Could Cows' Stomachs Hold the Key to Recycling Plastic?

When it comes to climate change, cattle are controversial. Although they account for just 2% of direct greenhouse gas emissions in the United States, they’re the No. 1 agricultural source of greenhouse gases worldwide, according to the University of California, Davis. The reason: flatulence.

Each year, UC Davis reports, a single cow will belch approximately 220 pounds of methane, which dissipates faster than carbon dioxide but is 28 times more potent with regards to global warming. But cows’ digestion isn’t just a cause of climate change. Also, it might be a solution.

So suggests a new study by Austrian researchers published this month in the journal Frontiers in Bioengineering and Biotechnology. Because bacteria in cows’ stomachs already are good at breaking down difficult materials—for example, natural plant polymers like cutin, a waxy, water-repellent substance found in the peels of apples and tomatoes—the researchers theorized that they might also be capable of breaking down synthetic materials like plastic, which is notoriously hard to process and recycle, and which has a chemical structure that’s similar to that of cutin.

To find out if they were right, scientists from the University of Natural Resources and Life Sciences, the Austrian Center of Industrial Biotechnology, and the University of Innsbruck engineered an experiment wherein they treated plastic with microbes from rumen, the first of four compartments in a cow’s stomach. When cows eat, they chew their food only enough to swallow it, at which point it enters the rumen for partial digestion. Once microbes in the rumen have broken it down sufficiently, cows cough the food back up into their mouths, where they chew it completely before swallowing it a second time.

Researchers harvested fresh rumen liquid from an Austrian slaughterhouse and incubated it with samples of three different types of plastic in the form of both powder and film: polyethylene terephthalate (PET), which is the type of plastic that’s used in soda bottles, food packaging, and synthetic fabrics; polyethylene furanoate (PEF), a biodegradable plastic that’s common in compostable plastic bags; and polybutylene adipate terephthalate (PBAT), yet another variety of biodegradable plastic. Within 72 hours, the rumen microbes had begun to break down all three types of plastic in both their powder and film forms, although the powders had degraded further, faster. Given enough time, scientists concluded, rumen microbes should be able to break down all three plastics completely.

In the next phase of their study, researchers plan to identify exactly which microbes in rumen in liquid are responsible for plastic digestion, and what enzymes they produce that facilitate it. If they’re successful, it may be possible to manufacture those enzymes for use in recycling plants and to genetically modify them in order to make them even more effective.

Of course, enzymes also could be harvested directly from rumen liquid. “You can imagine the huge amount of rumen liquid accumulating in slaughterhouses every day—and it’s only waste,” one of the researchers, Dr. Doris Ribitsch of the University of Natural Resources and Life Sciences, told The Guardian, which says Ribitsch’s rumen research is only the latest in a series of efforts to find and commercialize plastic-eating enzymes. Those efforts, however, have typically been laser-focused on recycling PET. The advantage of rumen is that it contains not just one enzyme that can be used to recycle one type of plastic, but many enzymes that could be deployed to recycle many types of plastics.

“Maybe we can find … enzymes that can also degrade polypropylene and polyethylene,” Ribitsch told Live Science.

While no solution compares to simply not creating so much plastic, the scale of the plastic waste problem necessitates a “the more the merrier” approach with regard to recycling solutions: According to The Guardian, more than 8 billion tons of plastic have been produced since the 1950s—which is approximately the same weight as 1 billion elephants.