Prepare an ice bath; set aside. Combine grapes and the water in a medium saucepan. Cook over medium-high heat until the liquid begins to bubble and grapes start releasing liquid, about 4 minutes. Reduce heat, and simmer until juices are dark purple and grapes begin to break apart, about 3 minutes.
Pass mixture through a food mill and then through a fine sieve into a large bowl set in the ice bath; stir frequently until completely cool. Stir in simple syrup and lemon juice.
Transfer mixture to an ice-cream maker, and freeze according to manufacturer's instructions. Transfer to an airtight container, and store in the freezer until ready to serve, up to 2 weeks.
To Cinnamon2- you know I'm not sure, but I don't think you have to be totally exact in terms of boiling it down to a certain amount. If you cook the grapes until the juice is really dark and the grapes are totally falling apart, that's a good point to stop and move on to the food mill stage.
How many cups of the grape juice do you end up with?
Made this last September and couldn't stop eating it... SO SO good! What a fantastic recipe. Healthy and packed with so much flavor. I can't wait for concord grape season again!