Remove the core from the pineapple and scoop out the flesh, keeping the
shells intact. Chill the shells in the fridge. Puree the pineapple flesh
in a food processor or blender. Dissolve the sugar in the water in a
saucepan, then bring to the boil and boil for 5 minutes to make a syrup.
Leave to cool. Mix the pineapple pulp into the syrup, pour into a freezer
container and freeze for 3 hours. Whisk the egg white until stiff, then
whisk in the ice cream. Return to the freezer container and freeze until
firm. Transfer to the fridge 10 minutes before serving to soften the ice
cream, then scoop into the pineapple shells to serve.