The following is an excerpt from Mary Roach’s Gulp: Adventures on the Alimentary Canal.

Of the four half-pints on the table between us, Langstaff prefers the lightest, a strawberry wheat beer. I like the IPA best, but to her that’s not a “sitting and sipping” beer. It’s something she’d drink with food.

I ask Sue Langstaff—sensory consultant to the brewing industry for twenty-plus years, twice a judge at the Great American Beer Festival—what she’d order right now if she had to choose between an IPA and a Budweiser.

“I’d get Bud.”

“Sue, no.”

“Yes!” First exclamation point of the afternoon. “People pooh-pooh Bud. It’s an extremely well-made beer. It’s clean, it’s refreshing. If you’re mowing the lawn and you come in and you want something refreshing and thirst-quenching, you wouldn’t drink this.” She indicates the IPA.

Of all the descriptors in the Beer Flavor Lexicon I brought with me today, Langstaff would apply just two to Bud: malty and worty. She warns me about equating complexity with quality. “All that stuff you read on wine bottles, in wine magazines, where they throw out a dozen descriptors? That’s not sensory evaluation. That’s marketing.”

Taste—as in personal preference, discernment—is subjective. It’s ephemeral, shaped by trends and fads. It’s one part mouth and nose, two parts ego. Even flavors that professional evaluators agree are “defects” can come to signify superior taste. Langstaff mentions a small brewery in northern California that has been taking its beers right up to the doorstep of defective, adding strains of bacteria known for their spoilage effects. Whether through exposure or a desire to ride the cutting edge, people can acquire a taste for pretty much anything. If they can come to like the smelly-foot stink of Limburger cheese or the corpsey reek of durian fruit, they can come to enjoy bacteria-soured beer. (One assumes there are limits, however. Leaving olive oil in contact with rotting sediment at the bottom of a tank can create flavors enumerated on Langstaff’s Defects Wheel for Olive Oil as follows: “baby diapers, manure, vomit, bad salami, sewer dregs, pig farm waste pond.”)

Because it’s hard for people to gauge quality by flavor, they tend to gauge it by price. That’s a mistake. Langstaff has evaluated wine professionally for twenty years. In her opinion, the difference between a $500 bottle of wine and one that costs $30 is largely hype. “Wineries that sell their wines for $500 a bottle have the same problems as wineries that sell their wine for $10 a bottle. You can’t make the statement that if it’s low-cost it’s not well made.” Most of the time, people don’t even prefer the expensive bottle—provided they can’t see the label. Paul Wagner, a top wine judge and founding contributor to the industry blog Through the Bunghole, plays a game with his wine-marketing classes at Napa Valley College. The students, most of whom have several years’ experience in the industry, are asked to rank six wines, their labels hidden by—a nice touch here—brown paper bags. All are wines Wagner himself enjoys. At least one is under $10 and two are over $50. “Over the past eighteen years, every time,” he told me, “the least expensive wine averages the highest ranking, and the most expensive two finish at the bottom.” In 2011, a Gallo cabernet scored the highest average rating, and a Chateau Gruaud Larose (which retails from between $60 and $70) took the bottom slot.

Unscrupulous vendors turn the situation to their advantage. In China, nouveau-riche status-seekers are spending small fortunes on counterfeit Bordeaux. A related scenario exists here vis-à-vis olive oil. “The United States is a dumping ground for bad olive oil,” Langstaff told me. It’s no secret among European manufacturers that Americans have no palate for olive oils. The Olive Center—a recent addition to the Robert Mondavi Institute for Wine and Food Science, on the campus of the University of California at Davis—aims to change that.

It starts with tastings. I don’t know which vineyard first ushered wine-tasting off the palates of vintners and into the mouths of everyday consumers, but it was a stroke of marketing genius. Wine-tastings spawn wine enthusiasts, wine collecting, wine tourism, wine magazines, wine competitions, (wine addictions,)—all of it adding up to a multibillion-dollar industry. Olive trees grow in the same climate and soil conditions as grapes. The olive oil people have been up in Napa Valley all along, going, “Hey, how do we get a piece of this action?”

In addition to hosting tastings, the Olive Center has hired Langstaff to train a new UC Davis Olive Oil Taste Panel. Taste panels (or flavor panels, as they are more accurately called) have typically been made up of industry professionals. Langstaff wants to open it up to novices, for the simple reason that know-nothings are easier to train than know-it-alls. The center has a call for apprentice tasters on its website. The “tryouts” are coming up. At least one know-nothing will be there for sure.

