Combine sugar and gelatin in a small sauce- pan; gradually add milk, srirring well.
Cook over low heat, srirring consranrly, unril sugar and gelatin dissolve.
Remove from heat, and cool.
Stir in crushed pineapple, lemon juice, and cream de menthe.
Beat egg white (at room temperature) in a small bowl until stiff peaks form.
Fold into pine-apple mixture.
Spoon mixture into an 8-inch square baking pan; cover and freeze 4 hours or until slushy, stirring at 1 hour intervals.
Spoon mixture into container of an electric blender or food processor, process until smooth.
Cover and freeze 8 hours or overnight.
Process mixture again in electric blender just prior to serving.
Scoop into individual dessert bowls; garnish with fresh mint sprigs and pineapple chunks, if desired.