The raw materials for the manufacture of gelatine and glue may be
classified according to their origin. The preliminary treatment, which
comprises chiefly purifying and cleansing operations, is varied
according to type of manufacturing process for which it is a
preparation.
In the case of hide or =skin gelatine=, the raw material is a
bye-product of the leather industry. After the hides or skins have
passed through the preparatory processes which convert them into "pelt"
(see Part I., Section II.), they are so trimmed that all that is left
will make a useful leather. These "trimmings" or "roundings" include
ears and noses, the udders of cows and heifers, and also include parts
from the butt, belly and shanks which are collectively termed "pieces."
The operation of fleshing (Part I., Section II.), in which fat and flesh
are cut from that side of the hides and skins which was next the flesh,
also involves cutting into the collagen to some extent, and these
"fleshings" comprise another very large class of raw material. The
fleshings obtained by hand labour contain distinctly more hide
substances than those obtained by machine work, and their commercial
value to the gelatine manufacturer is of course proportionate to the
collagen content. Some hides and skins are split in the pelt (Part I.,
Section IX.; Part II., Sections II., III. and IV.), and the "flesh
split," though sometimes made into leather, is also used in making
gelatine, a high quality being obtained from such material. Minor
sources of material are tendons and cartilages, and also hides and skins
which have been too much damaged by partial putrefaction or by accidents
to make sound leather. Of course the material from the hides for heavy
leathers form the greater bulk of raw material for skin gelatine which
is thus derived principally from ox hides but sheep and goat skin pieces
have also an important place. The skins of other animals, such as dogs,
cats, hares and rabbits not usually made into leather can also be
depilated and used for making skin gelatine and glue. Horse hide
fleshings and pieces are sometimes used, but are notorious for the poor
quality of their product. They seem to contain less [alpha] gelatin. All
these materials are of course readily putrescible and must be put "into
work" without much loss of time. When it is impossible to convey them
from the tannery to the gelatine factory quickly enough, _e.g._ foreign
material, the "glue stock" is dried out completely and sold in that
condition. In the manufacture of pickers from limed pelt there is some
superfluous material, and this is cut into shavings and dried. This
"picker waste" also forms a useful source of raw material. Skin gelatine
material is not very strong in gelatine-substance. The fleshings,
pieces, etc., contain much water, even up to 80 per cent. This, however,
is very variable, and only a practical test or a hide substance
determination can indicate the commercial value of any particular
material. This value, moreover, is determined not only by the yield and
quality of the gelatine which can be obtained, but also by the yield of
grease, the valuable bye-product.
The preliminary treatment of material for skin gelatine consists
essentially of liming and of washing. The object of each process is to
purify. Liming has much the same action on hide pieces, etc., as on
hides, and indeed the liming treatment is somewhat superfluous on
cuttings from well-limed hides. The material is plumped up and the
partially hydrolyzed products are taken into solution. Lime also acts as
mild antiseptic, stops any putrefaction and liberates ammonia formed by
fermentation in transit to the factory. When plumping is particularly
wanted (as in wetting in dry stock) caustic soda is sometimes used as an
assistant (_cf._ dried hides). Sodium sulphide has also been used
for this purpose. The liming is in brick pits, an excess of undissolved
lime being always used. It is advantageous frequently to disturb or
agitate the goods in the lime pits. Up to ten weeks liming has sometimes
been given, but about three weeks is now generally considered
sufficient, and the tendency is to shorten the time. The lime and soda
have also a detergent action on soiled stock, and they probably assist
in hydrolyzing the pigments of the hair roots and sheaths. They also
saponify and emulsify the grease, and it is obvious, therefore, that
liming can be carried too far. Slaked lime, of course, must always be
used.
