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 refractometer. 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 1.1875 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.’ 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 
1Tf 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. 
