298 Report of the Department of Chemistry of the 



the acidity as measured by titration is increased over what it would 

 be if no such reaction occurred. These facts serve to explain some 

 results obtained by us in connection with the study of certain prob- 

 lems relating to milk. 



We have found that when we titrate whole milk with alkali, 

 in the usual way and then similarly titrate the serum obtained 

 by filtering the milk through a porous procelain filter, the titration 

 figure given by the whole milk is about double that obtained with , 

 the serum. For example, 100 c.c. of whole milk may show an acidity 

 of 17 c.c. of r!r alkali, and 100 c.c. of serum, 8 c.c. This difference 

 has ordinarily been interpreted as being due to the acidity of milk 

 casein, but in a future paper we shall show that casein is present 

 in fresh milk as a calcium caseinate that is neutral to phenolphthalein. 

 The other constituents removed from the milk by filtering through 

 porous porcelain are fat and dicalcium phosphate, both of which 

 are also neutral to phenolphthalein. From the illustration given 

 above, the titration figure of the residue on the filter would appear 

 to be 9 (17—8) for 100 c.c. of milk, though in reality the reaction 

 is neutral. We believe that the cause of this discrepancy is to be 

 found in the dicalcium phosphate which is present in the whole milk 

 but which is not present in the serum. Its presence in the milk 

 permits the formation of relatively large amounts of phosphoric 

 acid and tricalcium phosphate, requiring the use of the increased 

 amounts of £> alkali (17 c.c.) to neutralize the milk, as compared 

 with the amount (8 c.c.) needed to neutralize the serum. We have 

 been led by such results to believe that the acidity of milk, as usually 

 determined, is about twice what it should be. 



The disturbing influence of calcium salts in the presence of phos- 

 phates has been studied by Folin 3 in connection with the deter- 

 mination of acidity in urine; he was able largely to overcome the 

 difficulty by the addition of neutral potassium oxalate, by which 

 the calcium is removed in the form of the insoluble oxalate. He 

 showed that by this preliminary treatment correct titration figures 

 could be obtained for monocalcium phosphate which, without 

 such treatment, gives figures that are remote from the calculated 

 acidity. 



METHOD FOR DETERMINING THE ACIDITY OF MILK. 



Making use of Folin's procedure, and, before titrating with 

 alkali, adding to milk some saturated solution of neutral potas- 

 sium oxalate, we are able to obtain figures which conform more 

 closely to the results indicated as accurate by other considerations. 



The method, as modified by us for the determination of acidity 

 in milk, whether fresh or otherwise, is as follows: 



Measure 100 c.c. of milk into a 200 c.c. Erlenmeyer flask, add 50 



3 Amer. Journ. of Physiol., 9:265. 1903. 