The Olive Center is smaller than its name suggests. It consists of a single office and a shared receptionist on the first floor of the Sensory Building at the Robert Mondavi Institute. Bottles of oil and canned olives line the tops of cabinets and have begun to colonize the wall-to-wall. There’s no room in the center to hold the tryouts, so they are taking place next door in the Silverado Vineyards Sensory Theater, the building’s lecture hall and classroom tasting facility. (Silverado helped fund it. Additionally, each seat has a sponsor, with the name engraved on a small plaque.)

Langstaff makes her entrance burdened like a pack mule. Three tote bags hang off her shoulders, and she wheels a multitiered cart crammed with oils, laptops, water bottles, and stacks of cups. She wears dun-colored pants, black sport sandals, and a short-sleeved shirt in the Hawaiian style, though without an island motif. She calls roll: twenty names. Of them, twelve will make the first cut, and six will go on to apprentice.

Langstaff lays out the ground rules for future apprentices: be here, be on time. Be agreeable. “We will be evaluating some nasty oils. You will have to put them in your mouth.[1] For the good of science. For the good of olive oil. We are here to help the producers, to tell them, What attributes does the oil have, does it have defects, what can they do differently next year—treat the olives better, pick them at a different time, et cetera.” There will be no pay. No one will reimburse for the seven-dollar parking-garage fee. The existing panelists are known to have some prickle, to borrow an official olive-oil sensory descriptor.

“You may be thinking, wow, I really don’t want to be on this thing.” The faint of heart are invited to pack up and go. No one moves.

“All right then.” Langstaff surveys the room. “Shields up.” She is referring to removable panels used to partition the room’s long tables into private tasting booths. This way, you aren’t influenced by the facial expressions (or test answers) of the people seated next to you. Hired sensory-science students move along the rows, pulling the panels out of slots in the front of the tables and sliding them into place, like helpers on a game-show set.

A plastic tray is set in front of each of us. The trays hold eight small lidded cups: our first test. Each cup holds an aromatic liquid. Swirl, sniff, identify. A few seem easy: almond extract, vinegar, olive oil. Apricot required two full minutes of deep thought. Others remain unfamiliar no matter how many times and how deeply I sniff. According to the journal Chemical Senses, a “typical human sniff” has a duration of 1.6 seconds and a volume of about two cups. I’m sniffing twice as hard. I’m sniffing the way clueless Americans try to make non-English speakers understand them by shouting. One aroma will turn out to be olive brine—the water from a bottle or can of olives. Reflecting the preponderance of olive people trying out today, an impressive thirteen out of twenty get this right.

Next is a “triangle test”: three olive-oil samples, two of them identical. Our task is to identify the odd one out. We are given paper cups of water for rinsing and, for spitting, large red plastic cups of the kind that litter the lawns and porches of frat houses on weekend mornings. The red here today perhaps serving as a warning: Do not drink! Langstaff sits at the front of the room, reading a newspaper.

It’s not going well here in the B.R. Cohn Winery seat. All three oils taste the same to me: a hint of freshly mown grass, with a peppery finish. I do not detect apple, avocado, melon, pawpaw, old fruit bowl, almond, green tomato, artichoke, cinnamon, cat urine, hemp, Parmesan cheese, fetid milk, Band-Aid, crushed ants, or any other olive-oil flavor, good or bad, that might set one of these oils apart. With time running out, I don’t bother spitting. I’m sipping oil like it’s tea. Langstaff glances at me over her glasses. I wipe my lips and chin with my palm, and a shiny smear comes away.

Our final challenge is a ranking test: five olive oils of differing degrees of bitterness. This proves a challenge for me, as I would not have described any of them as bitter. All around me, people make sounds like ill-mannered soup-eaters, aerating the oils to free the aromatic gases. I’m doing a mnyeh-mnyeh-mnyeh Bugs Bunny thing with my tongue, but it’s not helping. Well before the test period ends, I stop. I do something I’ve never done in my entire overachieving life. I give up and guess. I do this partly at the behest of my stomach, which is struggling to cope with the unusual delivery of a sizable amount of straight olive oil.

After everyone else leaves, Langstaff shares some of the group’s answers (with names removed). Those who performed well on the oil rankings—incredibly, several got it close to exact—also noted that aroma number 7, on the first test, was not just olive oil, but rancid olive oil. Four out of twenty people, all olive professionals, nailed that detail. (The oil smelled fine to me. I was right there with the numb-nose who wrote, on his answer form, “Oh, for a piece of good bread!”)

Here’s what I find interesting. The people who work with olives and olive oil, most of whom performed supernaturally well on the ranking and triangle tests, were occasionally stumped by some of the most common and, to me, obvious aromas. A woman who, in the initial sniff test, realized that the olive oil was “rancid, fusty” failed to recognize almond extract. She wrote, “Cranberry, fruity, sweet, aloe juice.” She described diacetyl, the smell of artificial (movie popcorn) butter, as “licorice, candy, bubble gum.” Those aren’t important flavors in the day-to-day of the olive world, so there’s no reason for her to know them. This supports what Langstaff said earlier. As with any language, proficiency builds with exposure and practice. (Though not quickly; the average training period for a sensory panelist is sixty hours.)

