1921.] Wright.—Chemical Technology of Meat Industry. 
101 
Tankage. (6, 8, 21, 26, 30.) 
The solid residue remaining alter the rendering of the tissues for the 
recovery of the fat for tallow is discharged from the digesters, after 
running off the tallow and the aqueous gluey liquor. During the process 
of rendering, in addition to the freeing of the fat from the thin envelope 
of skin in which it is enclosed, other important changes take place. 
Animal flesh and bones contain constituents, mainly nitrogenous, which 
are soluble in water, and under the conditions to which the meaty tissues 
and bones are subjected in the process of cooking these nitrogenous 
constituents pass into solution, leaving an insoluble residue of meat fibres 
and tissues and bones. This residue as it is discharged from the digester, 
hot, contains a considerable amount of water and some fat, the amount 
of the latter depending upon the efficiency of the rendering process. 
While still hot the material is put into iron cages and pressed in order to 
remove as much as possible of the water and fat remaining, the mixed 
water and fat being run into a series of settling-tanks and catch-basins 
in order to recover the fat. In some cases the material, instead of being 
pressed, is treated in a centrifugal hydro-extracter in order to eliminate 
the surplus water and fat. 
The pressed or centrifugal]'zed mass, which contains about 50 per cent, 
of water, is then dried in order to reduce the water content to an amount 
which will enable the resulting tankage to remain stable ; normally a 
maximum of 10 per cent, moisture may be present without risk of 
subsequent heating and change. 
Dor drying the material a variety of dryers are in operation ; in all 
the object in view is identical — namely, to remove efficiently and 
economically the surplus moisture without decomposing the material and 
losing any of the valuable constituent nitrogen. A study of many types 
of dryer for this purpose has led to the conclusion that the revolving 
steam-heated vacuum type of dryer is the most satisfactory; the 
advantages are economy in operation, while the material is dried at a 
temperature below that at which ammonia is disengaged from the nitro¬ 
genous constituents of the material, and any offensive gases are condensed 
and removed without causing a nuisance. 
The valuable constituents of tankage are the nitrogen and phosphates. 
In addition to these, however, as well as the moisture, is a certain per¬ 
centage of fat, and much attention has been directed towards the recovery 
of this constituent. 
Even with efficient rendering, and with pressing or centrifugalizing, 
an amount of fat is found in the dried tankage which may represent from 
12 to 15 per cent, of the tankage. In addition to the actual loss sustained 
by leaving the fat in the tankage a further disadvantage lies in the fact 
that, since the tankage is used principally as a fertilizer material, the fat, 
being decomposed with difficulty in the soil, and being coated upon the 
surface of the nitrogenous and phosphatic constituents, delays the action 
of the tankage in the soil and so detracts from its fertilizer-value. 
Many suggestions and processes have been put forward, but so far 
none has met with the success which has attended the use of volatile 
solvents for removing the residual fat in dried tankage. Two types of 
extraction plants for the recovery of fat by volatile solvents are in use, 
the one operating as a cold-extraction plant, the other by hot solvents ; 
but for tankage the hot-extraction plant is the more suitable. In this 
