Neurogastronomy: How the Brain Creates Flavor and Why It Matters

I have claimed that smell is the main component of flavor, and that it is stimulated through the retronasal route. What is the proof?

Psychologists and food scientists answer this question by devising sophisticated methods to introduce food volatiles at the back of the mouth, synchronized with expiration, to show activation of smell responses by the retronasal route. But there is a simpler test, one that is often used by psychologists when visiting schools and giving demonstrations of scientific principles in science class. It’s called the nose-pinch test, usually done with a piece of candy, but you can use any small, tasty morsel, even a dab of spicy sauce at the dinner table.

To perform this test, take the “tasty” bit and, while holding your breath, open your mouth and place the bit on your tongue while pinching your nose so that you can breathe only through your mouth. If you have good control, you can also prevent air from entering your nose by lifting the palate at the back of your mouth while you continue to breathe through your mouth. The requirement by any method is to prevent any air you are breathing out (exhaling) from going through your nose.

With the bit on your tongue, you will be able to identify certain senses. If it is candy you’ll be able to detect the sweet taste due to the sugar. You will also know from the sense of touch that the bit is resting on your tongue, and where it is. If it is a morsel of food, your sense of touch will tell you whether it is hard or soft, or maybe hot or cool. If you sense any more than this, you are probably not holding your breath!

Now unpinch your nose, or lower your palate. Immediately you will experience the flavor of the candy or of the meat or other food or spice you may have chosen. The sudden whiff of flavor can be surprising. Almost without realizing it, you will have let the air shoot from the back of your mouth the short distance through the nasopharynx to the smell receptors in the nasal cavity to stimulate your sense of smell. The faintest whiff of outward breathing will produce this result. In fact, the diffusion of the volatile molecules in the air of the nasopharynx will account for much of the sensation.

This experiment, and others by more scientific methods, tells you several key things about the role of smell in flavor:

1. It is clear that if there is no breathing out, there is no smell or flavor. You can confirm this by breathing in instead of out as soon as you unpinch your nose: there will be no sensation of flavor; it happens only with breathing out. This proves that the smell component of flavor is due exclusively to retronasal smell.

2. The ability to identify the type of flavor, such as lemon or strawberry, is clearly attributable to the sense of smell, which works in conjunction with the sweet taste and the senses of touch.

3. Perception of the retronasal smell is usually fused with the taste and touch so that we are unaware of it as smell. This is why the importance of smell in flavor has gone unrecognized for so long.

4. You have just proved that smell is the major component of flavor. Although sweet is a strong sensation associated with candies and other foods that contain sugar, sugar by itself is sweet and little else. The actual flavor of the candy depends on the sense of smell.

5. Even though by unpinching your nose you proved that the flavor of the candy is due to the smell you sense in your nose, when the smell is fused with the taste and touch of the candy in your mouth it appears to come from the mouth. Not only is smell not recognized as a part of flavor, the flavor is not even recognized as coming from the nose. Rather, it is perceived as coming from the mouth. The mouth has taken all the credit!

Why should this be? Where else do we have a sense that is divided into two and one of them is hidden among other senses? Scientists are only beginning to realize that this is an interesting problem for psychology, for neuroscience in general—and for determining the food we buy and consume.

We do not know the answer, but it will surely involve the fact that when we ingest food, we put it in our mouths. Although we take this simple act for granted, when it is looked at from the perspective of animal behavior it is seen as being symbolically and practically a highly significant behavior. Something is taken from the outside world and inserted into the body. The animal has acquired it as a potential foodstuff—by killing another animal, or consuming some grass, or picking some fruit. The foodstuff looked as though it would be flavorful and nutritious, but it needs to be tested.

The touch system tells us the food is in our mouths; our attention is therefore focused on what is in our mouths as we make an assessment of it. That assessment involves, first, assuring ourselves that this is something we want to take into our bodies. Does it have anything unpleasant about it—a fish bone, a rotten taste? Is it too salty or sour or possess a bitterness that might signal a toxic or poisonous substance? Is it too hot or cold? The list is long. Second, if it passes the aversion test, is it something we like? If so, we lavish even more attention on it, because a desired flavor is one of the greatest of human joys.

Because our attention is on the food in our mouths, our perception follows our attention. It appears that this is what happens to retronasal smell. The attention is on the mouth, and the smell perception, due to the molecules released from the mouth, is merged with the other senses in making the assessment.

Because the smell in the nose is sensed as if it is coming from the mouth, it is a property of the nervous system known as referred sensation, which occurs when a sense appears to be in one place but actually arises in another. In this case, the sensation is not only referred to another place, but hidden in the senses from that place.

Does it matter? Assessing smell within flavor gives us new insight into how complicated our sense of smell is. We have seen that smell is actually two senses, one for breathing in and one for breathing out. The one for breathing in is the sense of smell we all recognize as a single sense emanating from our nose. However, the one for breathing out is never recognized as a distinct sense, because it is always fused with two other senses, taste and touch, to form a third sense—flavor, which is referred to another part of the body, the mouth. Retronasal smell is unique among all our senses in these respects. If, as Brillat-Savarin maintained, “Tell me what you eat and I’ll tell you what you are” is true, it follows that retronasal smell is the hidden factor that makes us what we are.

From the point of view of flavor, the unique properties of retronasal smell pose new challenges in understanding the flavors of the food we eat. Our nose-pinch test shows that you can explore these challenges yourself to reveal more clearly the contribution of retronasal smell to the flavors of what you are consuming. Separating retronasal smell from overall flavor would seem to be especially of interest to a food scientist or a gourmet cook, who wants to understand the flavor of the dish that is being cooked by analyzing each sensory property and how it arises from the molecular composition of the dish. This is one of the avowed aims of the new field of “molecular gastronomy.” I think we can see the beginnings of a close collaboration with neurogastronomy.

Given that retronasal smell is stimulated by the release of smell molecules from our ingested food, one would suppose that we would sense the smell of our food separately from the stimulation of the taste buds on the tongue and the stimulation of touch receptors in the mouth. The nose-pinch test has shown that the smell can be sensed separately only by holding your breath and then releasing it to isolate the smell component of the flavor. Otherwise, the only thing you sense when eating your food is the flavor as a unified sense.

Flavor is thus the unified percept that we strive for in cooking a dish. We try to determine whether it was made this time with too much or too little salt, or enough curry, or was cooked too long or not enough, or any of the dozens of variables possible. Smell has the property of being, in general, “synthetic”; that is, a mixture of several smells makes a new unified smell. It is not “analytic,” the way taste is: sweet and sour tastes sweet and sour rather than being a new unified taste.

The synthetic quality of smell challenges us to explain how the smell pathway discriminates its stimulus molecules, so that we can understand its contribution to flavor. I start with an introduction to the basic types of flavor molecules in the foods we eat that activate the sense of retronasal smell.