The conversion of a gelatine sol into cakes of gelatine has been much
simplified by the advent of the evaporator. Before this machine was used
much trouble was experienced with putrefaction, and in hot and thundery
weather, especially on the Continent, it was often necessary to suspend
operations. Evaporation has, however, materially contributed to the
possibility of rapid and satisfactory cooling and drying.
From the time the weak sol is decolorized and bleached, the finishing
processes consist essentially in the removal of water. This is now
usually done partly by evaporation of the sol, and partly by the
desiccation of the gel. There is an obvious elasticity in method, and
factory practice does actually vary considerably in the relative
proportions of these two alternatives. Some factories evaporate to a 20
per cent. sol, approximately, and rely upon drying sheds and lofts to
complete the desiccation: other factories evaporate up to a 55 per cent.
gelatine sol, and so can manage with less shed room. Something depends
upon local conditions, but the main issue is between the cost of steam
in evaporation and the cost of land and buildings required for sheds. On
the whole the modern tendency is to evaporate more, for this course has
the additional advantage of speed, involving both a quicker turnover and
less liability of putrefaction. Lower-grade products need relatively
greater evaporation to form a gel of equal rigidity.
After evaporation and bleaching, the concentrated sol is first cooled
rapidly until it has set to a stiff gel, then cut up into cakes
according to the size required, these being dried out on network frames
arranged in tiers, through which a draught of air is usually forced or
induced. This general description is of course applicable to many
factories with innumerable variations in detail, most of which
variations originate in local convenience and are unessential parts of
the manufacture.
An essential principle is that the cooling or gelation should be done
rapidly, not only to avoid putrefaction but also to avoid the action of
heat on the elasticity of the gel. A hot sol or gel is liable to
hydrolysis and loss of setting power, and should have its temperature
quickly reduced, but a warm sol or gel (say 100° F.) is most liable to
putrefaction, so that the cooling should be continued quickly. On the
other hand, the gel should not be frozen. For cooling purposes a copious
supply of cold water is most usually employed, but some factories have
installed refrigerators. These plants operate by the rapid evaporation
of liquefied gases such as carbon dioxide, sulphur dioxide, or ammonia,
so arranged as to cool a solution of common salt, which forms the
circulating liquor and is returned after use to the refrigerator. Where
such plants are used, it is natural that their use should be extended to
the drying sheds to cool the air entering in the height of summer. In
some factories the cooling is attained neither by cold water nor cooled
brine, but merely by cold air.
The kind of vessel in which gelation is induced varies widely in
different factories. For lower-grade products metal boxes are used,
heavily galvanized iron being the most common material. If the liquor be
muddy, deep boxes are preferred, but if clear, rapid cooling is best
attained by having them long and shallow, and so exposing a relatively
greater area to the cooling action. In either case the boxes may contain
up to 1/2 cwt. of jelly. Lambert mentions boxes 24" × 6", which are 5"
deep; Cavalier suggests rectangular moulds holding 30 litres. In place
of galvanized sheet iron, boxes of sheet zinc or of wood lined with zinc
are sometimes used. In any case the most scrupulous cleanliness should
be observed in all cooling-house work, and in some factories the most
elaborate precautions are taken for cleansing vessels, tools, floors,
etc., and even for their disinfection and sterilization. Iron, tinned
iron, and copper cooling vessels are ruled out on account of their
tendency to rust and tarnish, and the last is unjustifiably expensive.
Many of these vessels are unsuitable for pure food gelatines in which
traces of copper, zinc and arsenic are held to be very objectionable.
For the best gelatines, therefore, a very shallow vessel (1/4" to 1/2"
deep) with a sheet glass bottom is preferred, and the concentrated sol
is run on to this for gelation.
Glue (or gelatine) which has set in this way is sometimes called "cast
glue." That which sets in metal boxes in blocks is termed "cut glue,"
because the blocks of jelly need subsequently to be cut into slabs of
the desired size and shape. Jelly blocks may be cut by hand with the
"wire knife" which yields a characteristic wavy appearance to the
finished product. This may also be done by machinery, the block of gel
being placed on a series of correctly spaced wires and forced through
the network by hydraulic pressure. A cutting machine (Schneible) has
also been used to cut up blocks of jelly into slices of the required
thickness, but these machines have not made great headway in this
country. It will be clear that cast glue is cooled more rapidly than
glue in blocks; it is therefore not surprising to note Lambert's
statement that the former comprises the larger proportion on the market.
The cut or cast cakes are next placed upon network frames, and a series
of such frames are placed on a bogey. The bogey is run along tram lines
into the drying tunnel, through which air is forced or induced by a fan.
Many such bogeys are, of course, passed into each tunnel, and as many
tunnels as required may be constructed. Care is necessary to expose the
cakes evenly to the action of the air. It is mostly necessary to warm
the air at the inlet by means of steam pipes and so increase its drying
power. This is especially necessary in winter or wet weather. In summer,
however, it is often arranged that the air is cooled before entering
the sheds. This is accomplished by passing the air through pipes from a
refrigerator. When heated air is used, it is stated by Lambert that the
maximum temperature should be 25.5° C. (78° F.); Rideal considers 21° C.
(70° F.) should be the maximum. In all cases the drying power of the air
is easily ascertained from a wet-and-dry bulb thermometer, and the
amount of air passing along the shed from a wind gauge. Lambert states
that drying normally occupies four to five days. The final product is
still a gel, of course, and contains from 10 to 18 per cent. of water.
It appears, however, very hard and solid. The dried cakes are removed
from the frames and transferred to the warehouse, where they are sorted
according to quality and packed in bags or tin-lined boxes. Some
material is ground to powder.
The network of the drying frames has been made from many materials.
Cotton or string netting is very common, but is liable to sag and to get
dirty. It also has a short life. Ordinary galvanized iron soon loses its
galvanizing cover, and the iron then is liable to rust. Attempts have
been made to use sheet zinc and other alloys, which are cut or punched
into nets with square or diamond-shaped holes. These were found to warp
and break. Rideal's conclusion, which is confirmed by the general
experience, is that the best material is a heavily galvanized iron wire
netting. He suggests that it should have 15 to 25 per cent. of its
weight of zinc, and that it should be strengthened by stiffer ribs
arranged both longitudinally and transversely.
Many attempts have been made, and many patents taken out, with the
object of making the cooling, cutting, and drying processes as
continuous and as quick as possible, and with a view to saving labour,
which is rather costly at this stage. These attempts, however, have only
met with indifferent success. A common idea is that a continuous supply
should fall upon a revolving appliance, and be instantly congealed in a
thin state, which last lends itself to more rapid desiccation. Vacuum
drying has also been attempted.
REFERENCES.
"Glue and Glue Testing," S. Rideal, D.Sc., pp. 68-74.
"Glue, Gelatine, and Allied Products," T. Lambert, pp. 30-35.
_Chem. Zeit._, 1911, 85, 17 (Cavalier).
PATENTS.
Eng. Patent (1894) 11,426 (Hewitt).
Eng. Patent (1898) 2,400 (Brauer).
Fr. Patent (1909) 398,598 (Lehmann), _J.S.C.I._, 1909, 897.
U.S. Patent (1912) 1,047,165 (American Glue Co.).