376 MILK BACTERIOLOGY 



Second Stage. This stage extends from the end of the germicidal 

 period to the time of curdling. There may be a gradual increase 

 during this time of many species, but the predominating types are 

 the Bad. lactis acidi. These rapidly produce lactic acid which 

 exerts a suppressing influence on many species. When the milk 

 reaches an acidity of .75 to .80 per cent, it usually curdles. The lactic 

 acid organisms seldom produce more than 1.25 per cent. acid. 



Third Stage. This stage extends from the time of curdling until 

 the neutralization of the acid. The acidity becomes so great that 

 the action of the lactic acid bacteria is checked and their number, 

 which at first may be as high as 100,000,000 per c.c., rapidly 

 decreases. The predominating species become O'idium lactis, certain 

 species of molds and yeasts. The proteins are broken down with 

 the formation of ammonia which neutralizes the acidity. 



Fourth Stage. The liquefying and peptonizing bacteria which 

 remained inactive in the sour milk find suitable conditions in the 

 alkaline media for their growth. They rapidly decompose the 

 casein. 



Abnormal Changes in Milk. At times foreign undesirable organ- 

 isms find their way into milk and produce abnormal and objectionable 

 changes. The B. coli communis and the Bad. lactic aerogenes types 

 produce considerable gas and disagreeable odors and flavors in the 

 milk. B. lactis viscosus produces a slimy or ropy condition of the 

 milk. The slimy condition is supposed to be due to the mucin con- 

 taining capsule which surrounds these bacteria. Milk may be normal 

 in color when first produced, but on standing may turn blue due to 

 B. cyanogenes or red due to B. erythrogenus or B. prodigiosus. At 

 times a bitter taste develops in milk some time after it has been 

 drawn from the udder. This, according to Conn, is due to a micro- 

 coccus. 



Although these changes are very objectionable when considered 

 from the standpoint of the dairymen, they are not known to be the 

 cause of illness. However, when milk putrefies with the production 

 of a bitter alkaline milk illness often does result from its use. This 

 may be due to the poisonous action of the ptomaines which it con- 

 tains, or probably more often to the bacterial infection. 



Classes of Bacteria. The bacteria found in milk are a hetero- 

 geneous lot but, according to Hastings, may be roughly divided 

 into five classes, as follows: 



1. Acid-forming Bacteria. There are constantly present in milk 

 many acid-forming bacteria. These vary in morphology, cultural 

 characteristics, and products of fermentation. They may be divided 

 into five groups. The number and kind vary greatly in milk, 

 depending upon the methods of handling. 



(a) The most important organism of this group is Bad. lactis 

 acidi. The group, however, includes a number of organisms. They 

 produce no gas, a mild acid flavor, and are desirable. 



