as one-fourth of the protein may escape digestion and thus be useless for nourish- 

 ment. From one-sixth to one-tenth of the protein of wheat flour, corn meal, beans, 

 and peas may in like manner be assumed to escape digestion, or rather to leave the body 

 without being used for nutriment. These estimates assume that the materials are cooked 

 and eaten in the usual way. Under the same circumstances, from nine-tenths to the 

 whole of the protein of milk, meats, and fish are assumed to be digested. The digestibility 

 of the fats is likewise variable. Sometimes a large part of the fat of the food fails of 

 digestion. In general it may be assumed that about 5 per cent, of the fat of milk, meat, 

 eggs, butter, and lard, and a considerably larger proportion of the fats of some vegetable 

 foods, will usually escape digestion. When, however, the diet contains a very large 

 amount of fat — for instance, when it consists largely of fat meat — the digestion is less 

 complete. One way in which the fat of ordinary foods is digested is by being made into 

 an emulsiQn in the intestine. The fat of milk is an extremely fine emulsion, and is thus 

 in a sense " pre-digested," or in a partly digested form before it is taken into the 

 stomach. This may help to explain why it is so easily digested. 



" The carbohydrates, which make up a large part of vegetable foods, are in general 

 very digestible. Cane sugar is believed to be completely digested, and this is assumed 

 to be the case with the sugar of milk. 



" The animal foods have in general the advantage of the vegetable foods in 

 digestibility in that they contain more protein and their protein is more digestible. 

 Milk ranks among the most digestible of the animal foods in respect to all its 

 ingredients." 



Effects op Cooking. 



Cooking changes the texture of a food material and affects its digestibility to 

 a greater or less extent. In general, it increases the digestibility of vegetable 

 food materials. In the case of milk the experience of different persons with cooked 

 and uncooked milk is quite varied, and the results of experiments are conflicting. 

 The most common experience seems to indicate that cooking or boiling the milk 

 makes the proteids somewhat more difficult to digest. There are, of course, ex- 

 ceptions. 



Skim Milk. 



The average skim milk contains nearly 10 per cent, of milk solids or nutritive 

 ingredients, while whole milk contains about 13 to 14 per cent. The cliief 

 material removed from the milk in skimming is the fat. Thus, naturally, the 

 skim milk must be richer in the valuable protein materials than the whole milk. 

 The amount of fat left in the skim milk must of necessity vary with the com- 

 pleteness of the skimming, and frequently varies from less than one-tenth of one 

 per cent, to as much as three-tenths or even four-tenths of one per cent. 



The value of skim milk as a food is not generally appreciated. Taken alone 

 it does not satisfy the sense of hunger, for very large quantities would be re- 

 quired to furnish the food materials desired : but it is a cheap source of very 

 digestible proteins, and when taken with bread or used in cooking it forms a very 

 nutritious addition to the diet. Quoting again from Dr. Langworthy's writing 

 we have the following comparisons : — 



"A pound of lean beef (round steak, for example) contains about 0.18 pound of 

 protein and has a fuel value of 870 calories. Two and a half quarts, or 5 pounds, of skim 

 milk will furnish nearly the same amount of protein and have about the same fuel value 

 as the pound of round steak. Two quarts of skim milk has a greater nutritive value 

 than a quart of oysters; the skim milk has 0.14 pound of protein and a fuel value of 

 470 calories. The nutriment in the form of oysters would cost from 30 to 50 cents, 

 while the two quarts of skim milk would have a market value of from 4 to 6 cents, and 

 a value on the farm of from 2 to 4 cents. An oyster stew made of one part oysters and 



