ACID FERMENTATION OF MILK 187 



changed for many days; but, in the normal course of 

 events, it invariably becomes seeded with a variety of 

 organisms, which in a short course of time are able to de- 

 velop and produce the characteristic fermentative by-prod- 

 ucts that bring about tin- usual chants noted. These 

 changes are not produced by a specific organism, but by a 

 i!Toup of widely dissimilar species as far as form is con- 

 cerned, which, however, are able to produce varying 

 amounts of acids, particularly lactic acid. While this fer- 

 mentation product is produced in such quantities as to 

 characterize the change involved, yet other by-products are 

 likewise formed, as other acids, gases, and various sub- 

 stances. Some of these flavor- forming products may be of 

 value, as in the case of those producing agreeable flavors 

 in butter. 



The curdling of milk. The casein of milk is not in 

 actual solution, but exists in a colloidal condition in combi- 

 nation with calcium. As the bacteria multiply, a portion 

 of the sugar that they use is changed to acid, which com- 

 bines with the calcium, leaving the casein free, in which 

 condition it is precipitated in the form of curd. 



The bacteria that are primarily concerned in the acid 

 ferment at ion of milk may be divided into two groups. The 

 name liact. hicti* acid I is applied to the typical representa- 

 tive of the first group. The natural habitat of this organ- 

 ism is unknown. It is a facultative non-spore-forming or- 

 ganism. Its optimum temperature is from 86 to 95 F. 

 It grows rapidly at from 60 to 68 F., and some strains 

 at 50 F. Owing to its facultative nature, it grows 

 throughout the entire mass of milk, causing a uniform 

 curdling. Normally it is not introduced into milk in as 

 large numbers as many other kinds of bacteria ; but, as it 

 finds in this medium an exceedingly favorable environment, 

 it is responsible for most of the decomposition that is noted 



