(FORTUNE Magazine) – The room is white--pristinely white. We're inside the sensory panel room at Wendy's headquarters in Dublin, Ohio, where employees of the $7.3 billion fast-food giant get to taste-test new products before they're rolled out to the masses. Here, as in other corporate test kitchens across the country, the future of fast food is decided.

Eight people file into the room, which receives regular blasts of fresh air from its own separate ventilation system so as not to sully the testers' nostrils with scents from elsewhere in the building. Each person sits in a small partitioned space, akin to a voting booth, with a touchscreen at eye level. Testers use the screen to provide immediate feedback on various criteria, including appearance, aroma, texture, and overall liking. Water is provided to cleanse their palates.

On the menu today is not a new kind of burger or fries but something much more exotic: mandarin orange slices. And if you're wondering why the nation's second-largest fast-food chain would be thinking about putting orange slices on its menu, then you haven't been paying much attention. The $144 billion fast-food industry, which serves almost a third of all American adults every day, is in the midst of a tectonic shift. Or so, at least, it appears. After all, two-thirds of American adults are now officially overweight, and the fast-food industry has been targeted as the primary villain in the obesity crisis. In the past few years it has been slapped with numerous lawsuits on behalf of overweight kids and has been the subject of powerful polemics in the media, such as the 2002 bestseller Fast Food Nation and the recent documentary Super Size Me. Is it any wonder, then, that Wendy's wants to put orange slices on its menu?

But it turns out to be awfully hard to devise healthier menu items that can work in a fast-food context. Fast food has to be, well, fast. It has to taste good. And it has to taste the same in California, Maine, and all points between. Just getting that much accomplished can be an operational nightmare--never mind whether the food is healthier or not.

Consider again those orange slices. They are intended to be an alternative to French fries--and originally Wendy's had planned to offer melon cubes instead of oranges. But the company couldn't find a dependable year-round supply of fresh melon, and the amount of time that would be consumed slicing the melons each day was unacceptable by fast-food standards. So Wendy's switched to oranges, which it already had in its supply chain for its salads. Although oranges bring their own set of operational difficulties, those seemed --at least on this day in early May--ultimately solvable.

Yet even assuming that the logistical difficulties can be worked out, the company will face the toughest test of all: Will customers actually order orange slices in place of French fries? Or will orange slices turn out to be just one more flop in the long history of healthy fast food that no one wanted?

For here is the most powerful stumbling block of all when it comes to getting people to eat healthier fast food. Despite the popularity of the Atkins and South Beach diets, the more stringent recommendations about cholesterol levels, and the ongoing concerns about an obesity crisis in America, most of us don't really want to eat healthier when we walk into a fast-food establishment. The great paradox of consumer behavior--that we don't necessarily do what we say we're going to do --seems almost doubly true when it comes to fast food. Even now, the overwhelming choice of fast-food customers are burgers, fries, and soda--or rather, cholesterol-laden burgers, fatty fries, and sugary soda--just as they've always been, according to market researcher NPD Foodworld. That's a truth not lost on the menu developers who work for McDonald's, Burger King, Wendy's, and the rest.

This is a dilemma the industry is trying to finesse in a half-dozen ways. It's adding new menu items that are genuinely healthier--knowing they're likely to be poor sellers. It's trying to make the greasy stuff marginally healthier through the wizardry of food science, though thus far without much success. But it's also offering menu alternatives that create the illusion of being healthier even though they aren't. And all the while it's continuing to peddle burgers, fries, and soda. Because after all, the customers are always right. Aren't we?

If you want to understand why the fast-food industry is so stuck on greasy burgers and fries--and why it's so reluctant to fully embrace health trends--a little history will help. In 1981 a new fast-food chain called D'Lites hit the scene, offering lean burgers on multigrain buns and a big salad bar. The Atlanta-based chain's founder, Doug Sheley, thinking that healthier fast food would resonate with an increasingly fitness-crazed America, boldly predicted that he would have 1,000 locations and sales of $1 billion by 1991.

While some were skeptical--"fat, er, slim chance," sneered one business magazine--D'Lites generated enough buzz to warrant an IPO in September 1984, and its stock zoomed on the first trading day. The fast-food giants took notice. Wendy's soon offered up a multigrain bun, Burger King tried a low-sodium pickle, and McDonald's even tested a Lite version of its signature Big Mac. "Everyone is getting on the bandwagon," said a USDA nutritionist at the time. But not for long. Customers may have been intrigued by the novelty of D'Lites, but they didn't come back for seconds. D'Lites started posting losses in 1985 and filed for Chapter 11 in August 1986; it dissolved in 1987. "Their audience wasn't as broad as they had hoped," deadpanned one analyst.

