There are two chemical steps in the making of vinegar. First, sugar in a liquid is converted into alcohol.  Yeast does that.  Second, the alcohol is converted into acetic acid.  Bacteria do that.  What the bacteria (called "acetobacter") do is incorporate oxygen into the alcohol to form acetic acid and water.  Thus the process requires both the right bacteria and oxygen.   The amount of acetic acid in vinegar ranges from 4% to 6.5%; wine vinegar typically has 6.5%.

     There are three basic methods for making vinegar: the Orleans process, the Boerhaave method, and submerged fermentation.  Submerged fermentation was first used in the early 1950's, the Boerhaave method dates from the 18th century, but the Orleans process is the oldest by far.  In the Orleans process, barrels of about 50 gallon capacity are used.  They are positioned on their sides with two air holes drilled into the upper end to let air in.  A barrel is filled with wine (about 30 gallons) almost up to the air holes and then a mass of vinegar bacteria, called "mother of vinegar", is added.  A mass of bacteria grows over the surface of the wine because the supply of air is greatest there.  The film of bacteria is called "mat."  It takes three or four months to complete the aerification process, the turning of alcohol into acetic acid.  No heat is used to speed up the process. When the concentration of acetic acid reaches about 5%, most of the vinegar is drawn off through a spigot at the lower end of the barrel.  Approximately 5 gallons is left for the next batch.  New wine is then very carefully poured into the barrel so as not to break the film or mat of bacteria.

The process is time consuming and thus is appropriately called the "slow process."  Also, because of the length of time and exposure to air, a fair amount of liquid is lost to evaporation.  This means that the Orleans process is the expensive way to make vinegar.  However, it is also the most natural and best. It produces superior vinegar for both aroma and flavor.