CULTURAL TECHNIQUE 17 



0.18 per cent of lactic acid, equaling + 1.3 to + 1.6 per cent 

 on the arbitrary scale ordinarily employed for media. 



On heating, the acidity is reduced by the precipitation of 

 the calcium salts and the expulsion of the C0 2 , so that a 

 milk having an acidity of 0.18 per cent calculated as lactic 

 acid will show not more than + 1.2 per cent when tested 

 after sterilization by the usual method. It is possible to neu- 

 tralize milk showing a high degree of acidity, but it is not 

 advisable, since within certain limits the acidity is a measure 

 of purity, and even under the best of conditions milk is very 

 difficult to sterilize. The whole milk may be allowed to cream 

 in a refrigerator by gravity, and the skim milk siphoned off, 

 or, preferably, the milk may be skimmed in a centrifugal 

 machine or cream separator. 



Milk is used either as plain milk or in the form of litmus 

 milk prepared by the addition of a litmus solution. The 

 litmus solution should be added to the milk before tubing, 

 and in sufficient amount to give it a decidedly blue color. 

 Milk is preferably sterilized by the discontinuous method. 

 Litmus is easily reduced at 100 C. by the action of the milk 

 sugar. Thus when litmus milk is removed from the sterilizer, 

 it may appear almost colorless. The color soon reappears on 

 exposure to the air, due to the oxidation of the reduced in- 

 dicator. The sterility of the milk must be tested by allowing 

 it to stand a week, if possible, before using* 



Preparation of sugar media. Gelatin, agar, and bouillon 

 are often amended by the addition of sugars (saccharose, dex- 

 trose, and lactose). Sugar media should be sterilized by the 

 discontinuous method, and the reaction should be neutral to 

 phenolphthalein. Saccharose is, however, the only one of the 

 three sugars easily affected, being changed in part (inverted) to 

 dextrose and levulose when heated in acid solutions. Lactose 



