use our eyes to judge the heat of a flame and, you know, how brown the surface of a piece of meat has gotten. But, you know, if you're doing something in the oven, you know, you're toasting and the toast didn't get quite brown enough the first time around, so you just push the button again. And then a minute and a half later, you start to smell smoke. Then your nose is telling you that things have gone too far. So just attentiveness to what's going on and imagining what's going on inside the food. We can see the surfaces, but what we have to be thinking about is whether the inside is being maltreated.

GROSS: You mentioned how the pandemic is changing, you know, what you eat and how much you cook because you're not, you know, going out to restaurants. Has it affected how you shop for food?

MCGEE: Yes, it has. My food buying habits have changed a lot since the pandemic. I don't go to supermarkets hardly at all these days, partly to try to support the smaller operations that are still going and to support restaurants that have turned themselves into little markets to survive. But then there's also this wonderful network of local fishermen who bring in their catch directly to neighborhoods. And we pay the fishermen directly, which means that we get fish that have been caught either the same day or the day before. They've been handled beautifully. The money that we pay goes directly to them and not to any middlemen. And so that's something that I feel very lucky to be able to participate in both to support the fishermen, but also because the quality of the fish is just beyond belief.

GROSS: In the years that you have written about the science of cooking, what do you think is one of the principles that you've proven or disproven that you think has had the most impact on other chefs and home cooks?

MCGEE: The one thing that I've heard about most often from other cooks, and particularly from chefs, is finding that - I wrote about back in the 1980s the idea that searing a piece of meat, cooking a piece of meat at a very high temperature to begin with, cauterizes, seals the outside of the meat so that when you then cook it through, you don't lose all the juices and the moistness and the tenderness that you would lose if you cooked at high temperature the whole time. So the idea was, sear a piece of meat at a very high temperature. Then turn the temperature down and let it cook through.

And it turns out that was not true. And that was actually shown many, many decades before I was writing about cooking. I just discovered that information and relayed it to the food world. And what I've heard from chefs in particular is that they had all been taught - this is now in the '80s and '90s - by their mentors, all of them professional chefs taught in the old-fashioned apprentice - master-apprentice way, that searing meat does seal in juices, that that was just, you know, one of the foundational ideas of meat-cooking in, particularly, continental restaurants.

And so what they told me was that to find out that that was not true didn't just mean that they now had better control over cooking meat, it meant that they realized that everything that they did in the kitchen had to be questioned. They couldn't rely on the master chef for accurate information on every last, little subject - that they had to think for themselves. They had to do their own experiments to make sure that what they were being taught or what they'd heard through the grapevine was actually true. So it changed their way of thinking. And I think that's something that I'm very happy about.

GROSS: Harold McGee, thank you so much for coming back to our show. It's really been a pleasure.

MCGEE: For me, too, Terry. Thanks so much.

GROSS: Harold McGee's new book is called "Nose Dive." After we take a short break, John Powers will review the new season of the Netflix series "The Crown," starring Olivia Colman as Queen Elizabeth II, with Gillian Anderson joining the cast as British Prime Minister Margaret Thatcher. This is FRESH AIR.