278 FERMENTATIONS IN MILK 



2. The texture of the curd thrown down. 



3. The separation of whey. 



4. The presence or absence of gas bubbles. 



5. The odour of the fermented milk. 



Do any of the organisms peptonize or soften and digest the 

 curd? 



(c) With portions of the different colonies isolated above 

 Inoculate an agar slant. 

 a potato slice. 

 ,, a gelatine stab. 



a tube of glucose broth in a Durham's tube. 

 lactose 



saccharose 



Test their nitrate reducing power (Exs. 74 and 82). 



indole production (Ex. 63). 



Try Voges and Proskauer's reaction with each (Ex. 65). 

 Record the characters of each in accordance with the scheme 

 given on pp. 60-6 1. . 



2. Slimy or Ropy Fermentations of Milk. Many 

 liquids, such as grape juice, beer, cider, and various plant 

 infusions which contain sugar, sometimes become ropy 

 or slimy, so much so that they may be drawn into long 

 gelatinous strings. A similar production of a slimy 

 substance is not unfrequent in milk and cream, and is 

 brought about by the action of bacteria, most of which 

 appear to be introduced into the dairy in the water 

 supply or in dust from hay and other food materials. 

 The chemical nature of the ropy material has not been 

 exactly determined. It appears to arise, however, 

 either : 



(1) from the milk sugar which is changed into a gum- 



like carbohydrate ; 



(2) from the production of a slimy protein substance 



