This section is from the book "A Manual Of Home-Making", by Martha Van Rensselaer. Also available from Amazon: A Manual of Home-Making.
Protein is a name given to a large variety of substances in foods. Casein in milk, albumin in egg, milk, and meat, gluten in wheat, are illustrations of types of protein. Protein is a substance necessary to the building of all living parts of the body.
All proteins have not the same value. Some are much inferior to others as tissue builders, and in some foods the proteins are of such poor quality that they cannot build tissue until a food containing a better quality of protein is combined with them.
Some of the day's protein should be supplied by animal foods, as these contain the best quality of protein. Animal foods used for their protein may replace one another and will supplement the poor quality of less valuable protein foods. Animal foods that are valuable sources of protein are milk, eggs, cheese, and meat.
Much of the remainder of the protein may be supplied by certain plant foods. None of the foods here listed, with the exception of the soybean and the peanut, may be regarded as complete substitutes for an animal food, since they contain an inferior quality of protein. Some animal food should be used with them to improve the quality of their protein. Plant foods that may be used to supply part of the day's protein are: cereals of various kinds, such as breakfast foods, breads, macaroni; legumes, such as dried peas, beans, and lentils; and nuts.
A part of the day's protein should be supplied by fruits and vegetables. The quantity of the protein supplied by vegetables is small, but it helps to improve the quality of proteins from plant foods mentioned above.