Combine sugar, warm water and lemon rind in a saucepan. Cook over medium heat, stirring for 4-5 minutes or until sugar is dissolved. Continue to simmer without stirring for 10 minutes or until mixture starts to thicken. Take saucepan away from the heat and leave aside to cool completely.
Strain the syrup into a bowl and stir in lemon juice. Pour this mixture into a stainless steel bowl. Cover and freeze for 1-2 hours or until the mixture is almost firm.
Transfer the set mixture to a food processor. Add egg white and process until smooth. (Do not allow it to melt.)
Pour mixture into a deep loaf tin. Cover and freeze again for 3 hours or until firm. Scoop out sorbet with an ice-cream scoop and serve immediately.