Grapefruit-Buttermilk Sherbet

Bring 1 cup water to a boil in a small saucepan. Add rind to pan; reduce heat, and simmer for 2 minutes. Drain, reserving rind. Combine rind, sugar, and juice in pan; bring to a boil, stirring constantly until sugar dissolves. Reduce heat, and simmer 20 minutes. Remove from heat; pour into a bowl. Chill for 1 hour. Strain juice mixture, and discard solids. Combine the juice mixture, buttermilk, and vodka in the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions.