A cow has tested positive for mad cow disease in America for the first time since 2006, the USDA announced Tuesday. Officials were quick to assure the public that the slaughtered former dairy cow was located at a rendering plant, and that its flesh was never going to enter the human food supply. If you’re not going to eat a dead cow’s meat, what are you supposed to do with it?
Make pet food, floor wax, and explosives, among many other things. Rendering plants take animals or animal parts that are unsuitable for human consumption and separate them into two streams: fat and protein. There are innumerable uses for those basic building blocks. Most of the dry, proteinaceous matter is sprinkled onto livestock feed as a nutritional supplement. (Cattle protein cannot be fed to other cattle due to concerns over mad cow disease, but farmers do feed it to other animals.) As for the liquid fat and oil, some enters the livestock food chain along with the protein—it increases caloric content and reduces the dustiness of plain corn or soy feed. A large portion of the liquids, however, are sold on to refineries that reduce them into chemicals to make crayons, shaving cream, detergent, and a long list of other products. Glycerin, one of the many chemicals that can be derived from cow fat, is an ingredient in dynamite.* In recent years, rendered cow fat has been increasingly used to make biofuels, and researchers are experimenting with adding animal byproducts to concrete and plastics.
Americans produce an astonishing quantity of cow leftovers. U.S. slaughterhouses kill more than 34 million cattle annually, with each individual weighing approximately 1,250 pounds. Humans are only willing to eat 51 percent of a cow or bull’s body, leaving behind 10.5 million tons of hide, hair, hoofs, horns, bones, blood, and glands to deal with. That back-of-the-envelope calculation is likely an underestimate of the total cattle rendering stream, though, because many diseased cattle are discarded and rendered in their entirety. (The animal identified this week seems to have fallen into this category, although there is no indication that it was showing any particular signs of mad cow disease prior to slaughter.)
Leftover cow parts like hooves and hair aren’t worth very much in their whole form, so renderers grind them into a paste or powder and load that into a cooking vessel at a steady rate while 300-degree heat, pressure, and steam break it down. The renderer might add other, non-animal waste products into the cauldron, such as used vegetable oil. Around one-half of the paste is water, which cooks off during this process. The lumpy soup that emerges from the other end of the cooker is then separated into liquid fats and solid proteins, using either a centrifuge or a press.
A small amount of rendered beef ends up in human food. The now notorious “pink slime” that many food chains had been putting into their products is made of fat that has been trimmed from beef and put through the rendering process. The USDA monitors rendered-cow byproducts intended for human consumption more closely than floor-wax-to-be.
While many Americans find the process foul, and some worry about the industry’s safety, renderers argue that their work provides a use for a potentially enormous waste stream. It also lends a small economic boost to ranchers. Cattle byproducts sell for 37 cents per pound (about 13 percent as much as a farmer gets for beef).
Got a question about today’s news? Ask the Explainer.
Explainer thanks Tom Cook of the National Renderers Association.
Correction, April 26, 2012: This article originally stated that glycerin is an ingredient in TNT. It is an ingredient in dynamite. (Return to the corrected sentence.)
Video Explainer: How Many Megabytes Does Your Brain Hold?
This video was produced from an original Explainer by Forrest Wickman. Want more questions answered? You can now watch video Explainers at Slate's News Channel on YouTube.