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.
But the outstanding characteristic of the vacuum system is the increased yield of fat forthcoming. No mechanical system, whether it be pressure or high-speed whizzing, can extend completely satisfactory efficiency results. As is well known, the fat entering into the constitution of animal matter is contained in myriads of minute cells which are surrounded by tissue. The walls of these cells are exceedingly elastic and of prodigious strength. They may be compressed to an inordinately intense degree in a press, or distorted and stretched by recourse to centrifugal action without breaking. It is this circumstance which reacts against a high recovery of fat by recourse to pressing and whizzing because the cells cannot be induced to burst.
When a vacuum is applied a totally different result is recorded. The application of heat causes the fat and air within the tiny cells to expand, and in this manner the walls of the cells become distended to the limits of their elasticity. The removal of the surrounding air within the vessel by the vacuum pump completely upsets all equilibrium. The air pressure within the cells is higher than that applied from without, and consequently there results an accentuated expansive effort within the cells. But the tissue has already been stretched to its utmost limit, and so being unable to withstand the increased strain imposed collapses, thus releasing the imprisoned air and fat. Under the vacuum process the disruption of the fat-carrying cells is complete, and this explains why an augmented yield of fat is obtained by this method.
Under the open steam vacuum process the actual practice is to apply the vacuum three times at intervals during the operation. The first application serves to remove the obstructive air to facilitate and expedite cooking of the contents. The second brings about the disruption of the cells and the release of the fat which they contain. The third application of the vacuum, which is effected towards the end of the process, effects the withdrawal of the foul vapours arising from the digesting operation and their discharge into the fire.
Owing to the steam being admitted to the digester and being allowed to come into direct contact with the mass, the residue upon withdrawal is wet. The grease, which has been rendered fluid in the process, has escaped from the digester through a suitable draining pipe into a tank where settlement and clarification are carried out. But all the grease cannot be recovered in this manner. A certain proportion, notwithstanding the disruption of the fat cells, is held up in the mass and can only be recovered to an appreciable degree by submitting the residue to treatment in a press. In this way the greater part of the remaining fat suffers expulsion and recovery. The wet cakes upon removal from the press then have to be dried and disintegrated.
The dry vacuum process, which is essentially adapted to the rendering of edible fat, has many advantages over the wet steam method. Whilst the plant employed is broadly similar to that employed in the process already described, there is one notable difference. The digester is enveloped in an outer shell or jacket, and the steam is circulated through the space between the two walls. It is not brought into contact with the contents of the digester at any stage of the process. The action taking place within the vessel during the operation is precisely the same as when the steam is brought into direct contact with the refuse, the fat being rendered fluid by the heat and the cells undergoing disruption by the creation of the vacuum. A high vacuum is maintained throughout the whole rendering process. Consequently the moisture inherent to the raw material is withdrawn as rapidly as it is converted into steam, resulting in the production of a fine edible fat totally free from moisture. Moreover, the residue withdrawn from the digester at the end of the process, known as “crackling” or “greaves,” is likewise quite free from moisture, although, as in the case of that resulting from the open steam process, an appreciable proportion of fat is held up in the mass which can only be recovered to a pronounced degree by the application of pressure.
The dry steam or jacketed vacuum process is especially adapted to the treatment of fresh fat waste, the reclaimed product of which is primarily intended for the preparation of edible foodstuffs, such as oleo-margarine. By carrying out reclamation without bringing the steam into contact with the fat several distinct advantages are obtained, the most important being the retention of the natural properties of the fat, and no loss of glycerine which otherwise is inevitable to a certain degree. Consequently, it is an ideal process for the treatment of the “Premier Jus.” There is no need to mince the fat finely, as in the orthodox rendering process, it being necessary only to cut the waste roughly for charging the digester.
A special press has been devised for the treatment of the crackling or greaves. It is of the cage type which allows the fat, during pressure, and which operation is carried out while the residue is very hot, to be expressed between the bars of the cage to fall into a trough for recovery. The cakes, after pressing, are dry, excellent in quality, light in colour and of attractive flavour, a result due to the fact that the tissues have not been scorched or charred in any way during the rendering process. The greaves constitute an excellent ingredient for the preparation of kennel and poultry foods, and enter extensively into the manufacture of dog-cakes. In a few instances the dry greaves, owing to their high nutritive value, are served to the kennel in the straight form as they issue from the press.
While the dry vacuum process is certainly efficient, it does not fully comply with the latest ideas pertaining to the recovery of fats from organic waste. The press is the weak link, because thereby it is only possible to recover a certain proportion of the fat held up in the mass, even when the cellular construction has been completely broken up. It is stated, as a result of accumulated experience, that the amount of fat left in the greaves may run up to as high as 10 per cent. of the original fatty content of the offal: in many instances it has been found to range as high as 20 per cent. The fact that this remaining fat defying reclamation by pressing must be relatively high is evident from the readiness with which certain waste exploiters will buy up the greaves, not to turn them into kennel and poultry foods, but to submit them to further treatment in order to wring out still more of the fat which they carry.
This manifestation of enterprise has been rendered possible by the advance of the science of fat recovery from offal to such a level as to enable 9 per cent. of the fat remaining in a 10 per cent. greaves to be extracted. It is the prevailing high price commanded by fats which renders such additional treatment upon an extensive scale so attractive and eminently profitable.