DEHYDRATED CULTURE MEDIA 65 



of "Standard Methods for the Examination of Dairy Products." ^-^ This medium 

 is also used for the isolation and cultivation of yeasts and molds in other dairy 

 and food products. "Standard Methods for the Examination of Dairy Products"^ 

 specifies a reaction of pH 4.5 for Potato Dextrose Agar in determining the mold 

 content of dried milk. 



In a study of comparative methods and media used in the microbiological 

 examination of creamery butter, Shadwick^ investigated a number of media and 

 found that Bacto-Potato Dextrose Agar gave the most consistent and highest 

 count of yeasts and molds in salted and unsalted butter. 



To rehydrate the medium, suspend 39 grams of Bacto-Potato Dextrose Agar in 

 1000 ml. of cold distilled water and heat to boiling to dissolve the medium 

 completely. Distribute in tubes or flasks and sterilize by autoclaving for 15 minutes 

 at 15 pounds pressure (121°C.). The medium will have a final reaction of 

 pH 5.6. 



It is frequently desirable in making yeast and mold counts to inhibit bacterial 

 growth by acidifying the medium, and "Standard Methods" recommends that 

 the reaction of the medium be reduced to pH 3.5±:0.1 subsequent to sterilization. 

 The label of each package of Bacto-Potato Dextrose Agar specifies the quantity 

 of sterile tartaric acid (10 per cent solution) that should be added to each 100 

 ml. of the sterile melted medium to adjust the reaction to pH 3.5. After the 

 tartaric acid has been added to the medium, it is mixed well and poured plates 

 are prepared as usual. The medium should never be heated after the acid is 

 added inasmuch as heating in the acid state will hydrolyze the agar, destroying 

 its solidifying properties. 



One pound of Bacto-Potato Dextrose Agar will make 11.3 liters of medium. 



1 Standard Methods for the Examination 2 Standard Methods for the Examination 



of Dairy Products, gth Edition: 157: 1948. of Dairy Products, gth Edition: 181: 1948. 



3 Food Research, 3:287:1938. 



BACTO 



TRYPTOSE AGAR 



DEHYDRATED 



"Standard Methods for the Examination of Dairy Products," Ninth Edition 

 recommends the use of Bacto-Tryptose Agar for the detection of Brucella 

 organisms in milk. A complete discussion of Bacto-Tryptose Agar is given on 

 page 111. 



BACTO 



MALT AGAR (B24) 



DEHYDRATED 



Malt Extract, Difco 30 g. 



Bacto-Agar 15 g. 



Bacto-Malt Agar is recommended for the detection and isolation of yeasts and 

 molds from dairy products, food, and other materials. For the mold count of dry 

 milk "Standard Methods for the Examination of Dairy Products"^ specifies the 

 use of dehydrated Bacto-Malt Agar or Bacto-Potato Dextrose Agar. This medium 

 is also recommended for carrying stock cultures of yeast and molds used in 

 microbiological assay procedures. 



Malt media for yeasts and molds have been in use for many years. Reddish^ in 



