Why Did My Bitter Orange Tart Filling Not Set?

Our answer

Nigella's Bitter Orange Tart (from SIMPLY NIGELLA) has a gingernut (gingersnap) crumb base that is filled with a curd made from the juice of bitter Seville oranges. You can also use a combination of sweet orange and line juice as an alternative to the Seville orange juice.

The curd should be softly set but not completely solid, so it may just be that you expected a more solid filling such as for a lemon meringue pie. If the filling is runny and flowing out of the tart when you cut into it, then it is likely that the curd was not cooked for quite long enough. You need to be patient when cooking citrus curds as if you heat the mixture too quickly then it can curdle and become lumpy. You need to keep a low heat and stir constatntly. The time it takes for the curd to thicken can vary accoding to the size of the pan and the heat used. To test if the curd is cooked properly, dip a spoon into the mixture then run your finger through the mixture on the back of the spoon. The line made by your finger should hold and remain clear, without any sign of the curd flowing back into the space. If the curd looks like it is flowing back then return the pan to the heat, cook for a couple of minutes longer and test again. If you have a sugar, candy or digital thermometer then the curd should thicken between 77c and 82c (170-180F), but we would still recommend checking the thickness of the curd as described. The curd should thicken more as it cools.