Albumen, an organic nutritive principle, is a constituent of all animal fluids and solids. The white of eggs contains 12 per cent, of albumen, and the fluid portion of blood [serum] 7 per cent. It occurs also in the flesh, in the brain, and more or less in all serous fluids. Fibrin also may be regarded as coagulated albumen. It occurs in the vegetable kingdom, in the sap or juice of many plants, such as the potato, turnip, carrot, cabbage, in the green stem of peas, in the seeds of the cereal grasses, and in many nuts.
There are two modifications of albumen, soluble and insoluble. The former occurs in the animal body, but the insoluble modification may readily be prepared from it by the action of heat. This property of becoming insoluble, or "coagulating," as it is termed, by the action of heat, is especially characteristic of this substance, and constitutes its chief value for technical applications.
Animal albumen is always associated with certain inorganic salts and free soda. It exists in the animal body in solution, in the form of an alkaline albuminate. If the white of eggs, or the serum of blood, or any animal liquid containing albumen be incinerated, the residue is chiefly carbonate of soda. This alkali may readily be removed, and the albumen rendered insoluble, or coagulated, by the action of heat. Exposed to a gentle heat, soluble albumen gives off a peculiar, characteristic odour. It can be dried at 104° F. (40° C;) without being rendered insoluble, and in this form is usually met with in commerce. On raising the heat to 130° F. (54° C), white fibres of insoluble albumen begin to appear; at 160° F. (70° C.) it becomes a solid, jelly-like mass; and at 212° F. (100° C.) it dries up, turning yellow and brittle, like horn. When in this condition, 5 times its weight of water will redissolve it, bringing it once more to its original consistency. The only change which the albumen undergoes during the process of coagulation is the removal, by the hot water, of the alkali and soluble salts.
Its chemical composition remains the same throughout.
Albumen of good quality is recognized by its transparency when in flakes, by its flavour not being disagreeable, and by leaving no odour of putrefaction. Constantly stirred in cold water, it should dissolve entirely. For practical purposes, it is best dissolved in warm water, at a maximum temperature of 113° to 122° F. (45° to 50° C). The albumen should be added gradually, and the liquid constantly stirred. The water should on no account be added to the albumen. The liquid, after straining through a fine silk sieve, is usually mixed with a small proportion of ammonia, turpentine-oil, etc, in order to prevent frothing, and make it work smoothly. Turpentine also tends to prevent putrefaction, but an addition of about 1 per cent, of arsenious oxide is said to be the best preservative. Commercial albumen is very liable to adulteration (especially the dearer eggalbumen) with gum-arabic, dextrine, flour, sugar, etc. Allen gives some very useful tests for commercial samples in the Analyst for December 1882, pp. 210-11.
The uses of albumen are numerous and important. For clarifying wines and liquors it cannot easily be replaced; in sugar refining it is extensively used for a similar purpose. In dyeing and calico printing it holds a high place as a mordant for "fixing" certain colours; in this sphere it has a competitor in caseine, which the trade have misnamed "lactarine." Photographers use large quantities of albumen for preparing paper for silver-printing. In varnish making, both egg- and blood-albumen are employed; for this purpose they are sometimes replaced by caseine, and will probably soon have to compete also with legumine (vegetable albumen).
According to Wagner and Witz, albumen which has been coagulated (rendered insoluble) may be restored to the soluble (coagulable) state by means of treatment with pepsin. By Wagner's plan, 12 to 13 oz. of insoluble albumen are placed in contact with 1 oz. of calf's stomach, cut into little pieces, and distributed through 1} pints of water, previously treated with 1/3oz. concentrated hydrochloric acid, and having a temperature of 100° F. (37 1/2°C). After 24 to 36 hours' standing, the whole is passed through a fine sieve, and the filtrate neutralized with ammonia.
Witz uses sheep's stomach, and over 4 oz. of dry insoluble albumen to 1 3/4 pints of acidified water, digesting for 40 hours at a temperature of 95° to 104° F. (35° to 40° C), whereby about half the albumen goes into solution. This portion is removed by filtration, and the insoluble residue is again treated in the same manner to yield a second portion of soluble albumen. Pigs' stomachs are even more active than sheep's. The solution of albumen obtained by Witz is odourless and colourless, and, after the neutralization with ammonia, coagulable either by heat or alcohol. More, it does aot gelatinize, even after long standing.
The addition of the hydrochloric acid is essential to success with the pepsin processes. Indeed, dilute hydrochloric acid (1 part of 1.169 sp. gr. in 100 water) alone, at a temperature of100° F. (38° C.), after some days effects the solution of insoluble albumen, affording a solution which will coagulate on boiling. - (See Dingier's Jl., ccxix. 166-71.)
Blood-albumen occurs in commerce in various forms. The lowest quality is packed in casks in the liquid state, and consists merely of blood which has been defibrinated by whipping. The purer forms of blood-albumen are prepared from the blood of slaughtered animals, which after coagulation, is filtered: 5 oxen, or 20 sheep, or 34 calves are said to yield the same quantity of dry albumen - viz. 2 lb. In producing blood-albumen for commerce, the objects borne in mind are the attainment of a substance whose solution is free from colour, possesses perfect coagulation, and which is cheap. The superior price of egg-albumen has led to various attempts to prepare a blood-albumen of a similar whiteness and quality. In these attempts, animal charcoal, incipient coagulation, air rendered ozonic by means of electric induction, have severally been employed, but none proved capable of producing an albumen at all comparable in value to egg-albumen.