Preparation Purée half of grapes in a blender until smooth, then force through a fine-mesh sieve into a bowl, discarding solids. Repeat with remaining grapes to yield 3 cups purée. Whisk in sugar until dissolved. Chill until very cold, 3 to 6 hours. Freeze in ice cream maker, then transfer to an airtight container and put in freezer to firm up, at least 2 hours. Cooks' note:Sorbet keeps 1 week.
Preparation Purée half of grapes in a blender until smooth, then force through a fine-mesh sieve into a bowl, discarding solids. Repeat with remaining grapes to yield 3 cups purée. Whisk in sugar until dissolved. Chill until very cold, 3 to 6 hours. Freeze in ice cream maker, then transfer to an airtight container and put in freezer to firm up, at least 2 hours. Cooks' note:Sorbet keeps 1 week.