The chemistry of the various constituents of skin is still very imperfectly understood, but Beilstein, in his great handbook of organic chemistry, places gelatin, albumens and keratins in the “aromatic” series, and implies therefore that they contain the “benzene” ring. It is at least certain that all are very complex. The epidermis structures belong to the class of keratins, which are closely related to coagulated albumin; while the white fibres of the corium (or true skin) are either identical with gelatin, or only differ from it in their molecular condition or degree of hydration. This gelatinous tissue constitutes the bulk of the corium, but it also contains albumen as a constituent of the lymph and blood which supply its nourishment, keratins in the epithelial structures of the blood and lymph vessels, and “yellow fibres,” which are perhaps allied to the keratins, but which cannot well be isolated for analysis. The white connective tissue of the corium is converted into gelatin (glutin) by boiling with water. Owing to the impossibility of obtaining unaltered hide-fibre free from the other constituents, and still more to that of deciding to what point it should be dried to remove uncombined water, it is impossible to prove by analysis whether its composition is identical with that of glutin; but as the white fibre constitutes by far the largest part of the corium, and the other constituents do not differ largely from it in their percentage composition, an analysis of carefully purified corium is practically identical with that of the actual fibre. The following analyses of hide and gelatin are therefore of interest. The analyses of Von Schroeder and Paessler [22] Ding. Polyt. Journ., 1893, cclxxxvii. pp. 258, 283, 300. Analyses of Purified Corium.
Analyses of Gelatin (free from Ash).
[23] Contained also 0·7 sulphur. Journ. Physiol., xii. p. 23. It will be noted that the above analyses of skin differ more widely among themselves than their average does from that of the gelatin analyses, though on the whole the nitrogen is somewhat higher in the latter. The molecular weight of gelatin must be very high,
[24] Paal (Berichte D. Ch. Ges., xxv. (1892) pp. 1202-36, and Ch. Soc. Abst., 1892, pp. 895-7) calculates a molecular weight of about 900 from physical (freezing, boiling point) methods. [25] Annales de Chimie [5] xxvi. p. 18. [26] Compt. Rend., lxxxii. pp. 262-4. The addition of a molecule of water would make a difference in the percentage composition indicated by these formulÆ which would be less than their probable experimental error, and the change may therefore be one of hydration. Gelatin certainly contains both carboxyl and amido-groups, and is capable of combining with both acids and alkalies (see p. 84). Reimer [27] Ding. Polyt., ccv. p. 164. Hofmeister [28] Bied. Centr., 1880, p. 772. So far as our present knowledge goes, we may regard hide-fibre as merely an organised and perhaps dehydrated gelatin. Gelatin or glutin (not to be confounded with the gluten of cereals), when pure and dry is a colourless, transparent solid of horny toughness and of sp. gr. 1·3. It begins to melt about 140° C., at the same time undergoing decomposition. It is insoluble in hydrocarbons, in ether, or in strong alcohol. In cold water it swells to a transparent jelly, absorbing several times its weight of water, but does not dissolve. In hot water it is soluble, [29] See Pascheles, ‘Versuche Über Quellung,’ Archiv fÜr ges. Path., Bd. 71. [30] See Prollius, Ding. Polyt. Journ., ccxlix. p. 425, who employs a 1 per cent. solution; also StÜtzer, Zeit. Ann. Ch., xxxi. pp. 501-15. Solutions of gelatin from skin and bone are powerfully lÆvorotatory to polarised light. At 30° C. (A)D = -130°, but temperature and the reaction of the solution have much influence on the value found. Gelatin is precipitated from aqueous solution by the addition of strong alcohol and concentrated solutions of ammonium sulphate and some other salts. Many other colloid bodies such Decompositions.