MILK 



327 



scale of the lactoscope which is level with the surface of the diluted milk. This 

 reading represents the percentage of fat present in the undiluted milk. Pure milk 

 should contain at least 3 per cent of fat. 



3. Total Solids. 1 Introduce 2-5 grams of milk into a weighed flat-bottomed 

 platinum dish 2 and quickly ascertain the weight to milligrams, Expel the major 

 portion of the water by heating the open dish on a water-bath and continue the 

 heating in an air-bath or water oven at 97-iooC. until the weight is con- 

 stant. (If platinum dishes are employed this residue may be used in the 

 determination of ash according to the method described below.) 



Calculation. 3 Divide the weight of the residue, in grams, by the weight of 

 milk used, in grams. The quotient is the percentage of solids contained in the 

 milk examined. 



4. Ash. Heat the dry solids from 2-5 grams of milk, obtained according to 

 the method just given, over a very low flame 4 until a white or light gray ash is 

 obtained. Cool the dish in a desiccator and weigh. (This ash may be used in 

 testing for borates according to directions on page 324.) 



5. Proteins: Nephelometric Determination of Proteins, Casein, 

 Globulin, and Albumin in Milk. Method of Robert Principle. The 

 proteins are precipitated with sulphosalicylic acid and the precipitate 

 estimated nephelometrically (see discussion of nephelometric methods, 

 page 290). 



Procedure. Five c.c. of milk are carefully measured into a 250 c.c. flask 

 and after adding 200 c.c. of distilled water and 10 c.c. of decinormal sodium 

 hydroxide solution, water is added to the mark and the mixture shaken. Ten 

 c.c. are put with exactly 2 c.c. of ether in a centrifuge tube which is then tightly 

 stoppered with a cork and vigorously shaken. Allow to separate and withdraw 

 5 c.c. of the aqueous layer without contamination with ether. Dilute to 50 c.c. 

 Take 10 c.c. of this solution and add 10 c.c. of 3 per cent sulphosalicylic acid. A 

 suspension of casein is obtained which can be matched accurately with the 

 following standard : i volume (5 c.c.) of a o.oi per cent casein solution* to which 

 is added 2 volumes (10 c.c.) of 3 per cent sulphosalicylic acid. 



The protein obtained with this reagent is not all casein, and in order to obtain 

 the exact amount of casein the casein is precipitated according to the "official 



1 Shackell's method for the vacuum desiccation of frozen preparations may be used 

 where great accuracy is desired (see American Journal of Physiology, 24, 325, 1909). 



2 Lead foil dishes, costing only about one dollar per gross, make a very satisfactory 

 substitute for the platinum dishes. 



3 The percentage of total solids may be calculated from the specific gravity and percentage 

 of fat by means of the following formula which has been proposed by Richmond: 



8=0.25 L-f-i. 2 F+o.14 

 S = total solids. 

 L = lactometer reading. 

 F = fat content. 



^ 4 Great care should be used in this ignition, the dish at no time being heated above a 

 faint redness, as chlorides may volatilize. 



6 Kober: /. Am. Chem. Soc., 35, 1585, 1913. 



6 Standard Casein Solution. Dissolve with stirring o.i gram of casein or its equivalent 

 in i c.c. of o.i N NaOH, add 95 c.c. of distilled water, add 2 c.c. of toluene, shake thoroughly 

 and make up to 100 c.c. This is the stock solution which keeps three or four days or longer. 

 The standard solution is made up fresh every day by making 10 c.c. of the stock solution 

 up to 100 c.c. with water. The standard is controlled by total nitrogen estimations using 

 the factor 6.38 for casein. 



