Fresh watermelon, sugar and a stater culture is all you need to make a lacto-fermented watermelon soda!
Strain out in a strainer covered with cheesecloth to get the watermelon juice.
Add the juice and the whey to the sugar water in a 2 quart glass ball jar or demijohn
Let it ferment on your counter for at least 2 days and up to 10 days. The longer you let it ferment the less sugar there will be as the bacteria will consume and convert the sugar during fermentation. Swish the soda around once a day to prevent mold from forming on the surface.
The carbonation will increase rapidly. Be sure to release the pressure that builds every day until you’re ready to transfer them to the fridge. Do this over the sink or outside as the geyser effect is common.