382 



MICROBIOLOGY OF MILK AND MILK PRODUCTS 



(C2H6.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 diflSculty. 



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. CoU-aerogenes Group* — This group includes 

 a considerable! variety of organisms, which differ in piorphology, 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 

 group, the vegetation range being 15° to 45°. They are to be classed as 

 facultative anaerobes. / 



The conditions for development are less narrow than for the Bad. 

 lactis .acidi group, growth occurring on all the ordinary culture media 

 and in the absence of carbohydrates. Indol and hydrogen sulphide 

 are often formed and nitrates are reduced. The growth is usually pro- 

 fuse, the colonies large and surface growth occurring in stab cultures. 

 Gelatin is not usually liquefied. 



Lactose, dextrose and saccharose are fermented, with the production 

 of varying amounts of gas in which have been found carbon dioxide, 

 hydrogen, methane, and free nitrogen. The maximum amount of acid 

 produced in any culture medium is quite similar to that formed by the 

 members of the previous group. The relative proportions between the 



* Prepared by E.,G. Hastings. 



