While the majority of U.S. fast food consumers don't know what portobello mushrooms are, they're willing to pay nearly $5 for a burger with them on it.

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Leading the pack of fast food chains going upscale is the Carl's Jr., which did market research before introducing its $4.59 Portobello Mushroom $6 Burger last month. It's now the chain's best-selling burger, despite the ignorance, USA Today reported.

"We did focus groups and found that consumers don't know what a portobello is, but they know it means quality," said Brad Haley, marketing chief at Carl's.

Quiznos began selling the $4.49 Chicken Milano sandwich this summer, which is on rosemary Parmesan bread with sun-dried tomato pesto, Tuscan basil mayo and spring-mix lettuce.

But at Burger King, research found their regulars prefer "ooey, gooey" processed cheeses to a natural Swiss it recently tested for a new burger, said Denny Post, chief concept officer.