i MILK 83 



This method is, however, not scientifically correct, for milk 

 contains small quantities of other nitrogenous substances, and 

 these appear in the result as protein. To avoid this error, the 

 following method of analysis may be employed. 



(1) Ritthausen's Method for Estimating the Total Protein. 1 



Twenty-five c.c. of milk are weighed out and diluted with 

 400 c.c. of water, and to this 10 c.c. of Fehling's copper solution 

 (34-63 g. CuS0 4 per 500 c.c.) and 6'5 to 7'5 c.c. of caustic potash 

 or caustic soda (14'2 g. KOH or 10*2 g. NaOH per litre) solution 

 are added. When the precipitate which forms has subsided, the 

 liquid ought to be feebly acid or neutral, but under no circum- 

 stances alkaline. Where there is only a slight excess of alkali, 

 the liquid remains cloudy after the precipitate has settled. 

 The liquid is then decanted through a dried weighed filter, the 

 precipitate washed several times by decantation and then 

 brought on to the filter, where it is washed with alcohol to free 

 it from water, and then extracted in a Soxhlet apparatus with 

 ether to get rid of the fat. The residue which remains after 

 the extraction with ether is again washed with alcohol and then 

 dried, first of all in a desiccator over sulphuric acid and then in 

 the drying-oven at 125 C., or even as high as 150 C., until the 

 weight is constant. After the precipitate has been weighed, it 

 is ignited, a moderate heat being employed at the commence- 

 ment. The loss of weight on ignition gives the amount of 

 protein. 



This method is, however, not entirely free from error, for 

 some phosphoric and sulphuric acid arising from the phosphorus 

 and sulphur of the protein are weighed along with the ash. 2 

 Fleischmann 3 states that this error for milk containing 3 per 

 cent, protein is 0114 per cent., for 3'5 per cent, protein 

 0133 per cent., and for 4 per cent, protein 0152 per cent. 



Instead of burning the precipitate, the nitrogen in it may 

 be estimated by Kjeldahl's method, and multiplied by 6'37, to 

 give the amount of protein. If this modification is adopted 

 the precipitate and filter can be heated with sulphuric acid 



1 Fresenius, Ztitschrifi fur analyt. Chem., 1878, p. 241. 



2 See note, p. 10. 



3 Lthrbuch der Milchwirtschaft, Leipsic, 1907, p. 70. 



G 2 



