File’ (pronounced fee-lay) spice powder is a unique ingredient used primarily for making a good gumbo, and it is a necessity for cooking authentic Cajun cuisine. Quite simply, file’ is the powdered leaves of the sassafras tree. When ground, it has a faint smell like eucalyptus. Long before the use of file’ powder for Creole and Cajun cooking, Choctaw Indians of the Gulf coast pounded sassafras leaves into a powder and added them to soups and stews. In addition to contributing an unusual flavor, the powder also acts as a thickener when added to liquid.
Typically I will add the file’ powder toward the end of making a gumbo, but before I put it into the hot gumbo I will mix the fine file’ powder with a small amount of cool water until it is dissolved creating a thick slurry. I then fold in the file’ slurry into the hot gumbo. This prevents the file’ powder from clumping and keeps it smooth in the gumbo. File’ does really help thicken up a gumbo and it adds a unique flavor profile too.
Here is a link to an interesting article on General Horticulture.com about how to make your own file’ powder. It has some good background on the sassafras plant.