Combine sugar and water in a small saucepan and heat until sugar is melted and mixture is clear. Remove and chill. In a glass bowl, combine chilled sugar syrup, orange juice, lemon juice, zest and liqueur. Mix thoroughly, and then fold in the egg whites using an over-and-under motion. Continue folding until no egg white streaks remain in the mixture. Pour mixture into an ice cream maker and freeze until firm, according to manufacturer's instructions.