Hi! The other day I made a coconut cake used called for coconut milk and brown sugar. I substituted the brown sugar with demerera sugar and followed the recipe accordingly. It all came out well but for the fact that it became too crumbly. The cake was too brittle even when cooled and cut. I was wondering what could cause this? Thank you.
Without seeing the recipe it can be difficult to pinpoint the exact problem but the different sugar could be one of the problems. Demerara sugar (turbinado sugar) is a raw cane sugar and comes in cryastallized form. Brown sugars are soft and are made with a combination of crystallized sugar and molasses. The molasses have the quality of being able to attract moisture (they are hygroscopic) so baked goods made with molasses will tend to be more moist than those made with just regular crystallized sugar. Consequently using the demerara suagr probably caused your cake to be more crumbly, rather than moist.
If you don't have soft brown sugar to hand then you can make a substitute by using 1 tablespoon of molasses for every 200g (1 cup) caster (superfine) sugar. You can add the molasses when you add the sugar (for example if creaming butter and sugar or melting the sugar with other items) or you can mix the molasses and sugar together in a mixer or food processor first. If you don't have molasses then honey, golden syrup or corn syrup are also hygroscopic sugars and will help to add moisture, but they will have a very different flavour to the molasses.