The Principles of Leather Manufacture

by H. R. Procter

Wood is of the opinion that the ferments found in bran do not originate in the drench itself, but come from the bated skins, as the drench-bacteria soon die out without finishing the fermentation, and constant renewing of the nutrient material is necessary (cp. [p. 18]).

Bating and puering, though differing practically in many ways, are identical in theory, and most of what follows applies to both of them. The action is much more complex than that of the drench, involving both chemical reactions and those of organised and unorganised ferments, and it is a matter of no little difficulty to say what proportion of the observed effect should be ascribed to each of these agencies.

Formerly, the principal effect was attributed to organic salts of ammonia and its homologues, and to amido-acids which combine with lime. Phosphoric acid is also present, and if any exists in the form of soluble salts, it will combine with lime, and render it insoluble and inactive. It is probable, however, that most if not all the phosphoric acid is already in the form of tricalcium phosphate, and therefore without effect.

It is now, however, recognised that the effects of these chemicals are of no importance as compared with the products of bacterial action, and the researches of J. T. Wood have cleared up much that was until recently quite inexplicable.[101]

[101] Journ. Soc. Chem. Ind., 1894, p. 218; 1895, p. 449; 1898, pp. 856, 1010; 1899, pp. 117, 990.

Much effect has been ascribed to the digestive ferments, such as pepsin and trypsin, which are present in fresh dung. It is known that the animal organism secretes these in considerable excess of its requirements, but it is doubtful whether any exist undecomposed, even in fresh dung; though they are apparently more resistant to putrefaction and decomposition than would a priori have been expected of such complex organic compounds, and there is therefore a possibility of their existence in the dung, even as it comes to be used in the tannery. Both pepsin and trypsin are enzymes (see [p. 16]), and belong to the great class of albuminoids. They are soluble in water, but insoluble in alcohol, and hence are precipitated by the addition of the latter to their solution, but are not altered by it, and regain their activity on solution in water. By heat they are coagulated and decomposed, and their activity permanently destroyed.

Pepsin is the active principle of the secretion of the glands of the stomach, and large quantities are prepared for medical use as an aid to digestion from the stomachs of pigs. Pepsin only acts in slightly acid solution, and, though fresh bate liquor is slightly acid to litmus, it speedily becomes alkaline from the lime of the skins and the ammonia present, so that the action of pepsin in a bate can only be a very limited one. Wood[102] compared the action of a 1 per cent. solution of pepsin, acidified with 0·2 per cent. of hydrochloric acid, with that of a dogs’ dung puer liquor, both at the temperature of 40° C. At the end of one hour the skin in the pepsin-solution was considerably fallen, but that in the puer-solution was almost dissolved. Since the solution here employed was much stronger than is likely to occur in practice, and the conditions much more favourable to its action, it may be assumed that the practical effect of traces of pepsin in the bate may be neglected.

[102] Journ. Soc. Chem. Ind., 1894, p. 220.

Trypsin or pancreatin[103] if present, is more likely to have an effect, since it is active in neutral and in alkaline solutions. It is the product of the pancreas, and is largely concerned in intestinal digestion. Chemically it much resembles pepsin, but is more resistant to heat, retaining its power of digestion after heating to a temperature of 160° C. in a dry condition. Its warmed solution dissolves fibrin almost instantly, and in large quantity, and peptonises gelatin and hide-fibre, so as to render them soluble in water. Wood found that a 1 per cent. solution of pancreatin acted far more rapidly than a solution of pepsin of equal strength. At 40° C. in neutral solution, the skin fell rapidly, and the action continued even in the cold. In 15 hours the liquid was swarming with minute bacteria. At the suggestion of the Author, the experiment was therefore repeated, with the addition of 15 per cent. of chloroform, which prevented the development of bacteria, while it did not stop the action of the pancreatin. The skin fell as before, but in neither case had it the peculiar touch of puered skin, nor were the characteristics of the leather produced from it the same. We may therefore conclude that, though trypsin may contribute to the action of the bate or puer, it can only do so in a minor degree, and that the principal effect of the bate or puer is due to other causes. It is certain, however, that fresh bird-dung, and probably that of all animals, contains ferments capable of liquefying gelatin. An instance of this is found in the observation, common in glue manufacture, that if the dropping of a sparrow falls on a cooler full of solidified gelatine size, it will liquefy a track quite down to the bottom of the cooler. Trypsin, or at least the secretion of the pancreas, as well as the gall from the liver, have great power of wetting and emulsifying fats, and this has possibly something to do with the action of the bate in enabling the skins to be cleansed of fat.

[103] Loc. cit. and Beilstein, iii. p. 1308, 2nd ed.