[46] Ding. Polyt. Journ., ccv. p. 153.
Hide Albumin.—The fresh hide contains a portion of actual albumin, viz. that of the blood-serum and of the lymph, which is not only contained in the abundant blood-vessels, but saturates the fibrous connective tissue, of which it forms the nourishment. This albumin is mostly removed from the skin by the liming and working on the beam, which is preparatory to tanning. Probably for sole-leather, the albumin itself would be rather advantageous if left in the hide, as it combines with tannin, and would assist in giving firmness and weight to the leather. It is, however, for reasons which will be seen hereafter, absolutely necessary to get rid of any lime which may be in combination with it. The blood must also be thoroughly cleansed from the hide before tanning, as its colouring matter contains iron, which, by combination with the tannin, produces a bad colour.
The albumins form a class of closely allied bodies of which white of egg may be taken as a type. They are also related to the casein of milk, to fibrin, and more distantly to gelatin. A good deal of information on the class may be found in Watt’s Dict. of Chem., 2nd ed., article ‘Proteids,’ and Beilstein’s article ‘Albuminaten,’ and in Allen’s ‘Commercial Organic Analysis,’ vol. iv.
The most characteristic property of albumins is that of coagulation by heat. The temperature at which this takes place differs somewhat in different members of the group, egg and serum albumin coagulating at 72-73° C. Dry albumins become insoluble if heated to 110° C. for some time. Traces of acid tend slightly to lower, and traces of alkali to raise the temperature of coagulation. Sodium chloride and some other neutral salts favour coagulation. Solutions of albumin become opalescent at a temperature slightly below that at which flakes form.
Albumins are also coagulated by alcohol and by strong mineral acids. Coagulated albumin is only soluble in strong acids and alkalies by aid of heat, and strongly resembles keratin ([pp. 56], [68]).
Solutions of albumin are lævorotatory to polarised light.
“Acid” and “Alkali” Albumins are formed by the action, in the cold, of dilute acids (such as acetic, hydrochloric) and alkalies on albumin solution. They are uncoagulable by heat, and are precipitated by careful neutralisation, but are soluble in excess of either acid or alkali, or alkaline carbonates. They are thrown out of solution by saturation with sodium chloride or magnesium sulphate. It is doubtful whether albumins combine with either acids or bases, and it is probable that the “acid” or “alkali” albumins are identical with the parapeptones formed in the first stage of peptic digestion.
On putrefaction, or on more severe treatment with acids and alkalies, albumins break down in a way similar to gelatin, and yield almost identical products (see [p. 57]); amido-acids of the acetic series, and tyrosin (para-oxy-α-amido-phenyl-propionic acid) and aspartic (amido-succinic) acid, being the most important.
Treatment with alcoholic soda (see [p. 62]) yields peptones similar to those of gelatin.[47]
[47] Paal, Ch. Zeit., 1895, p. 1487.