THE RELATION OF MICROORGANISMS TO MILK 385 



although many grow under anaerobic conditions. Gelatin may be 

 liquefied or not. Milk may or may not be curdled, the curd often 

 resembling that formed by rennet-like enzymes. They produce no 

 lactic acid, but only acetic, propionic, butyric and caproic acids; 

 and hence cannot be classed as lactic bacteria. 



Bacteria Having No Appreciable Effect on Milk. — ^This 

 group is made up of many different forms. They produce no changes 

 which can be detected either by the eye or the taste. They do not 

 develop very rapidly in milk, and some species gradually disappear 

 while others increase in numbers. Many of the organisms in this 

 group are chromogenic, orange and lemon yellows being among the 

 more common forms. They are mostly cocci and do not liquefy gelatin. 

 From the standpoint of the commercial milkman these organisms 

 are of little significance and this is probably also true from the stand- 

 point of the consumer. 



The Casein-digesting or Peptonizing Bacteria. — These organ- 

 isms digest the casein either with or without coagulation. Many of 

 them coagulate the casein with an alkaline reaction. They liquefy 

 gelatin. Most of the organisms of this group are rods of various shapes 

 and sizes, some of them being the largest rods found in milk. Some are 

 motile and some non-motile. Some representatives of this group 

 produce little or no odor, but many of the species develop very strong 

 putrefactive odors. Barny or cowy odors or other off-flavors sometimes 

 found in milk and dairy products may be caused by the action of this 

 type of bacteria. They are associated with filth and their presence 

 in milk indicates insanitary conditions of production or handling. 



Pathogenic Organisms. — This group includes all those species 

 which may gain access to milk, which are capable of causing specific 

 diseases in human beings. They are of the greatest importance to the 

 consumer. They do not appreciably affect the physical or chemical, 

 properties of the milk, or produce any changes in its appearance, 

 flavor, or keeping quality which would indicate their presence. 

 Some of them do not even develop in milk, as is the case with the Bad. 

 tuberculosis. Others, as the diphtheria bacteria and t)rphoid fever 

 bacilli, may grow in milk with great rapidity. This group also con- 

 tains' certain species which produce diarrhceal disorders, especially 

 in infants and young children. Some of them are probably organisms 

 which are also included in the peptonizing group. The specific 



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