82 FEEDING EXPERIMENTS WITH ISOLATED FOOD-SUBSTANCES. 



milk powder thus produced as above described left about 14.5 per 

 cent of inorganic matter on ignition. This includes not only the 

 inorganic constituents of the milk, although by no means in the com- 

 bination in which they occur in the mammary secretion, but also the 

 inorganic salts which were formed by the addition of the hydrochloric 

 acid used to precipitate the casein and also the sodium salts which 

 resulted from neutralizing the milk serum with sodium hydroxide 

 solution. 



EXPERIMENTS WITH ISOLATED PROTEINS AND " PROTEIN-FREE " MILK. 



The use of this product (which we shall designate as protein-free 

 milk) as an adjuvant to isolated proteins to furnish the inorganic 

 elements of the diet has succeeded beyond our expectation. When 

 employed, for example, in combination with various proteins, in the 

 proportion in which its ingredients occur in the complete milk food 

 already used (see page 76), it induces normal growth. Added during 

 the periods of nutritive decline to food mixtures which no longer 

 suffice to maintain rats, recovery has manifested itself in practically 

 every case. Where, as in the case of zein, gliadin, or hordein feeding, 

 no advantage has been obtained by the use of the protein-free milk, 

 it has become obvious that the protein per se is the defective food 

 constituent. Thus at length we have found a method of controlling 

 or furnishing some of the most essential non-protein factors in the 

 diet, so that the value of the individual proteins can be investigated 

 under much more favorable conditions than formerly. 



Numerous charts (see p. 103 fig ) present the graphic records 

 of feeding experiments with casein, edestin,* glutenin,* glycinin,* 

 gliadin,* hordein,* ovalbumin,! and lactalbumin,| showing appropri- 

 ate growth, or maintenance, according to the age at which the animals 

 were started on the use of the protein-free milk as the non-protein 

 component in place of the earlier inorganic salt mixture. 



It might be objected, after superficial consideration of these re- 

 sults, that the favorable outcome (especially for growth) is due to 

 milk protein contaminating the ' ' protein-free milk ' ' component of the 

 diet. Aside from the fact that the amount of possible contamination 

 is at most small, evidence of the untenability of such a theory is 

 available from several sources. In the first place, growth has not 

 followed the use of all proteins when the protein-free milk was added 

 to them. 



*For the preparation of these vegetable proteins see T. B. Osborne: Darstellung der 

 Proteineder Pflanzenwelt, Abderhalden's Handbuehderbioehemischen Arbeitsmethoden, 

 1909, ii, p. 270. 



fThis was prepared by Hopkins's method and was free from conalbumin. Cf . Osborne, 

 Jones, and Leavenworth: American Journal of Physiology, 1909, xxiv, p. 252. 



JThe preparation of this is described on p. 81. 



