Place the chopped rhubarb in a large non-reactive container. Add the water, sugar, and lemon juice, stirring until sugar dissolves. If using, add the raisins or ginger.
Cover with a clean cloth secured with a rubber band. Leave at room temperature, stirring twice daily. You should see bubbles forming within 2-4 days as wild fermentation begins.
Ferment for just 2-3 days total once bubbling starts - this shorter fermentation keeps the alcohol content lower (around 0.5-1% compared to 2-4% in the yeasted version) while still developing lovely flavour and gentle fizz.
Strain through muslin, squeezing well. Taste - it should be lightly tart and just beginning to fizz. Bottle in swing-top bottles with 5cm headspace.
Leave at room temperature for 12-24 hours only to develop light carbonation, then refrigerate immediately. This version is more delicate and best consumed within 1-2 weeks.
The wild version will taste more nuanced and less "clean" than the yeasted versions - you'll get subtle funky notes, floral complexity, and a gentler, more sessionable fizz.
