1 cup (250 mL) rhubarb, washed and chopped, at room temperature (I used frozen, but the fresh stuff is coming up now)
3 or 4 juniper berries (for the yeast on their skin)
¼ cup (60 mL) Uncle Berwyn’s Pure Yukon Birch Syrup, at room temperature
4 cups (1L) water at room temperature – if tap water, let stand for a couple of hours in order for the chlorine to evaporate
Place the rhubarb in a bowl deep enough to allow vigorous stirring and wide enough to give maximum exposure to air. A 2L mixing bowl will do the trick.
Dissolve the birch syrup in the water and pour over the rhubarb. Stir vigorously and cover with cheesecloth secured by an elastic or string.
For the next week stir vigorously every 2 to 3 hours. After a few days, the mixture should start to bubble in small, fizzy bubbles that gather around the rim of the bowl. You might see white yeast gathering on the surface. Don’t worry about the yeast, it’s not a bad thing, however more vigorous stirring may be called for. If mould starts to form, spoon it off.
(I tasted the vinegar every second day. At first it tasted like birch syrup. After three days or so it took on a sharpness that eventually became both sharp and sour.)
After a week to 10 days, strain the mixture and discard the rhubarb. Return the strained liquid to the cleaned bowl or a screw top jar. Cover with cheesecloth.
Continue to stir or shake vigorously (put the lid on the jar before shaking, and take it off again afterwards) for three or four days. Taste your vinegar. If you are happy with the sharpness and sourness, stop the show! Cover the jar and put the vinegar in the fridge. I was happy with mine after 2 weeks, though in Wild Fermentation Sandor Katz says fermentation generally takes 3 to 4.