DEHYDRATED CULTURE MEDIA 



193 



medium cools and absorbs oxygen. Overheating during sterilization results in the 

 carmelization of the milk sugar. This is to be avoided since this resulting dis- 

 coloration may give an atypical appearance of the sterile medium and may be 

 reflected in the appearance of growing cultures. The final reaction of the medium 

 will be pH 6.8. 



One pound of Bacto-Litmus Milk will make 4.3 liters of medium. 



BACTO 



ULRIGH MILK (B251) 



DEHYDRATED 



Bacto-Skim Milk 100 g. 



Bacto-Methylene Blue 0.005 g. 



Bacto-Chlor Phenol Red 0.015 g. 



Bacto-Ulrich Milk is recommended for propagating and carrying stock cul- 

 tures of the lactic acid bacteria associated with dairy products, and for deter- 

 mining the action of bacteria in general on milk. Ulrich,i in 1944, described an 

 indicator system for use to replace litmus in Litmus Milk. His system consists of 

 a mixture of chlor phenol red (dichlorophenolsulfonphthalein) and methylene 

 blue. Chlor phenol red is a hydrogen ion indicator, yellow at pH 4.8 and more 

 acid reactions, and pink to red at pH 6.5 and more alkaline reactions. Methylene 

 blue in the concentration employed is used to indicate changes in the oxidation- 

 reduction potential (Eh) in the medium, being blue when oxidized and colorless 

 when reduced, by growth of organisms and when heated to boiling, or when first 

 removed from the autoclave following sterilization. The combination of the two 

 indicators in milk gives information similar to Litmus Milk, plus indication of 

 alkalinity and acidity in the lower (reduced) portion of the tube. 



In his descriptions of the color changes occurring in his medium due to the 

 growth of various types of lactic bacteria Ulrich used the Ridgeway^ color classi- 

 fication. The table showing the Ridgeway color description, the Munsell^ color 

 system, and the Inter-Society Color Council and National Bureau of Standards 

 terminology, which uses familiar terms, is given below to show typical color 

 changes of some of the more common biological reactions in Ulrich Milk. Slight 

 variations in color, due to the variable alteration of the milk during sterilization, 

 and to changes in pH or Eh of the growing cultures, may occur. 



^We are greatly indebted to Dr. H. J. Conn, Biological Stain Commission, Geneva, 

 New York, and to Prof. F. L. Dimmick, Hobart College, Geneva, New York, for 

 this color terminology. 



