The chief principle of extraction is so to arrange the process that both the material and the extracted liquor are maintained at high temperatures for the shortest possible time. As we have observed, gelatine is readily hydrolyzed by hot water, and as hot water is needed for its extraction or solution, care must be taken to remove the solution as soon as possible from the source of heat. In practice this can only be done somewhat imperfectly, as it is necessary to obtain a gelatine sol of several per cent. strength before removing it from the extraction vessel. The stronger this sol is made before removal, the less the time, trouble and expense is incurred in evaporation subsequently, but the more is the exposure to heat with consequent weakening of the gelatine. Hence in practice it is necessary to compromise. The matter is complicated further by the necessity of obtaining a clear sol, for which it is desirable that the sol obtained in extraction should not be too concentrated, as impurities settle and filter much more readily from weaker and less viscous sols.
It will be understood, therefore, that whatever material is being extracted, the most favoured procedure is to extract in fractions. The first fraction, which is least exposed to hydrolytic decomposition, produces the highest quality products, and the subsequent fractions (nearly always two more, and sometimes several) yield products which gradually become of inferior quality owing to the number of times the raw material has been re-heated.
Within limits, the precise temperature of extraction does not have the importance one would expect. Lambert suggests the temperature of 185° F. as suitable for both skin and bone gelatine, and most manufacturers would, on the whole, endorse this. If, however, a higher temperature be preferred, the hydrolytic action is increased in intensity but decreased in its time of operation, whilst if a lower temperature be adopted the decomposition is retarded in speed, but is increased in totality because of the longer time needed to obtain a suitable strength of liquor. Thus, with care, much the same result is obtained by extraction at near boiling-point for a short time as by extraction at 160° F. for a long time. The higher temperatures have the definite advantage of speed, whilst the lower temperatures have the advantage that one may choose to be satisfied with a weaker extract, and so gain a little in the strength of the gel, by throwing more work on the evaporator. One other point should, however, be borne in mind in this connection, viz. that a gelatine sol kept at temperatures above 185° F. begins to deteriorate in colour. Whilst, therefore, much depends upon the precise class of material, it is broadly true to say that the higher temperatures are advantageous for glue, whilst the lower temperatures are preferable for the highest quality gelatine.
Extraction in open vats is used both for skin and bone gelatine. It is usually preferred when it is intended to extract at the lower temperatures, and it is usually adopted also when the material is such that the extraction is comparatively rapid, as for example in the case of skin gelatine and bones by the acid process. The vats themselves are often constructed of wood, in which case they are heated by a copper (or brass) steam coil. They may be constructed also of iron, cast or wrought, the former being cheaper, less liable to corrosion, but more liable to fracture. In the case of iron vessels the heating may also be done by a steam coil beneath a false bottom, but it is sometimes arranged that iron vats are heated by a steam jacket, and even by a hot-water jacket. Heating in either wood or iron vessels has been brought about by direct application of raw steam, but the results are both uncertain and unsatisfactory owing to local overheating. Whatever appliances are used agitation of the material or liquor is advantageous.
Extraction in closed vats is also used. This is generally associated with extraction at higher temperatures, and more often also with the manufacture of glue than of gelatine. It has been used on the Continent for skin glue, and in this country for bone gelatine and glue by the "boiling" process. In this system of working the vessels are usually made of ⅜inch steel plates, and will take a charge of 3 to 5 tons of material. It is claimed for the system that there is a lessened steam consumption as well as lesser manipulation, that strong liquors are more easily and quickly obtained, and that the material may be more thoroughly exhausted. Extraction is sometimes made by steam and water playing alternately on the material, but many manufacturers prefer the use of direct steam, keeping the pressure at 15 lbs. for about 2 hours. The pressure is then reduced considerably and the process finished off by spraying the material with water. From such a procedure a 20 per cent. glue sol may be obtained. It is common to work such extractors in couples or in batteries of four to six. It will be readily understood that the process is suitable for making bone glue when the phosphate has not been dissolved. The high temperature is in this case almost necessary to ensure thorough extraction. It will be equally clear that the process is not so suitable in the manufacture of a strong gel.
As alternatives to the systems of fractional extraction, several processes have been devised in which the extraction is continuous.
Amongst these is the tower system, in which the material is placed upon a series of perforated shelves arranged inside a steam-tight cylinder or tower. Water is admitted from the top and trickles down over the material whilst steam is admitted from the bottom. Superheated steam is sometimes used. The material may thus be digested with a minimum amount of water, and the sol passes out of the apparatus and from the action of heat soon after it is formed. From bones the sol obtained is of good colour, but is somewhat dull. Several variants of this process have been patented.
Another continuous system of extraction is that involving the use of the Archimedean screw. The material is fed into one end of a cylinder carried along and discharged at the other end by the screw. The cylinder is of metal gauze and is steam jacketed. (Lehmann's patent, 1912.)
Continuous systems, involving a battery of digestors connected by pipes, have also been devised. Arrangements are made of course for admitting water and steam as required.
REFERENCES.
"Glue and Glue Testing," by S. Rideal, D.Sc., 2nd ed., pp. 47-56 and 61.
"Gelatine, Glue and their Allied Products," by T. Lambert, pp. 21-24, 40, 42-44, 49 and 51.
"Encyclopedie chimique," Fremy, tome x., p. 83.
PATENTS.
Edison: U.S.A. patent, 1902, 703204.
Bertram: English patent, 1892, 951.
Dorenburg: German patent, 1911, 239676.
Lehmann: French patent, 1912, 441548.