Cut pineapple into 2-inch pieces. Place pineapple and lemon juice in a food processor; process until smooth. Add sugar; process 1 minute or until sugar dissolves.
Pour mixture into the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions. Spoon sorbet into a freezer-safe container. Cover and freeze 1 hour or until firm. Garnish with mint sprigs, if desired.
If you don't have an ice-cream freezer, use a covered metal bowl. Freeze mixture 3 hours or until it is hard on the outside but slushy in the middle. Remove it from the freezer, beat it with a whisk until smooth, and return to the freezer, covered, for 4 hours until firm.