190 MILK AND DAIRY PRODUCTS PART 



results, but with the richer sorts it is sufficiently correct to 

 be used when a very high degree of accuracy is not needed. 



III. DETERMINATION OF THE NITROGENOUS SUBSTANCES. 

 (a) Estimation of the total Nitrogen. 



Two grams of cheese are analysed by Kjeldahl's method 

 (p. 80). 



(b) Estimation of the Water-Soluble Nitrogenous Compounds. 



Ten grams of cheese are rubbed up to a thin paste with water 

 at 40 C., and then transferred to a 250 c.c. graduated flask, which 

 is filled to the mark. The flask is allowed to stand for twenty- 

 four hours at room temperature, and during that time it is 

 frequently shaken. A few drops of formalin should be added 

 to prevent further decomposition. By evaporating 25 c.c. of 

 the filtrate the total water-soluble nitrogenous compounds are 

 obtained, and can be analysed also by Kjeldahl's method. In 

 another portion of the filtrate, 50 c.c., the soluble protein can 

 be determined by making acid with 30 c.c. of dilute sulphuric 

 acid (1:4) and precipitating with 20 c.c. of 10 per cent, phospho- 

 tungstic acid. After standing for twenty-four hours the 

 precipitate is collected on a filter, washed, and treated 

 according to Kjeldahl's method for the determination of 

 nitrogen. 



The phospho-tungstic acid not only precipitates the protein 

 b ut also any basic decomposition products, and these have to be 

 determined in another portion of the original filtrate by distil- 

 lation with magnesia, and deduction of the nitrogen so found 

 from that obtained with the phospho-tungstic acid. The water- 

 soluble nitrogen minus the soluble protein and the ammonical 

 nitrogen, is termed the amide nitrogen. 



IV. DETERMINATION OF THE ASH. 



This is done in the ordinary way by carefully igniting the 

 cheese in a platinum basin. If the common salt has to be 

 specially estimated, then the ash is dissolved in water and an 

 aliquot portion titrated with jY/10 silver nitrate solution 

 potassium chromate serving as an indicator. 



