242 ANIMAL PROTEINS 



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 con- 

 dition as practicable. The temperature is particularly 

 important during evaporation (see Section VI., p. 249). 



Fortunately for manufacturers of gelatine, the colour- 

 ing matter to be attacked is very susceptible both to re- 

 duction 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 oxida- 

 tion, 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 



