FOODS: THEIR CLASSIFICATION 391 



more or less yellow clastic tissue in flesh; it is indigestible and use- 

 less as food. 



When meat is cooked its white fibrous tissue is turned into 

 gelatin, and the whole mass becomes thus softer and more easily 

 disintegrated by the teeth. When boiled some of the protein 

 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 substance 

 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 precipi- 

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

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

 converted 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." 



