6 3 8 



MICROBIOLOGY OF ALCOHOLIC FERMENTATION 



Bacterium xylinum, A. J. Brown. This species forms a thick tough, leathery film, 

 the gelatinous substance of which stains blue with iodine and sulphuric acid. 

 B. acetigenus, B. oxydans, and B. industrius are motile species. 



All species are strictly aerobic and grow quickly only when freely 

 supplied with oxygen. This oxygen is necessary for the acetification 

 of the alcohol. Duclaux has calculated that one centigram of the 

 bacterial film is capable of uniting 1.3 g. of oxygen to alcohol, 130 times 

 its own weight. The optimum temperature for most species is about 



FIG. 158. Vinegar bacteria. A, Bact. aceti; B, Bad. pasteurianum; C, Bad. 

 kiitzingianum; D, Bact. pasteurianum, showing mucilaginous sheath. (After 

 Hans en.} 



34 and the range of temperature at which they grow is between 4 

 and 7 to 42. They all form acetic acid from ethyl alcohol, propionic 

 acid from propyl alcohol and most of them gluconic acid from dextrose. 

 B. industrius and B. oxydans, according to Henneberg, can form acids 

 from a large number of sugars and related substances, including 

 saccharose, maltose, starch, dextrin, glycerin and mannit. 



The presence of too much alcohol prevents the growth of acetic 

 bacteria, the limit being about 14 per cent under manufacturing con- 

 ditions. At 14 per cent and above, the film forms with difficulty, and 

 the oxidation of the alcohol is incomplete, aldehyde and irritating 

 products being formed. Acetic acid in amounts above 10 to 12 per 

 cent is, moreover, antiseptic to the bacteria. Below 14 per cent of 

 alcohol, the bacteria develop readily and produce in suitable solutions, 

 besides acetic acid, agreeable ethers which are more abundant when 

 the oxidation is slow. Below i or 2 per cent of alcohol, the bacteria 



