Whole blood is a liquid with a dry matter content of 18 to 20%. Animal type and age may slightly influence blood composition. Blood undergoes complex biochemical reactions (clotting, coagulation, syneresis) once it has been released. Uncoagulated blood can be fractionated into plasma (a pale yellow fluid) and red blood cells (a fraction containing hemoglobin). Serum can be obtained when clotted blood synereses.
About 3 to 4% of the animal's live weight is blood, so large quantities are produced daily at slaughter facilities, even small ones. As 90 to 95% of the dry matter in blood is protein, blood is an obvious source of high-quality protein. Whole blood and the processed fractions (especially plasma) can be used as food ingredients. The dark red color of whole blood and red blood cells usually limits the use of these products in foods (5-7), although several processes exist for manufacturing decolorized blood fractions. However, the high costs and the logistics of collecting blood hy-gienically usually preclude extensive use of blood for edible products, so it is usually processed into an inedible blood meal. Although this processing into blood meal produces some income, it is done mainly to reduce pollution treatment costs: whole blood has a high biological oxygen demand (250,000 mg/L) and therefore can be a major source of pollution. Blood is usually not collected hygienically, in which case it may contain urine and ingesta. As well, it usually has been diluted with wash water. As a result, the dry matter content of the whole blood can be reduced to as low as 10%, especially if there is poor water management.
Whole blood can be stabilized chemically by adding urea, ammonia, metabisulfite or sulfuric acid, and the preserved blood can then be held without refrigeration and fed to animals (8). However, in most commercial processes, the blood is heated to coagulate the proteins, and these coagulated solids are then separated and dried to produce blood meal with a moisture content of 8 to 10%. This meal is a cheap source of animal protein high in lysine. The three most common methods of processing inedible blood are as follows:
1. Apply indirect heat to the whole blood to boil off most of the water. This process is very energy inefficient and produces a denatured product with very low solubility. This method is often used at smaller older plants that have not updated their equipment.
2. Inject live steam to bring the temperature to about 90°C to coagulate the blood, then remove most of the water in a decanter and dry the solids (eg, in a ring dryer with hot circulating gases, a rotating drum cooker, or a batch dryer). This method uses less energy than the previously mentioned technique because about half of the water is removed mechanically. A denatured protein powder is produced, the solubility of which depends on the type of dryer used: powders produced in ring dryers usually are more soluble. The high temperatures and/or long times used during processing lower the nutritional value of the blood meal.
3. Concentrate whole blood by ultrafiltration, then dry the concentrate in a spouted-bed drier (9). A very soluble powder is produced.
The yields of dried blood depend on the processing parameters (10). Aging the blood (leaving for 12 or more hours before processing) can increase yields. Any water added to the raw blood reduces yields. If the steam coag-ulator is not working efficiently, the losses of protein in the "blood water" from the centrifuge can increase.
Blood and its components (serum, albumin, red blood cells, hemoglobin) can be used in food, animal feeds, laboratory reagents, medical preparations, and industrial uses and as fertilizer. Blood albumin can be a substitute for egg albumen. Dried blood meal is used as a protein supplement in livestock feed. It is deficient in the amino acids tryptophan and isoleucine but is high in lysine, although lysine availability is affected by the drying method. Laboratory uses for blood include as a nutrient for tissue culture media and as a necessary ingredient in some agars for bacteriological use. Many blood components are isolated from whole blood and used in chemical and medical analyses or as nutritive supplements. Industrial uses of blood include as an adhesive and for its film-forming properties in the paper, lithographic, plywood, veneering, fiber, plastics, and glue industries. As a fertilizer, blood meal contributes nitrogen, aids humus formation, and improves soil structure.
Blood char, or blood charcoal, is the carbon component of whole blood or blood meal. Blood char is produced by treating 20% of the weight of whole blood or 50% of the weight of blood meal with activating agents and heating in airtight containers to 650 to 750°C for 6 to 8 h. Blood char contains 80% carbon and is used for absorption of gases, as an industrial decolorant, and as an antidote for chemical poisoning.
Animal tissues are composed essentially of water, fat, and protein, with some minerals. The term "rendering" refers to a variety of processes that are used to separate the water, fat, and protein components, as far as is practicable, into commercial products. Until the 1850s, the highly perishable by-products from meat slaughtering were considered waste and were buried. The rendering industry developed to convert these materials into farm fertilizer and realize a profit from the operation. Proteinaceous byproducts yielded nitrogen fertilizer, whereas bone produced phosphate fertilizer. Today, the rendering industry produces many useful products that can be broadly classified as edible and inedible fats, fine chemicals, meat meals, and bone meals.
