I voided the warranty on my Sodastream today by farting up some carrot juice.
The internet says that the juice has to be at very least translucent in order to carbonate, and the internet was right! It took upwards of ten button pushes for any bubbles to appear in the opaque carrot juice, even after I cut it with water.
A cherry vanilla soda that my friend Micah came up with fared much better.
I may soon invest in a DIY carbonation rig like this.