In this and other series of lessons we have already discussed the food principles to some extent. Let us consider them now somewhat more in detail.
The proteids are more difficult to understand than the other food principles because different members of the class seem at first sight to have little in common. A few simple experiments that will isolate some typical proteids in a more or less pure state will serve to give a clearer image.
To a quarter of a cup of flour add very slowly a tablespoon of water and stir it until the flour is completely moistened, then work the dough in the hands until it becomes smooth and elastic, and finally wash it under cold water until fresh water added no longer grows milky. This will take from fifteen to twenty minutes. If a little iodine is at hand add a drop. If no blue color appears the starch is all washed out. There will be left in the hands a sticky, elastic mass, called gluten. Save part of this for comparison with other proteids and bake the rest in a hot oven.
With,a knife scrape a piece of lean meat until the tender muscle fibre is separated from the firm white connective tissue. The fibre represents one of the chief proteids of meat, called myosin. Beside the gluten, the casein, and the myosin, put the white of an egg, and you have before you the four chief representatives of the proteids of our food.
If we could add to them legumin, the proteid found in peas, beans, and other members of the pulse family, we should have a fifth important member of the class.
If we compare these substances, we shall find that although at first they seem very different, they yet have certain properties in common. All, for instance, to a greater or less extent, show the elasticity and tenacity that is so marked in gluten; all of them are toughened by a high temperature; and all when dried may be ground to powder similar in texture and appearance.
These physical likenesses, however, would hardly be sufficient to place these substances in one group. It is only when we consider the chemical composition of each and the function that each has in the body that we are justified in classing them together as proteids. Proteids are substances containing the elements carbon, hydrogen, oxygen, nitrogen, sulphur and frequently phosphorus. They alone of the food principles are able to supply nitrogen, one of the essential elements in all living things, whether animal or vegetable, and one that we are forced to obtain from our food, since, although we are surrounded by an atmosphere that is nearly four-fifths nitrogen, we cannot utilize it in this form.
Beside the true proteids, there are certain other substances which also contain nitrogen, but which are classed separately because they cannot alone supply the nitrogen needed by the body, though they can replace part of the proteid in the diet, and perform its function. Gelatin is one of the best known of these substances. They are called gelatinoids or albuminoids. Ossein, of which bone is largely composed, keratin, the horny material present in the hair and in the horns and hoofs of animals, collagen, forming the greater part of the connective tissue of meat, are all representatives of the same class of substances. All these named may be changed into gelatin by boiling.
Certain other nitrogenous substances called extractives, are present in some foods. These may help give the flavor to meat. They form the chief ingredient of the extracts of beef on the market; and it is these that give the chief value to beef tea and to clear soup. The extractives act as stimulants rather than as true foods since they neither build tissue nor act as fuel, but they seem to play some role in digestion.
The proteids, gelatinoids, and extractives, are sometimes classed together under the general name of protein. This is the usage of the United States Government pamphlets. The nomenclature applied to the nitrogenous substances is very confusing, since each author seems to have adopted his own. Albuminoid, for instance, is sometimes used to designate the true proteids, and sometimes is applied to the gelatin-oids. Proteid is sometimes used in a much more limited sense than we have given to it, including only certain classes of the substances ordinarily designated by the term.
In studying the subject, therefore, one must first of all ascertain the writer's use of terms.