A shot of bioplastics

Environmentally friendly bioplastics - which are made from chemicals produced by microbes - need to clear two obstacles before they can replace plastics made from oil.

First, researchers need to find inexpensive sources of carbon that the microbes can convert to bioplastic compounds. Second, engineers have to find a way of removing the compounds from the organisms and purifying them.

A University of Arizona graduate student is just finishing a thesis project that could solve the first problem. Once that first problem is cracked, the second should fall fairly easily to some straightforward engineering, says the student's advisor, Mark Riley, associate professor of Agricultural and Biosystems Engineering (ABE).

Luis Alva, an ABE master's student, has found that a recently discovered marine bacterium called Saccharophagus degradans 2-40 can degrade a tough, fibrous waste product generated during tequila manufacturing to produce polyhydroxyalkanoates - bioplastics.

Alva is working with one of Mexico's largest tequila makers, Tequila Herradura, which generates 45 tons of agave waste (bagasse) each day.

Bagasse is the tough plant byproduct that's left after the sugars are extracted from Agave tequilana. It contains cellulose, hemicellulose and lignin. Lignin, gives wood its strength and isn't easily broken down by most microbes.

The agave sugars are used in the distilling process, leaving the bagasse as a waste-product headache for the tequila industry. Currently, bagasse is composted, mixed with clay to make bricks, sun dried to make furniture and packing materials, or fed to animals.

None of these uses produce much income for Mexico's large tequila industry and they're done primarily to prevent huge piles of waste from accumulating. Mexico has lots of tequila manufacturers, some larger than Tequila Herradura, and hundreds of tons of bagasse are produced daily.

"If someone wanted to pick up the bagasse and cart it away, the tequila companies probably would be happy to give it to them," Alva said. "So I'm proposing bagasse as a cheap carbon source for producing bioplastics." A cheap carbon source would make bioplastics more economically competitive with plastics produced from oil.

Although Alva's research is promising on a lab scale, much needs to be done before the process can be scaled up to become commercially viable. "It's a long timeframe before we get to the point where we can handle tons of bagasse," Riley said. "There are still a lot of details that we need to pull together."

Alva has been working on the degradation of bagasse with Tequila Herradura Research Scientist Delia Orozco. He's also been working with Professor Jesus Nungaray at the University of Guadalajara on finding the optimum conditions in which S. degradans 2-40 will produce bioplastics. Two master's students and one PhD student at the University of Guadalajara will continue the work that Alva has started by further studying bioplastic production in microbes.

Riley noted that S. degradans 2-40 is an unusual bacterium that was found about 20 years ago in Chesapeake Bay. "It was downstream from a paper manufacturing and pulp plant," Riley said. "So it probably evolved degrading wood fibres."

The microbe is unusual in that it can degrade the major components of plant cell walls and many other chemicals. It can degrade complex polysaccharides (ICPs) that are produced by a wide variety of plants and animals in its ecosystem, such as marsh grasses, algae and fungi, Alva said. It also can degrade the exoskeletons of insects, zooplankton and crustacean.