Rendering is a process that converts waste animal tissue into stable, value-added materials. Rendering can refer to any processing of animal byproducts into more useful materials, or more narrowly to the rendering of whole animal fatty tissue into purified fats like lard or tallow. Rendering can be carried out on an industrial, farm, or kitchen scale.
The majority of tissue processed comes from slaughterhouses but also includes restaurant grease and butcher shop trimmings, expired meat from grocery stores, the carcasses of euthanized and dead animals from animal shelters, zoos and veterinarians. This material can include the fatty tissue, bones, and offal, as well as entire carcasses of animals condemned at slaughterhouses, and those that have died on farms (deadstock), in transit, etc. The most common animal sources are beef, pork, sheep, and poultry.
The rendering process simultaneously dries the material and separates the fat from the bone and protein. A rendering process yields a fat commodity (yellow grease, choice white grease, bleachable fancy tallow, etc.) and a protein meal (meat & bone meal, poultry byproduct meal, etc.).
An alternative process cooks slaughterhouse offal to produce a thick lumpy stew which is then sold to the pet-food industry to be used principally as tinned cat and dog foods. Such plants are notable for the offensive odour that they can produce and are often sited well away from human habitation.
The pressure tank made possible the development of the Chicago meat industry in the USA, with its huge concentration in one geographic area, because it allowed the economic disposal of by products which would otherwise overwhelm the environment in that area. At first, small companies that sprang up near the packers did the rendering. Later the packers themselves took up the industry once they saw the potential. Gustavus Swift, Nelson Morris, and Lucius Darling were among the early pioneers of the U.S. rendering industry with their personal backing and/or direct participation in the developing rendering industry. Technological innovations came rapidly as the 20th century advanced. Some of these were in the uses for rendered products and others were in the rendering methods themselves. In the 1920s, a batch dry rendering process was invented, in which the material was cooked in horizontal steam-jacketed cylinders that were similar to the fertilizer dryers of the day. Advantages claimed for the dry process were economy in energy use, a better protein yield, faster processing, and fewer obnoxious odours attending the process. Gradually, over the years, the wet "tanking" process was replaced with the dry process, so that by the end of World War II, most rendering installations used the dry process. In the 1960s, continuous dry processes were introduced, one using a variation of the conventional dry cooker and the other making use of a mincing and evaporation process to dry the material and yield the fat. In the 1980s high energy costs popularized the various "wet" continuous processes. These processes were more energy efficient and allowed the re-use of process vapours to pre-heat or dry the materials during the process.
Usually, materials used as raw materials in the rendering process are susceptible to spoilage. However, after rendering, the materials are much more resistant to spoiling. This is due to the application of heat either through cooking in the wet rendering process or the extraction of fluid in the dry rendering process. The fat obtained can be used as low-cost raw material in making grease, animal feed, soap, candles, biodiesel, and as a feed-stock for the chemical industry. Tallow, derived from beef waste, is an important raw material in the steel rolling industry providing the required lubrication when compressing steel sheets. The meat and the bones (which are in a dry, ground state) are converted to what is known as meat and bone meal. For many years meat and bone meal were fed to cattle. This practice is now prohibited in developed countries because it is believed to be the main route for the spread of bovine spongiform encephalopathy (mad-cow disease, BSE), which is also fatal to human beings. Meat and bone meal from cattle is, however, fed to non-ruminant animals and meat and bone meal from non-ruminant animals is fed to cattle in the United States. This may not prove to be a solution to the problem due to the resistant nature of the infectious agent of BSE, a misfolded protein called prion, therefore, even if cattle is fed to non-ruminant animals and vice-versa, it will not prevent BSE from occurring. The underlying cause is that the prion survives within the system of the animal that has been fed with meat and bone meal from different animals including cattle. These animals are then eventually rendered and fed to cattle, which also results in the development of the disease.
In the absence of the rendering industry, the cost of waste disposal of waste animal material would be very high and would place a significant economic and environmental burden on areas involved in industrial scale slaughtering. This cost may manifest itself through the expensive use of sanitary landfills, incinerators and other similar waste disposal techniques without yielding profit directly out of it leading to the incurrence of opportunity costs. Using substitute products to rendering products, may not necessarily prove to be lesser in cost. In spite of the advantages of the rendering process mentioned in this paragraph, it also poses certain economic disadvantages. One is when BSE and its cause has been discovered in the late 20th century. In effect, people were scared off and refrained from eating beef and other kinds of meat. Meat processors, especially beef processors, suffered losses due to a decrease in meat sales and the increase in difficulty in marketing their products, in effect, also causing losses to livestock growers through a decrease in sales. Another is that regular meat inspections discover infected products and animals, which have to be destroyed immediately to prevent further spreading of the disease. The epidemic also affected renderers, as lesser meat processors avail of their services making it difficult for them to allocate their raw materials and that they cannot avail of infected animals and products.
The widespread use of "boxed beef" in which the beef was cut up into consumer portions at the packing plant rather than at the retail level in local butcher shops and markets meant that the fat and meat scrap raw materials for renderers stayed at the packing plants and were rendered there by packer renderers, rather than by the "independent" renderering companies.
The rejection of animal fats by diet-conscious consumers led to a surplus of edible fats and their resultant diversion into soapmaking and oleochemicals, displacing inedible fats and contributing to the market volatility of this commodity.
The rendering industry is one of the oldest recycling industries, and made possible the development of a large food industry. The industry takes what would otherwise be waste materials and makes useful products such as fuels, soaps, rubber, plastics, etc. At the same time, rendering solves what would otherwise be a major disposal problem. As an example, the USA recycles more than 21 million metric tons annually of highly perishable and noxious organic matter. In 2004, the U.S. industry produced over 8 million metric tons of products, of which 1.6 million metric tons were exported.
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