Time to resurrect an old thread! I just got a Soda Stream (forked out for the KitchenAid model) and so far I'm very happy with it. One of the first things my wife said to me was "Now you can make Josta!" I was ordering Sassafras and Sarsaparilla root to make root beer, so I tossed some Guarana seeds into my cart and I'm looking forward to experimenting with it.
The main component to a "cola" syrup is a mix of spices and citrus. I imagine that I will simply include the Guarana seeds into this mix. The next component to making a Josta flavor will be the Dragon Fruit. I'm not sure if I should replace one of the citrus components in the cola syrup with dragon fruit? Or if I should simply add it to the already large list of ingredients. From what I read on Wikipedia, the Dragon fruit is relatively mild, so I'm wondering if there wasn't some other berry involved in Josta... perhaps some raspberries or something? That seems like it should be about the right flavor profile. Blackberries are too mild, and it certainly didn't use strawberry or blueberry.
I think my first attempt will be to make a straight up cola syrup. Once that is done I will make a guarana/dragonfruit syrup and simply mix those together. When I know how those go together I will then work on determining what components to replace in the cola syrup to replicate a single Josta syrup.