From the skin gelatine and glue trades the most valuable bye-product is the grease, which is obtained from the "fleshings" of the adipose tissue. These fleshings are themselves a bye-product of the leather trades. The recovery and purification of this grease has been dealt with previously (Part V., Section II., p. [227]). In the skin glue trade the only other bye-product is the residue from the extraction process (Part V., Section III., p. [230]). This residue is known usually as glue "scutch," and is composed of the proteins of the skin which are insoluble in hot water. These insoluble portions are obtained from all layers of the skin. There is much hair often in scutch, the hyaline or glassy layer (grain), and the elastic fibres of the corium are also insoluble, and a proportion is derived from the fibres of the adipose tissue on the flesh side. All these portions are fairly rich in nitrogen, and the scutch has, therefore, considerable value to makers of fertilizers. It is liable to contain also a percentage of grease, which is usually removed by steaming under hydraulic pressure. This process recovers a valuable bye-product and increases the manurial value of the scutch. There is always left in scutch some of the gelatinous skin substance which, strictly speaking, should have been removed during extraction. There is, however, a practical limit beyond which it does not pay to do this. When this limit is reached the cost of steam in extracting, and also in evaporating and drying, together with the loss of time and labour involved by occupation of the plant, is greater than the value of the possible product.
From the bone-glue industry, the grease is similarly a valuable bye-product, but there is also another of equal importance, viz. the phosphate of lime, which comprises about half the raw material. As previously described in Part IV., Section II., p. [225], this is usually extracted after the grease, by solution in weak hydrochloric acid. The solution is neutralized in lead-lined vats with milk of lime, a precipitate of di-and tri-calcium phosphates being obtained. Calcium chloride is left in solution, and the precipitate should be, therefore, well washed if it be desired to have dry phosphate. The bone-glue industry is, generally speaking, much more intimately connected with the fertilizer trades than the skin-glue trades, indeed the extraction of the bones for glue is not always advisable, in which case the protein matter as well as the phosphatic matter of the bones are employed for making "bone manures." For details of this industry the reader is referred to a companion volume in this series on "Chemical Fertilizers."
REFERENCES.
"Chemical Fertilizers and Parasiticides," S.H. Collins, M.Sc.
"Bones," Part II., Section V., p. 72.
"Precipitated Bone Phosphate," Part III., Section III., p. 157.
"Bone Manures," Part III., Section V., p. 173.
"Gelatine, Glue, and Allied Products," T. Lambert.
SECTION III.—FOOD PROTEINS
Although there are those who consider that animal proteins are both undesirable and unnecessary as foods, it is nevertheless true that man is almost universally a carnivorous animal. The animal world provides mankind with one of its chief sources of food, and especially of protein foods. Protein foods are unquestionably essential, and animal protein foods differ chiefly from those of vegetable origin in the fact that they contain generally much more protein. Of the proteins noted in our Introduction, the keratins have no value as foods; the gelatins have some value as culinary material, but little actual food value; whilst the albumins comprise practically all the useful animal food proteins. Whilst the actual flesh of animals is the principal source of food proteins—both as to quantity and food value—other parts of animals, e.g. kidneys, liver, blood, brains, tongue, are used and relished. The most important sources of animal food proteins are from fish, fowl, sheep, cattle, and pigs, the meat from these being roughly in the same sequence as to digestibility. There are, however, many other animals of which the flesh is quite edible, but most of the above are specially farmed and propagated primarily for their food value.
As the animal food proteins are exceedingly putrescible, they are usually consumed within a short time of the animal being killed. It is perhaps natural, therefore, that many efforts have been made to discover means of preserving such foods. These efforts form the basis of some important industries, and though they can hardly be included as chemical industries, it will not be out of place in this volume to point out that these efforts present analogies with, as well as differences from the methods used for preserving hides and skins (Part I., Section I., p. [12]). The curing of hides and skins is a temporary preservation from putrefaction until the opportunity is convenient for the permanent preservation (i.e. tannage). The preservation of meats is analogous to curing inasmuch as more drastic treatment might indeed make them non-putrescible, but would also render them indigestible and unsuitable for food. Thus drying, salting, drying and salting, pickling and freezing, are just as suitable for preserving food proteins as for hide and skin proteins. Hence we have dried meats, salt bacon, pickled beef, frozen mutton, etc. To a limited extent smoking (fish, bacon, etc.) has been employed as a cure. When it has been applied to skins it is usually combined with a fat tannage. There is, however, one method of preservation of proteins, inapplicable to skins, which has been eminently successful and useful for food proteins, viz. sterilization by boiling. The food has been placed in tins, hermetically sealed, and thoroughly sterilized. Hence have appeared corned beef, tinned tongue, sardines, etc., which merely illustrate the immense possibilities involved. A noteworthy advantage of this method of preserving animal food proteins, is that the food is already cooked and prepared for immediate consumption.
Another line of effort is the preparation of concentrated foods. Just as animal foods are on the whole more concentrated in protein than vegetable foods, so these prepared animal foods are more concentrated than animal flesh, and generally also more soluble. Such preparations of animal protein are obviously useful when there is difficulty in swallowing and when journeys are necessary into regions of poor food supply. It is a little doubtful, one must say, whether the concentration is as great in some cases as is claimed.
Yet another industry based upon the animal proteins is the manufacture of meat extracts. These are not merely concentrated extracts of animal flesh, but contain especially the stimulative properties of animal food proteins. There is now little doubt of the value of these preparations as stimulants, and it is claimed for them that they not only have food value, but also that they increase the food value of other foods used with them. Together with these products may be classed all the miscellaneous tonic foods, in which proteins are blended with carbohydrates and often also with drugs. These aim at the cure of specific disorders, such as nervous debility, sleeplessness, etc. Their claims are often extravagant. Amongst all the multitude of prepared foods, there deserve particular mention the partly predigested foods. In cases where the digestive functions are weak or disordered these products have been of real service.
One of the most useful and valuable of animal food proteins is obtained from hen eggs. The "white" of eggs is almost pure albumin, and there is much protein in the yolk also. Eggs are now produced and imported by the million, and form a most important item in the country's dietary, the protein being in a very easily digestible form.