250 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



Starch, flour, viscogen, etc., may be added to skimmed 

 milk or cream to increase the viscosity (see page 33) . 



The test for starch is made as follows: Add a few 

 drops of acetic acid to the milk or cream, boil and filter. 

 To the filtrate add a few drops of a dilute iodine solution. 

 If starch is present the filtrate becomes blue. 



The presence of viscogen is indicated by a high per 

 cent, of salts. 



DETERMINATION OF THE REFRACTION NUMBER 



The most satisfactory milk serum for this test is ob- 

 tained by Ackermann's calcium chloride method and the 

 refractive index is most conveniently determined by 

 Zeiss' dipping ref ractometer. The method is as follows : 



30 c.c. of the milk to be tested is placed in a test 

 tube of 75 c.c. capacity and mixed with 0.25 c.c. of a 

 1 to 10 solution of calcium chloride in distilled water 

 (specific gravity 1.1375 and refraction number 26 at 

 17.5 C.). The test tube is closed with a perforated 

 rubber stopper, then connected with a 22 c.c. condenser 

 and heated for 15 minutes in a bath of briskly boiling 

 water. 1 It is then placed in cold water and cooled. Dur- 

 ing this process the serum separates from the curd and is 

 poured off into a beaker. If the serum is not sufficiently 

 free of coagula to permit of the passage of adequate 

 light, it is an indication that acid fermentation has ad- 

 vanced too far for the milk to be investigated with the 

 refractometer, the loss of lactose suffered in such cases 

 being sufficient to render the result inaccurate. The 

 beaker containing the serum is placed in the water bath 



1 If the milk tube is not connected with a condenser the 

 result of the test will be nearly, if not exactly, the same and 

 the test is much simplified. 



