Chestnuts are covered with a membrane, known as a pellicle or episperm, which closely adheres to the fruit's flesh and must be removed because of its astringency. Marron nuts have a pellicle which is "superficially attached to the nut", making it easily removable from the fruit. Some chestnuts have two cotyledons usually separated with deep grooves penetrating nearly all the way through the fruit; this makes them too fragile for the necessary manipulations during the cooking process. There also are other grooves on the surface, which means more embedded pellicle that must be painstakingly removed. "Marron"-quality nuts do not have the separation into two cotyledons; it appears in one piece and it shows few very shallow grooves.