What is Rendering Really?

Updated on October 27, 2021

What is Rendering Really?

Rendering is a process that converts waste animal tissue or by-products into stable, value-added materials. It refers to processing of animal by-products into more useful substances, often the rendering of whole animal fatty tissue into purified fats (lard or tallow) or “tankage” that is used as a protein source in pet food (because “quality” for consumer identification is measured by protein level).

The development of rendering was primarily responsible for the profitable utilization of meat industry by-products, which in turn allowed the development of a massive industrial-scale meat industry that made food more economical for the consumer. Rendering has been carried out for many centuries, primarily for soap and candle making. The earliest rendering was done in a kettle over an open fire. This type of rendering is still done on farms to make lard (pork fat) for food purposes. With the development of steam boilers, it was possible to jacket the kettle to make a higher-grade product and to reduce the danger of fire. A further development came in the nineteenth century with the use of the steam “digester” which was simply a tank used as a pressure cooker in which live steam was injected into the material being rendered. This process was a wet rendering process called “tanking” and was used for both edible and inedible products, although the better grades of edible products were made using the open kettle process. After the material was “tanked”, the free fat was run off, the remaining water (“tank water”) was run into a separate vat, and the solids were removed and dried by both pressing and steam-drying in a jacketed vessel. The tank water was either run into a sewer or it was evaporated to make glue or protein concentrate to add to fertilizer. The solids were used to make fertilizer.

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 1920′s, 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 1960′s, continuous dry processes were introduced by The Dupps Company, 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 1980′s 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.

Materials that for aesthetic or sanitary reasons are not suitable for human food are the feed-stocks for inedible rendering processes. Much of the inedible raw material is rendered using the “dry” method. Most tissue comes from slaughterhouses, but also includes expired meat from grocery stores (including the packaging, which would be too expensive to remove); road kill, the carcasses of euthanised and dead animals from animal shelters, zoos and veterinarians (the tissue further tainted with pentobarbital, the barbiturate used to kill the animal); distiller fermentation waste, rancid restaurant grease and butcher shop trimmings. This material can include the fatty tissue, bones, and offal, or entire carcasses of animals condemned at slaughterhouses, and those that have died on farms, or collapsed in transit, etc., and their unborn fetuses.

The rendering process involves rupturing fat cells, either by heat or enzymatic- and solvent-extraction, and concurrently dries the material and separates the fat from the protein and bone, yields fat commodities, and animal protein meals (meat and bone meal, poultry by-product meal, etc.), which as concentrated protein products can be used to boost stated protein levels of inferior pet foods. Rendering plants also process materials such as slaughterhouse blood (blood meal), feathers, (feather meal) and hair as protein vendible.

Feather meal is a by-product of processing poultry; containing up to 12% nitrogen, it is used in formulated animal feed (for ruminants in “factory farm” environments) and as a source of slow-release / high-nitrogen fertilizer for organic farming. The bio-availability of this nitrogen, however, may be low.

In a pet food industry publication, (“Adding Value to Feathers”), a feather meal manufacturer (GoldMehl) states: “Processed by-products have become an important protein source for the industry …[as] an economical feed ingredient.” The statement continues: “Unprocessed feathers are high in crude protein (90 percent),” but notes that because they are “highly indigestible… feathers have to be processed.”

In order to have bio-availability (making the protein available), feathers need to be broken down to an amino acid level: the feathers are hydrolysed (separating chemical bonds by adding water) during rendering, and then dried for grinding into a powder. Other common methods are prolonged boiling in a strong acid (acid-HVP) or a pancreatic protease enzyme. Then, a palatant is added so that the feathers have “taste”.

Feather meal is mostly insoluble keratin (fibrous protein) with high cystine (an amino acid) content, which places stress on the kidneys: cystine may become concentrated in the urine, leading to the formation of crystals, then, kidney stones.
A 2012 study found caffeine, pharmaceuticals (including acetaminophen and fluoxetine) and residues of fluoroquinolones: broad-spectrum antibiotics banned for use in poultry production by the FDA in 2005. Still, despite that antimicrobials used in poultry production have the potential to bio-accumulate in poultry feathers as a “(toxicity) pathway”, feather meal itself is not directly tested as part of the U.S. Department of Agriculture’s Food Safety and Inspection Service and National Residue Monitoring Program(s).

After a mere seven-day feeding trial, GoldMehl determined that once processed, “digestibility for feather meal” was “comparable to regular poultry meal.” Royal Canin states that they are pursuing use of feather meal for an “An allergenic formula” as well as “worm meal” for pet foods in China: “For us, terms like ‘organic’ are irrelevant,” a company spokesman declares, since “… we take nutritional science to a whole new level;” adding that the matrix for sourcing protein ingredients is contracting: “We’re using something that would otherwise end up in a landfill… [and] it’s incredibly nutritious.”

The USA recycles more than 21 million metric tons annually of highly perishable and noxious organic matter into animal protein meals. The largest markets for these commodities are poultry (livestock) and pet foods / treats, the latter identified as a significant “growth market” by the US National Renderers Association. The cooked ingredients can be the most scandalous: diseased animals, diseased or cancerous animal tissues, and 4-D / downer animals.

The US National Animal Control Association estimated that each year about 5 million pets were shipped to rendering plants and recycled into pet food during the 1990s. Euthanised animals would generally be captioned as meat or bone meal in the ingredient lists.

Meat and bone meal are one of the ingredients US FDA testing determined to be associated with euthanised animals (the detection of pentobarbitol in pet foods suggesting that euthanised shelter animals were being rendered into pet food); and can also include diseased tissues and drug injection sites cut away from healthy slaughtered animals.

Presently, only 4 products: Friskies® Cat Food (Purina), Ol Roy™ Dog Food (Walmart), Kibbles & Bits® (Del Monte), and Pedigree® (Mars) disclosemeat (or beef) and bone meal” as an ingredient on their package labels, and this would not account for industry sales. Thus, it is reasonable to question whether some pet food companies are using meat and bone meal but label it as another ingredient.

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.

Considering the lack of local rendering facilities in South Africa, we have to assume that most of the dry ingredients used in the manufacturing of local pet feed comes from abroad, mainly the United States and China.

Once rendered, can you identify the source of protein?

There is no easy answer. We would need to turn to forensic pathology and Police Sciences for an answer. In forensic casework, it is vital to be able to obtain valuable information from burnt bone fragments to ascertain the identity of the victim. Burnt bones show significant alterations both in physical and in chemical properties, and these could be obstacles to anthropological tests and DNA profiling. Heat increases the difficulties of bone identification, depending on the exposure temperature. Recent progress of DNA analysis techniques is improving its discrimination power and sensitivity on an ongoing basis and now this technique is routinely applied to the identification of skeletal remains.

DNA profiling was expected to be a useful tool for identifying severely burnt bones when morphological tests would fail because of the deformation and fragmentation. However, casework encountered, and studies published on burnt bone DNA typing show the harsh reality of this application. The organic matrix disappears at a comparatively early phase in the burning process, and DNA is no exception. Studies experimentally burnt bovine compact bones at up to a maximum of 250°C using a variety of increments ranging from 10°C to 50°C in each study. The DNA was then extracted and subsequent polymerase chain reaction (PCR) processes targeted nuclear DNA and mitochondrial DNA, both with varied target lengths.

Their results indicated that the DNA consistently failed to be amplified, even at only 210°C for 2 hours and at 200°C for 45 minutes …

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