Neurogastronomy: How the Brain Creates Flavor and Why It Matters

I met a man from Denmark who had been to Disney World in Florida for a week with his friends.

“How did it go?” I asked. “Did you enjoy American culture?”

“Oh yes,” he said. “We had a great time.”

My wife is Danish, and I know how much a Dane loves food, so I kidded him by asking, “Did you enjoy eating hamburgers and hot dogs every day?”

“We never touched them,” he said.

“What in the world did you live off?” I asked.

“We brought with us a week’s supply of rugbrod [Danish pumpernickel] and Danish salami and Danish cheese. We even had some smoked eel.”

A week’s supply of that stuff would have weighed a ton. I know; I’ve lugged suitcases of rugbrod across the Atlantic myself when returning from trips to the Old Country. The deep dark aroma and bouncy chewiness of Danish rugbrod is unique. So is a bite of salty, meaty Danish salami, as well as the familiar flavor of Havarti cheese with dill. For me they’re a part of my adopted identity when I’m in Denmark. But for my Danish friends they were their native identity.

This story recognizes the old saying “Patriotism is a longing for the food of our homeland.” It expresses a loyalty to our home country based on the flavors of the food we were brought up on. When athletes in the Olympics play for the glory of their country, they also in a way play for the flavors of their country’s foods. Even in our age of globalization, when our daily fare may include dishes from other lands—think sushi in Los Angeles, pasta in New York, or “McDo” in Paris—the particular combinations we learn while growing up are part of our national identity. On a vacation trip in a foreign country, how strong is our desire, after tasting a few meals of the local food, for a burger, or fish and chips, or sushi, or lasagna, or… you fill in the blank. I like soya flavor, but on a trip to Japan, after days of around-the-clock soya, I yearned for a familiar flavor, so Grethe bought me small cartons of cold cereal and milk that I could have for my traditional breakfast.

Given this importance of the flavors we learn to like, it seems to me remarkable, and unfortunate, that most people are unaware that the flavors are due mostly to the sense of smell and that they arise largely from smells we detect when we are breathing out with food in our mouths. Few people know how modern research shows that an odor sets up patterns of activity—“smell images” in our brains—that are the main basis for our perception of flavors. These smell images are hidden factors that determine most of the pleasure we get from eating, and they share the blame for the problems we incur when eating foods that are not good for us. If we can understand better the central role that smell plays, we can understand better how to reduce the problems and increase the joy.

To appreciate the importance of retronasal smell in our lives, let’s step back and look at how far we’ve come in changing ideas that go back to the ancient Greeks.

Most people regard the sense of smell as not very important. The legacy of thinking of smell in this way began with Aristotle. In discussing the senses in De Anima (On the Soul), he observed that “our sense of smell is inferior to that of all other living creatures, and also inferior to all the other senses we possess.”

If you’re laboring under this misapprehension 2,500 years later, you’re not the only one. Like almost everyone else, you probably enjoy pleasant scents, such as attractive perfumes, fragrant flowers, and a steak barbecuing on the grill, and you do not like unpleasant smells, such as body or bathroom odors and polluted air. And that is about the limit of how much regard you give to smell.

These impressions seem minor compared with the important roles played by our sight and hearing. Against these, smell can seem trivial, although not if you suddenly lose your sense of smell because of an accident or infection. In her book Remembering Smell: A Memoir of Losing—and Discovering—the Primal Sense, Bonnie Blodgett has described the devastating loss of flavor that may occur. Most of that loss is due to retronasal smell. Still, if forced to give up one of the senses, who would choose a life without vision or hearing rather than smell? Sight and sound are obviously the essential senses for normal living and use of language.

But this is putting the question in the wrong way. What are the factors that shape our daily behavior? Which inputs to our brains are the motivating forces that determine the quality of our daily lives and that influence the decisions we make about health, diet, mates, and social relations? If we stick with only our obvious sensations, we miss the deeper factors. Among them, smell plays powerful but hidden roles.