Throughout the 1990s sporadic attempts to sell healthier fast food met a similar fate (see table at right). The McLean Deluxe burger from McDonald's, a 91% fat-free patty containing the seaweed derivative carra-geenan, was perhaps the most high-profile disaster. Taco Bell's Border Lights menu flopped too. One reason was taste. "A lot of companies were chasing the nutritional content and not the taste bud," says Credit Suisse First Boston analyst Janice Meyer. Another was price. Healthier items, more costly to produce, were often more expensive than the "bad" stuff. Then there was marketing. It turned out that words like "lean" and "light" meant, to consumers, "flavorless."

Fast-food marketers have wised up since then. They've learned that consumers often perceive an item that sounds higher quality as better for them, even if no mention is made of health or nutrition. That's why you see restaurants breathlessly shilling "applewood smoked bacon," even though it has the same amount of fat as plain old bacon. Kimberly Egan, a partner at the Center for Culinary Development in San Francisco, which has done menu development for McDonald's, Burger King, and Wendy's, rattles off words that give "quality" cues: "slow-roasted," "tender," "grilled," "spicy," "fresh-cut." Fast-food companies use these words "to make people think that fast food is healthier for them," says Egan. Witness Burger King's new Spicy TenderCrisp chicken sandwich, which is actually fried. (Not that it has helped BK much: The company is now on its third CEO in as many years.)

Talking about existing food in ways that make it seem healthier might work with some customers, but it won't wash with the food police. Activists such as the Center for Science in the Public Interest are especially concerned about the heart-unhealthy trans fat in frying oils; two-thirds of Americans are concerned about trans fats too, according to NPD. Fast-food restaurants have been cooking chicken and potatoes in trans-fat-laden hydrogenated vegetable oil for over a decade because it tastes good and has a long "fry life" (the length of time that an oil can be used in a fryer before it has to be replaced). But trans fat raises levels of LDL, the "bad" cholesterol, and decreases HDL, the "good" cholesterol. That's of particular concern now that the government has said that moderately high-risk people should have LDL levels of no more than 100, 30 points lower than previously recommended.

In September 2002, the activists cheered a big win. That's the month McDonald's, the world's largest fast-food company, announced plans to shift to a new frying oil that would cut trans-fat levels 48% by February 2003. McDonald's began a program to develop such an oil. But the company ran smack up against two basic facts. First, most unsaturated oils are unable to reproduce the crispy texture, savory taste, and pleasing "mouth feel" of a McDonald's fry. Achieving the right flavor is critical for McDonald's, whose beloved fries were once deemed "sacrosanct" by founder Ray Kroc.

Second, the bad-for-you oil is cheaper than the better-for-you kind. Here are some back-of-the-envelope calculations. Filling a typical fast-food fryer with 35 pounds of partially hydrogenated vegetable oil costs about $13; filling it instead with a reduced-trans-fat soybean oil might cost about $20. When you factor in fry life, the cost difference becomes even more dramatic. Hydrogenated oils are much more stable than less saturated oils; they stand up better to the 350-degree heat of a fryer. According to one oil supplier, partially hydrogenated oils last about twice as long in a fryer as trans-fat-reduced ones. Because a restaurant typically goes through 500 pounds of oil a week, switching to the healthier oil would cost more than $19,000 a year. Multiply that by the 13,000 McDonald's stores in the U.S., and you have an added cost of about a quarter of a billion dollars annually.

Some very smart people are struggling to come up with a solution to those problems. Colleen Zammer, for one. She runs the 15-person food and nutrition division at Tiax in Cambridge, Mass., formerly the technology and innovation arm of consulting firm Arthur D. Little. Companies like Procter & Gamble, Tropicana, and others quietly employ Tiax to take their foods, remove bad stuff like sugar or fat, and then rebuild them so that our mouths will never know the difference. Zammer and her colleagues are busy cooking up batch after batch of fries, testing new blends of lower-trans-fat oils--canola, soybean, sunflower, and others--from suppliers like ADM and Cargill. Once she has a suitable mix with optimal flavor and fry life, she hopes to work with one or more of the manufacturers to replicate it and sell it to the fast-food companies. She thinks she'll have a viable oil to pitch to restaurants by the end of the year. Why is it taking so long? Paying clients have kept her busy with demands for low-carb products, and Tiax isn't making any money on lower-trans-fat oils yet. "We've only just begun, really," she says.

