Why Won't The Bitter Orange Tart Filling Set?

Our answer

Nigella's Bitter Orange Tart (from SIMPLY NIGELLA) has a gingernut or ginger snap crumb base that is filled with a curd made from the juice of Seville oranges, or a combination of regular orange juice and lime juice. The juice, zest, sugar, butter and eggs are cooked together over a low heat until the mixture thickens enough to coat the back of a spoon and not flow back quickly if you run your finger through the curd on the back of the spoon.

The curd should have around 200ml (3/4 cup plus 1 tablespoon) juice to 3 whole eggs plus 2 egg yolks. This is usually sufficient to make a thick curd. However, sometimes you do need to be slightly patient when making a curd and use a low heat but keep whisking until the curd thickens. It only usually curdles if the heat is tuned up too high and you may be able to save the curd if it is only very slightly curdled by blitzing it with a hand blender and then passing it through a very fine sieve. If you have an instant-read or digital probe thermometer then you can also use this to help as the curd should start to set at around 75-76c (170F) and should not go above 85c (185F). You could also try following Nigella's method for making Passionfruit Curd, where the butter is melted first, as both methods work equally well for making a curd. However, the most important thing with both methods is to keep the heat low and be patient.