In my case, it won’t be happening any time soon. An e-mail from Langstaff arrives around nine that night. “Hi Mary. Hope you enjoyed the tryouts. Unfortunately you did not make the cut.”

Sensory analysis is not limited to the epicurean industries of Napa Valley. For any food or drink manufactured on a reasonably large scale, there are trained panelists and sensory descriptors. Poking around in the sensory-science journals, I have seen flavor lexicons for mutton, strawberry yogurt, chicken nuggets, ripening anchovies, almonds, beef, chocolate ice cream, pond-raised catfish, aged Cheddar cheese, rice, apples, rye bread, and “warmed-over flavor.”

The work entails more than just troubleshooting. Sensory analysts and panels help with product development. They keep the flavors of established products on track when a formula is altered—say, to lower the fat or salt content. They work with the market research staff. When focus groups of consumers prefer one version of, say, a ranch dressing over another (or over a competitor’s dressing), sensory evaluators may be brought in to figure out the salient attributes of the more popular item. The food scientists can then work backward from those attributes to tweak the formula.

Why use humans rather than lab equipment? Because the latter would yield dozens of chemical differences[2] between a pair of products. Without a human evaluator, it’s impossible to assign sensory meaning to them. Which of those dozens of differences in chemical makeup translates to a perceptible flavor shift, and which is below the threshold for human detection? Which ones, in short, make the difference in the consumer’s mouth and mind? “And you can’t ask the consumer,” says Langstaff. “You ask the consumer, ‘Why does it taste better?’ They say, ‘Because I like it better.’” The consumer’s flavor lexicon is tiny: yum and yuck.

Which product the sensory evaluator prefers, by the way, is irrelevant. He or she may not like any of them, or even the general category. (Langstaff, for instance, rarely drinks beer for pleasure.) “You don’t ask your gas chromatograph if it likes the olive oil it’s analyzing,” Langstaff told us at the tryouts. The goal is to be as neutral, as analytical—as “Mr. Spock”—as possible.

This perhaps explains how it was possible for a team of Canadian researchers to find nine men and women willing to create a canned-cat-food flavor lexicon and a set of tasting protocols. For humans. Tasting cat food. And they couldn’t be shy about it. The protocol for evaluating the “meat chunk” portion (“gravy gel” having its own distinct protocol) stipulated that the sample be “moved around mouth and chewed for 10 to 15 seconds, [and] a portion of the sample swallowed.”

The idea was to come up with a sort of code, a way to translate the mute preferences of cats. In theory, companies could use human tasters and sensory profiles of the foods cats like in order to predict the success of new formulations. In practice, the technique never really took off.

Because there was a concern that people with a “strong negative attitude” toward tasting cat food would drop out before the project ended, panel applicants at the initial screening were asked not only to describe the cat foods but also to rate them according to how much they liked them. (The average rating, I am gobsmacked to report, fell between “like mildly” and “neither like nor dislike.”) Thanks to this unusual data set, we now know that humans prefer cat food with a tuna or herbal flavor over cat food with the flavor descriptors “rancid,” “offaly,” “cereal,” or “burnt.”

But humans, as we are about to see, are not cats.

Mary Roach is the author of four previous books: Packing for Mars: The Curious Science of Life in the Void, Bonk: The Curious Coupling of Science and Sex, Spook: Science Tackles the Afterlife, and Stiff: The Curious Lives of Human Cadavers. Her writing has appeared in Outside, Wired, National Geographic, and the New York Times Magazine, among others. She lives in Oakland, California.

[1] It could be worse. In 1984, goat-milk flavor panelists were enlisted by a team of Pennsylvania ag researchers to sleuth the source of a nasty “goaty” flavor that intermittently fouls goat milk. The main suspect was a noxious odor from the scent glands of amorous male goats. But there was also this: “The buck in rut sprays urine over its chin and neck area.” Five pungent compounds isolated from the urine and scent glands of rutting males were added, one at a time, to samples of pure, sweet goat milk. The panelists rated each sample for “goaty” “rancid,” and “musky-melon” flavors. Simple answers proved elusive. “A thorough investigation of ‘goaty’ flavor,” the researchers concluded, “is beyond the scope of this paper.”

[2] Probably more. The Handbook of Fruit and Vegetable Flavors includes a four-page table of aroma compounds identified in fresh pineapple: 716 chemicals in all.