352 PEOTEIDS MILK SUGAR 



a sample of cows' milk are related to each other in such a manner 

 that it is possible to calculate the value of any one of these three 

 quantities if the other two are known. This can be done because an 

 increase in the solids-not-fat produces a rise in the specific gravity, 

 while the more fat there is present, the lower will be the specific 

 gravity. Many formulae have been devised to facilitate the calculation. 

 One of the most convenient is that of Eichmond : 



T = 

 where 



T percentage of total solids. 

 F = percentage of fat. 



G = " lactometer degrees" (i.e., sp. gr. x 1000-1000). 

 The results obtained for total solids from the fat and specific 

 gravity by this formula agree closely with actual determinations. 



Determination of Proteids. The total amount of albuminoids 

 in milk can most easily be deduced from the amount of total 

 nitrogen. This is conveniently found by the Kjeldahl method de- 

 scribed on p. 85. From 5 to 10 grammes of milk are taken and 20 

 or 25 c.c. pure sulphuric acid, the rest of the process being performed 

 as already described in the case of soils. By multiplying the total 

 nitrogen by 6'38 the total proteids are obtained, since both casein and 

 albumin contain 15*7 per cent of nitrogen. 



If separate determinations of the amounts of casein and albumin 

 be required, the casein must be precipitated. This can be effected by 

 the addition to the milk of twice its volume of saturated magnesium 

 sulphate solution and of the powdered salt until saturation is complete. 

 The casein can then be filtered off, washed with saturated magnesium 

 sulphate solution, and the nitrogen in it determined by the Kjeldahl 

 process. The percentage of nitrogen found, multiplied by 6-38, gives 

 the percentage of casein. The albumin can then be found by differ- 

 ence, for the amount of the other proteid of milk, the globulin, is so 

 small that it may be neglected. 1 



Another method of precipitation of casein is the following 2 : 

 Ten grammes of the milk, which must not be curdled, are diluted 

 to 100 c.c. with water and raised to 40. The casein is then precipi- 

 tated by adding 1-5 c.c. of a 10 per cent solution of acetic acid. The 

 whole is well stirred, allowed to stand for a short time, and the preci- 

 pitated casein washed three or four times with cold water. The 

 nitrogen in the precipitate is then determined as before. 



Determination of Milk Sugar. This can be done either by the 

 well-known Fehling method or by the use of the polarimeter. In 

 either case, previous removal of the fat and proteids from the milk is 

 necessary. If the Fehling method is to be employed, this can readily 

 be done by the method used by van Slyke above described, but in 

 addition, boiling the solution and filtering. For polarimetric examina- 



1 Sebelien, Zeits. fur Physiol. Chemie, 13, 137 and 160. 



2 Van Slyke, Jour. Amer. Chem. Soc., 15, 644. 