After liming the soaked, softened and plumped stock is washed as
thoroughly as possible. To do this it is necessary to supply repeated
batches of clean cold water. Some manufacturers, however, use the warm
water from the evaporators. Wooden vats or brick pits with arrangements
for agitation, for draining off and for inspection, are used for this
purpose. The agitation may be carried out by means of revolving shafts
or drums with projecting curved spokes or vanes. An American patent
(Hoeveler's glue stock washer) involves the use of a paddle wheel. It is
combined with a settling tank to gather particles of stock. In the
washing the chalk, excess lime, dirt, etc., are quickly removed and a
slow deliming process is commenced. The sediment from the washers and
wash waters has some value in making fertilizers. Deliming cannot be
carried on further than certain limits by water alone. Hence acid is
often added to finish off the process. Hydrochloric acid has the
advantage of forming soluble salts, but if they are not removed
completely their lyotrope influence is to weaken the gelatine. Sulphuric
and sulphurous acids are even cheaper, and the lyotrope influence of
their salts is in the opposite sense. The latter also has the advantage
of destroying sulphides, an important advantage for food gelatines.
Whatever acid is used, however, it is evident that an abundance of pure
cold water is the fundamental requirement of a pure product. It is a
sound maxim in gelatine manufacture to avoid, if at all possible, the
addition of any soluble substance, for it is always present in a more
concentrated state in the finished article. Thus if its solubility be
even moderate, one is likely to attain supersaturation in the "cake"
and consequently a dull product. Further, lyotrope influences can never
strengthen a gel very much, but may and often do weaken it very
considerably. Hence the aim of most manufacturers in the preliminary
treatment is so to delime that a nearly neutral and salt-free product is
obtained. An exception is the case of skin gelatine in which excess of
sulphurous acid is used. This process has for its object not only
deliming and purifying, but also a bleaching action.
In the case of =bone gelatine=, the raw material is such that there are
much longer and more elaborate preparatory processes. This arises from
the fact that about half the bones of animals consists of mineral
matter, chiefly calcium phosphate. Bones, of course, vary in composition
to some extent, and those from younger animals contain distinctly less
of the mineral constituents. Approximately speaking, bones have the
following average composition:--
Gelatinous matter 21-1/2 per cent.
Fat 12-1/2 " "
Calcium phosphate 48 " "
Calcium carbonate 3 " "
Alkali salts, silica, etc. 2-1/2 " "
Water 12-1/2 " "
--------
100 " "
It will be seen, therefore, that the manufacture of bone gelatine and of
a comparatively large proportion of phosphate involves the recovery and
purification of much fatty matter. The manufacturing processes are
naturally subject to considerable variation. One respect in which they
differ is the stage in which grease is removed. Sometimes this is simply
done as the need and occasion arise, and it is skimmed out in the acid
or water extractions, but it is now more usual to have a special
"degreasing" process. There are, moreover, two quite distinct types of
manufacture. In one of these (the boiling process) the routine bears
some resemblance to that for skin gelatine. In this process the bones
are washed and cleansed and then immediately subjected to extraction
with water. This removes the gelatinous matter and leaves the phosphate
and earthy matters behind. Grease may be removed before the water
extraction, but is also sometimes removed by skimming off during the
extraction, as is usual in the case of skin gelatine. This procedure is
now not much favoured unless only a low-grade glue is required. In the
other type of manufacture (the acid process) the material is first
degreased, and then the mineral matter is extracted or dissolved by
acids, leaving the gelatinous matter behind for subsequent refinement
and solution. The acid process has long been preferred for high-class
bone gelatine, and hence needs further discussion.
The degreasing operation was once brought about by steaming only, but is
now accomplished with the assistance of fat solvents.
The object of cleansing is not only to remove dirt, but also fleshy
matter which often adheres to the bones. This may contain a little
gelatine, but consists mainly of other proteins and insoluble fibre,
neither of which are wanted in the water extraction. The mill consists
of a large cylinder of stout wire gauze. This revolves round the axis of
the cylinder, and the bones are fed in at one end by a hopper and are
discharged at the other. The revolution of the mill causes the friction
which polishes off the fleshy matter. The dirt and flesh fall through
the gauze and are sent to the fertilizer factory. The polishings are
sometimes further separated by a similar machine. Raw bones may thus
yield nearly 60 per cent. of degreased bones, and about 56 per cent.
cleansed bones ready for extraction, and 3 or 4 per cent. "bone meal."
