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., 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.). 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.
It is also necessary to refer to the importance of cows' milk as a source of animal food protein. The amount of protein in milk (4-5 per cent.) is not large, but it is united with fats, carbohydrates, salts, and vitamines in such proportions, that milk is about the only article which may reasonably present a claim of being a complete food. Milk, moreover, forms the staple diet of infants and young children, so that its protein is certainly of great importance. As an infant food, cows'
milk is not altogether ideal. Even when the proportions of fat, carbohydrate, and protein have been adjusted to resemble human milk, there remains the difficulty that some of the proteins of milk (especially the casein) are too indigestible for young infants. This difficulty has been only partly surmounted by those industries engaged in manufacturing infant foods. Some claim to remove the bulk of the casein; others to have rendered it digestible by treatment with enzymes; others, again, simply claim to supply concentrated cows' milk. Tinned milk, generally concentrated to some extent, now forms a useful addition to animal food products. The casein of milk also finds some outlet for industrial purposes. When treated with formaldehyde it yields an artificial horn much used for the preparation of imitation tortoiseshell. Skim milk is treated with caustic soda or carbonate of soda, the casein precipitated by acid, pressed, impregnated with formaldehyde, and dried. The product is termed "galalith." It can be distinguished from real tortoiseshell by the action of fuming nitric acid (see _J.C.S.I._, 1909, 101).
The utilization of the blood of animals, which is very rich in protein, as a foodstuff has long been known, but has met with a good deal of prejudice in this country. This prejudice has arisen not merely from the objection to blood as food, but also from the fact that such foods have been particularly liable to putrefaction and hence to cause poisoning.
The shortage of all foodstuffs occasioned by the European War did much to overcome this prejudice, and there were considerable developments in the manufacture of black pudding and similar preparations of animal blood. The same circumstances made it necessary to consider more seriously the possibilities of other butchers' offal as human food, and resulted in new preparations of tinned animal proteins being placed on the food market.
The author would like to record his opinion that by no means the last word has been said on the question of drying as a method for preserving animal food proteins. There is much to be said for this method on every ground in theory, and it is evidently an increasing success in practice.
Dried milk has been followed by dried eggs, and in view of the success of the method when applied to fruits and vegetables, there seems a prospect of better success in respect of dried meats. After all, animal food proteins are chiefly lyophile colloids, and though desiccation presents some practical difficulties, the subsequent imbibition (assisted perhaps by lyotrope influences) seems to be the ideal method for restoring preserved protein to its original condition.
In conclusion, it will be interesting to note in the subjoined table, the relative importance of the different sources of supply of both animal and vegetable food protein. The figures are taken from the report of a Committee of the Royal Society. They show the average quantities of food materials (imported and home produced) available for the United Kingdom during the five years 1909-1913 inclusive, together with the amounts of protein, fat, and carbohydrate present and the energy value.
This information formed the basis of the Committee's recommendations as to economy of protein during the war shortage. These recommendations included the more economical production of meat by slaughtering cattle younger and the saving of 55,000 metric tons of protein annually by adopting cheese-making as a general practice in place of butter-making.
Metric tons. Energy +-------------------------------- value, Protein. Fat. Carbo- millions of hydrate calories.
---------------------------+----------+---------+-----------+----------- Cereals 549,000 63,000 3,628,000 17,712,000 Meat 356,000 799,000 -- 8,890,000 Poultry, eggs, game, 42,000 31,000 -- 461,000 and rabbits Fish 91,000 17,000 -- 531,000 Dairy produce, including 199,000 686,000 258,000 8,253,000 lard and margarine Fruit 9,000 14,000 222,000 1,077,000 Vegetables 120,000 10,000 1,031,000 4,812,000 Sugars (including cocoa, 5,000 18,000 1,572,000 6,633,000 etc.) Other cottage and farm produce 67,000 13,000 551,000 2,655,000
SECTION IV.--MISCELLANEOUS ANIMAL PROTEINS
The excreta of animals include animal proteins of great importance to agriculture and horticulture, forming the staple supplies of manure. The manure of animals should contain not only the solid waste material and undigested food, but also the urine, which contains much nitrogen, and hence makes considerable difference to the value of the product as a fertilizer. If the animals are fed on rich foods, the manure obtained is correspondingly richer, especially in its protein content.
The value of dung manures depends not merely upon the protein content, but also upon its content of phosphate and potash, as well as other organic matter. The protein breaks down into simpler nitrogenous compounds, and eventually, through ammonium carbonate, it becomes nitrate. Nitrogenous manures darken leaves and increase growth considerably. Dung manures are deficient in phosphates and potash and are of value partly as nitrogenous manures producing growth, and partly as dressings of organic matter for soil. From both points of view it is desirable that the manure should be well decayed. Fresh dung manures are both wasteful and injurious to soil, except perhaps to very stiff clays.
They are wasteful inasmuch as much ammonia escapes, and injurious inasmuch as they cause the "denitrification" of the valuable nitrates already in the soil. When possible dung manures should be kept under cover. Free exposure to air and rain will sometimes reduce its value by one half. It should be stored until "sweet," and until the straw has rotted and become "short." This takes usually several months. A ton of well-rotted farmyard manure contains very approximately 10-12 lbs.
nitrogen, about the same amount of potash, and about half that quantity of phosphates. It is, however, very variable. Horse manure is rather richer than cow manure, but more liable to loss on storage. Pig manure is intermediate between them. Sheep manure is distinctly richer in protein, and has therefore greater value as nitrogenous fertilizers.
Poultry droppings are richer still, perhaps partly because they include the urinary products. When fresh they contain 18-25 lbs. nitrogen, 12-24 lbs. phosphate, and 6-12 lbs. potash per ton. When dried they have about double the value. Pigeon manure is even richer, and the pigeon loft scrapings have a manurial value about double that of dry hen manure, and eight times that of farmyard manure. Guano is much decayed droppings of sea birds on the tropical coasts of Africa and America. The supplies are now quite exhausted, and the market guanos are chiefly artificial fertilizers.
There is one other animal protein which must be referred to before this volume is concluded, viz. silk. This is obtained from the cocoon of the "silkworm," which is the general name given to the larvae of certain bombycid moths. These larvae feed on the leaves of the mulberry, and when ready to pupate produce a considerable supply of a soft and delicate thread which is wound round about the larva itself. This is the raw silk, and it is unwound from the cocoon in a machine called the "silk-reel," and may then be wound into a thread. Two or more threads twisted together form "thrown-silk." Silk threads are also woven into cloth of characteristic texture and appearance. This protein thus forms the raw material of one of the most important textile industries.
From the fish trade there is much animal protein, which is useless for food purposes and which, to avoid nuisance, it is necessary to convert promptly in fertilizers. During the herring season there is the disposal in this way of the heads, tails, and the guts. Many fish are incidentally caught which, being valueless as food, are yet useful as manure. After the extraction of oil from fish livers the residue is suitable for a similar purpose. These residues are steamed, dried, and ground up, forming fish manure, rich in nitrogen and often also in phosphate.
REFERENCES.
"Chemical Fertilizers and Parisiticides," S. H. Collins, M.Sc.
"Organic Nitrogen Fertilizers," Part III., Section II., p. 105.
"Fish Manure," p. 110.