The Principles of Leather Manufacture

by H. R. Procter

The waste products of organisms are often poisonous to themselves, and for this reason fermentations frequently come to an end before the whole of the substance is fermented. Thus neither beer nor vinegar can be obtained of more than a certain strength by direct fermentation, the alcohol or acetic acid checking the growth of their respective ferments. A solution of glucose “set” with the lactic ferment of sour milk will only produce lactic acid to the extent of about half a per cent.; but if chalk be added, the lactic acid will be neutralised as produced, and the fermentation will go on till the whole of the glucose is converted into insoluble calcium lactate.[6] When this is accomplished the lactic ferment dies from want of nutriment, and its place is taken by another organism, of which some germs are sure to be present, which ferments the calcium lactate into calcium butyrate. If the nourishment fails, or the conditions become less favourable for one ferment than for some other which exists even in small quantity in a liquid, the former is quickly overgrown and killed, and the latter takes its place. Thus the ordinary ferment of the bran drench will die out rapidly unless constantly transferred to fresh bran infusions.

[6] For the practical preparation of lactic acid, the solution may contain 7¹⁄₂-11 per cent. of glucose, and some nitrogenous nourishment. The solution should be slightly acid. See Journ. Soc. Ch. Ind., 1897, p. 516.

Many of the products of bacteria (like those of some of the higher plants) are intensely poisonous both to animals and man. Many of the severe symptoms of disease are caused by these poisons produced in the body. Thus the tetanus-bacteria produce a poison similar in its effects to strychnine, and quite as virulent. Not only are such poisons produced by disease-bacteria in the body, but frequently also in the earlier stages of putrefactive fermentation. The latter are known as ptomaines, and when present in cheese and preserved foods are liable to cause poisoning. Such putrefactions are often unaccompanied by any disagreeable odour or flavour.

The fermentations which are most important in the tannery are, firstly, the ordinary putrefaction which attacks hides as well as other animal matter, and which is usually a complicated process carried on by many sorts of bacteria and other micro-organisms. This may be regarded as generally injurious to the tanner; but it is utilised in the “sweating” process for depilation and in the “staling” of sheepskins, in both of which advantage is taken of the fact that the soft mucous layer of the epidermis, which contains the hair-roots, putrefies more rapidly than the fibrous structure of the hide itself. In soaking also, use is made of the power of putrefactive ferments to dissolve the cementing substance of the hide, though in this case with doubtful advantage to the tanner. In the liming process putrefaction makes itself felt when the limes are allowed to become stale and charged with animal matter, softening the hide and finally rendering the leather loose, empty and inclined to “pipe.” Here the effect is in many cases useful if not carried too far.

In bating and puering, the action is almost entirely due to the enzymes and other products of bacterial activity, the original chemical constituents of the dung being apparently of minor importance. Naturally the liquid is adapted to the growth of many other organisms beside those acting most advantageously on the hide, and injury in the bates from wrong forms of putrefaction is very common, if indeed it is not always present in greater or less degree.

In drenching, the effect is, at first, entirely due to the weak acids produced by bacterial fermentation of the bran, but becomes complicated in its later stages by putrefactive and other fermentations which may be desirable or otherwise.

In the tanning liquors, fermentation is not so marked, but is of great importance owing to the production of acids by bacterial action from the sugars present in the material. The acids themselves are apt to be fermented and destroyed, principally by the oxidising action of Saccharomyces mycoderma and the higher moulds (see [p. 14]), which also act destructively on the tannins.

The effect of these acids on the hides is to swell them and to neutralise any lime they may contain. They also give to the liquors a characteristic sour taste, as a consequence of which, liquors containing acetic and lactic acids are usually known in the tannery as “sour liquors.”

It is doubtful whether the action of fungi is completely stayed even by the drying process. The heating of leather in the sheds is due to bacteria and the higher moulds, and Eitner considers their growth one of the causes of the “spueing” or “gumming” of curried leathers.

From what has been said, it is obvious that, with regard to fermentations, a double problem is presented to the leather manufacturer, since he desires to utilise those which make for his advantage, while controlling or destroying those which are injurious. The first step to a solution of these problems is a more complete knowledge of the organisms which serve or injure us, that we may, as it were, discriminate friends and enemies. We may then approach the question in two ways. Taking the drenching process as an example, we may on the one hand introduce a “pure cultivation” of the right ferment into a sterilised bran infusion, and so induce only the one fermentation which we require; or, on the other hand, as different ferments are affected in varying degrees by antiseptics, we may perhaps choose such as permit the growth of the organism we want, while killing or discouraging the rest. We may also arrange the nutriment, temperature, degree of acidity and other conditions, so as to favour one organism rather than another. All three methods have been applied in brewing with good results.