In a saucepan, combine the sugar, cornstarch, salt, lemon zest, and water. Bring to a boil over medium heat, stirring constantly until it thickens (the preparation must reach a boil to allow the starch to thickens). Reduce the heat and cook for 1 minute. Remove from the heat and set aside 75 ml (1/3 cup) of this mixture for the meringue.