1) Into an electric blender, add in the grapes. Puree to a fine consistency, then pass it through the strainer.
2) Add in 2 cups of dry cider. Mix well.
3) Into a pan, add in the remaining cider along with the sugar. Heat on low flame till the sugar dissolves completely. Slowly stir into the grape mixture.
4) Transfer the mixture into a freezing tray or shallow, rigid container. Freeze till it becomes thick.
5) In a bowl, beat the egg whites to stiff peak but not dry. Fold it into the grape mixture until mixed well.
6) To freeze the grape sorbet, transfer to a rigid container. Seal, label and freeze.
7) Serve the Grape Sorbet with slices of grapes for garnish, if desired.
8) Thaw the sorbet by placing in the refrigerator for 15 minutes till soft.
9) Serve as in step 7.