MILK MEDIA 71 
reaction of the solution to between +0.1 and +0.2 Fuller’s scale. 
Do not allow solution to become alkaline to phenolphthalein or over 
+0.2. If the casein is weighed accurately and the normal solution 
accurate the reaction will be about +0.2. 
The agar solution is prepared by dissolving 10 gms. agar in 500 c.c. 
of water. Both casein and agar solutions should be filtered, then mixed. 
Tube and sterilize in autoclave under pressure for twenty minutes; 
then cool the tubes quickly in cold water or ice water. The final 
reaction of the medium will be about +0.1 Fuller’s scale. If the 
medium is alkaline the bacterial growth will be restricted. If the 
medium is more than +0.1 some of the casein may be precipitated 
during sterilization. The casein egar should be clear and almost 
colorless when poured in a Petri dish. Sometimes the casein will be 
shghtly precipitated during sterilization or the cooling, but it is of no 
consequence since on pouring into plates the precipitate on account 
of its finely divided condition becomes invisible. 
Whey Agar. Add a few drops of acetic acid to boiling milk until 
the casein is precipitated. Neutralize, or bring to 1 per cent-+. Dis- 
solve 1 per cent peptone, 2 per cent dextrose and 1.5 per cent agar. 
Filter, tube and sterilize. 
Plain Milk. This may be made from fresh skimmed milk. It 
may be used in many of the different laboratory apparatus. Very 
often it will be found convenient to make it from desiccated milk. 
Desiccated milk... . . a . 100 gms. 
Distilled water .. Lees 1000 gms. 
The milk powder should be added to about 200 c.c. of the distilled 
water and thoroughly mixed. Beating with an egg beater often serves 
this purpose. It should then be made up to a liter. Sterilize in the 
Arnold. 
Litmus Milk. Add sterile litmus or azolitmin to plain skimmed milk 
until the color is a distinct lavender. Tube in any of the laboratory 
apparatus and sterilize. 
Litmus Whey (Petruschky). Fresh milk is slightly warmed and 
treated with dilute hydrochloric acid to precipitate the casein. This 
is filtered off and dilute sodium carbonate added to the filtrate to 
neutrality. This is then steamed for two hours to precipitate any 
casein which is changed to acid albumin by the dilute hydrochloric 
acid. After filtration a clear neutral solution should be the result. 
This is colored with litmus, tubed and sterilized. After growth the 
amount of acid may be determined by standard reagents. 
