After picking up a vegetable burrito on his way home from work, Mike Racanelli planted himself in front of his television and took a bite. The smell hit him immediately: cilantro.

Irate, the 29-year-old Chicago band manager drove 20 miles back to the Mexican restaurant where he'd bought the offending item, threw it on the counter, he recalls, and "raised hell," demanding a cilantro-free replacement "immediately."

Later,...