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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.

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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.

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