As discussed in the Introduction, the new evidence regarding these roles for smell comes from work in many fields. Taken together, all these studies reflect how the sense of smell and associated flavor engage an astounding extent of the human brain. This work is not only intriguing to the general public, but involves profound insights into our biological nature. Many research workers are realizing that the sense of smell is ripe for investigation as one of the most exciting frontiers in the brain. They are intrigued that this system may hold the key to unlocking many of the secrets of our body. This was already realized decades ago by the physician and essayist Lewis Thomas: “I should think we might fairly gauge the future of biological science, centuries ahead, by estimating the time it will take to reach a complete, comprehensive understanding of odor. It may not seem a profound enough problem to dominate all the life sciences, but it contains, piece by piece all the mysteries.”

Current studies are already revealing capabilities of human smell that go far beyond the traditional view. Rather than being weak and vestigial, human smell appears to be quite powerful. Some have even suggested that humans and their primate relatives are “supersmellers” among animals. It is time, therefore, for a new appreciation of this much maligned and neglected sense. The aim of this book is to show how the real power of human smell lies in its key role in human flavor.

Despite the dim view that Aristotle took of the sense of smell, this sense, through its dominant role in flavor, had its revenge by shaping the course of world empires during human history. Eric Schlosser, in Fast Food Nation: The Dark Side of the All-American Meal, was very clear about the dominant role of smell in flavor: “‘[F]lavor’ is primarily the smell of gases being released by the chemicals you’ve just put in your mouth.” And, he was also clear about the spell that flavor exerts over humans:

The human craving for flavor has been a largely unacknowledged and unexamined force in history. Royal empires have been built, unexplored lands have been traversed, great religions and philosophies have been forever changed by the spice trade. In 1492 Columbus set sail to find seasoning. Today the influence of flavor in the world market-place is no less decisive. The rise and fall of corporate empires—of soft drink companies, snack food companies, and fast food chains—is frequently determined by how their products taste.

Thus not only is flavor not recognized for the force it has been in human history, but smell has not been recognized for the dominant part it played. This despite the fact that flavorful herbs and spices were essential to Roman cuisine more than 2,000 years ago. As the quotation indicates, the quest for new flavors and aromas was one of the driving forces behind long-distance travel and the opening of trade routes. It is not widely appreciated that a millennium before Marco Polo “discovered” China, the spice trade of the Roman Empire stretched from the Mediterranean to China and the South Pacific, delivering to southern European tables delightful flavors and aromas to enliven their daily fare. The voyage of Christopher Columbus was not embarked on to prove that the world was round, but to find a more direct route to the sources of spices because of their flavors.

The voyagers who followed in the sixteenth and seventeenth centuries were seeking not just gold, but also control of spice trade and production. The atrocities committed over several centuries in the desperate attempts to control those sources are among the worst of any time. On the positive side, the case has been made that the organization of the great sea lanes for shipping tea and spices by fast clippers from Indonesia in the eighteenth and nineteenth centuries laid the basis for the British Empire, followed by the emergence of world powers and their economies in the nineteenth and twentieth centuries. We have inherited and adapted those global trade routes, from controlling the trade in tea and spices to controlling the sources of oil and spreading global capitalism in the past century.

There has thus been a contrast between the received wisdom that smell and flavor are of minor importance and the realities of the high value that humans put on them in their daily lives, as well as of the consequent economic forces that have driven human societies.

This book will provide new insights into why the smell and flavor of the foods we eat have such high value in human affairs. There have been some key steps along the way to our present understanding.

It was not until Jean Anthelme Brillat-Savarin that smell began to be appreciated, particularly for its role in taste, equivalent to flavor. Born in 1755, Brillat-Savarin was a lawyer and then a mayor. During the French Revolution, he was forced to flee France, spending two years in the United States. He then returned after the defeat of the Jacobins in 1796 to become a judge under Napoleon. His new position gave him plenty of free time to spend on writing about his main passion, eating well, otherwise known as being a gourmand. Shortly before his death in 1826, he published his reflections as a book whose full title is Physiologie du goût; Ou, meditations de gastronomie transcendante: ouvrage théorique, historique et a l’ordre du jour, Dédiée aux gastronomes parisiens, par un Professeur, Membre de Sociétés Litteraires et Savants (The Physiology of Taste; Or, Meditations on Transcendental Gastronomy: Theoretical, Historical, and Practical Work, Dedicated to Parisian Gastronomes, by a Professor, Member of Literary and Scientific Societies).

