Livestock have always been more than just slabs of meat. Traditionally, the inedible parts, like beef fat, were rendered to make products like soap and candles. It's a common refrain from sustainability advocates: "We used to use the whole cow," they'll say. Well, it turns out we still do. Rendering is a booming industry worth more than $10 billion today—the only difference is, the products the plants create have changed.

The Farmland Food Plant in Milan, Missouri, for example, uses the inedible parts of hogs—blood, glands, bone, fat, and skin—to produce everything from biofuels and fertilizers to insulin, pet food, and livestock feed. And why not? Americans only consume a little more than half of the meat produced by livestock, leaving the rest for either waste or reuse.

It's not just hogs rendered in factories across the U.S. either. Cows, sheep, and poultry also get the recycling treatment. Cows are particularly productive: Their protein can be used as a nutritional supplement in livestock feed, while their fats and oils can be used for crayons, shaving cream, detergent, and even dynamite (produced from glycerin in the fat). All in all, the rendering means that only about 4 percent of meat product is lost to waste in North America, as NPR pointed out a few months back.

"Rendering is an extremely green process," Jessica Meisinger, a director at the National Renderers Association, tells me in an email. "Rendering takes this extra [product] and uses it rather than sending it to the landfill or composting—both of which allow natural decay to occur which results in large amounts of greenhouse gases."

Plus, Meisinger notes, rendering reuses carbon, as well as nutrients like protein and minerals like phosphorus that would otherwise have to be found from non-renewable sources or additional farming.

But while reusing entire animals does contribute to cutting down the amount of waste from meat production, rendering is by no means a catch-all solution to making meat production sustainable. The process of raising livestock is a pollution source itself, according to the Natural Resources Defense Council, which highlighted livestock farms as an environmental issue area because of the toxic waste it can produce.

Of course, that first step—raising livestock to be used and rendered—is unavoidable if people want to eat meat. And even though meat rendering may be a process that produces biofuels, it's not as clean as it should be, making it another imperfect step in the U.S.'s ever-changing and complex food system.