The Truth About Biodegradable Plastics

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Of the purportedly biodegradable plastics we tested, only Novamont's Mater-Bi (top left), was compostable in typical home compost pile conditions.
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The Mater-Bi shopping bag, pre-compost.

Most of us are aware of how long-lived petroleum-based plastic bags and packaging are?—?we’ve seen the trash along roadsides and in our lakes and oceans. Some new “bioplastics” claim to be “100 percent compostable,” but testing commissioned by MOTHER EARTH NEWS reveals that most of these claims are misleading at best.

Basically, there are two kinds of “composting.” Composting at home usually involves small-scale piles with low temperatures and less-than-optimum humidity. Then there’s large-scale commercial composting, in which materials are shredded, mixed, and maintained at 140 degrees Fahrenheit?—?a much higher temperature than that of typical home compost piles. Many cities compost yard waste, but only a few sites?—?about 100 in the entire nation?—?will also accept these “biodegradable” plastics.

We tested five types of bioplastic bags to see how well they would compost. None of them broke down completely after 25 weeks in home compost conditions (77 degrees). A product from Italian bioplastic manufacturer Novamont came closest to what we would call truly compostable, with a product called Mater-bi. Mater-bi is “made of corn starch, vegetable oil derivatives, and biodegradable synthetic polyesters” In our tests, only Mater-Bi was compostable at typical home compost pile conditions.

Three other brands did fairly well in commercial composting conditions, but they showed little or no degradation in home compost conditions. One type, Oxo-Biodegradable, did not begin to break down even after 25 weeks at 140 degrees.

Most bioplastic products currently being marketed carry incomplete and/or misleading labeling, according to composting expert William Brinton of Woods End Laboratories, who conducted the testing for MOTHER EARTH NEWS. One exception is the packaging developed by Frito-Lay for its Sun Chips. The new bag (which we did not test) proclaims “100 percent compostable” on the front, and on the back it states the bag is made from “90 percent renewable, plant-based materials and it breaks down completely into compost in a hot, active home or industrial compost pile.” We applaud Frito-Lay for its accurate labeling and its ongoing efforts to develop better packaging. The company’s bags may well be more compostable than their competitors’. But what most consumers won’t realize is that most of these bags, at this point, are unlikely to reach a “hot, active home compost pile.”

The bottom line: Most plastic packaging that claims to be “biodegradable” or “compostable” will only partially break down under the conditions typical of most residential compost piles.

Click the following link to read the full biodegradable plastics report from Woods End Laboratories.

Cheryl Long is the editor in chief of MOTHER EARTH NEWS magazine, and a leading advocate for more sustainable lifestyles. She leads a team of editors which produces high quality content that has resulted in MOTHER EARTH NEWS being rated as one North America’s favorite magazines. Long lives on an 8-acre homestead near Topeka, Kan., powered in part by solar panels, where she manages a large organic garden and a small flock of heritage chickens. Prior to taking the helm at MOTHER EARTH NEWS, she was an editor at Organic Gardening magazine for 10 years.Connect with her on.