You might expect to find corn, soybeans and hemp on the shelves of your local organic supermarket, but not usually in a list of car parts.

Increasingly, however, carmakers are using bio-based materials in entry-level economy cars and high-end luxury vehicles. So what's going on?

Green cars

The main uses for these natural ingredients are door cladding, seatback linings, seat bottoms, back cushions, head rests, under floor body panels and flexible tubing.

"Natural fibers derived from wood can be used for package shelves, for example" said Wulf-Peter Schmidt from the Ford plant in Cologne. "We also use less traditional materials such as kenaf, which is a member of the mallow family."

Daimler Chrysler's Mercedes models are equally green, with door paneling that contains a whole range of renewable materials, including flax, sisal and hemp, as well as coconut fibers and latex in the seat upholstery.

"We recently introduced abaca fibers, which come from the Philippines," said press spokeswoman Karin Thiemann. "It's the first time that natural fabrics have been integrated into the exterior of a car -- this time on the mould for the spare tire."

Re-enforcement

Using these natural fibers makes sense in many cases. Those used in production are not used as raw materials but are combined with synthetic components such as polypropylenes to make what are known as natural fiber-enforced composites.

"The fibers give the materials structure and serve primarily as reinforcement," Schmidt said.

The natural fibers are subject to stringent safety controls, said Hans Schwager from BMW.

"We check susceptibility to rupture, reaction to climate change and high temperature -- the same tests we do on other materials," he said.

According to Thiemann from Daimler Chrysler, the tests are even more rigorous than those run on synthetic materials.

"But you have to take into account natural variations," she said, pointing out that while the problem of wear and tear might not apply to glass fiber, it does play a role with natural fibers, as does water-resistance.

Sustainability

The economic and ecological advantages are inextricably linked. Farming abaca, for example, helps create jobs and improve living conditions in the Philippines, while the car manufacturers themselves benefit from weight reduction (and therefore better gas mileage), and a cheaper production process.

"These materials require 60 percent less energy during manufacturing," Thiemann said. Other plus points include biodegradability and the ability to recycle them.

"Waste lands on the compost heap," she said.

Schwager from BMW pointed out that the composite fibers can be ground down and reused, while Schmidt from Ford is also keen to stress ecological expedience.

"While renewable raw materials are being cultivated they absorb carbon dioxide from the atmosphere, which then, as it were, flows into the car," he said. "It's a circular process."

While environmental protection groups welcome these developments, they're keeping a close eye on how the raw materials are farmed. They oppose the felling of rain forests and also want to see car manufacturers uphold corporate responsibility and social standards by refusing to employ children, for example.

But bio-based materials aren't a completely novel idea. In 1941, Henry Ford unveiled the first ever Soybean car. The idea might not have taken off then -- but perhaps it will now.