—When aqueous solutions of gelatin are heated under pressure, or in presence of glycerin and other bodies which raise the boiling-point, or more slowly at lower temperatures, they gradually lose the power of gelatinising on cooling, the gelatin being converted into modifications soluble in cold water, but still capable of being precipitated by tannin. Hofmeister [31] Bied. Centr., 1880, p. 772, and Ch. Soc. Abs., 1881, p. 294. Gelatin is also converted into soluble forms (peptones), perhaps identical with the above, by the action of heat in [32] Lorenz, PflÜger’s Arch., xlvii. pp. 189-95; Journ. Chem. Soc., 1891, A. p. 477. [33] Compare SchÜtzenberger, Comptes Rend., cii. pp. 1296-9; Journ. Chem. Soc., 1886, A. p. 818. Treatment with acids produces very similar effects. The first products are soluble peptones. Paal [34] Berichte, xxv. pp. 1202-36; Journ. Chem. Soc., 1892, A. p. 895. [35] See also Buchner and Curtius, Ber., xix. pp. 850-9; Journ. Chem. Soc., 1886, A. p. 635. The products of digestion of gelatin with gastric and pancreatic juice are peptones which do not differ materially from gelatin in ultimate composition, and the action is probably mainly hydrolytic. [36] Chittenden and Solly, Journ. Chem. Soc., 1891, A. p. 849. The earlier products of putrefaction are very similar. Many bacteria have the power of liquefying gelatin-jelly. This has been shown by Brunton and McFadyen Fahrion, By dry distillation of gelatin a mixture of pyrrol and pyridin bases are produced. This is commercially obtained by the distillation of bones, and is known as “bone oil,” or “Dippel’s animal oil.” Pyrrol, C4H5N, resembles phloroglucol in giving a purple-red colour to fir wood moistened with hydrochloric acid (p. 299). Reactions of Gelatin.—Gelatin is precipitated by mercuric chloride, in this respect resembling peptones, but not by potassium ferrocyanide, by which it is distinguished from albuminoids, and it differs from albumin in not being coagulated by heat. Solution of gelatin dissolves considerable quantities of calcium phosphate; hence this is always present in bone-glues. Gelatin and some of its decomposition products are precipitated by metaphosphoric acid. [41] Lorenz, PflÜger’s Archiv, xlvii. pp. 189-195. Gelatin is precipitated by all tannins, even from very dilute solutions; one containing only 0·2 grm. per liter is rendered distinctly turbid by gallotannic acid or infusion of gall-nuts; but some other tannins give a less sensitive reaction. The precipitate is soluble to a considerable extent in excess of gelatin, so that in using the latter as a test for traces of tannin care must be taken to add a very small quantity only. The addition of a little alum renders the reaction more delicate. Whether the precipitate is a definite chemical compound has been disputed, as its composition varies according to whether gelatin or tannin is in excess. BÖttinger [42] Liebig’s Ann. der Ch., ccxliv. pp. 227-32. Weiske [43] Bied. Centr., 1883, p. 673. Chondrin is the gelatinous body produced by the digestion of [44] Cp. Petri, Berichte, xii. p. 267; MÖrner, Skand. Archiv f. Physiol., i. pp. 210-243; and Journ. Chem. Soc., 1889, A. p. 736 and Zeit. Physiol. Chem., 1895, xx. pp. 357-364; and Journ. Chem. Soc., 1895, A. i. p. 254. See also Richter, Org. Chem., i. p. 559. Coriin.—Rollet 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-a-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] Paal, Ch. Zeit., 1895, p. 1487. 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] Globulin is an albumin soluble in dilute salt solutions, but insoluble in water. Yolks of eggs, preserved by the addition of salt, borax, or formalin, are used for dressing skins in the process of “tawing” (see p. 191). For the analysis of such yolks, see L.I.L.B., p. 159. Their most important constituent for the leather-dresser is egg-oil of which they contain about 30 per cent. Casein, the principal proteid of milk, may be mentioned here in connection with the albumins to which it is closely related, since, though in no way connected with the animal skin, since it is used to some extent as a “seasoning” or glaze for leather, for which it is well adapted, and it is now to a considerable extent a waste product of butter manufacture. It differs from albumins in being very incompletely if at all coagulated by boiling, but separates at once in curdy flakes on the addition of acids (hydrochloric, acetic, butyric), and by the action of rennet. The curd is easily soluble in small quantities of dilute alkalies, lime-water, and salts of alkaline reaction, such as sodium carbonate and borax. If no more than the necessary quantity of alkali is employed for solution, the compound has an acid reaction to phenolphthalein, and like the original milk, is curdled by rennet and dilute acids. Casein may also be dissolved by digestion with diluted mineral or organic acids. Hair, Epidermis and Glands.