The vinegar is already known to the ancients (traces of it can be found in both the Bible and some Egyptian papyrus) it is obtained from the fermentation of low-alcohol liquids due to the action of the acetic bacteria that transform ethyl alchool into acetic acid. The different types of vinegar are due to the different starting point: you can have, therefore, wine vinegar, alchool vinegar, apple vinegar and so on.
Common to all the different types, the gelatinous mass that is called the MOTHER of the vinegar: it is nothing but cellulose that can be found inside the bottles and it is produced as a “processing waste” of the active bacteria of acetic acid.
These filaments are traditionally used to produce vinegar and come in a stratified gelatinous form, generally placed on the surface of the containers as these bacteria need the constant supply of oxygen present in the air to act and remain active.
The “mother” was often “renewed” by taking it out, “leafing through it” and throwing away the old layers and washing it with wine vinegar to “regenerate” it: this procedure was made not so much by the cellulose itself, but simply by the fact that the vinegar brought new active bacteria, ready to work.
Since one of the intermediate elements of the process is the acetaldehyde which notoriously has a strongly unpleasant odor, at the time of opening the bottle if the vinegar still presents it tends to mean that the fermentation is not completed.
Finally it is important to underline that, although it is not very aesthetically attractive, the mother of the vinegar is absolutely harmless and it is possible to continue to use the vinegar in which it is contained, indeed given the process by which it is formed is in some way a sign and proof of the craftsmanship of the production process and the quality of the raw materials used.