Everything in nature is interconnected. But who would have thought cows’ stomachs would have anything to do with plastic recycling?
Cows and plastic both draw the ire of some environmentalists. Cows belch and fart methane. They are the top source of agricultural greenhouse gases worldwide. For that reason, some environmentalists consider them a climate change menace.
As it turns out, however, cows’ stomachs can potentially mitigate climate change by helping with plastic recycling.
The plastics used in a recent study
Researchers in Austria subjected plastic to the digestive juices in the rumen, the first stomach of cows. CNN reported that they found that they can break down hard-to-recycle plastics used for textiles, bottles, plastic bags, and food packaging
As it turns out, the research tested only three kinds of plastic. Polyethylene terephthalate (PET), the most easily recycled plastic PET is used for bottles and textiles. But not bags or most food containers. Polybutylene adipate terephthalate (PBAT), a biodegradable plastic, makes plastic bags. But not the kind most stores give out. Polyethylene furanoate (PEF), a bio-based plastic, was developed as an alternative to PET.
Polyester (PET plastic) is chemically related to cutin, a natural polymer and component of plant cell walls. Fungi and bacteria who feed on cutin produce cutinases, enzymes that break the polymer down into simpler units.
Earlier studies of plastic-eating bacteria have shown that enzymes made by bacteria degrade synthetic polyesters.
The Austrian researchers decided to investigate cutinases in the rumen of cows to see if they would also degrade polyester.
In nature, plastic only breaks down into microplastics. It is chemically unchanged. Plastic-eating enzymes break it down into simple monomers, which are no longer plastic. These monomers can make new plastic without using petroleum. So biological processes can be part of an advanced plastic recycling technique called depolymerization.
How cows’ digestive system works
In human digestion, we chew and swallow our food. It passes through the esophagus to the stomach where digestion begins, and then to the small intestine, where it continues. We can’t digest complex polymers such as cutin or cellulose.
Cows, on the other hand, represent a kind of animal known as ruminants. Cows’ stomachs are divided into four separate chambers. A cow grasps grass with its tongue and then chews just enough to be able to swallow it.
It enters the first chamber, the rumen. There, anaerobic bacteria produce enzymes that begin to degrade the cutin and cellulose.
All this material passes to the reticulum, which forces it up the esophagus and back into the cow’s mouth as cud. The cow chews the cud more slowly and carefully than it had the grass.
When it swallows the cud, it enters the omasum, and from there, the abomasum, which does the work of our stomach. In that way, cows can thrive on a diet that would not provide nourishment for any mammal besides another ruminant.
What the study discovered
The team obtained rumen fluid from an Austrian slaughterhouse. They soaked powders and films of the three kinds of plastic in it for one to three days. Then they took measurements to see whether the fluid degraded the plastic. It broke down PEF the best but worked on all three plastics.
Then they analyzed the rumen fluid to identify the specific plastic-eating bacteria in it. Pseudomonas and Acinetobacter predominated.
The same team had already patented a sequence of enzymes that breaks down polyester. Each enzyme works on only one polymer. So using multiple enzymes enables recycling blended textiles without first having to separate them into their components.
Rumen fluid is a waste material at slaughterhouses It provides an easy source of cutinases. Using it to recycle plastic turns it from a waste to a resource. Very little of slaughtered animals go to waste. If using cows’ stomachs for plastic recycling proves practical at industrial scale, it will reduce waste even more.
Previous research with plastic-eating bacteria has concerned only PET. Rumen fluid contains many bacteria and many enzymes. Maybe some of them can degrade other plastics such as polyethylene or polypropylene. That will require further research.
According to some estimates, the plastic produced since 1950 weighs about as much as a billion elephants. The best thing for the environment would be to produce less in the first place. Failing that, the more ways we can find to recycle it, the better off we’ll be. Plastic-eating bacteria in cows’ stomachs can join our tool set.
Further avenues of research on cows’ stomachs
Cows digest not only cutin but cellulose. The Austrian team specifically studied how cows’ stomachs could degrade plastic. I suppose other teams are studying it, too. Scientists have long been looking for inexpensive and efficient ways to make biofuels from cellulose.
I have previously reported on a British team’s investigation of gribbles. They’re a marine pest that eats wooden ships and piers. Science in general knows more about cows than gribbles. It could be that rumen fluid can speed production of cellulosic biofuels as well as recycle plastics.
Could cows’ stomachs hold the key to recycling plastic? / Matt Alderton, Treehugger. July 14, 2021
How cows eat grass / Adam I. Orr, US Food & Drug Administration. July 2, 2020
Microbes in cow stomachs can help recycle plastic / Nicoletta Lanese, Live Science. July 2, 2021
If anyone wants to read a more technical description of this topic, here is a link to the original research paper: Together is better: the rumen microbial community as biological toolbox for degradation of synthetic polyesters / Felice Quartinello et al., Frontiers in Bioengineering and Biotechnology. July 2, 2021