—These are all derived from the epithelial layer, and hence, as might be inferred, have much in common in their chemical constitution. They are all classed by chemists under one name, “keratin,” or horny tissue, and their ultimate analysis shows that in elementary composition they closely resemble the albumins. It is evident, however, that the horny tissues are a class rather than a single compound. The keratins are gradually loosened by prolonged soaking in water, and, by continued boiling in a Papin’s digester at 160° C., evolve sulphuretted hydrogen, at the same time dissolving to a turbid solution which does not gelatinise on cooling. Keratin is dissolved by caustic alkalies; the epidermis and the softer horny tissues are easily attacked, while hair and horn require strong solutions and the aid of heat to effect complete solution. The caustic alkaline earths act in the same manner as dilute alkaline solutions; hence lime easily attacks the epidermis, and loosens the hair, but does not readily destroy the latter. Alkaline sulphides, Elastic Fibres.—The elastic or yellow fibres of the hide are of a very stable character. They are not completely dissolved even by prolonged boiling, and acetic acid and hot solutions of caustic alkalies scarcely attack them. They do not appear to combine with tannin, and are very little changed in the tanning process. They are present in hide and skin to the extent of less than one per cent. Analytical Methods.—The reactions distinguishing the principal skin constituents are summarised in the following table:—
There is no simple method for the quantitative separation of the different constituents of skin. It is, therefore, customary to simply determine the amount of nitrogen which any particular portion of the material may contain, and, as gelatinous fibre, which constitutes by far the greater portion of the true skin, contains 17·8 per cent. of nitrogen, to base the estimation of the amount of skin present upon this figure (see p. 57). The most convenient process for the determination of the nitrogen is that devised by Kjeldahl, which is most easily carried out as follows:— A known weight of the substance which contains about 0·1 gram of nitrogen (0·5 gram of skin, or a corresponding quantity of liquor) is placed in a flask of Jena glass, capable of holding 500-700 c.c. together with 15 c.c. of concentrated sulphuric acid. The contents of the flask are then boiled over a small Bunsen flame for 15 minutes, or more, until all the water has been driven off and the material is quite disintegrated; and are then allowed to cool below 100°. 10 grams of dry powdered potassium persulphate is now added, and the boiling continued till the liquid has become colourless. The operation of boiling should be conducted in a good draught, or in the open air. Before the substance has begun to char it is advisable to place a small funnel in the neck of the flask to prevent, as far as possible, spirting and loss of sulphuric acid. Fig. 18.—Kjeldahl Apparatus. The colourless liquid is allowed to cool thoroughly, and the flask is then fitted with a tapped funnel and tube, as shown in Fig. 18. This tube must not be less than 4 mm. in diameter, and with the end in the flask cut diagonally to facilitate drops of liquid falling back again into the flask. It rises obliquely for a height of 12 to 15 inches, is then bent over as shown in the figure and connected by a rubber tube [49] The ends of the glass tubes should fit closely together, so as to expose the rubber as little as possible to the action of ammoniacal vapour. [50] “Bumping” is often very troublesome at this stage, and may be prevented by passing a current of steam from another flask, or ammonia-free air through a tube with a capillary opening into the boiling liquid; fragments of pure zinc, of platinum, or broken tobacco-pipe are much less efficient. It is an additional safeguard against the escape of ammonia to fix a small absorption-tube containing fragments of glass to the absorption-flask. The normal acid is run through this tube into the flask, so as to wet the broken glass, and is finally rinsed into the absorption flask before titrating its contents. [51] Cp. Procter and Turnbull, Jour. Soc. Chem. Ind., 1900, p. 130; also Nihoul, Composition des Cuirs Belges, p. 14 (Bourse aux Cuirs de LiÈge, Sept. 1901), who advocates the use of potassium permanganate in the oxidation; and Law (Jour. Soc. Ch. Ind., 1902, p. 847). In place of using 10 grm. of potassium persulphate as described, 10 grm. of ordinary potassium sulphate may be used, and potassium persulphate added in small quantities towards the end of the operation till a perfectly colourless solution is obtained. |