THE DETERMINATION OF LACTOSE IN MILK. 
By OTTO FOLIN anp W. DENIS. 
(From the Biochemical Laboratories of Harvard Medical School and of the 
Massachusetts General Hospital, Boston.) 
(Received for publication, January 25, 1918.) 
In this paper we describe two methods for the determination 
of the sugar in milk—a titration method and a colorimetric 
method. Of the two procedures we believe the titration to be 
the more accurate. The colorimetric method has, however, the 
advantage that by means of it a very large number of determi- 
nations can be made more or less simultaneously. 
I. Titration Method. 
In the sugar titration method of Folin and McElroy 40.4 mg. of 
anhydrous lactose have the same reducing value as 25 mg. of dex- 
trose. The lactose value was determined by us because it was 
thought that the method might prove quite as serviceable for the 
titration of the lactose of milk as for the titration of the dextrose 
of urine. Since albumin does not interfere with that titration it 
was hoped that the lactose of milk could be titrated directly, that 
is without any preliminary preparation of protein-free filtrates: A 
remarkably simple method for the determination of the sugar in 
milk would thus be secured. Our conjecture turned out to be cor- 
rect, for we have encountered no difficulties in titrating the lac- 
tose of milk: without first removing the protein and the fat.. By 
means of large numbers of check determinations, using protein- 
free filtrates and determining the sugar by titration, by the 
polariscope, as well as colorimetrically, we have satisfied our- 
selves that the preliminary removal of the protein materials is 
superfluous. Omitting the preparation of protein-free milk fil- 
trates eliminates not only the most tedious feature of the older 
methods for titrating the sugar in milk, but it also does away 
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