312 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



and in litmus milk reduction occurs. The maximum amount of acid 

 varies from 1.25 to 4.0 per cent. Some members of the group produce 

 dextro-, others laevo-acid, and it is claimed that racemic acid is formed 

 in some cases. The curd can be easily broken by agitation, and through 

 the solvent action of the acid is partially dissolved. The organisms do not 

 liquefy gelatin, but the casein of milk is partially changed into soluble de- 

 composition products, as was first shown by de Freudenreich, and later 

 confirmed by Hastings. 



It has been supposed by many that this group was confined to and 

 cha acteristic of certain of the fermented milks, especially those of eastern 

 Europe and western Asia, such as Yogurt and Matzoon. The work of 

 de Freudenreich has demonstrated their presence in Switzerland, and 

 more recently, Hastings has shown that similar organisms are constantly 

 present in milk and other dairy products in this country. They can be 

 demonstrated by placing a sample of milk in a corked bottle, and incu- 

 bating at 37. The acidity of the milk increases rapidly at first, due 

 to the growth of the members of the two previous groups. These ordinary 

 acid-forming organisms are soon inhibited by the appearance of free acid, 

 but the acidity of the milk nevertheless continues to increase slowly, and 

 with this continued increase a change in flora is noted, the short, plump 

 bacilli ceasing to predominate and long slender rods constantly increasing 

 in numbers. The source of this group is undoubtedly the alimentary tract 

 of the animal. 



Characteristics of the Coccus Group* This group is well represented 

 by the bacteria which form the characteristic flora of the udder. They 

 vary greatly in size and in other properties. They retain Gram's stain; 

 many are chromogenic, the color ranging from a white to a deep orange. 

 They grow slowly on all ordinary culture media, but the growth is not 

 necessarily meager. Generally they are aerobic, 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 in- 



* Prepared by E. G. Hastings. 



