DRENCHING.

In the manufacture of many kinds of light leathers, skins, after bating, are drenched. The process usually consists in placing the skins in a mixture of bran and water, half to one per cent. of bran being used (5 to 10grm. per litre, or 1/2 to 1lb. per 10 gall.) at a temperature of 29° to 35°C. This ferments vigorously for 18 to 24 hours, with evolution of a considerable quantity of gas, and the formation of weak organic acids.

The acids neutralize any lime which has not been neutralized in the bate, and they do this in a very gradual and mild way, being produced by bacterial fermentation exactly as required, in a similar manner to fermentations taking place in presence of calcium carbonate. The gases evolved bring the skins to the surface of the drench, and distend the fibres; the workman attending to the drenches puts the skins down under the liquor with a pole, but a better way is to have the drench vat fitted with a wooden paddle, which may be pulled round by hand when necessary; in this way the skins are put under the liquor without danger of tearing or trapping them; in the case of grain splits the paddle is a great advantage over the pole.

The skins are allowed to rise two or three times, according to the condition required. For many kinds of alum leather the bate is omitted and the drench only is used.

In English practice the bran is mashed in the vat, in which the drenching is to take place, at a temperature of about 95°F. (35°C.); the skins are then put in and the drench well stirred or paddled until the whole is uniform, the temperature is then usually down to 85° to 83°F. (29·5° to 28·2°C.), at which temperature the fermentation begins. The temperature allowed varies with the state of the goods and the prevailing exterior temperature; in winter it may be higher than in summer, but never usually higher than 90°F. (32·2°C.). In some continental works the bran is mashed in boiling water, allowed to stand for some hours to cool, and only the bran-water is used; in this case the fermentation is different to that in which the bran is mashed at a lower temperature. In the first process the starch of the bran undergoes changes to dextrin and glucoses, which are then fermented by bacteria with the production of acids and gases (see Chapter X.), in the latter case the starch, which has been gelatinized is fermented by other organisms, which probably secrete diastatic enzymes. Less gas is given off in this fermentation than where the bran is mashed at 95° to 100°F. The temperature of fermentation is kept low, usually 25°C. (77° F.). Bran contains a sufficient amount of nitrogenous matter,151 to furnish the bacteria with nutriment, and until this is exhausted there is no fear of the skins being attacked.

There is also a “sweet” bran drench used on the continent, in which fermentation is not allowed to take place, the skins being paddled in an infusion of bran in water for a short time only, two to three hours at the most. In addition to the mechanical cleaning action, which the fresh bran has upon the skins, there is also a softening effect due to some constituent of the bran, which is at present not understood. One reason for this, is that the usual drench employed being an acid one, research has practically been confined to the latter.

Another method of preparing the bran drench, consists in allowing the required quantity of bran to soak in cold water for several hours; a sufficient quantity of hot water is then added to bring the temperature up to 50°C.; at this temperature the diastatic enzymes in the bran act rapidly on the starch and transform it into easily fermentable sugars. The temperature is allowed to fall to 34° to 35°C., and then a quantity of an old drench is added to start the fermentation. The liquor becomes acid, and in two days usually reaches its maximum acidity, when it is ready for use. The bran is strained off and only the milky liquor used. Such a drench is largely employed in the manufacture of chamois leather, the goods (sheep fleshes) being paddled in it for some hours previous to pressing. The French also use a similar process in the preparation of suede leather. The bran is strained off and only the sour partly fermented and fermenting liquor used. The straining off of the bran also saves working over the beam which would otherwise be required to get rid of the bran adhering to the skins.

It has recently been proposed to employ the bacillus bulgaricus, the organism which causes the fermentation of milk in the preparation of Bulgarian and Turkish “Yoghurt” (Bibliography 124) for drenching. Bacillus bulgaricus is capable of producing up to 2·5 per cent. of lactic acid, or about three times the quantity of that produced by ordinary lactic bacteria. Dr. Hugo KÜhl (loc. cit.) proposes to cultivate the bacillus in the mother liquors, which are a by-product in the manufacture of milk sugar, and to use such liquor as a drench. Where skim milk or butter milk is to be had, this may also be used. A similar process is already in use in America, where the evaporated product is put upon the market under the name of Dermiforma (see pp.189 and 194).

As a substitute for the drench, and in order to further remove lime after erodin, the late Franz Kathreiner employed a solution of anticalcium, 1 in 500 water, in which the skins remained overnight. Anticalcium is a mixture of sulphonic acids derived from cresols, and has antiseptic properties, so that its action is quite distinct from that of the bran drench. It is put on the market by J. Hauff, Feuerbach, near Stuttgart.

Drench Damage.—Drenching is a very important and useful process in leather manufacture, but, like other fermentive processes, requires great care and experience in its application. Although not so risky as puering, a good deal of serious damage may occur in the drench. The writer knows of no fuller discussion of all the various forms of drench damage than the articles of Eitner and others which appeared in “Der Gerber” during a long series of years (1882–1900). In these articles perhaps every known form of drench damage is described. Procter152 also gives a concise account of drenching. It will suffice to mention briefly the chief accidents to which the drench is liable; these are generally known as the “turning” of the drench.

