196 MILK 



tate recognition of the end-point a solution of potassium ferro- 

 cyanid and acetic acid can be used or a piece of filter paper moist- 

 ened with this solution. 



A somewhat simpler method of determining the milk-sugar 

 by reduction of a copper solution is Pavy's method, as modified by 

 Long. The solution used in this test is made up as follows : 



Copper sulphate in crystals 8 . 166 grams; 



Sodium hydroxid (100 per cent.) 15.000 *" 



Glycerol : 25.000 c.c. 



Ammonia, specific gravity 0.9 350.000 " 



Distilled water to make 1000.000 " 



One cubic centimeter of this solution oxidizes 1 mg. dextrose or 1.9 

 mg. lactose in 0.2 per cent, solution. 



The technic is as follows: "Measure 50 c.c. of the solution into 

 a flask and dilute to 100 c.c. with water. Add enough pure white 

 paraffin to form a layer 3 to 4 mm. thick when melted. This is to 

 prevent escape of ammonia. Add a few pieces of porous plate to 

 prevent bumping. The tip of the butet is made long enough to 

 pass below the layer of paraffin. By boiling gently and adding 

 the weak saccharine solution slowly until the blue color disap- 

 pears very close and constant results can be obtained. If the 

 sugar solution is very strong, the reduced copper will not be held 

 in solution unless an inconveniently large volume of ammonia is 

 used. Some practice is necessary to show just how fast the sac- 

 charine solution may be safely added. If added too rapidly the 

 end-point may be overlooked and the sugar content appear 

 too low. 



" Instead of allowing the tip of the buret to extend below a 

 layer of paraffin, the following method may be employed : Fit a 500- 

 c.c. Kjeldahl flask with rubber stopper with two holes. Into one 

 hole insert the tip of the buret and into the other one a Bunsen 

 valve, and proceed as before." 



Lactose Determination by a Refractometer. The apparatus 

 needed for the use of Wollny's refractometer is shown in Fig. 42. 

 Five c.c. of the milk are placed in the sample bottle and 5 drops 

 of a 4 per cent, calcium chlorid solution added. The stopper is 

 then tied on with a string and the bottle placed in a water-bath 

 for ten minutes. After cooling, a small part of the serum is taken 

 up with a pipet and filtered. It is then examined in the refrac- 

 tometer at 17.5 C. 



DETERMINATION OF ACIDITY IN MILK 



Milk freshly drawn from the udder contains no free acid, but 

 gives an acid reaction when phenolphthalein is used as an indi- 

 cator. This is due to the presence of acid phosphates and per- 



