224 ANIMAL PROTEINS 



washed and cleansed and then immediately subjected to 

 extraction with water. This removes the gelatinous matter 

 and leaves the phosphate and earthy matters behind. Grease 

 may be removed before the water extraction, but is also 

 sometimes removed by skimming off during the extraction, 

 as is usual in the case of skin gelatine. This procedure is 

 now not much favoured unless only a low-grade, glue is 

 required. In the other type of manufacture (the acid 

 process) the material is first degreased, and then the mineral 

 matter is extracted or dissolved by acids^ leaving the 

 gelatinous matter behind for subsequent refinement and 

 solution. The acid process has long been preferred for high- 

 class bone gelatine, and hence needs further discussion. 



The degreasing operation was once brought about by 

 steaming only, but is now accomplished with the assistance 

 of fat solvents. 



The object of cleansing is not only to remove dirt, but 

 also fleshy matter which often adheres to the bones. This 

 may contain a little gelatine, but consists mainly of other 

 proteins and insoluble fibre, neither of which are wanted 

 in the water extraction. The mill consists of a large cylinder 

 of stout wire gauze. This revolves round the axis of the 

 cylinder, and the bones are fed in at one end by a hopper and 

 are discharged at the other. The revolution of the mill 

 causes the friction which polishes off the fleshy matter. The 

 dirt and flesh fall through the gauze and are sent to the 

 fertilizer factory. The polishings are sometimes further 

 separated by a similar machine. Raw bones may thus yield 

 nearly 60 per cent, of degreased bones, and about 56 per 

 cent, cleansed bones ready for extraction, and 3 or 4 per cent. 

 " bone meal." 



The next stage is the extraction of the mineral matters 

 by acid, for which purpose hydrochloric acid has proved 

 very suitable, as both phosphate and carbonate of lime are 

 dissolved by it. The usual counter-current system of 

 extraction is used [cp. Reaching and extract manufacture, 

 Part I., Section III., p. 35]. The process is methodical 

 and regular, the acid liquor passing successively through a 



