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, millions of calories. | |||
| Protein. | Fat. | Carbohydrate. | ||
| Cereals | 549,000 | 63,000 | 3,628,000 | 17,712,000 |
| Meat | 356,000 | 799,000 | — | 8,890,000 |
| Poultry, eggs, game, and rabbits | 42,000 | 31,000 | — | 461,000 |
| Fish | 91,000 | 17,000 | — | 531,000 |
| Dairy produce, including lard and margarine | 199,000 | 686,000 | 258,000 | 8,253,000 |
| Fruit | 9,000 | 14,000 | 222,000 | 1,077,000 |
| Vegetables | 120,000 | 10,000 | 1,031,000 | 4,812,000 |
| Sugars (including cocoa, etc.) | 5,000 | 18,000 | 1,572,000 | 6,633,000 |
| 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.