3 io THE CHEMISTRY OF DAIRYING 



per cent, of the theoretical amount of lactic acid is formed by 

 fermentation of sugar in milk ; the remainder appears in the 

 form of by-products, among which formic acid, acetic acid, 

 succinic acid, and carbon dioxide are the most important. 

 These products are formed by secondary or subsidiary reac- 

 tions. The amount of each depends upon the conditions of 

 the fermentation. 



The organisms by means of which these changes are pro- 

 duced develop very slowly, at temperatures below 50 F. 

 They are most active between 70 F. and 90 F. Higher 

 temperatures inhibit their action. They are also somewhat 

 sensitive to an acid reaction, and their development is arrested 

 when the lactic acid, produced by their action, amounts to 0*8 

 per cent, of the medium, i.e. of the milk. Calcium carbonate 

 reacts with lactic acid, forming a neutral salt ; and when this 

 substance is added to the milk, the acid is neutralised as it 

 is formed, and the process continues until all the sugar is 

 changed. 



Over one hundred different organisms have been men- 

 tioned by various observers as capable of inducing lactic fer- 

 mentation ; but all those commonly found in milk can be 

 referred to one or other of the four following types : l (i) strep- 

 tococcus lacticus; (2) bacterium acidi lacti; (3) micrococcus 

 lacti acidi ; (3) bacterium casei. 



Butyric fermentation generally succeeds the lactic fermenta- 

 tion. It is caused by a number of anaerobic forms. Butyric 

 acid may be formed from milk sugar either directly 



+ H 2 O -> 2C 4 H 8 O 2 

 or from the hexose sugars resulting from hydrolysis 

 C 6 H 12 6 -> C 4 H 8 2 + 2C0 2 + 2 H 2 



or it may be formed as a result of further change (fermentation) 

 from the lactic acid 



2 C 3 H 6 O 3 -> C d H 8 O 2 + 2CO 2 + 2H 2 

 1 Percival, "Agricultural Bacteriology." 