The next stage is the extraction of the mineral matters by acid, for
which purpose hydrochloric acid has proved very suitable, as both
phosphate and carbonate of lime are dissolved by it. The usual
counter-current system of extraction is used [_cp._ Leaching and extract
manufacture, Part I., Section III., p. 35]. The process is methodical
and regular, the acid liquor passing successively through a battery
of six vats in such a manner that the liquor richest in lime salts comes
into contact with the bones most recently charged; the fresh acid thus
acts upon the nearly extracted bones. The hydrochloric acid used is of 8
to 10 per cent. strength (5° to 7° Bé.). Stronger acid is apt to
hydrolyze ("rot") the gelatine, whilst weaker acid takes longer time.
The process takes 8 to 10 days, though up to 14 days is sometimes given,
and, on the other hand, the process has been occasionally reduced to 4
days. The gelatinous matter undissolved has the shape of the original
bone, but is much swollen. When the acid liquor is saturated with lime
salt, the liquor is drawn off from below the vats and sent to the
phosphate precipitation tanks. The phosphate is usually precipitated by
adding just sufficient milk of lime to neutralize the hydrochloric acid.
The precipitated phosphate is then well washed by decantation to remove
calcium chloride. It is then drained, and dried at a low temperature. As
a large bulk of phosphate is obtained it is often filter-pressed and
dried quickly in long revolving chambers through which a current of air
is passed. The phosphate is sometimes also precipitated by ammonia. It
is then more easily washed and dried, and the ammonium chloride is
recovered and may be used to regenerate ammonia, or be sold as a
valuable bye-product. Sometimes the acid liquor is not used for making
precipitated phosphate, but is evaporated with animal charcoal and
silica and then distilled to make phosphorus.
The next stage is the purification by washing of the gelatinous matter
which remains. The vat is filled up with pure cold water and the
material allowed to steep for six or seven hours. The acid and salts
remaining diffuse outwards into the water. This is drained off and
replaced by fresh water, and the procedure repeated half a dozen times
or as often as necessary. The end is said to be determined by the
absence of a precipitate on adding silver nitrate to the wash water, or
by the absence of any action on blue litmus paper. It will be seen,
however, that there are two actions involved, one being the removal of
calcium chloride and the other the removal of excess acid. The former
is the easier, and is almost necessarily brought about by the latter.
Hence in some factories the neutralization is brought about, therefore,
by the addition of a certain quantity of soda, or more usually by lime,
and the material is sometimes submitted to a veritable liming by which
it remains in milk of lime for about three weeks, the lime liquor being
renewed several times. The product is finally washed again to remove
excess lime. This is carried out in a rotating vessel through which
passes a continuous stream of water. If a slightly acid gelatine is
required, however, the lime and liming are both superfluous, and the
procedure is simply to wash as thoroughly as possible and then to
immerse the material in a 1 per cent. sulphurous acid solution for 3
hours to bleach, and then to proceed with the water extraction or
solution of the gelatine. The hydrochloric acid used for these processes
should be as pure as possible, and the degreasing as thorough as
possible, for, if not, a gelatine with a bad odour is liable to be
obtained.
Instead of using hydrochloric acid for the solution of mineral matter,
sulphurous acid is sometimes employed, and has the advantages that its
bleaching effect is thereby obtained throughout the process, and that it
is recoverable for subsequent use. The Bergmann process, most generally
favoured, is described very concisely by Rideal thus: "A sulphurous acid
solution is made to circulate over the bones in a series of closed
tanks, the solution being continually enriched with sulphurous acid from
a cylinder of the liquefied gas. The resulting liquor, containing an
acid calcium phosphate and calcium bisulphite, is heated by steam in a
leaden digestor, when the excess of sulphurous acid is liberated and
passes back to the tanks, while neutral calcium phosphate and sulphite
are precipitated. The latter is decomposed by an equivalent of
hydrochloric acid, setting free the remaining sulphurous acid, which is
returned to the tanks, leaving calcium chloride in solution, and neutral
calcium phosphate in suspension." Not more than 5 per cent. of
sulphurous acid is said to be lost in this process, and the gelatine is
more thoroughly bleached. It is subsequently well washed before
extraction.
