EXTRACTION 231 



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. L,ambert 

 suggests thl^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 com- 

 paratively 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 



