The adsorption law indicates that however much colouring matter is
removed from the volume concentration (continuous phase) there must
always be some left. After all that the decolorization processes can do,
there still remains much colour that can only be removed by a chemical
action of the ordinary sense. The amount of colouring matter of this
kind is not large, but it is a deep red-brown, and when the gelatine sol
has been evaporated and dried out the final product, if untreated,
possesses this typical colour, and is known as glue. If, before
gelation, a chemical bleaching action is applied to destroy this
pigment, the product may be then dried out in a nearly colourless
condition and is known as gelatine. Gelatine, therefore, is simply
bleached glue. Many other definitions have been given, and many
elaborate distinctions drawn, but the fact of bleaching is the essential
difference. In these days when gelatine is so valuable, the higher-grade
products are nearly always bleached, and the term "glue" is consequently
more often applied to a lower-grade product, and is sometimes used in a
sense implying this fact.
If it be desired to manufacture gelatine, it is fairly obvious that the
task is lightened by observing the axiom that prevention is better than
cure. If steps are taken to prevent the presence or development of such
colouring matter, a great advantage is attained, for not only is the
problem of bleaching easier, but also quicker and less expensive in
chemicals. The nature of the colouring matters is but imperfectly
investigated, but in the case of skin gelatine the pigment of the hair
roots and epidermis is doubtless one factor. A long liming is said to
assist in its destruction, possibly because this completes the loosening
of epithelial structures and possibly because the alkali causes some
hydrolysis of the pigment. In both skin and bone gelatine sols, however,
there is a considerable tendency to develop the brown colouring matter
typical of glue. This tendency is enhanced by an increase in temperature
and also by the presence of acid or alkali. These facts seem to indicate
that its development is associated with a partial hydrolysis of the
gelatine in some direction. Rideal says this colouring matter is allied
to caramel. In harmony with this is the experience that its development
is greatest in products which have been "burnt," _i.e._ subjected to
unusually high temperature. The practical maxims which arise from these
considerations are fairly obvious and widely known, viz. to conduct the
extraction and evaporation at as low a temperature as possible and in as
neutral a condition as practicable. The temperature is particularly
important during evaporation (see Section VI.).
Fortunately for manufacturers of gelatine, the colouring matter to be
attacked is very susceptible both to reduction and to oxidation, and
both types of bleach are widely used in practice. It is somewhat curious
that the same colouring matter should be destructible both by reduction
and by oxidation, but there is no doubt that each type gives a perfectly
satisfactory bleaching action and can result in a practically colourless
gelatine. On the other hand, the reduction is the more unstable
reaction, for the glue colour slowly develops again in the gelatine on
keeping it, even in a dried condition. Gelatine bleached by oxidation,
however, retains its colour quite well, and even tends to improve with
keeping. It is quite possible that quite different reactions are
involved in the two processes, but in the light of the above facts it is
somewhat surprising to observe Rideal's statement that reduction
followed by oxidation has been successful in practice.
Although there is a wide choice of reducing and of oxidizing agents,
those which are suitable for application to gelatine cover a very
limited field. This limitation arises not so much from the
ineffectiveness of the bleach, as from the other effects of these
substances upon the purity of the product and upon the elasticity of the
gel which it can yield. Especially important is the lyotrope influence
of the bleaching agent. Many reactive substances are ruled out simply
because they either insolubilize the gelatine or weaken the gel it
makes. Others are inadmissible on account of their poisonous nature. It
must never be forgotten that whatever is used in bleaching is, like the
gelatine itself, much concentrated during evaporation and drying. Its
possible percentage in the finished product should be considered, and
also the possibility that in these finishing operations what is present
may not remain in solution, owing to supersaturation.
=Bleaching by Reduction.=--Of all the reducing agents suggested,
sulphurous acid has proved to be much the most suitable and successful.
It has been used with equal success both for bone and for skin gelatine,
but on the whole has proved more suitable for the former.