As implied by the elaborate title, the book was not strictly a scientific treatise. However, in its thoroughness in considering from many angles the pleasures of eating, and in one delightful anecdote after another illustrating how human society is dependent on the social interactions that take place while eating meals together, Brillat-Savarin’s book became a classic. Part of this was due to his way with words; from him came such famous observations (in M. F. K. Fisher’s well-known translation) as

Tell me what you eat, and I will tell you what you are.

The discovery of a new dish does more for human happiness than the discovery of a new star.

Of most interest for our purpose was his passion for revealing the physiological and psychological processes that are responsible for the perception of taste. The taste in his title refers not specifically to the sense of taste, but to the combined perception of taste and smell, which we refer to as flavor. Brillat-Savarin acknowledged the dominant role of smell in taste and flavor:

I must concede all rights to the sense of smell, and must recognize the important services which it renders to us in our appreciation of tastes; for, among the authors whose books I have read I have found not one who seems to me to have paid it full and complete justice.

For myself, I am not only convinced that there is no full act of tasting without the participation of the sense of smell, but I am also tempted to believe that smell and taste form a single sense.

Brillat-Savarin thus identified clearly the important role of smell in taste, but unfortunately didn’t differentiate clearly between taste as a single sense and “taste” as a combined sense of smell and taste. That is why we will call the combined sense “flavor.” The close relation between smell and taste has obscured the dominant role of smell. A main aim of this book is to disentangle them.

Brillat-Savarin recognized that smell’s contribution to taste could come only from the smell arising at the back of the mouth and being swept into the nasal chamber by what we now call the retronasal route. He did not specify this route, however, or that it could be activated only by breathing out. He did declare in a colorful phrase that the back of the throat was the “chimney” of taste.

Recognition that smell’s contribution to flavor came from retronasal smell and breathing out was slow in coming. In his book What the Nose Knows, Avery Gilbert mentions Henry T. Finck, an American philosopher who in 1886 published the essay “The Gastronomic Value of Odours” in the magazine Contemporary Review, in which he described how swallowing pushes the aromas from the food in our mouths into the air in the back of our throats and how exhalation carries it through the nasal chambers as “our second way of smelling.” However, most interest in smell through the nineteenth and into the twentieth century was focused on trying to break down inhaled odors into a few basic categories, in analogy with the basic colors in vision. Odors, however, have been too numerous and difficult to categorize for this to be a practical venture.

The role of retronasal smell in flavor was finally put on the map by Paul Rozin, a psychologist at the University of Pennsylvania, in an article in 1982. As he phrased it, we need to recognize that smell is not a single sense but rather a dual sense, comprising orthonasal (breathing in) and retronasal (breathing out) senses. He devised experiments to show that the perception of the same odor is actually different depending on which sense is being used. Subjects trained to recognize smells by sniffing them had difficulty recognizing them when they were introduced at the back of the mouth.

Orthonasal smell is the one we commonly think of, and for good reason. It mediates a tremendous range of stimuli, evoking our sensations of the aromas of food, especially cooking food; the bouquets of wines; the fragrances of flowers; the scents of perfumes; the mysteries of incense. It also mediates the social odors: sweet scents of a loved one’s breath; body excretions in sweat; volatile compounds in urine and feces; pheromone-type molecules that send signals about gender, puberty, territory, and aggression. And there are alarm signals such as for fire or gas. With these functions, all of them obvious when we detect them during normal breathing or when sniffing the air, no wonder orthonasal smell has been the main type of smell recognized over the centuries.

Retronasal smell contrasts in so many ways with orthonasal smell that it can truly be considered a separate type of smell. To begin with, it arises from inside the mouth; it is the only distance sense that arises inside the body. Second, because it arises inside the mouth, it is always accompanied by stimulation of other senses inside the mouth, mainly taste and touch. Also, because the food is inside the mouth, retronasal smell always is accompanied by movements of the tongue, the jaw muscles, and the cheeks; it is an active sense by comparison with external smell and the other distance senses, vision and hearing. Chewing releases the smell molecules from the food to be carried in the smell-laden air to the nasal chambers to stimulate the smell receptors. And astonishingly, the sense of flavor produced is a mirage; it appears to come from the mouth, where the food is located, but the smell part, of course, arises from the smell pathway. No wonder it has taken so long to begin to realize what an amazing sense retronasal smell is.