FOODS: THEIR CLASSIFICATION 435 



matters of the meat pass out into the broth, and there in part 

 coagulate and form the scum; this loss may be prevented in great 

 part by putting the raw meat at once into boiling water which 

 coagulates the surface albumen before it dissolves out, and this 

 keeps in the rest, while the subsequent cooking is continued 

 slowly. In any case the myosin, being insoluble in water, remains 

 behind in the boiled meat. In baking or roasting, all the solid 

 parts of the flesh are preserved and certain agreeably flavored 

 bodies are produced, as. to the nature of which little is 

 known. 



Eggs. These contain a large amount of egg albumen and, in the 

 yolk, another protein, known as vitellin. Also fats, and a sub- 

 stance known as lecithin, which is important as containing a con- 

 siderable quantity of phosphorus. Lecithin, or rather a sub- 

 stance yielding it, is an important constituent of the nervous 

 tissues. 



Milk contains at least two proteins, lactalbumin and casein; 

 several fats in the butter; a carbohydrate; milk-sugar; much water; 

 and salts, especially potassium and calcium phosphates. Butter 

 consists mainly of the same fats as those in beef and mutton; but 

 has in it about one per cent of a special fat, butyrin. In the milk 

 it is disseminated in the form of minute globules which, for the 

 most part, float up to the top when the milk is let stand and then 

 form the cream. In this each fat-droplet is surrounded by a pellicle 

 of albuminous matter; by churning, these pellicles are broken 

 up and the fat-droplets then run together to form the butter. 

 Casein is insoluble in water; in milk it is dissolved by the alkaline 

 salts present. When milk is kept, its sugar ferments and gives 

 rise to lactic acid, which neutralizes the alkali and precipitates 

 the casein as curds. In cheese-making the casein is acted upon 

 by a ferment present in the extract of stomach used, and con- 

 verted into tyrein which is precipitated: this clotting does not 

 take place unless a calcium salt be present. Tyrein, which forms 

 the main bulk of a true cheese, is different from the curd pre- 

 cipitated from milk by acids; cheese made from the latter does 

 not " ripen." 



Vegetable Foods. Of these wheat affords the best; not that it 

 contains more of any particular nutrient but because of a peculiar 

 property of its protein. The protein of wheat is mainly gluten, 



