The second of the food principles, protein, is a complex and very important constituent of our food. The protein compounds differ from all others as to chemical composition by the presence of nitrogen; they contain carbon, oxygen, hydrogen, and nitrogen, while the fats and carbohydrates are composed principally of carbon, oxygen, and hydrogen, but no nitrogen. The so-called extractives or flavoring properties of meats are nitrogenous, and are consequently classed with the protein compounds.[10]
The body of an average person contains about eighteen per cent. of protein. The proteins of various kinds furnish nutriment for blood and muscle, hence the term "muscle-formers," which is sometimes given them. They also furnish material for tendons and other nitrogenous tissues. When these are worn out by use, it is protein which repairs the waste.
Most of the valuable work upon the analysis of food has been done in Germany. From estimates made by chemists of that country it has been decided that the amount of protein in a diet should not fall below four ounces daily. This is to represent an allowance for a man of average weight doing an average amount of work, below which he cannot go without loss in health, in work, or in both. Although protein is the most expensive of all food materials, one should endeavor to use at least four ounces each day. Meat, milk, eggs, cheese, fish of all kinds, but especially dried cod, wheat, beans, and oatmeal are all rich in this substance. The protein compounds are divided into three classes:
ALBUMINOIDS, GELATINOIDS, EXTRACTIVES.
Albuminoids. The most perfect type of an albuminoid is the white of egg. It is a viscous, glairy, thick fluid which occurs also in the flesh of meat as one of its juices, in fish, in milk, in wheat as gluten, and in other foods. It is soluble in cold water.
Exp. Mix some white of egg in a tumbler with half a cup of cold water. As soon as the viscousness is broken up it will be found to be completely dissolved. It is insoluble in alcohol.
Exp. Pour upon some white of egg double its bulk of alcohol. It will coagulate into a somewhat hard opaque mass.
Heat also has the power of coagulating albumen.
Coagulation of Albumen by Heat. Put into a test-tube some white of egg, and place the tube in a dish of warm water. Heat the water gradually over a gas-flame or an alcohol-lamp. When the temperature reaches 134° Fahr. it will be seen that little white threads have begun to appear; continue the heating to 160°, when the whole mass becomes white and firm. Now remove a part from the tube and test its consistency; it will be found to be tender, soft, and jelly-like. Replace the tube in the dish of water and raise the heat to 200° Fahr.; then take out a little more and test again; it will now be found hard, close-grained, and somewhat tough. Continue the heating, when it will be seen that the tenacity increases with rise of temperature until at 212° Fahr., the boiling-point of water, it is a firm, compact solid. When heated to about 350°, white of egg becomes so tenacious that it is used as a valuable cement for marble.