Sulphurous acid can fulfil in this instance a double function, viz. that
of acid solvent for the bone phosphate, and that of bleaching agent
also. As it penetrates the bone material, dissolving the phosphate, it
also exercises its bleaching influence on the gelatinous part of the
material. Changes of liquor tend to complete both actions, so that a
counter-current system is found most convenient. The "acid process" for
the manufacture of bone gelatine has been previously described (Section
II.), and the use of sulphurous acid in this connection is typified in
the Bergmann process. In this process bleaching is in effect merely a
continued treatment.
In the case of skin gelatine, also, sulphurous acid may fulfil a double
function, viz. that of deliming agent as well as of bleaching agent. In
such instance it is necessary to use excess of bleaching acid, some
acting as deliming material and the remainder as bleaching agent. As it
is desirable to get rid of the lime and soda salts, several changes of
liquor are given to the goods, possibly with intermediate washing. Here
again approximation to a counter-current system is of advantage, as the
employment of used bleach liquors for deliming purposes effects
considerable economy of sulphurous acid. Indeed, there need be no waste
acid at all.
Whether the material be for bone or skin gelatine, however, it will be
seen that the extraction is conducted in an acid condition and the
resulting sol is also acid. Most usually the decolorization and
filtration processes are also conducted with such an acid sol. From what
has been said (Section IV.) of the value of dibasic inorganic
acids as clarifying agents, it will be understood that the presence of
sulphurous acid at this stage is of great advantage in the production of
a clear and bright gelatine. Indeed, it is well known in trade circles
that sulphurous acid gelatines are usually of exceptional clarity and
brightness.
The disadvantage of sulphurous acid processes is also found in the same
fact that both sol, gel and cake are in an acid condition. To complete
the bleach it is sometimes necessary to add sulphurous acid to the sol
after extraction, or even after evaporation, but this is to be avoided
if possible. Usually the ideal attempted is that the bleaching action
should be as much as possible before extraction; the excess of
sulphurous acid is then washed off just before the extraction, as far as
practicable, and the rest is boiled off during extraction. The ideal is
practically never attained, for the acid is strongly adsorbed, and the
result is that the finished article is always an acid gelatine, and
sometimes indeed very decidedly such. The acid condition is
objectionable in the case of some forms of filter press on account of
the solvent action on the metals, and is objectionable in evaporation
for similar reasons. Acid gelatines are also objectionable for many
purposes for which gelatine is usually sold, and this limits the
commercial possibilities of the product thus obtained.
Sulphurous acid is itself, of course, a gas, and whilst the gas itself
has been used for treating the material (_e.g._ bones), it has been
found not only more convenient but also more effective to use an aqueous
solution. This is mainly because it is possible to attain a greater
adsorption in a liquor. Unfortunately, however, sulphurous acid is not a
very soluble gas, and although 8-10 per cent. solutions may be, with
great care, obtained, they are really supersaturated and readily yield
the gas, even with slight mechanical agitation. Solutions even of 2 to 3
per cent. strength are also liable to this, and the general experience
is that 1 to 2 per cent. solutions are most economical and convenient
for practical purposes. As the freight on weak solutions is prohibitive,
the manufacturer using sulphurous acid is faced with the necessity
either of purchasing cylinders of sulphur dioxide liquefied by pressure
or making the gas and solution himself. The former is the most
convenient course when only small amounts are required, but the latter
preferable for a gelatine factory of any size. Sulphurous acid is easily
manufactured by burning sulphur and leading the fumes by induced draught
up a scrubber down which water slowly trickles. Forced draught may also
be used, as in the Sachsenburg plant.
Of the other reducing agents which have been used, sodium hydrosulphite
(Na{2}SO{2}) deserves mention. It is a very powerful reducing agent,
and has been found most useful when employed as an assistant to
sulphurous acid. This reagent is usually added to the sol, after
evaporation and before gelation. It is sold as a white powder, usually
under trade names. Sometimes a mixture of bisulphite and powdered zinc
replaces it, but this is objectionable for pure food gelatines. Its use
also involves an impurity in the finished article, and a greater amount
of "inorganic ash."
