Boil the sugar, water and rinds of limes for 10 minutes. Strain and cool.
Cook the apples in lime juice. Liquidise and cool. Add the apple and lime purée to the syrup. Add the passion fruit pulp and place in the freezer. When it begins to set, whisk the egg whites and fold in. Freeze until set.
15 minutes before serving, transfer to a refrigerator to soften slightly.