Drain pineapple, reserving 1/2 cup juice. Combine pineapple, sugar, and lemon juice in a large mixing bowl. Set aside.
Dissolve gelatin in pineapple juice in a small saucepan. Bring to a boil, stirring constantly. Remove from heat; stir softened gelatin into pineapple mixture. Cool slightly. Gradually add milk, stirring constantly. Chill.
Beat egg whites (at room temperature) until stiff peaks form. Fold into pineapple mixture.
Pour mixture into freezer can of a 1-gallon hand-turned or electric freezer. Freeze according to manufacturer's instructions. Let ripen at least 1 hour before serving.