98 
The N.Z. Journal of Science and Technology. [June 
it is possible to reduce the production of objectionable odours to a 
minimum if the processes are carried out promptly, before decomposition 
of the animal-tissues can develop. 
In order to separate fat from animal-tissues, heat is necessary. The 
fats and oils of animals occur disseminated throughout the tissues in the 
form of minute globules enclosed within a thin skin. Pressure alone is 
not usually sufficient to break up the cellular tissue to enable the oily 
matter to exude ; the tissue must be heated, when it shrivels up as it loses 
its water, while the increasing temperature causes the fat to expand, burst 
the cell, and flow out as a liquid mass.' 
As has been shown, in the preparation of fats for edible purposes 
comparatively low temperatures are employed in order to free the fat 
without any secondary action upon the animal-tissue which would result 
in the production of substances giving an unpleasant flavour to the fat; 
in the manufacture of tallows such, precautions are unnecessary, and high- 
pressure steam is used, resulting in a greater yield of fat. 
For the purpose of rendering fats for tallow, the material is filled into 
digesters made of riveted steel plates, and capable of standing a working- 
pressure of several atmospheres. The digesters are provided with safety- 
valves, and discharge through either a gate valve at the bottom or through 
a door near the bottom. A false bottom is fitted within the digester, 
and the steam for cooking the material is admitted at the bottom of the 
vat under the plate, thus enabling the steam to permeate the whole 
mass. The digester is filled within a foot or so of the top with the 
material to be rendered, which consists of the products not utilized for 
edible purposes. 
The details of cooking the vat vary in different establishments and 
with, the class of material to be rendered, and need not be discussed here. 
The amount of water to be first placed in the digester, the length of time 
during which the pressure-steam is admitted, and the amount of pressure 
to be given are all matters of individual opinion and experience. In 
general, however, it may be observed that prior to cooking the vat it is 
important to heat the material thoroughly in order to drive out of the 
vat any air and gases contained within the spaces of the material and in 
the vat, replacing such air and gases with steam. Otherwise a false record 
will be observed upon the steam-gauge and the material may be insuffi¬ 
ciently rendered. It must be borne in mind that with saturated steam 
the pressure indicates the temperature. 
As the gases and fumes which are evolved during the heating are more 
or less offensive, it is now customary to lead these fumes through water 
condensers, which discharge into the sewers. As has already been pointed 
out, however, if the material is rendered while fresh and before decom¬ 
position has occurred much of the offensive odour can be prevented. 
While the details of the actual rendering will not be discussed here, 
it may be stated that in practice the material is subjected to steam- 
pressure up to 40 lb. in some cases. This practice varies, of course, but 
in general from 25 lb. up to 40 lb. pressure is maintained, equivalent to 
a temperature of from 267° to 287° F. The length of time and also the 
practice of leaving the vent slightly open during the cooking are matters 
of individual experience. The high temperature which has been used 
has the effect of thoroughly disintegrating the tissues which hold the 
fat-cells, and of liberating the fat, which is of a darker colour and possesses 
