1.Stir lemongrass, sugar and the water in medium saucepan over high heat until sugar dissolves; bring to a boil. Reduce heat; simmer, uncovered, without stirring, 5 minutes. Transfer to large heatproof jug, cool to room temperature; stir in juice and food colouring.
2.Process sorbet mixture with egg white until smooth.
3.Pour sorbet mixture into 14cm x 21cm loaf pan, cover tightly with foil; freeze 3 hours or overnight.
Return to pan, cover; freeze until firm.Note