Dear Nigella Team, I found some glorious ruby red grapefruits at my market and was thinking of using them instead of the clementines in the Clementine Cake. I just ate one and they are fairly sweet, so maybe just a little more sugar than listed in the original recipe could offset that. But I wasn't sure about the skin and pith from the grapefruit as compared to that of the clementine. Please advise and thank you very much.
Nigella's Clementine Cake (from How to Eat and on the Nigella website) uses whole clementines that are cooked until soft and then pureed (with the zest and pith). It is the white pith that is bitter and fortunately most clementines have very thin skins with little pith, which makes it possible to use them for this recipe.
Grapefruits generally have a much thicker skin than clementines and a much greater ratio of pith to zest. We feel that on balance this is likely to make the grapefruits a little too bitter to use whole even with some extra sugar added, and if you add too much extra sugar then it will affect the texture of the cake. However if you would like to showcase the grapefruits in a cake then we would suggest using them as a substitute for lemons in Nigella's Lemon Polenta Cake (from Kitchen and on the Nigella website) or her Lemon-Syrup Loaf Cake (from Domestic Goddess). Both of these cakes will use the zest and juice of the grapefruits, without the bitter pith.