The second division of the Combustible Compounds is called nitrogenous, or flesh-forming foods. Nitrogen is the flesh-forming element common to all foods. It enters largely into the composition of the body, forming sixteen per cent of the animal tissue. A liberal supply is necessary to form and repair tissue. Although the atmosphere is four fifths nitrogen, we get no supply from that source. It must be supplied in a state of combination, not as an element, from such compounds as have been produced under the influence of life. We require organic nitrogenous matter, and not pure nitrogen. This is sometimes derived from vegetable sources, but is most abundant in animal substances. Animal food is richer and more nutritious than vegetable food; but the latter, if taken in large quantities, yields the same amount of flesh-forming material. Nitrogenous substances in plants and animals are identical in composition; and, from whichever source they are taken, the most important consideration is, to digest them and make them into blood.
Nitrogen is an essential part of some of our most powerful medicines, like quinine and morphine, and of our most dangerous poisons, such as strychnine and prussic acid.
Nitrogenous foods are also called albuminous, because albumen is their common element, though it is called by different names in different things. Albumen is from albus, meaning white. The principal varieties of albuminous food are lean meat, fish, eggs, milk, cheese, peas, beans, oatmeal, flour, rye, and corn. These are treated more fully in other parts of the book, in chapters on Bread, Meats, Fish, Eggs, Milk, and Vegetables.
The albuminous portion of meat is the juice, or albumen, and the fibre, or fibrine. In eggs it is the white. In milk it is the caseine, or the curdy part that separates when milk has soured. In peas and beans it is called vegetable caseine. In flour it is gluten, - the sticky, glutinous substance which is left after squeezing or washing out the starch.
Albumen exists in two states, - one soluble in water and one insoluble. The soluble may be changed to insoluble by heating to 120°, or by adding nitric acid. It is the most easily digested of all flesh-forming foods.
Albuminous substances have the property of coagulation; but all albuminoids do not coagulate in the same way. The albumen of eggs and the juices of meat coagulate by heating to the boiling-point. The fibrine of the blood coagulates when exposed to the air. Milk coagulates by the addition of an acid.
Albuminous substances also have the property of fermentation. This occurs principally in substances which are rich in sugar, starch, and gluten, like flour, milk, etc. The fermentation in flour and milk is explained in the chapter on Bread-Making.
Fermentation will not take place without air or moisture, and a moderate degree of heat. Therefore, if albuminous substances be excluded from air and moisture, and kept very hot or very cold, they will not ferment. Fermentation is a change in the elements of a body composed of oxygen, carbon, and hydrogen. Sugar is composed of carbon, oxygen, and hydrogen in equal parts; when sugar ferments, it decomposes and then reunites in different proportions, forming different compounds, - alcohol, carbonic acid, and water.
Albuminous substances are the only substances which putrefy. Putrefaction resembles fermentation, and is due to the presence, in large proportion, of the fickle element, nitrogen, and also to the large number of elements combined in all albuminous sub-stances. Complicated machinery is always more easily deranged than simple; and in all chemical combinations, the more complex they are, the more unstable. Nitrogen has a very weak affinity for other elements, and forms very unstable compounds. All substances rich in nitrogen, when exposed to the air, soon pass into a state of decomposition, or putrefaction. The oxygen of the air has a greater affinity than the nitrogen for the other elements, and unites with them very easily. In putrefaction the oxygen unites with the carbon to form carbonic acid, and with the hydrogen to form water. The hydrogen and nitrogen unite and form ammonia, and this occasions the peculiar, unpleasant odor of all putrefying bodies. In substances rich in sulphur and phosphorus, the hydrogen unites with them and forms sulphuretted and phosphoretted gases which are very offensive. Therefore, if we exclude the air or oxygen from such substances, we can arrest decomposition. This is done by keeping them in air-tight vessels, thus removing the oxygen from the outside; and by boiling or drying, to remove that which is diffused within. Freezing will have the same effect; also salting or preserving. The salt draws out the moisture, hardens the albumen, and prevents the access of oxygen. In preserving, by the use of a strong solution of sugar, the watery juices are drawn put and formed into a thick syrup which excludes the air.
There are some albuminous substances, such as isinglass and gelatine, which are taken as food; but, strictly speaking, they are not flesh-formers. Isinglass is obtained from the sound, or swimming bladder, of the sturgeon, and is imported from Russia. It is not actually gelatine, but is transformed into it by boiling water. Gelatine is obtained from bones. Ossein is that part of the bones to which their strength and elasticity are due. It is insoluble in cold water, but is slowly dissolved and changed into gelatine by being boiled gently under a pressure sufficient to prevent the escape of steam. Gelatine is also obtained from tendons, calves' feet, fish scales, stag's horns, etc.
There are other varieties of nitrogenous food which are also carbonaceous. These are commonly included under the general term Beverages; namely, tea, coffee, cocoa, and chocolate. The nitrogenous principle of tea is theine; of coffee, caffeine; and of chocolate, theobromine. Tea also contains iron and manganese.
Drinks, Beverages, and Liquid Foods are classified as follows: water, including rain, well, and mineral spring water; mucilaginous, farinaceous, or saccharine drinks, including toast water, Irish moss, and barley water, sago, tapioca, arrowroot, and other gruels; aromatic or astringent drinks, including, tea, coffee, chocolate, cocoa, and herb teas; acidulous drinks, including lemonade, raspberry vinegar, and other fruit syrups; animal broths, or drinks containing gelatine, including soups, broths, and beef tea; emulsive drinks, including milk; alcoholic and intoxicating drinks, including wines, cider, beer, ale, porter, brandy, and whiskey.
Water is discussed under Non-Combustible Foods. Receipts for mucilaginous and acidulous drinks, animal broths, and herb teas are given in the chapter on Cookery for the Sick. The aromatic drinks are included in a separate article under the head of Beverages. Alcoholic drinks will not be discussed for want of space. Milk is food as well as drink, and deserves especial consideration.