Pour the puree into a medium saucepan over medium-high heat. Bring just to a simmer, then reduce heat to medium-low. Continue stirring occasionally, then more often, until mixture is very thick and has lost most of its moisture, about 35 to 40 minutes. NOTE: Mixture may splatter if you don't stir often enough, so don't walk away from the stove. Remove saucepan from heat and set aside.