1. Acid “turning.”—This usually occurs when the atmosphere is charged with electricity—during thunderstorms. It appears to be an extremely rapid form of butyric fermentation, which has not been fully investigated. The skins become swollen, transparent and tender, and, unless the fermentation is stopped, soon dissolve to a jelly. The only way to save the skins from destruction is to add salt to the drench; this reduces the swelling and, in fact, pickles the skin. I have pointed out elsewhere that this is really the origin of the modern pickling process, which is still known by the name of “rising.” A very acid drench will swell the skins almost the same as dilute sulphuric acid, and if they are then placed in a salt solution, or salt is added to the sour liquor, it is absorbed by the skins, and they are preserved almost as effectually as by the use of mineral acids.

2. Putrid “turning.”—This occurs under very similar atmospheric conditions to 1, and may be almost as rapid. Instead of becoming acid, the liquor turns slightly alkaline, frequently becoming bluish black, due to the presence of certain chromogenic bacteria. The goods fall as in a puer. Both the chromogenic and other bacteria present are peptonizing organisms, which obtain the upper hand owing to favourable conditions, and rapidly digest the skin unless the action is stopped. The best remedy is to remove the skins from the drench and pickle them without delay by the usual acid and salt process.

3. “Pinholey” drench.—This is the same as the Germans call “pikiren,” and is generally due to a too rapid evolution of gas both in the drench and in the capillary spaces of the skin itself. The gases form under the hyaline layer, and finally burst through in small holes. A damage very similar in appearance is also caused by colonies of gelatin liquefying bacteria developing on the grain. Each colony forms a small hole. The trouble is usually due to the setting of the drench at too high a temperature.

4. Spongy leather may be produced by leaving the goods too long in a sound drench, if they are allowed to rise too often. In this case experience alone can determine the proper duration of the drenching process.

A special case in which the hyaline layer is slightly attacked and its brilliance destroyed has been described by Eitner.153 A slimy film was formed on the surface of the drench and also on the grain of the skins. This film was found to consist of Bacillus megaterium (the potato bacillus), and where it was growing on the hyaline layer the latter was etched as it were, by the peptonizing enzyme secreted by the bacillus, so that when tanned the grain was covered with dull patches very much resembling “blast.”

Fermentations in the Drench.—Compared with the dung bate, or puer, the fermentation taking place in the bran drench is a simple one; as will be shown in detail in the next chapter, about half the weight of the bran consists of starch; this becomes hydrolysed by the enzymes in the bran (and in some cases secreted by the bacteria) into glucoses. According to the following equation:—

(C₆H₁₀O₅)_n + H₂O = n C₆H₁₂O₆
starch glucose

This expresses the change in its simplest form; in reality it is much more complex, various dextrins and sugars being formed simultaneously.

The sugars are then fermented by a variety of bacteria with formation of various organic acids, in accordance with the equation

C₆H₁₂O₆ = 2 CH₃CH(OH)COOH

which represents the formation of lactic acid.

In addition, formic and butyric acids are produced along with small quantities of other bodies, as will be shown. It is pretty certain that the butyric acid is formed by the further fermentation of the lactic acid and lactic salts in the liquid; at the same time CO₂ and hydrogen are evolved as follows:—

2 C₃H₆O₃ = C₄H₈O₂ + 2 CO₂ + H₂

Fig.27 is a drawing of the organisms seen in the drench, under a magnification of 1000 diameters. This was taken from a drench which had been set about 3 hours, and was just beginning to ferment; the bacteria form chains of varying length: the short chains have a slow motion in the liquor. As the fermentation proceeds, these form longer chains (Fig.28), some of which are very beautiful objects, especially when viewed by dark ground illumination. The other organisms present are bacteria and bacilli from the puer, and at a later stage, as the nutrient material for the bran ferment becomes less, they begin to develop, first causing butyric and finally putrefactive fermentation.

A fairly pure culture may be obtained by inoculating a tube of bran infusion, previously sterilized, by means of a freshly made capillary pipette, which is pushed into the tube without removing the cotton-wool plug. The tube is placed in the incubator at 30°C., and the organisms causing the fermentation develop rapidly; as soon as the tube is observed to become cloudy, indicating the development of the bacteria, another tube is inoculated in the same manner, using a very minute quantity of liquid; this second “attenuation” is then carried on to five or six more tubes. In the fifth tube will usually be found a pure culture of the bran fermenting organisms, which I have called Bacterium furfuris. The bacteria (see Fig.30) are mostly in the form of dumb-bells or pairs, each cell 0·75µ ×1·25µ: they vary slightly in size, some forming chains; those on the surface become surrounded by a kind of jelly, and form an iridescent pellicle on the surface of the liquid, the zoogloae form. When the nutriment of the liquid is exhausted or the acidity becomes too great,154 this film sinks to the bottom and the liquid above becomes comparatively clear. There is no spore formation, and, consequently, the culture soon dies unless inoculated into a fresh nutrient medium. The liquor, after the fermentation both in the drenches and tubes of pure cultivations, is always slightly ropy.

