292 SCIENCE AND PRACTICE OF CHEESE-MAKING 



responsible for many of the defects found in Amer- 

 ican cheddar cheese will be discussed only briefly here, 

 because their relations to cheese-making are fully 

 treated from a practical standpoint in Part II, pp. 

 I 15-130. 



LACTIC ACID FERMENTATION 



The ordinary souring of milk is due to the forma- 

 tion of lactic acid, which is produced by the action 

 of lactic acid bacteria (Bacillus lactici acidi. Fig. 44) 

 upon the sugar in milk. A large number of different 



kinds or types of bacteria 

 are able to produce lactic 

 acid from milk-sugar. 

 Some interesting work 

 has been done recently 

 (Bull. No. 42, Mich. 

 Agr. Coll. Exp. Sta.) 

 which shows that other 

 micro-organisms are often 

 associated with the micro- 

 organisms of lactic fermen- 

 tation and that these 



FIG. 44 — TYPICAL LACTIC- 

 ACID BACTERIA (Rogers) 



micro-organisms 



associate 



often have the power of furnishing products that 

 exert a decided influence upon the rapidity of the 

 growth of the lactic micro-organisms. 



We have already (p. 150) called attention to 

 the fact that the sour taste of milk is not due to the 

 presence of uncombined lactic acid, since little or no 

 free lactic acid is present in sour milk until it has 

 quite a high degree of acidity ; but is due to acid phos- 

 phate of calcium, which is formed by the action of 

 lactic acid upon the insoluble -calcium compounds in 



