FERMENTATIONS WITH NO SOLUBLE FERMENTS 207 



secretion of a specific ferment. We will now describe the best-known 

 examples of such fermentations. 



I. Lactic acid fermentation. From an industrial point of view this 

 fermentation is of very great importance, as will be seen in the next 

 chapter, and in point of distribution is probably more widespread than 

 any other. Lactic acid may be produced by the fermentative decom- 

 position of many sugars, including milk-sugar, cane-sugar, mannite, and 

 sorbite. It is formed when plant infusions, wort and other liquids are 

 left exposed to the atmosphere, and it has also been detected in the 

 stomach and other parts of the body. The organisms that are 

 responsible for these changes are numerous and widely distributed. 

 The best known are Bacillus acidi lactici, Bacillus lactis aerogenes, 

 Streptococcus acidi lactici, Micrococcus acidi lactici, and Micrococcus 

 lactis I. and II. In the decompositions of these bacteria, lactic acid is 

 the chief product. There are other bacteria which produce this acid as 

 a subsidiary product, or under exceptional circumstances as a chief 

 product. Among these are the pathogenic Bacillus coli communis, 

 Bacillus typhosus, and Vibrio cholerae. 



Although no ferment has as yet been extracted, this fermentation in 

 some of its characteristics shows evidence of the probability that such 

 a ferment does exist. Thus, some of the lactic-acid bacteria, when 

 grown for a long time in a medium devoid of sugar, if now transferred 

 into a sugar-containing medium, are found to have lost the power of 

 changing sugar into lactic acid. But if, after losing this capacity, the 

 bacteria are once more grown continuously in a sugar medium, the 

 capacity of changing sugar into lactic acid is gradually regained. 

 Again, the addition of certain metallic salts, e.g. corrosive sublimate or 

 copper sulphate in very small quantities, causes an increase in the 

 intensity of fermentation, but a decrease in the power of multiplication. 

 This is the case in all fermentations in which the secretion of a definite 

 ferment has been demonstrated. 



II. Acetic acid fermentation. When an alcoholic liquid is left 

 exposed to the atmosphere it acquires a sharp acid taste. This is due 

 to the formation of acetic acid by the agency of various bacteria which 

 fall into the liquid and create this fermentation. The manufacture of 

 vinegar is an example of such a fermentation. The best known acetic 

 acid bacteria are Bacillus aceti (Fig. 123), Bacterium Pastorianum 

 (Fig. 124), Bacterium xylinum, and Bacterium Kiitzingianum (Fig. 125). 

 The decomposition which takes place is the same as that effected by 

 chemical oxidising agents, viz. a conversion of the alcohol into the alde- 

 hyde, and afterwards a further oxidation of the latter into acetic acid. 



