222 THE ANALYSIS OF SOLID MILK PRODUCTS. 



and melted at a low temperature. A solvent for the fat, of 

 which ether is perhaps the best, though chloroform, amyl alcohol, 

 and others may also be used, is poured on, the whole mixed well, 

 and allowed to stand in a warm place till the solvent is quite 

 clear. The solution is decanted carefully and a fresh portion 

 of the solvent poured on the residue, and, when clear, poured 

 off. Four or five successive treatments are sufficient to remove 

 the whole of the fat. With a little practice the operation may 

 be so performed that none of the solids not fat are poured away 

 with the solvent. The residue is placed in the water-oven, 

 and dried to constant weight ; the weight represents solids not 

 fat and salt. 



Salt. -To estimate the salt the residue is treated with hot 

 water and filtered, the filter together with the residue washed, 

 and the filtrate, or an aliquot portion of it, is titrated with a 

 standard silver nitrate solution, using potassium chromate as 

 indicator. It is essential that the solution should be cold before 

 titration, and the silver nitrate solution should be standardised 

 on pure sodium chloride. The strength should not be deduced 

 from the amount of silver nitrate present, as Hazen, and, later, 

 W. G. Young, have pointed out that the amount of silver used 

 is always greater than that theoretically required to combine 

 with the chlorine. From the amount of silver nitrate solution 

 used the weight of salt is readily calculated. It is convenient to 

 make the silver nitrate solution of such strength that 1 c.c. 

 = O005 gramme of sodium chloride. 



Solids not Pat. The weight of salt found by titration is 

 subtracted from that of the residue left after the extraction of 

 the fat, and the difference represents the solids not fat. 



Pat. The fat is best estimated by subtracting the total of the 

 water, salt, and solids not fat from 100 ; though the solvent may 

 be evaporated and the fat actually weighed, if desired. 



Curd. An estimation of the actual curd present can be made 

 by submitting the residue, left after estimation of the fat, to 

 Kjeldahl's process for the estimation of nitrogen (p. 171), and 

 multiplying the nitrogen found by 6-39. The milk-sugar may 

 be estimated in a portion of the solution used for the titration 

 of the salt by one of the methods given for the determination 

 of milk-sugar (p. 154). These determinations are rarely 

 required. 



Casein is estimated by extracting the solids not fat with dilute 

 ammonia till no lumps are left, filtering the solution, and washing 

 the residue ; the filtrate is made acid with acetic acid, and the 

 precipitated casein collected on a tared filter or Gooch crucible, 

 and weighed as in the estimation of casein in milk (p. 174). The 

 extraction of the fat, and ignition may be, however, omitted, as 



