If I'm brewing a high gravity beer with liquid yeast, one yeast vial does not provide enough cells for a proper fermentation. Rather than buying more vials, I propagate the yeast by adding it to a 1-4 liter starter of unhopped wort. If the starter is on the larger side, I typically decant the liquid and add only the yeast slurry at the bottom of the starter. This means up to 3 or 4 liters of unhopped beer down the drain.
While this decanted liquid is pretty bland on its own, it is exactly what malt vinegar is made from. Therefore, I've decided to start making my own vinegar. For the first batch, I added Bragg's raw apple cider vinegar to kickstart acidification, at a ratio of 1 part cider vinegar to 3 parts unhopped beer. For future batches, I will add samples from previous batches instead, so the cider content will be negligible after a few generations.
After decanting the liquid off the yeast and adding the cider vinegar, I let fermentation continue for two weeks in a gallon jug covered with tinfoil, occasionally swirling the jug to aerate. Then I transferred the finished malt vinegar to a swingtop bottle for longer term storage. This particular starter was fermented with a Belgian yeast (WLP570), so I'll be interested to see if any of the yeast flavors remain in the vinegar.
In the future I may experiment with adding oak chips or fruit to my malt vinegars. (Raspberry malt vinegar = delicious raspberry vinaigrette?) I could also add caramel or liquid malt extract to make darker, sweeter vinegar.