448 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



others are not injured. This ability to destroy microorgan- 

 isms is lost when milk is heated above 80 c. It should be 

 emphasized that the germicidal action in milk, while of 

 some assistance in holding down the number of bacteria, 

 can never be depended upon to destroy the disease-pro- 

 ducing germs which have gained access. 



The Development of Lactic Acid. The organisms re- 

 sponsible for the formation of lactic acid have been dis- 

 cussed in the chapter on lactic acid fermentation. In milk 

 usually these bacteria develop rapidly, particularly if the 

 milk is not kept chilled. Milk which contains 0.4 per cent 

 of lactic acid has a decidedly sour taste and the milk curdles 

 when the acidity reaches about 0.8 per cent. 

 ) Disappearance of the Acid. Sour milk exposed to the 

 air soon comes to have a coating or scum composed very 

 largely of Oospora lactis and other molds. These oxidize 

 the lactic acid, that is, utilize it as food, forming carbon 

 dioxide and water. Part of the acid also combines chem- 

 ically with some of the proteins of the milk, particularly the 

 caseinogen. 



Decomposition of Protein. Some of the lactic acid 

 bacteria produce small quantities of proteolytic enzymes 

 and lead to some digestion of the curd. This usually, how- 

 ever, is not apparent to the eye. Upon the neutralization or 

 oxidation of the acid, putrefactive forms become active. 

 Certain molds growing upon the surface of the milk pro- 

 duce proteolytic enzymes in considerable quantities. The 

 proteins, particularly the caseinogen of the milk, are soon 

 digested and transformed into peptones, polypeptids, 

 amino acids, etc. 



ABNORMAL MILK FERMENTATIONS 



Usually when milk is allowed to stand, the lactic acid 

 bacteria are present in sufficient numbers, so that their 

 multiplication prevents the development of other forms of 



