The plant necessary to the vacuum system is simple. It comprises a cylinder or boiler called a digester, into which the offal to be treated is placed. Under the wet steam process and after the vessel has been closed a vacuum is created. Open steam then is admitted into the digester and in such a way as to enable the steam to pass upwards through the mass, thereby thoroughly permeating it. Naturally the hot steam renders the fat fluid, that which is free running readily to the attached tanks. Rendering is conducted under a pressure varying from 20 lb. to 40 lb. as the case may be, but the lower the pressure the better. The application of the vacuum to the process constitutes the crux of the invention. At first sight the advantages of the principle may not be readily apparent, but they may be simply explained. In the first instance the creation of vacuum conditions effects the removal of the greatest obstruction to the influence of heat, namely air. If this be eliminated cooking can be conducted at a much lower temperature than would otherwise be practicable. Fat, indeed all animal matter, carries a certain proportion of moisture and this must be withdrawn before the actual release of the commodity can be effected. In vacuum water boils at a temperature below one-half of that required at ordinary atmospheric pressure. In other words, instead of the boiling-point of water being 212 degrees Fahrenheit, as is the case with the kettle on the hob, it will boil at less than 106 degrees Fahrenheit. Consequently, if a high vacuum be established within the digester the latent water can be converted into steam to assist in the melting process proper, which then can be conducted unhampered. Temperature, moreover, exercises a decisive influence upon the quality of the product, this being very superior in quality when the recovery is carried out at a low degree. Another point to be noted is that all noisome odours which are thrown off during cooking, and which cannot be avoided, are exhausted from the vessel. They are not allowed to escape into the open air, but are led to the furnace to be discharged into the hottest part of the fire. They have to ascend through the incandescent fuel resting upon the fire-bars, and, since they are not allowed to become mixed with air, must undergo complete combustion. Consequently no pollution of the atmosphere can possibly result from the treatment of even the most rancid offal. It being impossible to construe the operation into a nuisance, the plant can be installed at any convenient point even in a densely-settled area in safety, because the system fully complies with all the rigid requirements of the local sanitary authorities and health officers. This is a most important feature and one which will be readily appreciated when one recalls the insufferable conditions precipitated by the recovery of fats and greases from refuse under the old systems.