Starting from Hard Apple Cider
Fill a thoroughly cleaned wide-mouthed glass jar (a 700 ml mason jar will due fine) with about 500 ml of 1.05 specific gravity fresh cider. Fall(winter) apples, they have a higher sugar content if no water is aded after apples are pressed.
Add 50 ml of unpasteurized and unfiltered organic apple cider vinegar which contains some mother of vinegar (Available at most health food stores).This will quick-start the vinegar making process.
Cover the jar top with two layers of cheesecloth, this will allow vinegar bacteria and oxygen from the air to get to the surface of the cider without being contaminated with fruit flies and other pests.
Place the jar in a warm room but in a dark place away from sunlight, which will interfere with the action of the bacteria. The optimum temperature for vinegar making is about 75 F.
After about 2 weeks there will be a gelatinous white film floating on top of the liquid, this is the mother of vinegar, which is produced by the vinegar bacteria as it converts the alcohol into vinegar (acetic acid).
Allow the reaction to proceed for at least 4 to 8 weeks, then, if you started with a hard cider with 6% alcohol content, you should have a vinegar with about 5% acetic acid.
The age-old method for determining if the vinegar is complete is to simply smell and taste it. No odor or flavor of alcohol should be present.
A far more accurate way is to measure the acid content by titration. Inexpensive titration kits can be found at your local wine and beer making shop and are easy to use.
Once completed, store the apple cider vinegar into clean long necked glass containers
equipped with plastic screw-type caps, and discard the thick mother of vinegar film or reuse it to start-up a new batch.
This works well for me.