Heated for some days with dilute nitric acid (1 : 2) all proteids, including albumins, gelatin and keratins, yield yellow flocks of “xantho-proteic acid,” a substance of somewhat indefinite composition, soluble in ammonia and in fixed caustic alkalies with production of an orange-red or brownish-red colour.
Millon’s reagent gives an intense red coloration when heated with albumins, keratins, or gelatin. The reagent is made by dissolving 2·5 grm. of mercury in 20 c.c. of concentrated nitric acid, adding 50 c.c. of water, allowing to settle and then decanting the clear liquid.
Albumins, previously purified by boiling with alcohol and washing with ether, when dissolved in concentrated hydrochloric acid (sp. gr. 1·196) by aid of heat, give a violet-blue coloration, but the reaction is often somewhat indefinite. Gelatin, chondrin and keratins do not give this reaction.
Treated with a trace of cupric sulphate and excess of caustic potash solution, albumins give a violet, and gelatin and peptones a pink solution (biuret reaction).
Dissolved in glacial acetic acid and treated with concentrated sulphuric acid, albumins and peptones give a violet and feebly fluorescent solution. A somewhat similar reaction is obtained if sugar solution be substituted for acetic acid.
A solution of albumin rendered strongly acid with acetic acid is precipitated by potassium ferrocyanide, salt, sodium sulphate, lead acetate, mercuric chloride, tannin and picric and tungstic acids.
Egg-Albumin is contained in the whites of eggs in membranes which are broken up by beating with water and can then be removed by filtration. When fresh its reaction is slightly alkaline, and it is lævorotatory.
According to Lehmann, white of egg contains 87 per cent. of water, and 13 per cent. of solid matter, the latter being almost entirely composed of egg-albumin. This latter coagulates and becomes insoluble in water on heating to 60° C.
Vitellin (the albumin or globulin[48] of the yolk) is insoluble in water, and is obtained as a white granular residue on extracting undried egg-yolk with large quantities of ether. It closely resembles myosin, the chief globulin of muscle, but differs from other globulins in being soluble in a saturated solution of common salt. A neutral solution of vitellin in very dilute brine coagulates at 70-75° C.
[48] Globulin is an albumin soluble in dilute salt solutions, but insoluble in water.