I'd like to use royal icing on a Christmas cake to get a nice 'snowscape' effect. What is the shelf life of a decorated cake made using royal icing with eggs whites on the one hand, and pasteurised egg whites or meringue powder on the other? I'm worried about the icing going off!
The type of icing used for "Snowscapes" (peaked icing) on Christmas cakes is called Royal icing and is made with egg whites, lemon juice and icing sugar (confectioner's sugar). The icing seems to have quite a long shelf life, particularly once it is fully dried, regardless of which type of egg white is used. The main thing to consider is that the longer the icing stands, the more it will dry out and the harder it will become. We would suggest that the icing it at its best within a month of being made and put on the cake. It should also be noted that over time the icing may discolour as oils from the marzipan layer on the cake will start to seep into the royal icing. The icing is still edible but will not look as white.
Fresh egg whites can always carry a risk of salmonella and so you may prefer to use pasteurized egg whites from a carton or dried egg whites or meringue powder, using the quantities suggested on the packaging. If you live in the UK then we would suggest trying the Royal Icing Powder that can be found in the baking aisle, as this is both easy and convenient to use.