homebrew apple vinegar
A couple of weeks ago, I bought a jug of unfiltered organic apple juice from Trader Joe's. I poured the juice into one of the seed sprouting jars, but instead of using the mesh cap, I covered the mouth of the jar with a cloth and rubber band. I don't think that let in enough air. The juice hadn't changed much after several days.
So, I poured the remains of some Bragg's apple vinegar into the jar (though perhaps not enough, given the volume of juice). Then I capped the jar with the mesh lid. Later, I added some of the remains of the "pombucha" (kombucha fermented with pomegranate molasses) that I'd just bottled. That pombucha was fairly sour, so I thought that it might augment the process with the apple juice.
More than a week later, it seems to be working out. There's definitely more of a mother growing, the flavor has become much more tart, and the whole brew froths when I stir it every day. Other then the few minutes that I take to stir, I keep the jar in a dark cabinet.