EarthTalk: How green is corn-based plastic?

Pro: It’s compostable. Con: But only in an industrial composting facility.

Plastics made out of Cargill Dow's corn-resin pellets break down in an industrial compost facility in about 45 days.

Q: What are the environmental pros and cons of corn-based plastic as an alternative to conventional petroleum-based plastic?
Laura McInnes, Glasgow, Scotland

A: Polylactic acid (PLA), a plastic substitute made from fermented plant starch (usually corn), is quickly becoming a popular alternative to traditional petroleum-based plastics.

As more and more countries and states follow the lead of China, Ireland, South Africa, Uganda, and San Francisco in banning plastic grocery bags responsible for so much “white pollution” worldwide, PLA is poised to play a big role as a biodegradable replacement.

Proponents tout the use of PLA – which is technically “carbon neutral,” in that it comes from renewable, carbon-absorbing plants – as another way to reduce emissions of greenhouse gases in a warming world. PLA also does not emit toxic fumes when incinerated.

But critics say that PLA is far from a panacea for dealing with the world’s plastic waste problem. For one thing, although PLA does biodegrade, it does so very slowly. According to Elizabeth Royte, writing in Smithsonian magazine, PLA may well break down into its constituent parts (carbon dioxide and water) within three months in a “controlled composting environment,” that is, an industrial composting facility heated to 140 degrees F. and fed a steady diet of digestive microbes. But it will take far longer in a compost bin or a landfill packed so tightly that no light and little oxygen are available to assist in the process. Analysts estimate that a PLA bottle could take anywhere from 100 to 1,000 years to decompose in a landfill.

Another issue: Because PLA differs from regular plastic, it must be kept separate when recycled, lest it contaminate the recycling stream. Being plant-based, PLA must go to a composting facility, not a recycling facility. And that points up another problem: Only 113 industrial-grade composting facilities exist across the US today.

Another downside is that PLA is typically made from genetically modified corn, at least in the US. The world’s largest producer of PLA is NatureWorks, a subsidiary of Cargill, which is the world’s largest provider of genetically modified corn seed. With increasing demand for corn to make ethanol, let alone PLA, it’s no wonder Cargill and others are experimenting with plant genetics to produce higher yields. But the environmental and human-health impacts of genetic modification are still largely unknown.

PLA has promise as an alternative to conventional plastic once the means of disposal are worked out. For now, unless you’re certain that PLA bottle will end up in an industrial recycling plant, PLA may not be much better than plain old plastic. Meanwhile, consumers might do well by switching to reusable containers: using cloth bags, baskets, and backpacks for grocery shopping (most chains now sell reusable bags for less than a dollar apiece), and refillable bottles for beverages.

Got an environmental question? Write: EarthTalk, c/o E – The Environmental Mag­­azine, Box 5098, Westport, CT 06881. Or:

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