=Bleaching by Oxidation.=--Many oxidizing agents have been suggested for
bleaching gelatine, but most of them have some practical disadvantage.
Most of them contradict the maxim (previously noted) that it is
desirable to avoid adding any soluble substance, as this involves a
permanent impurity, possibly concentrated to supersaturation in the
finishing processes, and possibly involving a disadvantageous lyotrope
influence. There is another objection to oxidizing agents also; whilst
their bleaching action on the pigments is undoubted, some of them have
also a special action upon the gelatine itself which is in reality akin
to tanning, and may indeed involve an insolubilization of the gelatine.
Thus, chlorine gas (which Meunier patented for tanning) has been used
for bleaching gelatine, but the conditions of success have not yet been
thoroughly elucidated, and it is problematical indeed whether the
process is consistent with best results. Hypochlorites and bleaching
powder have also a similar action, which has been utilized with some
success in practice. Rideal suggests that a suitable concentration for
these reagents is 1:2000, and emphasizes the care necessary. An
advantage of all these chlorinations is the formation of the strongly
antiseptic chloramines, which preserve the gelatine from putrefaction.
Ozone has also been tried as an oxidation bleach for gelatine, but not
successfully, partly on account of difficulties in controlling the
quantity used. Peroxide of soda has also been used, but it is not only
alkaline, but liable to contain sodium hydrate and carbonate as
impurities, and this involves neutralization either before use or in the
gelatine sol, and the consequent presence of sodium salts in the
finished article. Peroxide of calcium is open to the same objections,
except that calcium is more easily removed from the sol than sodium.
Rideal's suggestion for removing this lime, viz. precipitation by a
current of carbonic acid, merits attention in this and in other
directions also. Rideal also states that in the case of an acid bone
gelatine, a good peroxide of lime is almost an ideal reagent for
bleaching, inasmuch as "the lime carries down phosphate, several
impurities and colouring matters." It thus acts as bleach, as
neutralizing agent, and as precipitant, and the precipitate itself is a
strong adsorbent. On account of its freedom from bases, and because its
residue is simply water, peroxide of hydrogen has been found of great
service in practice, and in most factories it has shown itself superior
not only to the other peroxides, but also to all other oxidizing
agents. Its application is simple, a concentrated solution being added
to the gelatine sol before or after evaporation. It is the most
"fool-proof" of all the oxidizing agents used in bleaching, and it
yields the purest product. Its bleaching action is perfectly
satisfactory, but only in a non-acid sol. Hydrogen peroxide is
moderately stable in acid solution, and its bleaching action is best in
slightly alkaline solution. An acid sol bleaches too slowly, or not at
all; an alkaline sol induces evolution of oxygen and consequent waste.
The great disadvantage of peroxide of hydrogen is its great expense,
which is enhanced by an increasing demand for it in other industries. A
minor disadvantage is its instability, which leads to loss in transit
and storage. It is sold usually in strengths indicated by the volume of
oxygen obtained from unit volume of the solution, when treated with
permanganate in a nitrometer (_e.g._ "15 vols. peroxide").
It is a fortunate feature of both the oxidizing and reducing agents
usually employed in bleaching, that they have considerable antiseptic
power. This assists materially in preserving the gelatine from
putrefaction during the critical period between extraction and
concentration.
REFERENCES.
"Glue and Glue Testing," S. Rideal, D.Sc., 2nd ed., pp. 61-66,
78-82.
"Gelatine, Glue, and Allied Products," T. Lambert, pp. 29, 30, 49,
51.
"Chemical Engineering," _J.R. San. Inst._, No. 2, 1910. S. Rideal.
On adsorption phenomena:
1. "Chemistry of Colloids," Dr. W. W. Taylor.
2. "Chemistry of Colloids," V. Pöschl.
3. "Chemistry of Colloids," Zsigmondy and Spear.
4. "Chemistry and Physics of Colloids," E. Halschek.
5. "Surface Tension and Surface Energy," Willows and Hatschek.