Dissolve the sugar in the water over a low heat, then bring to the boil and boil for 2 minutes. Remove from the heat, add the lemon zest, cover with a thin cloth then leave to cool and infuse for 30 minutes.
Add the lemon juice to the sugar mixture, then strain into a shallow container. Freeze until slushy, about 2 hours.
Whisk the egg white until it rises up into a fluffy foam, and stir into the sorbet (make sure the sorbet is slushy enough or the egg white will stay on the top; but if it not frozen enough just freeze it for about another 1/2 hour and mix it in). Mix the mixture then place it in a container and freeze for 6 hours minnimum.
When you want to eat your sorbet, place it at room temperature for 10 minutes. There you have it - a delicious sorbet.