Vinegar stocks are made using a base material that is fermented with yeast to create alcohol. Brewer's yeast is used for cereals, grains, and molasses. Wine yeasts are used for fruit juices and honey. A sugar concentration in the range of 10-18 percent is considered ideal for making vinegar stock. This is because concentrations of alcohol at 9-12 percent are considered optimal for vinegar production. Acetobacter aceti is the bacteria used to create vinegar from alcohol. It is widely found in nature and requires oxygen to function. The bacteria are added to the vinegar stocks to begin the second fermentation process. Vinegar can be made from stock with alcohol concentrations as low as 5 percent, although these conditions can result in over-oxidation, while at higher concentrations, over 15 percent alcohol, acetobacters are inhibited by the amount of alcohol present and can even be killed off by it. If you're using leftover wine, the alcohol levels range from 10 percent to 15 percent. Not to worry, if you don't want to mess about with testing kits and you're working with some leftover wine at the high end of the scale, say a California cabernet clocking in at 15 percent or more, just add a splash of water and let it ferment. It may take longer than something in the optimal range, but it will become vinegar eventually and you will be rewarded for your patience by the flavor of the finished vinegar. .