194 MODERN DAIRY PRACTICE. 



quality of the skim-milk, is lost and the applicability of the 

 milk decreased. 



The length of time in which this milk will keep 

 slightly acid varies greatly. I have noticed cases where 

 such milk has kept evenly sour for several days, and others 

 where it assumed a rancid taste within twenty-four hours. 

 The different conditions to which the milk has been pre- 

 viously exposed will largely decide how long the sample will 

 keep. These conditions exert the same influence on the 

 skim-milk as on any other milk, and we find that if the 

 new milk has been handled carelessly the skim-milk ob- 

 tained from it will possess a very inferior keeping quality. 



The manner in which the sour milk is kept has also 

 decided influence on its keeping quality. Proper clean- 

 liness must above all be observed; the acidity should not 

 be allowed to develop too far. Strong cooling or pasteuri- 

 zation need not be applied, but the temperature should be 

 kept at a point that will allow the lactic-acid fermentation 

 to continue, but not to develop excessively. This seems to 

 take place best at 60 F. The lactic-acid bacteria then 

 multiply easily (although not as rapidly as at 77 F.), and 

 so strongly that they may hold in check the other bacteria 

 found in the skim-milk. This is easily accomplished by 

 keeping the sour skim-milk in a light room and in not too 

 deep pans, and by stirring the milk occasionally. By these 

 precautions, and by mixing some sour skim-milk with it 

 every day, it is possible to keep such skim-milk of about 

 even acidity for several weeks, as I have often seen done. 



In the modern methods of creaming the cream is always 

 removed in a sweet condition, and the skim-milk is also as 

 a rule sweet. The problem of its keeping quality is there- 

 fore entirely different from that of sour skim-milk. In 



