ACETIC FERMENTATION 113 



Temperature maxima, and some fine differences of shape, distinguish the 

 three species from one another. B. aceti is stained entirely yellow by iodine, 

 but in the two other species the gelatinous mass that holds the cells together 

 in the pellicle takes on a blue tint, the cell remaining yellow. Whether 

 a carbohydrate is present is doubtful, and must remain so as long as we are 

 ignorant of the composition of the cell membrane from whose outer layers, 

 by inhibition of water, the jelly is produced. Cellulose is said not to be 

 present. 



The optimum temperature for acetic fermentation is 34°C, the maxi- 

 mum 42 , the minimum 4°— 7 . Approach to the maximum gives rise in 

 all three species to involution forms (Fig. 14, c and d, p. 27). The borders 

 of neighbouring cells become indistinct, the cells themselves swell up to pear- 

 or spindle-shaped bodies, and short branches arise on the filaments. Even 

 at the optimal temperature involution forms arise as soon as acetic acid 

 begins to accumulate, and when about 14 per cent, is present all growth 

 stops, and the bacteria die. As a concrete example (83) of the activity 

 of these bacteria it may be mentioned that B. Pasteurianus in 12,5 c.c. of 

 lager beer (3-7 per cent, by volume of alcohol) formed in seven days, at 

 34° C, 4-2 grm. of acetic acid. The alcohol had disappeared, and the acetic 

 acid was finally oxidized to C0 2 and water. In a parallel culture, after 

 twenty-one days, only 0-7 grm. acid was present. In the manufacture of 

 acetic acid the process must, of course, be stopped at the proper time in 

 order to prevent loss of acid by further oxidation. 



In vinegar factories the so-called vinegar plant consists of a zoogloea 

 mass of acetic bacteria, of which there are doubtless many races. 



The usual methods of vinegar-making all aim at bringing the alcoholic 

 .fluid as much as possible into contact with the air, either by running it into 

 large vats or by letting it flow over wood-shavings in barrels. These 

 methods originated at a time when the process was supposed to be a purely 

 chemical oxidation effected by the air. But the bacteria lying upon the 

 surface of the shavings are the cause of the change ; the free access of air 

 promoting it only because the vitality of the bacteria is increased thereby. 



Any alcoholic fluids, such as cider, dilute brandy, or wine, can be used 

 to make vinegar, but the taste varies according to the nature of those con- 

 stituents of the liquids that remain unaltered by the process. 



As a by-product, acetic acid arises in many other fermentations, the 

 vinous, the lactic, and the butyric, for example. 



In fluids containing no alcohol, however nutritious these may be, the 

 acetic bacteria cannot thrive. They can make use of ammonia salts as 

 a source of nitrogen, but obtain this element in the liquids used for vinegar- 

 making from proteid substances. Whether the alcohol is used for respira- 

 tion only, or whether its carbon is in part seized upon by the bacteria as 

 food, is not known. 



