Their Digestion And Assimilation

The proteins are highly complex compounds containing - in addition to the carbon, hydrogen, and oxygen, which we have seen are the elements constituting fats and carbohydrates - nitrogen and sulphur. They may be divided into the following classes, beginning with the simplest: -

(1) The protamines in the roe and milt of fishes, the only proteins not known to contain sulphur.

(2) The histones in blood corpuscles.

(3) The albumins in blood serum, egg and milk (soluble in water alone).

(4) The globulins in blood and white of egg (soluble in water with the aid of a neutral alkali salt).

(5) The sclero-proteins, e.g., collagen of connective tissue, gelatin, keratin, elastin, ossein, chondrin.

(6) The phospho-proteins, e.g., vitellin in egg-yolk, caseinogen of milk.

(7) The conjugated proteins: -

(a) Nucleo-proteins, combinations of protein with nucleic acid, which on decomposition yield purins,

(b) Gluco-proteins, e.g., mucin, combinations of protein with carbohydrates. The carbohydrate substance is not always sugar, but usually a nitrogenous substance called glucosamine, with a similar reducing power to sugar and a similar formula to glucose, HO being replaced by NH2, thus: C6H41O5,NH2.

(c) Chromo-proteins, e.g., haemoglobin, combinations of protein with chromogenic compounds. (8) The derivatives of the proteins, produced by hydrolysis, as in digestion and otherwise, are usually considered to form another class by themselves. These are: -

(a) Metaproteins.

(b) Proteoses.

(c) Peptones.

(d) Polypeptides.

(e) Amino-acids.

This classification refers almost entirely to animal proteins. The vegetable proteins are really in a class by themselves, for although many of them may be ranged under the above headings - leucosin in wheat being an albumin, and edestin in hemp a globulin - the glutelins and gliadins are quite distinctive. The glutenin of wheat gluten is an example of the former class, and, like its associates, is insoluble in water but soluble in dilute alkali; the gliadins differ from all other proteins in being soluble in alcohol alone, in containing no lysine amongst their cleavage products, and in yielding a very high percentage of glutamic acid, although all the vegetable proteins give a much higher yield of this amino-acid than any of the animal proteins.

Effects Of Cooking

As with other food-stuffs, the application of heat exercises a mechanical as well as a chemical action. The former is manifested in the case of meat, e.g., chiefly by the formation of steam between the fibres, which tends to loosen them and render them more accessible to the digestive juices. The insoluble collagen of the connective tissue is converted into soluble gelatin, the protein of meat is coagulated, and odorous substances (extractives) are developed, which impart flavour and stimulate the appetite. Although the proteins are rendered more insoluble by cooking, it is undeniable that the process is on the whole favourable to digestion, because of its concomitant counterbalancing advantages.