=Recovery and Purification of Grease.=--The degreasing operation, which
is applied usually to bones and to skin glue scutch, was once brought
about by steaming only, but is now accomplished with the assistance of
fat solvents, though in the latter case steaming together with
mechanical centrifugal force has proved sufficiently successful. On the
Continent carbon disulphide was once largely used as solvent, and in
this country benzene has been employed, but their low volatility and
high inflammability, as well as their expense, make both these
substances somewhat unsuitable, and it is now usual to make use of
petroleum oils, whether Scotch, American or Russian. A fraction which
boils about the same temperature as water is usually employed, and all
of it must be volatile under 280° F. Before the actual grease extraction
the bones should be sorted over and unsuitable substances (horns,
gravel, iron, etc.) removed. They are also usually put through a mill
and roughly crushed or broken. The actual grease extraction plant
consists of large copper vessels which will each take 5 tons of bones.
These extractors are arranged in sets so that the degreasing is
proceeding in some whilst the others are being emptied and recharged.
The doors for charging and emptying must be securely fastened. When the
extractor is charged the solvent is run in and heated by a steam coil
which eventually causes it to distil. After some hours the remainder,
which has dissolved much grease, is run off, and a fresh lot of solvent
is added and heated up. After four such extractions only about 1/4 per
cent. of grease remains in the bones. To remove the remainder of the
solvent high-pressure steam (80 lbs.) is blown through the bones. The
extractor is then opened and the degreased and somewhat dried bones are
mechanically conveyed to the cleansing mill. The grease solutions
obtained are subjected again to steam with a view to removing the
solvent and obtaining it for repeated use in this sense. The efficient
distillation and recovery of the solvent is indeed an essential element
in the success of the process.
The greases obtained, whether by the use of fat solvents or by skimming
off during extraction, or in any other way, are mixed together as is
appropriate to their origin and purity, and subjected to further
purification, the object of which is to remove gelatinous and albuminous
matters, and to decompose lime or soda soaps. The precise methods of
purification are, of course, dependent mainly upon the impurities known
to be present, but the readiest method is to give the grease further
steaming or boiling with water, and so effect by washing and by solvent
action the elimination of non-fatty matters. In many cases it is found
advantageous to employ mineral acids or oxidizing agents to assist the
process. The process may be repeated as often as is desired.
The recovered and purified greases are often of a high standard of
purity, and the best are quite fit for edible purposes. The large
extension of the margarine industry in this country has indeed caused a
larger proportion than ever of this bye-product to be so used. In some
cases it is found commercially advantageous to submit the grease to
action of the filter press, and so to separate it into solid and liquid
portions, the former containing a much larger proportion of stearin, and
the latter of olein. Much of the grease from the gelatine trade is also
found suitable for soap manufacture, and is therefore a valuable source
of glycerine.
=Other Raw Materials.=--Whilst hide pieces and fleshings, and animal
bones, comprise the principal raw material for the manufacture of
gelatine and glue, there are also minor sources of raw material which,
though often not suitable for gelatine manufacture, will yield a
satisfactory glue. Thus the skins, bladders and bones of fish form the
source of "fish glue." Sole skins, indeed, when deodorized by chlorine
and decolorized by animal charcoal, are made into gelatine. The bladders
of some fish (_e.g._ the sturgeon) are washed, purified and dried with
rolling to make "isinglass," a form of natural gelatine in which the
original fibrous structure is retained. There is a limited demand for
this material for clarifying purposes by brewers, wine merchants and
cooks.
Leather waste may sometimes be used to make a low-grade glue.
Vegetable-tanned leather offers much difficulty unless very lightly and
recently tanned. The tannage must be stripped by drumming with weak
alkalies, _e.g._ borax, sodium sulphite, or weak soda. Chrome leather
may be stripped easily and completely by Rochelle salt and other salts
of hydroxy acids (Procter and Wilson), and also by ammonia acetate,
oxalate and similar salts (Bennett), also by certain organic acids
(Lamb). Processes are patented by which chrome leather is digested with
lime to make glue, the chromium hydrate being insolubilized. Viscous and
tenacious substances are also obtained from some vegetable matters and
are called "glue."
REFERENCES.
"Glue and Glue Testing," S. Rideal, D.Sc., 2nd ed.; Skin Gelatine
and Glue, pp. 25-48; Bone Gelatine and Glue, pp. 59-66.
"Gelatine, Glue and their Allied Products," T. Lambert, pp. 11-52.
"Encyclopedie chimique," Fremy, tome x.