GENERAL CONSIDERATIONS.
Improved Modes of Extraction. Animal Fats.—One of the most important operations under this head is the manufacture of tallow and lard from animal fats. The process is usually termed "rendering," as applied to the fats themselves. These fatty matters are contained in animal tissue, into whose composition water largely enters. The simplest method of separating them from each other is by heating the fat in an open pan over a fire, at a temperature considerably above the boiling-point of water, 100° (212° F.), constantly stirring the mass, whereupon the animal tissue dries and cracks, allowing the pure fat to run out, and become separable by mere straining. This process, however, requires careful watching, as the tallow is apt to be discoloured by overheating, and, unless the fat be very new, is certain to evolve noxious odours.
The methods employed on a large scale may be divided into two, according as it is desired to save the animal tissue in a solid form, or not. If it be so desired, a very suitable apparatus is that used by Dole, of Bristol, and made by Miles, engineer, Bristol, shown in Fig. 1022. The apparatus consists of a strong iron cylinder a, provided above with a charging hole b, closed by a eliding cover ; a man-hole near the bottom, for tho discharge of solid refuse; two taps c for drawing off pure fat ; water feed-valves d; steam feed valves e; and water draw-off valve f. Steam at a pres sure of 60-80 lb. is introduced at e, and circulates in the coil g below the perforated false bottom h, d sup plying water when necessary. The vent-tap i regu lates the pressure, and permits the blowing off of steam. The apparatus being charged, steam is introduced for 6-8 hours. With a heavy charge, the fat can be drawn off by the upper of taps c, being floated to that level if necessary by letting in water. With a light charge, it is considered better to draw off all liquids together by the lower tap c, and skim off the fat when cold. The appa ratus is strengthened by a stay-bolt h, and has a stage at /. • Another modification, which produces a very pure tallow, is described under Butterine (p. 1362). Its dis advantages are the necessity for the comminution of the fat, and the length of time required by the process, i. e. the large amount of plant necessary to "render" a few tons of rough fat. Its advantages are the almost com plete freedom from noxious vapours (hence it is strongly recommended by Dr. Ballard), and the great purity of
the produot.
When the saving of the animal tissue is a matter of no moment, it is better to "render" the fat in the presence of water and steam. This can only be completely done under slight pressure (boiling the comminuted fat upon water in an open vessel only extracts part of the tallow, &c.), and is most suitably effected in a vessel such as that shown in Fig. 1023. The apparatus consists of a series of steam-tight cylinders of 1200-1500 gal. capacity, formed of boiler-plate, with a length about 24 times greater than the diameter, and provided with false bottoms. The operation proceeds thus :—The cylinder is fed through the man-hole K with crude fatty matters to within about 24 ft. of the top. The man-hole is secured, and steam is admitted by the foot-valve into the perforated pipe C. The safety-valve 0 is set at the required pressure, and frequent testing of the state of the contents of the cylinder is made by opening the try-cock R. An excess of condensed steam in the cylinder will be indicated by the spurting ejection of the fatty matters, when the regulating-cock X must be opened, and the condensed steam be drawn off into the tub T, till the escape of fatty matter from R has ceased. After 10-35 hours' continued steaming, the steam is shut off, and such as remains uncondensed in the cylinder ie let out by the try-cock and safety-valve. After due rest, the fatty matters separate out and form the uppermost layer, and are drawn off through the cocks p into ordinary coolers. The fat being emptied from the cylinder, the cover F is raised by the rod G from the discharging-hole E, and the residue falls into the tub T. Should the residue retain any fat, it is returned to the cylinder with the next charge. The pressure of steam commonly used is 50-75 lb. a, sq. in., sometimes advancing to 100 lb., though this last figure is excessive, and calculated to injure the qualify of the fat by decomposing the animal matters present. The yield obtained is considerably greater than by the ordinary methods, being stated at 12 per cent. extra for lard, and 6 per cent. for tallow. On the other hand, it is almost necessary to wash the fat with fresh water, and remelt and settle it, to remove the last traces of animal matter held in suspension by the unseparated water, and hav ing a tendency to putrefy.