systems depend on cooker volume, cooker heat-transfer capability, and the characteristics and feed-rate of the raw material. Cooker contents are discharged into a percolator to remove free-draining fat; then the solid material is pressed (a continuous operation) or centrifuged (a batch operation) to remove additional fat. The material is then ground into meat meal. The fat is polished in disc centrifuges to remove fines and moisture.

Batch dry rendering has the following advantages:

• Little material is lost.

• Cooking, pressurizing, and sterilizing can be carried out in the same vessel.

• Different cookers can be assigned for processing different raw material, and hence for producing different grades of fat.

• Heat can be recovered from the vent steam.

Raw material

Raw material

Tallow Meal to mill

Figure 3. Semicontinuous wet-rendering system.

Tallow Meal to mill

Figure 3. Semicontinuous wet-rendering system.

The disadvantages are as follow:

• The fat is usually of poorer quality than that from wet or LTR systems.

• The high temperatures used produce fines, which can pass into the fat, degrading its quality, and which can be lost in the effluent from the polishing centrifuges.

Raw material

Meal Tallow

Figure 4. Dry-rendering system.

Meal Tallow

Figure 4. Dry-rendering system.

• The meal has a higher fat content than meal from wet and LTR systems.

• Raw material must be washed (producing effluent) to produce good-quality fat.

• Indiscriminate hosing and inadequate draining of washed material add extra water, increasing evaporation loadings and, hence, energy requirements.

• High-protein, low-fat material, such as slaughter wastes from calves and young lambs, is difficult to process.

• The end point of the cooks is difficult to control, so fat quality can be variable.

• It is difficult to keep the processing area clean and tidy; the process is not completely enclosed, so cooked products can be recontaminated.

Energy usage is high, especially if vent steam is not recovered as hot water; dry-rendering cookers are not efficient dryers.

• The process is labor intensive.

Continuous dry rendering has most of the advantages of batch dry rendering, uses less labor, and requires less space. It has the disadvantages that the system cannot be pressurized, so sterilization and hydrolysis cannot be done, and tallow quality tends to be lower than with batch operations because end-point temperatures are often higher.

Low-Temperature Rendering (LTR). In the wet- and dry-rendering systems discussed so far, the raw material is subjected to high temperatures for long times. In dry rendering, near the end of the cooking process, when most of the free water has evaporated, the solids (cracklings) are essentially "frying" in the fat. With such high temperatures, the raw material must be washed to ensure that paunch contents and other "dirt" does not downgrade the color of the fat. Operators often overdry the cracklings to make pressing easier, so dry-rendered meal can be high in fat and low in moisture.

LTR systems were developed in the late 1970s to overcome some of the disadvantages of dry-rendering systems. Heat treatment is minimized and phase separation is carried out at low temperatures (70 to 100°C). Systems are usually continuous, so material flow through the size-reduction equipment, heating units, phase separating units, evaporators, and dryers must be balanced using surge bins and/or variable speed drives. Raw material, which can be unwashed, is minced, then heated to 70 to 95°C in a unit called a coagulator, preheater, melting section, or rendering vessel. The phases are then mechanically separated in decanters or presses.

In a MIRINZ low-temperature-rendering (MLTR) system (Fig. 5) processing 5 t of raw material per hour, the rendering vessel volume is only 1 m3. Despite this, the residence time for the raw material, along with some recycled fat, to reach 80 to 90°C using indirect heat (in coils in the vessel) is just 6 to 8 min (11). The resulting liquor and solids are separated in a decanter, and the wet solids are dried, often in a direct-fired rotary drier, to give a low-fat meal. The liquor is separated into high-quality fat and an aqueous phase (stickwater). Product loss in the stickwater should be low.

In other low-temperature systems (eg, the Atlas or Stord Bartz wet pressing systems), raw material is heated indirectly to about 90 to 95°C over a 30- to 60-min period in a preheater, then pressed in a twin-screw press (Fig. 6). Drainings from the preheater and press are further heated to coagulate any soluble protein, then centrifuged to remove fines. The fines and pressed solids are dried, usually in contact driers.

The stickwater from LTR systems can be concentrated by ultrafiltration or in evaporators, then dried. The steam side of the evaporator is often supplied with waste vapors from the cooker/drier to save energy. Heat can be recovered from the stickwater and from the vapors from the drier and used to preheat incoming raw material, or to provide hot water.

The advantages of LTR systems are as follow:

• Energy requirements are usually about half those of dry-rendering systems.

• The raw material may not need to be washed.

• A low heat treatment is used, so high-quality fat is produced.

• The meal has a low fat content and has a high nutritional value because heat treatment is minimal.

• Labor requirements are low.

• The system can be easily automated.

However, the systems have high capital costs, may have high repair and maintenance costs, and usually require highly trained technical operators.

Other methods of treating waste by-products from the meat industry (and also the fish and other protein indus-

Figure 5. MIRINZ low-temperature rendering (MLTR) system.

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