I have been fermenting ginger beer using a natural yeast culture for the last couple years. Recently, sad at the lack of commercial availability, I decided to brew some grapefruit soda. This was a bit of an experiement, as there was not much guidance online.
Knowing that the citrus could make the product bitter I added more sugar than usual, which turned out to be a good bet. I boiled down a simple syrup with the sugar, water and zests of the grapefruit and lemon. This, combined with the grapefruit juice and the yeast culture, left for a couple days in a kilner bottle.
The final product was so good, better than any grapefruit soda I have managed to source in the UK.

• IngredientsGrapefruit, Lemon, Sugar, Ginger Yeast Culture