In Nigella's Autumnal Birthday Cake (from How to be a Domestic Goddess), the icing ingredients include 1/4 teaspoon cream of tartar. Is this necessary or can it be done without? What does it do? I live in Holland and have not been able to find it here. Is there an alternative ingredient that can be used instead? Thanks a million! Natalia
The icing used for the Autumnal Birthday Cake is a meringue-type frosting, often known as seven minute frosting as it is whisked for about 7 minutes. Cream of tartar is an acid and the reason it is added to the the frosting is that the acid reacts with the proteins in the egg whites (called "denaturing") and helps the whites to whisk to a greater volume, to give a lighter frosting.
If you can't find cream of tartar then you can use a different acid in the same quantity. Lemon juice is a good alternative or you can use a mild flavoured vinegar such as white wine vinegar or cider vinegar. Also make sure that the bowl and whisk that you use are free of any traces of grease, as if they are greasy the egg whites will not whisk up properly.