But the brewer and vintner have long known that the fermentation is frequently interfered with by irregularities.  The troubles which arise have long been known, but the bacteriologist has finally discovered their cause, and in general their remedy.  The cause of the chief troubles which arise in the fermentation is the presence of contaminating bacteria among the yeasts.  These bacteria have been more or less carefully studied by bacteriologists, and their effect upon the beer or wine determined.  Some of them produce acid and render the products sour; others make them bitter; others, again, produce a slimy material which makes the wine or beer “ropy.”  Something like a score of bacteria species have been found liable to occur in the fermenting material and destroy the value of the product of both the wine maker and the beer brewer.  The species of bacteria which infect and injure wine are different from those which infect and injure beer.  They are ever present as possibilities in the great alcoholic fermentations.  They are dangers which must be guarded against.  In former years the troubles from these sources were much greater than they are at present.  Since it has been demonstrated that the different imperfections in the fermentative process are due to bacterial impurities, commonly in the yeasts which are used to produce the fermentation, methods of avoiding them are readily devised.  To-day the vintner has ready command of processes for avoiding the troubles which arise from bacteria, and the brewer is always provided with a microscope to show him the presence or absence of the contaminating bacteria.  While, then, the alcoholic fermentations are not dependent upon bacteria, the proper management of these fermentations requires a knowledge of their habits and characters.

There are certain other fermentative processes of more or less importance in their commercial aspects, which are directly dependent upon bacterial action, Some of them we should unhesitatingly look upon as fermentations, while others would hardly be thought of as belonging to the fermentation industries.

Vinegar.

The commercial importance of the manufacture of vinegar, though large, does not, of course, compare in extent with that of the alcoholic fermentations.  Vinegar is a weak solution of acetic acid, together with various other ingredients which have come from the materials furnishing the acid.  In the manufacture of vinegar, alcohol is always used as the source of the acetic acid.  The production of acetic acid from alcohol is a simple oxidation.  The equation C2H6O + O2 = C2H4O2 + H2O shows the chemical change that occurs.  This oxidation can be brought about by purely chemical means.  While alcohol will not readily unite with oxygen under common conditions, if the alcohol is allowed to pass over a bit of platinum sponge the union readily occurs and acetic acid results.  This method of acetic-acid production is possible experimentally, but is impracticable on any large scale.  In the ordinary manufacture of vinegar the oxidation is a true fermentation, and brought about by the growth of bacteria.