From a technological point of view, there are two well defined methods for vinegar production: traditional (slow) processes and submerged (quick) methods.
The first one is the so called surface culture fermentation, where the acetic acid bacteria is placed on the air–liquid interface in a direct contact with atmospheric air (oxygen). The presence of the bacteria is limited to the surface of the acidifying liquid and hence, it is also considered as a static method. Nowadays, this method is employed for the production of traditional and selected vinegars and a very long period of time is required to obtain a high acetic degree. As a consequence, production time and costs are increased. These systems permit simultaneous acetification and ageing.
Wine vinegar is mostly produced in countries with enological tradition. Besides vinegar from white and red wine there are special products such as vinegar from Jerez (Sherry vinegar) in Spain or balsamic vinegar elaborated in an specific region in Italy (Modena).
Sherry wine vinegars are produced by peculiar traditional methods: the ‘solera’ system and the static method (Fig. 1). A solera system consists on a series of butts arranged in steps whose number may vary from three to eight. The substrate arrives at the step on the top of the system and the final product is withdrawn from the step at the bottom which is the most aged, but the volume transferred will never exceed one-third of the total volume. Barrels in stage 1 are immediately filled with vinegar from stage 2, which, in turn, are filled with the content of barrels in step 3. In this way, a great homogenisation throughout the whole system is accomplished. This is a dynamic method of production in contrast with the static method in which vinegar is produced in a single butt