Recently I decided to try making Nigella's cupcakes with royal icing. I assumed that the icing would set firm fairly quickly, but alas it took about 12 hours! Is that normal? And just how hard is it supposed to set?
Nigella makes cupcakes with royal icing in Domestic Goddess (p267) and Christmas (p216). In the UK these use an instant royal icing mix whereas in the US the roayal icing is a more traditional combination of icing (confectioners') sugar and powdered egg white (meringue powder). The time royal icing takes to set will mainly be dictated by the thickness of the icing. If the icing is applied thinly then it should take 4 to 6 hours to dry completely (though the surface will be touch dry quite quickly) but if it is a very thick layer (such as "snowcene" icing on a christmas cake) then it can take a few days to dry completely.
The "hardness" of the icing will be determined by the amount of egg white in the mixture and this can vary between 1 and 3 egg whites per 500g icing sugar (5 cups confectioners' sugar), plus 1 tsp lemon juice (to make the icing brighter and slightly offset the sweetness). A higher proportion of egg white will give a harder icing. Instant royal icing mix already contains powdered egg white. If you are using powdered egg white or meringue powder then you should follow the conversions or instructions on the packaging. If you are using an instant royal icing mix then it should already set quite hard but you can and some extra powdered egg white to make it even harder. If you want the icing to remain slightly soft then you can also add 1/2 teaspoon liquid glycerine to each 500g/5 cups sugar.
we would suggest that powdered egg white or meringue powder is used instead of fresh egg whites if the icing is to be consumed by the very young, elederly or anyone with a weakened immune system.