Gut Wrench: Human Digestion 101

This interview originally aired live on April 3, 2013. An edited version was re-aired on August 9, 2013 as part of a special episode of The Brian Lehrer Show. 

Mary Roach tells you everything you every wanted to know (and maybe some stuff you didn’t) about human digestion. In Packing for Mars, she wrote about space toilets and for RadioLab she stuck her hand inside a real-live cow’s stomach to experience digestion from the inside. Her new book is Gulp: Adventures on the Alimentary Canal.

EVENT: Mary Roach reads from Gulp tonight at the Union Square Barnes & Noble on 17th St. at 7PM.

Excerpt from Gulp (a "taste" of Chapter 1)  Nose Job: Tasting Has Little to do with Taste

The sensory analyst rides a Harley. There are surely many things she enjoys about traveling by motorcycle, but the one Sue Langstaff mentions to me is the way the air, the great and odorous out-of-doors, is shoved into her nose. It’s a big, lasting passive sniff .* This is why dogs stick their heads out the car window. It’s not for the feeling of the wind in their hair. When you have a nose like a dog has, or Sue Langstaff, you take in the sights by smell. Here is California’s Highway 29 between Napa and St. Helena, through Langstaff’s nose: cut grass, diesel from the Wine Train locomotive, sulfur being sprayed on grapes, garlic from Bottega Ristorante, rotting vegetation from low tide on the Napa River, toasting oak from the Demptos cooperage, hydrogen sulfide from the Calistoga mineral baths, grilling meat and onions from Gott’s drive-in, alcohol evaporating off the open fermenters at Whitehall Lane Winery, dirt from a vineyard tiller, smoking meats at Mustards Grill, manure, hay.

Tasting—in the sense of “wine-tasting” and of what Sue Langstaff does when she evaluates a product—is mostly smelling. The exact verb would be flavoring, if that could be a verb in the same way tasting and smelling are. Flavor is a combination of taste (sensory input from the surface of the tongue) and smell, but mostly it’s the latter. Humans perceive five tastes—sweet, bitter, salty, sour, and umami (brothy)—and an almost infinite number of smells. Eighty to ninety percent of the sensory experience of eating is olfaction. Langstaff could throw away her tongue and still do a reasonable facsimile of her job.

Her job. It is a kind of sensory forensics. “People come to me and say, ‘My wine stinks. What happened?’” Langstaff can read the stink. Off-flavors—or “defects,” in the professional’s parlance—are clues to what went wrong. An olive oil with a flavor of straw or hay suggests a problem with desiccated olives. A beer with a “hospital” smell is an indication that the brewer may have used chlorinated water, even just to rinse the equipment. The wine flavors “leather” and “horse sweat” are tells for the spoilage yeast Brettanomyces.

The nose is a fleshly gas chromatograph. As you chew food or hold wine in the warmth of your mouth, aromatic gases are set free. As you exhale, these “volatiles” waft up through the posterior nares—the internal nostrils* at the back of the mouth—and connect with olfactory receptors in the upper reaches of the nasal cavity. (The technical name for this internal smelling is retronasal olfaction. The more familiar sniffing of aromas through the external nostrils is called orthonasal olfaction.) The information is passed on to the brain, which scans for a match. What sets a professional nose apart from an everyday nose is not so much its sensitivity to the many aromas in a food or drink, but the ability to tease them apart and identify them.

Like this: “Dried cherries. Molasses—blackstrap.” Langstaff is sniffing a strong, dark ale called Noel. We are at Beer Revolution, an amply stocked, mildly skunky†bar in Oakland, California, where I have an office (in the city, not the bar) and Langstaff has a parent in the hospital. She could use a drink, and we have four. For demonstration purposes.

In general, Langstaff isn’t a talky person. Her sentences present in low, unhurried tones without italics or exclamation points. The question “Which beer do you want, Mary?” went down at the end. When she puts her nose to a glass, though, something switches on. She sits straighter and her words come out faster, lit by interest and focus. “It smells like a campfire to me also. Smokey, like wood, charred wood. Like a cedar chest, like a cigar, tobacco, dark things, smoking jackets.” She sips from the glass. “Now I’m getting the chocolate in the mouth. Caramel, cocoa nibs . . .”

I sniff the ale. I sip it, push it around my mouth, draw blanks. I can tell it’s intense and complex, but I don’t recognize any of the components of what I’m experiencing. Why can’t I do this? Why is it so hard to find words for flavors and smells? For one thing, smell, unlike our other senses, isn’t consciously processed. The input goes straight to the emotion and memory centers. Langstaff’s first impression of a scent or flavor may be a flash of color, an image, a sense of warm or cool, rather than a word. Smoking jackets in a glass of Noel, Christmas trees in a hoppy, resinous India pale ale.

