ZYMOGENIC BACTERIA 213 



A large number of species of bacteria which are present in 

 milk produce in the process of fermentation other acids be- 

 sides lactic acid, and large amounts of gases such as carbon 

 dioxide, methane, nitrogen, and hydrogen. Bacteria do not 

 produce as much acid in cream as in milk. 



In the process of cheese manufacture, the presence of 

 gas-producing bacteria may cause the curd to " float " and 

 produce a poor cheese, due to the holes formed by the gas. 



A curd due to the rennet enzymes of bacteria may also 

 be produced without the formation of any acid. Occasion- 

 ally milk does not sour rapidly and produces a soft, slimy 

 curd in a weak acid solution. The condition is known to 

 dairymen as " sweet curdling." The bacterial enzymes 

 which produce this condition are very similar to rennet 

 enzymes. The curd produced may be eventually softened 

 and digested by the action of the bacterial proteolytic 

 enzymes. Duclaux calls this digesting enzyme "casease." 

 The chemical reactions in these zymogenic processes are very 

 complex. 



Butyric acid fermentations usually occur in milk which 

 has become sour due to the formation of lactic acid. This 

 process has been discussed above. The formation of butyric 

 acid may take place, however, in a neutral or faintly alkaline 

 medium. In butyric acid fermentation of milk, the bacteria 

 secrete their enzymes and produce the chemical changes in 

 the milk after the oxygen has been abstracted by lactic acid 

 bacteria and other aerobic species. Butyric acid fermenta- 