Animal by-products used by Tenderers consist of fatty tissue, trimmings, bones, hooves, and soft offals (viscera). Processing carcasses into cuts and boneless meat increases the amount of material for rendering through the bones and trimmings that result. The material for rendering can represent 30 to 60 percent of the weight of a slaughtered animal. In developed countries, with centralized slaughter of large numbers of animals, a large volume of material is available for Tenderers. This has led to the development of sophisticated equipment and processes.
The basic purpose of rendering is to produce stable products of commercial value, free of disease-bearing organisms, from raw material that is often unsuitable or unfit for human consumption. Most of the fat comes from fatty tissue, which can be located anywhere within connective tissue, and is made up of cells containing fat. This fat is deposited when animals have a surplus of dietary energy. The fat cells are surrounded by reticular fibers, and the fat cannot be released from animal tissue until the supporting structure has been broken. The two basic processes in rendering are separation of the fat and drying of the residue. The most common method used to rupture fat cells is heat, although enzymic and solvent-extraction rendering processes also exist.
A large proportion of the raw material for rendering is viscera (soft offal) and its associated contents. Because paunch and gut contents contain chemicals that can adversely affect fat quality, and solids that can downgrade meal quality, paunches are usually opened and emptied. The viscera are then cut and may be washed. Size reduction increases mass and heat transfer, although in some rendering processes it may be difficult to get even heat transfer in the processing vessel if the particles are too small. Both hard and soft offals may undergo size reduction. This size reduction is usually done in devices with rotating knives or anvils (termed hogors or prebreakers when producing large particles, grinders or mincers when producing fine-particle material) or with rotating hammer devices (hammer mills). Hooves and bones (hard offals) need to be reduced in size but usually do not require washing. The raw material is then processed to separate the fat from the nonfat phase.
A rendering process can generally be classified as wet or dry, depending on whether the fat is removed from the raw material before or after the drying operation. Processes can operate in a batch, semicontinuous, or continuous mode, and some of the newer rendering systems are classified as low-temperature-rendering (LTR) systems because of their milder heat treatment. Table 1 lists the best-known and most-used rendering systems, divided ac cording to process type. Connected systems produce products of similar quality when processing similar raw material. There is no one "best" process for all applications; the most appropriate method will often depend on the application.
Profitability of any rendering system can be maximized by ensuring maximum yields and by obtaining the best possible product quality. Regular and planned maintenance of equipment will minimize repairs and maintenance costs. Processing additional water (from excessive washing and hosing) is wasteful, because more energy and a larger rendering capacity is required for a given throughput of solid material.
Wet Rendering. Wet rendering is an old processing method. In older systems, the preground raw material is cooked in a closed vertical tank (termed a digestor, autoclave, or cooker) under pressure by direct steam injection, usually to 380 to 500 kPa for 3 to 6 h (Fig. 2). Operators use past experience to gauge the end point. The pressure is then slowly released, and the liquid and solid phases are allowed to settle. The fat that has floated to the top is drawn off and can be "polished" in disc centrifuges to remove residual water and particles (fines). The water phase (liquor) in the cooker is drained off, then the solid material (greaves) is removed and can be pressed or centrifuged to remove additional liquid (stickwater) before being dried. The liquor and stickwater may be further processed to remove fines, which can be recycled to the greaves, and residual fat. Wet rendering produces good-quality fat (but only if the viscera are cut and washed). However, it requires very long cook times, is very labor intensive, and has significant losses (up to 25% of the solids may be lost in the stickwater). It is energy intensive, although heat can be recovered from the vent steam.
A more modern wet-rendering system is semicontinuous and involves cooking the raw material in a conventional dry-rendering cooker under pressure (to ensure sterilization) for a short time, then processing the cooked material in decanters to separate the liquor from the wet solids (Fig. 3). The meal is dried in continuous driers, and the fat is separated from the liquor in disc centrifuges. Process water is evaporated in multiple-effect evaporators and the concentrate added to the wet solids. The system produces high-quality fat and low-fat meal and uses less energy than conventional wet- or dry-rendering systems. However, capital costs, repair costs, and maintenance costs are high.
Dry Rendering. Both batch and continuous processes exist (Fig. 4). The material is heated in a horizontal, steam-jacketed vessel until most of the water has evaporated. The vessel has an agitator, which also may be steam heated. The evaporated water is usually condensed to recover heat and reduce atmospheric pollution. In batch systems, the raw material can be subjected to 200 to 500 kPa for some specified time to sterilize the material and/or hydrolyze wool and hair. It may take up to 3 h to produce a "dry" material, and the end-point temperature is often 120 to 140°C. The stage at which pressure is applied can influence the ease of further processing. Cooking times in continuous
Table 1. Best-Known and Most-Used Rendering Processes
Cooker temperature, °C
Cooker temperature, °C
Table 1. Best-Known and Most-Used Rendering Processes
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