PRESERVATION OF MILK 91 



either by the exact dilution method or by the loop method. Deter- 

 mine the percentage of bacteria destroyed by the pasteurizing process. 



An examination should also be made, if possible, of milk before 

 and after pasteurization in a continuous machine, and the efficiency 

 of the two methods compared. 



Exercise. EFFECT OF HEAT ON THE CREAMING OF MILK. Heat 

 some whole milk over a water bath to 80 C. for fifteen 'minutes, 

 stirring constantly during the heating. Cool at once to at least 20 C. 

 by placing in cold water ; compare the creaming power of the heated 

 sample with that of raw milk by placing a certain amount of each in 

 large test tubes or graduated cylinders and placing in the ice box for 

 twenty-four hours. 



In Denmark and in several of the states of the United 

 States it is required by law that all creamery skim milk shall 

 be heated to 80 C. before it can be returned to the farms. 

 This regulation is for the purpose of preventing the spread of 

 contagious diseases such as tuberculosis and foot and mouth 

 disease, the causal organisms of which are often found in the 

 mixed creamery skim milk. In order to determine whether 

 the creameries are obeying the law, some means of control 

 must be employed, and the Storch test is used for determin- 

 ing whether or not the milk has been heated to the required 

 temperature. 



Detection of heated milk. Storch 's reaction. The test 

 is made by adding to 5 cc. of milk in a test tube one drop 

 of a 0.3 per cent solution of hydrogen peroxide (the commer- 

 cial product contains 3 per cent of the peroxide), and at once 

 adding 2 drops of a 2 per cent solution of paraphenylendia- 

 min. Mix the reagents thoroughly with the milk. If the 

 milk becomes an intense blue at once, either it has not been 

 heated at all or if heat was applied it was not higher than 

 78 C. If the color appears more slowly, after the lapse of 

 half a minute, and becomes a clear grayish blue, the milk 

 has been heated to 79-80 C. If the milk retains its white 



