1. Combine all ingredients in a medium saucepan over medium heat; bring to a boil, stirring occasionally. Remove from heat; let stand 15 minutes.
2. Strain the mixture through a fine sieve over a bowl, reserving soaking liquid (do not press berries or the mixture will be cloudy). Reserve berries for another use. Pour mixture into an 8-inch square glass or ceramic baking dish. Cover and freeze until partially frozen (about 2 hours). Scrape mixture with a fork, crushing any lumps. Freeze for 3 hours or until completely frozen, scraping with a fork every hour. Remove from freezer; scrape mixture with a fork until fluffy.