8 



The chief function of food in the adult is to repair tissue and to furnish 

 energy to do work. Energy and heat are closely related, and in order to have 

 some measure for expressing the amount of heat that a given food is capable of 

 producing, the calorie is taken as a unit. Roughly speaking, this is the amount 

 of heat required to raise the temperature of one pound of water 4 degrees Fahren- 

 heit. One pound of sugar or starch would, if burned and all the heat utilized, 

 raise 1,860 pounds of water 4 degrees in temperature. The fuel value of protein 

 as it ia ordinarily burned in the body is very nearly the same as that of one 

 pound of carbohydrates, but fats have a fuel value of two and a quarter times 

 that of protein and carbohydrates, or 4,220 calories per pound. Thus, M^iile 

 the protein of meat, eggs, etc., are the only materials in the food that will pro- 

 duce muscle, they have no greater value as an energy-producer than carbohy- 

 drates. Thus, the problem in arranging balanced dietaries is to use just enough 

 protein to do the work which they alone can do, and use the cheaper fats and 

 carbohydrates as the energy producers. 



Very few, if any of our food materials form a complete and balanced food 

 in themselves. The mother's milk is for the infant, but for the adult it is too 

 bulky. Bread is as nearly complete as any of our ordinary foods, but it is a little 

 low in the protein compounds. Milk used along with bread improves it in this 

 respect and the two form a fairly complete and balanced diet. The best foods or 

 diets are -those which perform their function in the most thorough and complete 

 manner; that is, with as little waste as possible and with the best physiological 



results. 



There are, however, several other factors which must be considered in judging 

 of the value of a food. These are: digestibility, palatability ; they must agree with 

 the system, and they must be reasonably cheap. 



COMPOSITION OF MILK. 



Milk, as supplied to the consumer, may vary widely in composition. But the 

 average milk is slightly heavier than water, its specific gravity ranging form 1.029 

 to 1.034 at 60" F., and the following is possibly a fair average of the composition : 



Water • ^7.5 per cent. 



Protein: Casein ^-^ „ 



Albumin -I ,. „ 



Fat I'l .. .. 



Sugar ^-^ .. „ 



Ash ;' , , 



Fuel Value per 100 gms 67 calories 



The proteins amount to about 25 per cent, of the total solids, which is a much 

 larger proportion than exists in many foods, including meats. The casein is the 

 most abundant constituent, and is readily coagulated by an acid, either by that 

 found in the milk when it sours, or when an acid, as vinegar for example, is 

 added. It may also be curdled or coagulated by rennet, and the curd thus ])ro- 

 duced is utilized in the manufacture of cheese. It differs from many other form^ 

 of proteins in the amount of phosphorus it contains. Besides the casein, there is 

 a small amount of albumin present, which is very similar to that which occurs in 

 blood and the white of egg. 