It’s this too: Humans are better equipped for sight than for smell. We process visual input ten times faster than olfactory. Visual and cognitive cues handily trump olfactory ones, a fact famously demonstrated in a 2001 collaboration between a sensory scientist and a team of oenologists (wine scientists) at the University of Bordeaux in Talence, France. Fifty-four oenology students were asked to use standard wine-flavor descriptors to describe a red wine and a white wine. In a second round of tasting, the same white wine was paired with a “red,” which was actually the same white wine yet again but secretly colored red. (Tests were run to make sure the red coloring didn’t affect the flavor.) In describing the red-colored white wine, the students dropped the white wine terms they’d used in the first round in favor of red wine descriptors. “Because of the visual information,” the authors wrote, “the tasters discounted the olfactory information.” They believed they were tasting red wine.

Verbal facility with smells and flavors doesn’t come naturally. As babies, we learn to talk by naming what we see. “Baby points to a lamp, mother says, ‘Yes, a lamp,’” says Johan Lundström, a biological psychologist with the Monell Chemical Senses Center in Philadelphia. “Baby smells an odor, mother says nothing.” All our lives, we communicate through visuals. No one, with a possible exception made for Sue Langstaff, would say, “Go left at the smell of simmering hotdogs.”

“In our society, it’s important to know colors,” Langstaff says over a rising happy-hour din. We need to know the difference between a green light and a red light. It’s not so important to know the difference between bitter and sour, skunky and yeasty, tarry and burnt. “Who cares. They’re both terrible. Ew. But if you’re a brewer, it’s extremely important.” Brewers and vintners learn by exposure, gradually honing their focus and deepening their awareness. By sniffing and contrasting batches and ingredients, they learn to speak a language of flavor. “It’s like listening to an orchestra,” Langstaff says. At first you hear the entire sound, but with time and concentration you learn to break it down, to hear the bassoon, the oboe, the strings.*

As with music, some people seem born to it. Maybe they have more olfactory receptors or their brain is wired differently, maybe both. Langstaff liked to sniff her parents’ leather goods as a small child. “Purses, briefcases, shoes,” she says. “I was a weird kid.” My wallet is on the table, and without thinking, I stick it under her nose. “Yeah, nice,” she says, though I don’t see her sniff. The performing-chimp aspect of the work gets tiresome. While not discounting genetic differences, Langstaff believes sensory analysis is mainly a matter of practice. Amateurs and novices can learn via kits, such as Le Nez du Vin, made up of many tiny bottles of reference molecules: isolated samples of the chemicals that make up the natural flavors. A quick word about chemicals and flavors. All flavors in nature are chemicals. That’s what food is. Organic, vine-ripened, processed and unprocessed, vegetable and animal, all of it chemicals. The characteristic aroma of fresh pineapple? Ethyl 3-(methylthio) propanoate, with a supporting cast of lactones, hydrocarbons, andaldehydes. The delicate essence of just-sliced cucumber? 2E,6Z-Nonadienal.The telltale perfume of the ripe Bartlett pear? Alkyl (2E,4Z)-2,4-decadienoates.

*A few words on sniffing. Without it (or a Harley), you miss all but the most potent of smells going on around you. Only 5 to 10 percent of air inhaled while breathing normally reaches the olfactory epithelium, at the roof of the nasal cavity. Olfaction researchers in need of a controlled, consistently sized sniff use anolfactometer to deliver “odorant pulses.” The technique replaces the rather more vigorous “blast olfactometry” as well as the original olfactometer, which connected to a glass and aluminum box called the “camera inodorata.” (“The subject’s head was placed in the box,” wrote the inventor, alarmingly, in 1921.)

* An Internet search on the medical term for nostrils produced this: “Saveon Nasal Nares! Free 2-day Shipping with Amazon Prime.” They really are taking over the world.† “Skunky” is between “rotten egg” and “canned corn” on the Defects Wheel for Beer. (Langstaff designed diagnostic wheels for off-flavors in wine,beer, and olive oil.) In the absence of skunks, a mild rendition of skunkiness is achieved by oxidating beer, that is, exposing it to air, as by spilling it or leaving out half-filled glasses.

* In 2010, inventor George Eapen and snack-food giant Frito-Lay took the comparison beyond the realm of metaphor. They patented a system whereby snack-food bags could be printed with a bar code allowing consumers to retrieve and download a fifteen-second audio clip of a symphonic interlude,with the different instruments representing the various flavor components. Eapen, in his patent, used the example of a salsa-flavored corn chip. “A piano intro begins upon the customer’s perception of the cilantro flavoring. . . . The full band section occurs at approximately the time that the consumer perceives the tomatillo and lime flavors. . . . A second melody section corresponds to the sensation of the heat burn imparted by the Serrano chili.” U.S. Patent No. 7,942,311 includes sheet music for the salsa-flavored chip experience.

Reprinted from Gulp: Adventures on the Alimentary Canal by Mary Roach. Copyright © 2013 by Mary Roach. With permission of the publisher, W.W. Norton & Company, Inc.