Book Review

Neurogastronomy

How the Brain Creates Flavor and Why It Matters

By
June 18, 2012

Have you ever tried to describe why you adore certain foods — cilantro, say — that some of your friends find cloying, dull, or disgusting? Why is taste so individual? What, exactly, are we experiencing when we taste?

Ask a scientist for help, and face another conundrum: Taste, for a scientist, is a very limited perception. There are five tastes, all sensed through the tongue: sweet, salty, sour, bitter, and umami. Most people, in my experience, can’t identify umami (the savory taste of the amino acid glutamate, discovered in 1908), and many can’t even clearly identify bitterness; they confuse it with sourness or astringency. The latter is a manifestation of “mouth feel,” which includes texture, temperature, and pungency, and is wholly different from “taste” as defined by food scientists.

Basic taste preferences are universal — even newborns smile when given sweet foods and show disgust at bitter foods — but we humans vary in our ability to taste particular compounds. Some of us are more sensitive tasters than others, because we have more taste receptors in our mouths. And although most people like crunchy foods, we vary in our preferences among other kinds of mouth feel — in our tolerance for very hot or cold food, for example, or for the fiery bite of chiles.

The whole experience that non-scientists call “taste” is, to a food scientist, better described as “flavor.” According to Gordon M. Shepherd, the author of Neurogastronomy: How the Brain Creates Flavor and Why It Matters, the most important component of flavor isn’t taste or mouth feel, but odor.

We smell our food in two ways, he explains. If you’ve ever taken a class in wine tasting, you’ve experienced this in a deliberate way. Start with a simple sniff, and you’re taking in smell molecules by the orthonasal route. Next, take a sip. Roll the liquid around in your mouth, bathing your taste buds, then swallow or spit. As you breathe out, you experience the flavor of the wine — or, actually, its aroma, sensed retronasally, as vaporized molecules travel through the back of your mouth into your nasal cavity. There they meet the same receptors that take in smells through sniffing.

A chart from Shepherd’s book.

The retronasal smells are a little different, though, because they exclude large molecules. And since they are experienced along with so many mouth sensations — in the lips, cheeks, tongue, palate, and jaws — our brains “refer” retronasal smells to our mouths.

According to Shepherd, flavor begins with the five tastes, the aromas, the touch sensations, and even the look and sound of food. But the whole flavor experience is actually created from all these sensations by the brain.

Shepherd has made a career of studying how this happens. Connected to the receptors in the nasal cavity is the adjacent olfactory bulb, a small protrusion at the base of the brain. In this bulb are a thousand or so glomeruli, each a site of convergence for nerve fibers from about 25,000 smell receptor cells. Each glomerulus is activated differently by different odors, so through various technologies beyond my ken, a smell can be mapped as a unique image.

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The olfactory bulb is connected to the olfactory cortex, an inch or so away on the underside of the brain, and here smells are remembered and compared. The olfactory cortex is connected in turn to the orbitofrontal area of the neocortex, just above the eyes. (The neocortex is where we carry out our highest cognitive functions, such as spatial reasoning and language.) Cells in the orbitofrontal cortex combine olfactory signals with other sensory signals stimulated by food in the mouth, and share them with other parts of the neocortex, such as the amygdala, which is involved in emotion, and parts of the prefrontal cortex that are involved in learning and decisions.

The human nose is far less sensitive than the snouts of quadripeds; we have only 35 percent, for example, of the functional smell-receptor genes of the lemur. But with our complex mental processing equipment, we are very good at analyzing smells and the other sensations involved with eating.

We naturally judge every food as somewhere in the range from delicious to foul. We can learn to like new flavors, and we may tire of old favorites. We learn which smells and tastes are complementary. From our innate and learned preferences derive entire cuisines.

As James Boswell wrote in the late 18th century, man is a "Cooking Animal," and Richard Wrangham explored this idea at length in his 2009 book Catching Fire. Cooking, writes Shepherd, has allowed us to take in more energy faster, increased our pleasure in eating, and driven us to wander the world over in search of new flavors. Our preference for cooked food has impelled us to organize societies to hunt, gather, and raise food; to store and protect it; to create vessels and utensils to prepare it; and to eat together.

Whether the search for flavor initiated the expansion of the primate brain, or whether the expansion of the primate brain turned humans into gourmets, our big brains and cooking skills developed together.

After struggling through the neuroscience in Shepherd’s book, I was happy to reach Part IV, “Why It Matters.” Here Shepherd explores the connections between flavor and emotion, memory, addiction, and nutrition. Searching hopefully through other scientists’ studies, though, he turns up disappointingly little.

He points out, unsurprisingly, that the same brain regions are stimulated by cravings for food as for mind-altering drugs. But why do some people overeat while others with abundant opportunity do not? The neurotransmitter dopamine is involved, somehow. So are hormones in the gut and bloodstream, though Shepherd doles them only a sentence.

He shares a psychologist’s diagram of the “sensory control system” for drug addiction and overeating; the obese person has enlarged circles for “drive” and “saliency” (the attractiveness of a stimulus) and a shrunken circle for “control.” But the brain circuits involved go unidentified.

Likewise, a study that found subjects can learn, and quickly unlearn, to detect glucose at low concentrations doesn’t merit the inference that eating a lot of sugar causes people to want even more.

Shepherd echoes Eric Schlosser’s thesis that fast-food meals maximize sensory stimulation and so cause caloric overload, but he overlooks values — such as speed, thrift, and security — that complicate our food choices. And he neglects to consider that a plastic-wrapped hamburger can seem dull beside a vibrant, aromatic, healthful bowl of pho.

Future developments in neurogastronomy will no doubt further inform our discussions of why we eat what we eat. Shepherd’s book is a good — if occasionally wobbly — step in that direction.

Linda Ziedrich has written several cookbooks. She keeps a large organic garden on her family homestead east of Albany, Oregon.

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1. by anonymous on Jun 27, 2012 at 3:01 PM PDT

I’ve come to believe that my sensitivity to hoppy, harsh, bitter beers (and horseradish and Dijon mustard, which taste the same to me) is because I have more taste receptors on my tongue. I find that most filtered IPAs sear my tongue and I can’t taste anything for quite awhile afterwards. But that means that I’ve tasted a lot of great other styles: ambers, browns, stouts, and Belgians. However, I wish there was a pill I could take so I could tolerate the flavors that others love, but I find harsh and offensive.

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