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MICROBIOLOGY OF MILK AND MILK PRODUCTS 



The fermentation of lactose is usually expressed as follows: 

 Ci 2 H 22 On + H 2 O = 4C 3 H 6 O 3 . 



Thus 342 parts of lactose should yield 360 parts of lactic acid. The 

 theoretical yield of lactic acid is never obtained, for the action of the 

 organism on the carbohydrate is much more complex than is represented 

 by the equation given. In the following table are given data obtained 

 by a number of investigators. 



These data signify that other compounds than lactic acid are 

 formed in the fermentation of lactose by these acid-forming bacteria. 

 Acetic acid (CH 3 .COOH); formic acid (H.COOH); propionic acid 



(C 2 H 5 .COOH); traces of alcohols, aldehydes and esters have been 

 found. The lactic acid formed is the dextro modification. It is be- 

 lieved that the fermentation is due to an enzyme, lactacidase, one of the 

 intracellular enzymes that can be demonstrated only with difficulty. 



Milk fermented by members of this group has a mild acid taste, an 

 agreeable odor, and the curd can be so finely divided by agitation as to 

 produce almost as perfect an emulsion as in raw milk. The organisms 

 are to be classed as desirable from the standpoint of the dairy manu- 

 facturer, and the fermentation produced by them may be called a true 

 lactic fermentation. 



Characteristics of the B. Coli-aero genes Group.* This group includes 

 a considerable variety of organisms, which differ in morphology, in cul- 

 tural characteristics and undoubtedly in the character and amounts of 

 their by-products. They are more distinctly bacilli than the members 

 of the preceding group; are motile or non-motile; none produces spores 

 and they are usually negative to Gram's stain. The optimum growth 

 temperature, 35 to 40, is somewhat higher than for the preceding 



* Prepared by E. G. Hastings. 



