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 10 grm. per litre, or 1/2 to 1 lb. 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.
[151] Bran examined by Kjeldahl’s process contained 2·2 per cent. N,
equivalent to about 13–14 per cent. proteids.
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. Procter[152]
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.
[152] Principles, pp. 166–170.
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.”
[153] Der Gerber, 1898, p. 204.
*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:--
(C_{6}H_{10}O_{5})_{n} + H_{2}O = n C_{6}H_{12}O_{6}
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
C_{6}H_{12}O_{6} = 2 CH_{3}CH(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 CO_{2} and
hydrogen are evolved as follows:--
2 C_{3}H_{6}O_{3} = C_{4}H_{8}O_{2} + 2 CO_{2} + H_{2}
[Illustration: Fig. 27.--Organisms in Bran Drench × 1000.]
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.
[Illustration: Fig. 28.--Chains of Bran Bacteria × 1000.]
[Illustration: Fig. 29.--Bran Fermentation. Advanced Stage.]
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 0·75 µ × 1·25 µ: 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.
[154] Claflin (92) gives the limits of acidity for lactic bacteria
from 0·02 to 0·5 per cent. Reynolds Green (97, p. 346) gives the
upper limit for B. acidi lactici as 8 per cent. lactic acid.
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.
Eitner[155] 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 1893[156] 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
2·34 grm. total acids per 1000 c.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 11·44 grm.
total acids per 1000 c.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.
[155] Der Gerber, 24, 570.
[156] Jour. Soc. Chem. Ind., 1893, p. 426.
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 0·1 grm. to 0·3 grm.
lime (CaO) per litre. After the wheel has been in use for a week, it
rises to 0·5 grm. to 0·8 grm. 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 observed[159] 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.
[157] Der Gerber, 1895, p. 193, etc.
[158] J.S.C.I., 1897, p. 513.
[159] Ibid., 1890, p. 28.
[160] Ibid., 1893, p. 422.
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.