CONDITION OF CASEIN AND SALTS IN MILK* 



LUCIUS L. VAN SLYKE and ALFRED W. BOSWORTH. 



SUMMARY. 



i. Milk contains two general classes of compounds, those in 

 true solution and those in suspension, or insoluble. These two 

 portions can be separated for study by filtering the milk through a 

 porous earthenware filter like the Pasteur-Chamberland filtering 

 tube. 



2. Serum prepared from fresh milk is yellow with a faint green- 

 ish tinge and slight opalescence. The following constituents of 

 milk are wholly in solution in the milk-serum: Sugar, citric acid, 

 potassium, sodium and chlorine. The following are partly in solu- 

 tion and partly in suspension: Albumin, inorganic phosphates, 

 calcium, magnesium. Albumin in fresh milk appears to be adsorbed 

 to a considerable extent by casein and therefore only a part of it 

 appears in the serum. In serum from sour milk and milk to which 

 formaldehyde has been added, nearly all of the albumin appears 

 in the serum. 



3. The insoluble portion of milk separated by filtration through 

 the Pasteur-Chamberland filtering tube is grayish to greenish 

 white in color, of a glistening, slime-like appearance and gelatinous 

 consistency. When shaken with water it goes readily into suspen- 

 sion, forming a mixture having the opaque, white appearance of 

 milk. Such a suspension is neutral to phenolphthalein. When 

 purified, the insoluble portion consists of neutral calcium caseinate 

 (casein Ca 4 ) and neutral di-calcium phosphate (CaHP0 4 ). The 

 casein and di-calcium phosphate are not in combination, as shown 

 by a study of 16 samples of milk from 13 individual cov/s, and also 

 by a study of the deposit or " separator slime " formed by whirling 

 milk in a cream separator. By treating fresh milk with formalde- 

 hyde and whirling in a centrifugal machine under specified con- 

 ditions, it is possible to effect a nearly complete separation of phos- 

 phates from casein. 



4. Both fresh milk and the serum from fresh milk show a slight 

 acid reaction to phenolphthalein but are strongly alkaline to methyl 

 orange, indicating that acidity is due, in part at least, to acid phos- 

 phates. In 8 samples of fresh milk, the acidity of the milk and of 

 the milk-serum was determined after treatment with neutral 

 potassium oxalate. The results show that that acidity of the whole 

 milk is the same as that of the serum and that, therefore, the con- 



Reprint of Technical Bulletin No. 39, December. 

 20 [305] 



