Where will you find diners with the hardest palates to please? Try 30,000 feet above ground.

As airlines struggle with higher fuel prices and intensifying competition, they’re trying to offer more satisfying in-flight food in an attempt to boost passenger loyalty. Carriers like Delta Air Lines Inc. are rolling out celebrity-chef recipes on more flights while US Airways Inc. is investing in higher-quality ingredients. Still, it is hard to make meals tasty because airline chefs face challenges their counterparts at ground-level restaurants do not.

“We have restrictions as far as what we can do,” said Boston celebrity chef Todd English, who customizes sandwich and salad recipes for Delta. That means English’s in-flight meals wind up less adventurous than the interpretive rustic Mediterranean cuisine he built his reputation on: “The most progressive thing we did was the black olive spaghetti salad.”

The list of hurdles airline chefs have to overcome is long. For one, airline chefs have to add more seasoning because passengers’ ability to discern taste is blunted 15 percent to 40 percent at 30,000 feet. On top of that, most meals need to be cooked hours before takeoff and reheated onboard in a convection oven for 20 minutes, which can dry out natu ral juices. And butter and cream sauces break apart when reheated, so those often are left out.

Still, airlines in the last year or so have been trying to make their meals tastier. Delta is partnering with another celebrity chef, former Food Network star Michelle Bernstein, who is adding flavor by cooking sweet potatoes with candied ginger. And US Airways is tossing grilled slices of fresh chicken breast into its salads, rather than relying on precut chunks of frozen poultry.

The renewed focus on food comes after more than a decade of deteriorating in-flight meal service, which worsened when most US airlines scrapped free meals in coach on domestic flights after the Sept. 11 terrorist attacks sent the industry into a financial tailspin. The money US airlines have spent on food and beverages has plummeted 43 percent since 1992, when it was $5.92 per passenger. By 2006, the nine largest airlines were spending only $3.40 per passenger, according to the US Bureau of Transportation Statistics.

Airline cuisine has long been the butt of jokes, and some say the reputation is well deserved. Weekly flier Alan E. Gold of Burlington distinctly remembers a meal he ate in December – “one of these wrap sandwiches – I’m guessing it was sun-dried tomato. It was mushy stuff. It was prepackaged crud, and all the liquid settled to the bottom.”

Some passengers have even become a bit obsessed with airline food. At airlinemeals.net, travelers on 536 airlines have uploaded 18,821 snapshots of their onboard eats since late 2001 and critiqued the tastes, textures, and portions. A passenger on Alaska Airlines showed evidence of a “miniscule” and “lackluster” breakfast burrito. Meanwhile, a US Airways diner who ate a chicken lunch on a 2006 international flight complained the “entree was hot enough but was WAY too salty” and the accompanying almond cannoli was “too sweet.”

It’s tricky to compensate for dulled taste buds without overdoing it. Sampling meals onboard can be key. “I took a flight and tasted the food. I couldn’t believe it was the same thing,” Bernstein said. “On the ground, what you might find to be salty and spicy and ├╝ber-flavorful pales in comparison when you’re on a plane.”

Discuss What should airlines do to improve inflight meals?
more stories like thisAs a result, “I put shallots and garlic in almost everything,” Bernstein said.

The mechanics of creating airline meals can kill a chef’s creativity. Bernstein gave up on trying to pair hot and cold foods. The reason? There’s not enough cabin space or time for flight attendants to top reheated fish with cool salsa. And while she whipped up a white gazpacho that airline taste-testers liked, the dish likely won’t make it onto the menu. “Logistically, it’s so hard,” she sighed. “When the planes go up, the gazpacho might come out of the cup.”

Even creating good coffee proved a feat for Dunkin’ Donuts, which has offered its brand on JetBlue Airways Corp. flights for two years. Water boils at lower temperatures at higher altitudes, but the onboard coffee machine settings can’t be changed. In-flight coffee also can taste funny after being brewed from water that’s grown stale in the belly of a plane. So, Dunkin’ Donuts had to adjust the ratio of water and grounds for in-flight coffee and use water that is run through an onboard filtration system.

Despite the challenges, chefs argue that the food has improved. “I remember the days when you got a piece of mystery meat covered in sauce with vegetables,” said Bob Rosar, corporate executive chef with Gate Gourmet, the world’s second largest in-flight caterer. “Those days are long gone.”

A reporter who sampled airline food on the ground found American Airlines Inc.’s quiche filled with Portuguese sausage, shiitake mushrooms, and Monterey Jack cheese to be hearty, creamy, and savory. The signature dish of Hawaiian celebrity chef Sam Choy is served in first and business class on some flights. English’s food was also good enough to eat every day, particularly the moist apple butter croissant sandwich with cheddar, turkey, and bacon for breakfast and a Mediterranean salad with grilled shrimp for lunch.

One of Bernstein’s Delta dishes – braised short rib in red wine – is so popular that it’s bringing fliers into her Miami restaurant, Michy’s. For variety, Delta will soon replace the entree with a new one – perhaps Bernstein’s fish braised in ginger, green mango, tomatoes, a pinch of curry, jalapeno, and a bit of unsweetened coconut milk.

Bernstein likes the fish dish so much she added it to Michy’s menu. Still, “I changed it a little bit,” she admitted. The version served at sea level is topped with a chilled green-papaya salad, a pairing of hot and cold that “I don’t think I could do on the Delta flight.”