B. furfuris probably belongs to the group of coli bacteria, some of which ferment milk and other sugars with production of lactic acid and gases. This explains the ease with which the fermentation takes place, since the skins bring in immense numbers of bacteria from the bate, among which are the organisms described.

Eitner155 states that the action of the bran drench cannot be considered a chemical one, and that the chief action is a dynamical one, resulting from the gases generated in the liquid and in the skin. While I do not deny that the gas has such action—indeed I pointed out in 1893156 that the gas acted by floating and distending the skins, and so enabling them better to take up the acids—I maintain, however, that the chief action is a chemical one and due to the weak organic acids produced; this was confirmed experimentally. The researches on bran drenching commenced in 1887 by myself, and later in conjunction with Dr. W.H. Willcox, prove conclusively that the amount of acid produced, and especially the lactic acid, is sufficient to produce a very decided action on skin. In actual working we found from 1·07 to 2·34grm. total acids per 1000c.c., while in artificial cultures, with addition of calcium carbonate, inoculated with pure cultures of bacteria, obtained from drenches, we obtained from 4·53 to 11·44grm. total acids per 1000c.c. Now traces of lime in the skins act in just the same way as the calcium carbonate; they neutralize a portion of the acid produced by the bacteria, and so allow a greater amount of acid to be produced than if it accumulated in the liquid. By preparing solutions of the above acids of suitable strength and agitating skins in them, results were obtained in 1 1/2 hours equal to a drenching of 14 hours. The defect of such artificially prepared liquids in practice is due to the fact that they do not contain any particles of bran or flour; these take up dirt from the skin mechanically, and so produce a better-coloured leather. Where the skin has been bated in a perfectly clean bate, and then paddled in such an artificial drench, the leather is in every way equal to that prepared by means of the dung bate, followed by a bran drench.

By far the most important action for the tanner is the solution of the last traces of lime not dissolved by the bate, and this is the chief reason for its use, where colour is a desideratum, and where sweet liquors are used; for some kinds of leather where it is desirable not to lose pelt, the drench is used alone for this purpose. A dung bate, such as most light leather tanners use, is seldom run off more than once a week, and contains a considerable amount of dissolved lime; the amount in a freshly made-up puer wheel is from 0·1grm. to 0·3grm. lime (CaO) per litre. After the wheel has been in use for a week, it rises to 0·5grm. to 0·8grm. per litre. It will be readily seen that a liquid which already contains such considerable quantities of lime is not an especially good medium to use for its complete removal from the skins, hence drenching is necessary to get rid of the lime left in the skins.

Eitner states that the fermentation is alcoholic at the commencement, and then becomes an acid fermentation with generation of gas. In tan liquors there is no doubt that this is the course of the acetic fermentation, as has been shown by Andreasch,157 but in the usual type of bran drench the action is certainly a different one. The acetic acid, as far as we can ascertain, is produced directly from the dextrose, without the previous production of alcohol, since the presence of the latter is not shown by tests in any stage of the fermentation.158 No yeasts were observed159 and no alcohol was found in any stage of the fermentation.160 J. O’Sullivan remarks that although it has not been noticed it is possible that in this case the production of alcohol and its oxidation into acetic acid are simultaneous, but that there is no preliminary fermentation by yeasts followed by acetic fermentation by bacteria such as Eitner indicates is certain.

To sum up the conclusions arrived at from the experiments above quoted.

The starch of the bran or flour is first transformed into glucoses and dextrin by the action of an unorganized ferment called cerealin. The glucoses are then fermented by certain species of bacteria, Bacillus furfuris being perhaps the chief, with the formation of lactic, acetic, formic and butyric acids, and the evolution of carbon dioxide, hydrogen, nitrogen, and a small quantity of sulphuretted hydrogen.

The principal acid produced is lactic; the acetic acid is produced directly from the glucoses by the above-mentioned bacteria without any preliminary alcoholic fermentation by yeasts.

The mode of action of the drench on the skins may be summed up as follows:—

1. The solution of the last traces of lime which has not been removed by the bate, by the organic acids produced by the fermentation, and the subsequent swelling action of these on the skin fibres. The acids also dissolve a small amount of skin substance.

2. Simultaneously with (1) the distension and floating of the skins by gases produced by the fermentation, so enabling them better to take up the acids.

3. The mechanical absorption of dirt by the particles of bran or flour in the drench.

It is not proposed to enter into further detail, since the next chapter, consisting of the original papers of Dr. W.H. Willcox and the author, contains a pretty complete account of the analysis of the drench and theory of the drenching process.