It may not surprise you, then, to learn that McDonald's did not fulfill its promise to reduce trans fats by February 2003. In fact, it still hasn't done so. One exception: Last year the company did reduce trans fats in its Chicken McNuggets and in two chicken sandwiches by more than 15% by switching to a different oil. Why doesn't the company just use that oil for everything else too? A McDonald's spokeswoman says, "Fries are a different product." The company postponed the across-the-board oil switch because testing is "taking longer than anticipated," says the spokeswoman. She says there's no timetable for when a switch might take place.

While it was treading water on the trans-fat front, what was the fast-food industry doing to entice healthy-food lovers? Plunging into salads. Wendy's made a big splash in March 2002 with the launch of its Garden Sensations salads; McDonald's followed a year later with its own new salad line, and Burger King introduced Fire-Grilled salads last May. Given the rigid parameters of fast food--limited space, limited time, and a customer with a limited budget--the launches were rife with challenges. The four salads that Wendy's initially offered, for example, required 19 new ingredients to be sourced, shipped, and stored, including field greens, red onions, grape tomatoes, and mandarin oranges. Frozen French fries and burgers can be tossed into a restaurant freezer and left there for months until ready to be plopped in the fryer or on the grill, but fresh fruits and veggies don't work that way. They spoil more easily and thus need to be delivered more frequently; they usually come precut (teenage workers + sharp knives = liability), which adds to their cost; and they often require added refrigerator space. "My job has become a lot harder," says Tammy Bailey, a VP of menu marketing at Jack in the Box, which also has a new line of salads.

What's more, fresh food can throw a wrench into a well-oiled fast-food machine. Witness the chaos that ensues one sunny day in May when NYU nutrition professor Marion Nestle visits a McDonald's in New York City's Rockefeller Center with two companions to taste the fast-food giant's new Go Active! adult Happy Meal. The hapless cashier has to recruit two other staffers to help prepare the meals (each of which includes a salad, a package of Newman's Own dressing, bottled water, and a step-counting pedometer), fold together the cardboard packaging, and figure out how to ring it all up. The whole process takes more than five minutes--an eternity by fast-food standards. "This is a complete operational nightmare," Nestle sighs. (McDonald's says that operations are smoother now.)

Okay. But at least fast-food salads are good for you, right? Well, some are better than the typical fried fare if you use low-fat dressing and eschew some of the toppings. But Wendy's Chicken BLT salad with honey-mustard dressing and garlic croutons has more calories (710), more sodium (1,610 milligrams), and more cholesterol (120 milligrams) than Wendy's Classic Single burger, and twice as much fat (47.5 grams). Doesn't that pose a bit of a marketing problem? Wendy's gets around it by being careful not to make any explicit claims that its salads actually are healthier. Says Wendy's CEO Jack Schuessler: "We don't say [Garden Sensations] is healthy or nutritious."

Clearly, Wendy's and the other fast-food companies understand something fundamental, and fundamentally contradictory, about consumers: No matter how much they tell pollsters they care about healthy food, they don't really give a hoot once they've crossed the threshold of a fast-food joint. Just ask Jarrett Paschel, a sociologist at consumer-research firm the Hartman Group, whose clients include McDonald's, Coca-Cola, and Kraft. We tagged along with him one day recently at a Seattle-area Red Robin, one of 200 restaurants in the burger chain. Paschel is accompanying Sarah (not her real name), a slim, 30-ish mother of two. He has pegged Sarah as a "core wellness consumer," meaning that she's slightly fanatic about stuff like organic produce and yoga.

Wait a second: If Sarah is such a health nut, shouldn't we be in some vegan restaurant? That's just the point. In Sarah's fridge, a carton of soy milk stands next to a bottle of Coke. She talks knowledgeably about pesticides and then in the next breath says, "I'm a McDonald's addict." She has no desire to order a salad there. Burgers are her treat, and she doesn't see that as inconsistent with a healthy lifestyle.

Paschel isn't surprised. Consumers, he says, "are messy." Sarah embodies a key finding in Hartman's recent study on obesity: Consumers want healthy food options, but they also want indulgent options, and they refuse to have to choose between the two. Quite often they're perfectly happy just knowing that the healthier option is there at restaurants, but good luck getting them to order it. Wendy's likes to trumpet that 10% of its sales come from salads, up from 3% in 2001. But it's the exception. According to industry data from NPD, Americans ordered main-dish salads for lunch nearly twice as often way back in 1989 (9.3% of all meals included one) as they did just last year (5.2%). "No matter what we say, we like burgers," says Harry Balzer, VP at NPD. And until that changes, don't expect fast-food companies to change much either.

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